The Science of Everything Podcast - Episode 161: Dinosaurs and Other Ancient Reptiles

Episode Date: June 1, 2026

A survey of extinct animal species from the Permian through the Cretaceous periods. We discuss important differences between anapsids, synapsids, and diapsids, and then review diverse species from all groups, including pelycosaurs, therapsids, parareptiles, euryapsids, pterosaurs, pseudosuchia, and dinosaurs. We also consider aspects including bipedal gait, endothermy, gigantism, and the cause of the end Cretaceous mass extinction. Recommended pre-listening is Episode 160: The Evolution of Animals. If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

you're listening to The Science of Everything podcast, episode 161, dinosaurs and other ancient reptiles. I'm your host, James Fodor. So this episode is going to be continuing from our discussion of evolutionary history. So the recommended pre-listening is the prior episode, 160, the evolutionary origins of animals. So in that episode, we discussed the evolutionary development of animals from the Tonian period, around 760 million years ago. go with their split from early single-cellular, single-celled protests, right up until the development of the fissopods, so the first sort of tetrapods that came out of oceans onto land, and we discussed a bit about the early amphibium-like terrestrial animals that exist in

the carboniferous. And we sort of, so we finished up late carboniferous early Permian at the time of the origin of the amniotes. And so in this episode we're going to pick up from there and talk about the amniotes that dominated terrestrial ecosystems from the in the Permian period, as well as in the subsequent Triassic, Jurassic, and Cretaceous periods. So mostly the Mesozoic era, which is those three, but also the Permian, which was the final period of the Paleozoic era. So I'm going to talk about all of the major terrestrial amniote animals that existed at that time. Recall the amniotes are the clade of animals that are more adapted for life on land, so they're less dependent on the water. Having readily access to water, they have

tougher skin, they can live in dry environments. They don't need to lay their eggs in water like amphibians do, so we talked about those adaptations last time. So those are the animals we're going to focus on now. Current day living amniotes include reptiles, mammals, and birds. And so in this episode, we're going to discuss they are ancestors during the very end of the Paleozoic and then throughout the Mesozoic eras. So the title of this episode, obviously we will focus a lot on dinosaurs but I have other animals that I want to discuss too and the title is not quite strictly speaking accurate because while many of the animals that we will discuss are reptiles not all of them technically are the most accurate title that I could

come up with was extinct pre-Senozoic amniotes but I don't think anyone would have known what that meant so other reptiles will have to do and if you'll forgive me for a little bit of inaccuracy there. So to begin with, what I'm going to do is give an overview of the different main classes of animals that we're going to be talking about and how they relate to each other and how they developed over evolutionary time in broad brushstrokes. And then I'm going to go through and talk about some of the diversity within each of the clades of animals. So before we begin, I should comment on classification in biology. I've sort of touched on this in previous episodes, but it will be important here. So the current preferred methodology is to use cladistics.

A clade is defined as a set of animals like over evolutionary time that includes a single common ancestor and then all of the descendants of that ancestor, like all of the subsequent species that evolved from that. So an example of a clade would be mammals, another one is birds. Traditional classification systems weren't as focused on this, particularly they didn't have genetic information to use and didn't have a stronger sense of the importance of sort of evolutionary relationships. This non-cleristic classification methods rely more on morphological similarities or similar niches or other things like this or traits that organisms held in common, even if they're not necessarily evolutionarily related. That leads us then to the definition of a reptile

because traditionally reptiles were defined as snakes, lizards, turtles and crocodiles. And it turns out that those do not form a clade because that does not include all of their ancestors who are now extinct. And in addition, it turns out that birds are actually descended from the same ancestors that give rise to, for example, crocodiles. And, I mean, if you go far back, all of the other reptile groups as well. So if you want to turn traditional group reptilia into a clade, you need to include the birds in that as well, as well as a... wide number of now extinct organisms. So this complicates the discussion a bit because as you may be aware, birds are descended from dinosaurs. We'll discuss this later. So in order to make dinosaurs a clade, you need to include the birds within that. Otherwise, you're not including all of the

descendants and that technically becomes a paraphylitic grouping, not a monophyletic grouping, which is a clade. So the problem with this preference for clodistic classification, then is that sometimes, and in fact, in my reading of the sources quite often, we still want to refer to non-cleristic groupings. A very common one being non-avian dinosaurs. So all of the dinosaurs except the birds, because we often want to talk about mesozoic dinosaurs and not talk about contemporary birds. And so that's a non-cleristic grouping, and so we need to use a sort of an alternative language for that. And so the phrase non-avian dinosaurs is typically used. But you'll see others as well that we will discuss later on.

So one of the reasons I'm talking about this is because although I'll try to highlight when I do this, I do make use of non-cludistic groupings in order to describe various organisms and like classes of organisms. And there are sometimes debates about whether particular groupings are monophyletic or not, such as the parareptiles. We'll get to those later. I'm less concerned about this because what I'm trying to explain is the range of different organisms that existed and how they relate to which other evolution. in broad terms. And for that purpose, it's less important whether, for example, a group like the parereptiles was exactly one clade or might have had multiple common ancestors. That turns out to be a very difficult question to answer in a lot of cases, particularly when we have, we don't have as good fossil records. So I don't necessarily focus as much here on whether establishing

every grouping as being a clade. I'll try to highlight when it's not. But just bear that in mind, because I think it's useful to understand morphological similarities as well and not solely focus on whether a grouping is a descendant from a common ancestor and includes all of those descendants. Okay, so that being said, I'm now going to introduce you to three major groupings of amniotes. So amniotes originated in the late Carboniferous and fairly quickly diversified into three major groupings, particularly after the, you may recall that Carboniferous, the Carboniferous period, terrestrial ecosystems were dominated by these huge rainforests that covered most of Pangaea at this time, and the organisms that dominated those were largely amphibians,

not technically amphibians in the modern sense, but sort of close relatives of those, like ancestors of those and cousins on slightly different branches, but morphologically similar in the sense that they were not amniotes, so they weren't as close as well adapted to living purely on the land. Near the end of the Carboniferous period, there was a worldwide climactic changes we discussed in the previous episode leading to the collapse of many of these rainforests and the proliferation and divergence of many of the amni- species within amniotes.

Amniotes pre-existed the collapse, but they really diversified and sort of took over afterwards and into the Permian period, which followed the Carboniferous. So Lake Carboniferous, we see the divergence of amniotes into three major groupings. This can be broadly defined in terms of features of their skulls that systematically vary. and particularly what we're talking about are the openings or holes on the sides of the skulls. Skulls often fossilize fairly well, it's relative to many other parts of the animal, and there are important sorts of information, particularly with regard to diet, for example. So they're very commonly studied, and they provide a lot of important information.

So classifying animals based on features of their skull makes a lot of sense, and historically that was one of the, I mean, it still is, but particularly historically, that was one of the ways that this was done. The three major groupings of amniotes are based primarily on these openings that are found on the back part, so the posterior part of the sides of their skull. And these are called temporal fenestray. So their openings are on the temples, like on the essentially the sides of the skull. And basically, it comes down to how many of them are there. I should say, if you look at a skull or a picture of a skull, there are quite a few openings.

One of the larger ones you'll see around the center of the side of the skull is the orbit, so that's where the eye. is there's often one for the anterior which is related to the nose and there can be others around the front as well but we're looking around the back the posterior part of the skull so behind the orbit how many openings are there in that location in most cases there are there are none one or two and so this corresponds to then the three major groupings anapsids which have no openings synapsids which have one low opening and diapsids which have two openings one on top of the other. So anapsids, synapsids, and diapsids. So you can associate these three major groupings based on the skull, the temporal thanistrate in the skull, with three fairly well-known

lineages of animals. One are the synapsids, which you can think of as essentially the mammals plus their ancestors. So we are synapsids. In fact, mammals are the only surviving synapsids, but there are other non-mammalian synapsids which we will discuss. So synapsids with one opening at the back of the skull, that's the mammalian lineage. The diapsids are the reptiles. So that includes birds as well as crocodiles and turtles and so forth. So that is the reptilian lineage. And then the anapsids, this is the parareptile lineage.

It's thought that anapsid was the, like having no temporal fenestrade, was the most primitive condition. So amphibians are also anapsids, but in this definition I'm not including them. We're just talking about amniotes here. And so it's thought that the synapsids and then diapses then developed those temporal fenestray subsequently. Oh, I should mention, so parereptiles were important in the Permian and then Jurassic periods, but they are now extinct, so there are no extant parereptiles. That's maybe why you haven't heard of them. There's also, as I said, some dispute as to whether this is a monophyletic grouping.

The sources that I've consulted are indicating that the leading towards that it is monophyletic. I'm not so interested in that for our purposes here. We'll still group them together for convenience. So anapsids, no temporal fenestray, these are the power reptiles, synapsids, one low temporal opening, this is the mammalian lineage, and diapsids, they have two openings, they're the reptile lineage. Now, before I get angry emails, there are some complications here. So I think for a first approach, it's helpful to make that association. It is, unfortunately, as things often are more complicated than this, because

because some of some animals within the diapstid clade have secondarily lost their temporal fenestray. And so are anapsids from a morphological point of view, but not from a classification point of view. So this is why it gets confusing. So in particular, turtles, turtles have an anapst skull, but it's thought that this was acquired secondarily, and they are classified with the diapsids. So turtles are classified with the reptiles, but it's thought that their ancestors did have the two openings, but they've subsequently lost those. So there's not a perfect association between

anapsids and the parareptiles, but it's a useful first approximation to think in this way. But remember, the turtles are the exception. Now there's another exception, which is a group called the uriapsids. So they have one high opening at the back of the skull, distinct from the synapses which have one low opening. And uriapsids are also classified with the reptiles. So again, in this case, it seems they secondarily lost the lower opening, even though they're answering. sisters had two. So unlike the turtles which are still around it, uriapsids are extinct, but they include many of the famous marine reptiles like the Ictheosaurs and Pleasiosaur and placodons. So we'll talk about those a bit later. So the uriapsids are a subcategory

within the diapsids, which have a different pattern of temporal openings. There may be some other exceptions that I'm not aware of, but primarily from a, you know, because it's complicated, from the first brush, we can think of the anapsids with no openings as the power reptiles, synapsids with one as the mammals plus their ancestors, and the diapsids with two openings as the reptiles plus birds and, you know, and their ancestors. So now that we have our three main classes or more our three main clades of animals, we can place them in evolutionary time. So I mentioned that all of these originated around Lake Carboniferous Early Permian and diversified following the rainforest collapse in the Lake Carboniferous.

So the way it happened is that during the Permian era, the synapsids and the parareptiles were dominant. So the species from these groupings played a dominant role in the terrestrial ecosystems. Diapsids or reptiles existed, but they were fairly rare, they were small, they weren't very dominant. At the end of the Permian, there was mass extinction. as I've talked about previously. And most of the parereptiles went extinct.

Most of the early synapsids went extinct, but not all of them, some of them survived. And following the Permian extinction, there was a radiation, diversification of the diapsids, the reptiles. And the Triassic, then, the period following the Permian, represents a kind of a transitional stage, where the reptiles were developing, they were diversifying, they were becoming more prominent, but the parereptiles and the early synapses were still around. So it was this sort of transition as it would turn out.

And then what happened at the end of the Triassic is that there was another extinction, not quite as big as the N-Permin extinction, but still quite a large one. And that end triassic extinction sort of marks the end of this transition period because by the end of the Triassic all of the parapses were extinct. And essentially all of the synapsids also went extinct by that time, with the exception of a small grouping called the synodonts. And these are basically mammals. Technically mammals only evolved in the Jurassic period,

but it's sort of a gradation of increasing accumulation of mammal-like characteristics. And so it sort of depends on exactly where you define the beginning of mammals. For our purposes, we can think of the synodons as pretty close to mammals, certainly by the late Triassic. And the other thing is that the synodons were not very diversified. there weren't very many species during the Mesozoic era in general of synodons. And so they trundled along the mammals and their immediate ancestors during the Mesozoic, like following the Triassic extinction.

But they weren't very diverse and they weren't very dominant in the ecosystem. So the point is the end-Triassic extinction marks the complete extinction of all parereptiles and nearly all the synapses with just the mammals' ancestors just hanging on by a thread there. And so who took over? Well, it was the reptiles. So the reptiles had already begun to diversify during the Triassic, but they just exploded at the end of the Triassic and then during the subsequent Jurassic and Cretaceous period. So that's why we think of the Mesozoic era in general as the time of the dinosaurs, as well as some of the other related reptiles, which we would get to.

Then what happened at the very end of the Cretaceous, there was another mass extinction. That's when the asteroid hit. And that marked the extinction of all of the non-avian dinosaurs, as well. as all of the marine reptiles, the ureapses we talked about, as well as the flying reptiles, the pterosaurs, and significant reductions in the other reptilian groups as well. And then following the Cretaceous mass extinction event, the mammals exploded in diversity. And you see the rapid emergence of many of the most of the major mammalian groupings around the 10 million years or so period immediately after the Cretaceous extinction event. So basically what we have then,

when we look at this whole period from the Permian to the present, is this sort of alternation. Initially, it was the parareptiles and the early synapsids, which were dominant. Reptiles were fairly small. Then there was a transition period during the Triassic, where reptiles were rising and the others were just sort of hanging on. And then at the end of the Triassic, all of the parapses went extinct, and nearly all the synapsids went extinct. There was just a small mammalian lineage that hung on throughout the Jurassic and Cretaceous periods, where the reptiles were dominant. and then at the end of the Cretaceous, many reptiles were wiped out, and those that survived severely reduced, and so then the mammals took over in the Cenozoic era.

So you can think of the Cenozoic as a sort of like a revenge of the synapsids, where they nearly went extinct at the end of the Triassic, and reptiles took over for a couple of hundred million years, but then they sort of came back with a vengeance in the Cenozoic era. And maybe at some point in the future, the reptiles will have their own revenge, and they'll become dominant again, although, So, you know, we'll have to wait and see what happens there. But anyway, that's the broad brushstrokes picture of evolutionary time from the beginning of the Permian down to the present. So that's a 300 million year period. So now what we're going to do is discuss in more detail the different groupings of animals that come under these headings.

So we're going to start with the synapsids that will work through the para-reptiles, and then I'll talk about the major groupings of reptiles that exist, particularly during the Mesozoic. All right. So let's start by talking about the early synapsids. synapsids. So remember, synapsids means that they have one temple fenestray at the lower back of their skull. This is the mammalian lineage, but early synapsids were not mammals. They were more reptile-like, but sort of progressively became more mammal-like over time, as we will

see. The first group of synapsids that diversified and lived during the Permian are traditionally called pelicosaurs. And this group is also sometimes called mammal-like reptiles. but I'm going to call them pelicosaurs. Now that term is problematic as well because it's not a single clade. So there's many outbranchings that came off that split off sort of the synapsid evolutionary tree before you get to the mammalian lineage. And we sort of group all these together and call them pelicosaurs. So they're not monophyletic, but it's still useful because it's handy to have a term to refer to

essentially reptile-like permian non-therapids synapsids, which is. what pelicosaurs essentially is, and so the books still use this, and they just put it in quotes, so I'm going to use it as well. So forgive me for using this older terminology. If you look at these organisms, they're quite morphologically diverse. To the untrained eye, they look similar to a dinosaur, really. They were all quadrupedal, as far as I'm aware. Many of them had these large spine sails on their backs. So you may be familiar with Dimetrodon, which is the probably most well-known of the Pelicosaurs. So it has this very big sail on its back, which is thought to have been probably bright colored. And these sails, we don't entirely know what they are for. They are formed from

elongated vertebral spines that then has flesh that grows between and forms this big sort of sail that sticks up from the animal's back. The two main proposed functions for these are either thermoregulation, so to help regulate the body temperature or to use as a mating display, or possibly to scare off predators or something. We don't know exactly, but we actually find similar sails on some dinosaurs and some other other animals as well. But they're particularly prominent in many of the pelicosaur species. But not all of them had these large sails. Some of them didn't. As I said, if you sort of look at them from an untrained eye, they look like very large lizards. They had a sprawling gate. So this means that they walked on all four, so they're quadrupedal,

but they walked with their legs out to either side and elbows bent. So that's similar to how modern-day, most modern-day lizards and crocodiles walk. So crocodiles probably the closest analog here. We'll talk more about crocodiles in a little bit because crocodiles can actually walk in a somewhat more upright gate, but they often walk using the sprawling gate. And so that is how these early synapsids walked, and that's more of an ancestral gate. So the amphibians also have a similar type of gate. The more upright gate where the, it's called an erect posture, where the legs, instead of being sort of elbows bent out to the other side, the legs are straight underneath the body.

So that type of gait is found in dinosaurs and mammals and some other groups as well, which we'll discuss. So in this sense, the pelicosaurs were more primitive in the sense that they had more traits consistent, they had more traits in common with the earlier, their ancestral forms rather than the later traits. These early synapsids looked quite a lot like reptiles and probably had many sort of reptilian traits and wouldn't be recognized by most people as related to mammals, but they are actually more closely related to mammals than any surviving reptiles.

So we'll then move on and talk about a different grouping which came off, which developed from these early synapsids, called the therapsids. I mentioned those a few minutes ago. So the mammalian heritage is a little bit confusing because in the reptilian case, there are many different groupings that branched off of the early ancestors and that still have. have descendants today. So you've got lizards and snakes, you've got birds, you've got crocodiles, you've got turtles. They all branched off at somewhat different times and have their descendants still alive today. And so it's more of a sort of a bushy structure spreading out in many directions.

The mammalian lineage from, at least from synapsids down to the beginning of the Cenozoic, is much less bushy. It's sort of much more lopsided. So you've got these early synapsid forms, for example, the pelicosaurs, but those all went extinct by the end of the Permian and didn't really give rise to many success. Well, I mean, none that survived the Permian, right? But there is one group of synapsids which did survive the Permian extinction, and they're called Therapsids. And there are a number of different early Therapsid groups, but again, all of them went extinct by the end of the Triassic, and only one offshoot survived, and they're called the synodonts. And the synodons, I mentioned earlier,

that they sort of trundled along during the Jurassic and the Cretaceous periods, not very diverse, not very many species, but they still existed, and eventually developed into mammals, then following the Cretaceous extinction, the end Cretaceous extinction, the mammals then diversified massively, and now we have a very bushy tree of mammals, but prior to that, prior to that diversification, what you have is essentially only, for a long time,

only one line of the synapsid lineage survived. So, you know, there were these early pelicosaurs, but they all went extinct, and it was just the Therapsid line that survived. And then there were some early Therapsid forms, but by the end of the Triassic, they were all extinct, and it was only the cyanadon line that survived. And then from that, only the mammals survived. So it's a bit confusing because you have these sort of progressively nested classifications, but really only like one endpoint, because it's only the mammals that ended up surviving. And many of these other organisms didn't have nothing much branched off them, at least that we know about. Now we're going to talk about the non-mammalian thurapsids, or the early therapsids. So this is a group which includes the mammals, but again, we're looking at the earlier

offshoots here. Therapsids developed around the beginning of the Permian, and so there are a number of forms that lived during the Permian and became extinct at the end of the Permian. So the therapsids lived alongside the pelicasaurs. But they did outlive the pelicosaurs because there were some non-mammalian therapsid groups which continued throughout the Triassic in a reduced form, and then all of the rest of the therapsids, apart from the synodont lineage, became extinct by the end of the triassic. So what were the early therapsids like?

So essentially they represent a kind of a transitional form between reptiles and mammals, as would be expected. So they're quite an interesting group, actually, because, as I said, if you look at the pelicosaur, the pelicosaurs, they look quite reptilian and had many reptilian traits. If you look at the synodonts, they look more or less like contemporary mammals. But the early therapsis are very interesting because they have a combination of two traits. And so you look at them and they look a bit strange because they look like a cross between a mammal and a reptile, which in some sense they are. Although they are more closely related to mammals than they are to reptiles. So some of the morphological changes that we see include changes in the skeletal structure.

So that brings the limbs underneath the body, resulting in a more upright structure. So remember I said the pelicose had a sprawling gait while the early therapsid lineage brought that into a more erect posture, which allowed them to move more efficiently. and this helps them to also be able to breathe at the same time as walking, whereas amphibians and I think also early pelicasaws would have, the way they walked sort of involves contorting their body sort of from side to side, which changes the pressure inside their lungs, which means that they can't breathe separately from walking, they sort of do one or the other, which limits the efficiency with which you can get oxygen and therefore limits how far and fast you can run. The development of this more erect posture allowed what's called for a parasagital gait, which is essentially just the way we think of as running in a mammal, where your legs go side to side underneath the body, rather than sort of contorting your body to the left and to the right, which is more how amphibians run.

This allows for breathing and running at the same time, and therefore much more efficient use of oxygen and longer and faster running. So that required changes in the skeletal structure, which we see with the early therapsids. Another thing that we see is the beginnings of the differentiation of teeth. So reptiles, I'm pretty sure all reptiles today, have essentially a single type of teeth. So they have at least little differentiation. Although of course birds don't have teeth at all, but the rest of them have a single type of teeth.

Whereas mammals, one of the defining characteristics of mammals is that they have differentiated teeth. This is called heterodontism, just like different teeth. We see the development of this in the early therapses that their teeth begin to differentiate into canines, incisors, and moles. Another feature that we see developed in the early therapies was change in the integument. So that's essentially like the skin or scales or outer layer of the organism. So we talked about how one of the important developments with amniotes was the development of either scales or at least tougher skin that is waterproof and prevents the organism from drying out when it's away from water for long periods of time. That's an important development that was needed to be able to operate for long periods of time away from water. So we think that the early reptiles, probably also the parareptiles and the pelicosaurs, the early synapses, probably had scaly skin, like tough, scaly-like skin, kind of like modern reptiles.

The development of mammalian softer skin as well as fur or hair came later, and it's thought that that probably developed, it's thought that that probably evolved during the early Therapsid lineage. Now, the oldest organisms that have unambiguous evidence of fur are synodons from much later on, but it's thought that there probably was some amount of fur or hair, or at least the antecedent forms of these in the early therapies. There's sort of equivocal evidence of this that have some skin impressions, but they may have looked more like elephant skin with a little bit of proto-fur or fuzz in some parts of the organism. So they probably weren't furry all over, but they probably also looked different to like the scaly skin of reptiles.

So the sort of transitional form. The skull was also differently shaped. So the skulls of therapses were generally robust and squarish rather than the more slim form that you tend to have, at least in many of the pelicosaurs or other more reptilian form that has, think of like a crocodile head that's sort of like longer and slender. Mammals have sort of differently shaped skulls. Typically, part of the reason for that is because their jaws are different

and they have the heterodont teeth. So there's changes that evolved in the musculature of the jaws and things like that. You'll see if you look at images that the early therapsids skull shape is just sort of a bit different. I mean, obviously there's a lot of variation among reptiles and their ancestors, but you see some changes there too. So some of the more prominent forms of the early therapsids were the gorgonopsids. So they lived in the Permian, and they're quite an interesting organism. They look a little bit like a dog, but if you see the depiction,

I mean, they're quite large. They'd be like a larger breed of dog. And they don't look quite right for a mammal. They still have sort of a thicker tail. They're lacking ears. The ears are actually a relatively modern evolutionary development, the mammalian ears. They have actually evolved from part of the jaw of reptiles or what were earlier reptiles. And as I said, it doesn't have fur all over. So it looks like a bit of a strange-looking dog, which is like some sort of vaguely reptilian features. Very interesting if you want to look these up. Another similar sort of looking animal is pherosophalia, the name meaning beastheads, because they have these large canines indicating they were probably carnivores, and look a little bit dog-like. But again, without all of the fur and the shape of the body, sort of just isn't quite right from comparison to contemporary mammals. Another one is dynosophalia, which is a group that lived during the Permian. And they are quite diverse. They included herbivores, carnivores, and omnivores. and many of them had thickened skulls with sort of knobs and bony projections on them.

So they look a little bit like weird rhinoceruses in some ways. But again, with the sort of thicker reptilian tail and just not quite fully vertical gait, at least the depictions that I've seen. Anyway, so these are the early therapsids. So they flourished mostly during the Permian. Some of them did survive into the Triassic, but they all went extinct by the end of the Triassic. So the only synapsid lineage that survived the end of the Triassic were the synodonts.

And by the late Triassic early Jurassic, these look quite a lot like mammals. And so these sort of small dog-ish-like-looking proto-mammals continue to exist in the Jurassic and then Cretaceous period before you have the real diversification of mammals, which begins, you know, just at the end of the Cretaceous and then really peaks in the beginning of the Cenozoic, which happened just after the extinction. but we'll talk about that in a future episode where we talk about the evolution of mammals and human evolution. Here we're focusing on the earlier forms. All right, that concludes our discussion of the early synapsids. We're now going to move to talk about the anapsids, or specifically the parareptiles. So this is the second grouping.

So parareptiles maintained more of the sort of primitive characteristics of the ancestral amniote form. So they had very robust, low-slung bodies, so walking closer to the ground, more of that sprawling form. we talked about, and they had no temporal fenestreias we talked about, so no temporal openings. And they were quite dominant in the Permian period. So Permian ecosystems had many power reptiles, and they were fairly morphologically diverse. So one of the most well-known, commonly depicted forms is the Pereasora Morpha. I may not have pronounced that quite right, but it looks a little bit like a large rhino or sort of an elephant, but not as erect as an elephant. again, a bit hard to describe. They still have the sprawling posture, the thicker reptilian tails

and the different skull shape. Another form which looks much more like a contemporary amphibian was called the prokulophonids. So they're quite sprawling and they look similar to like a salamander in many ways. Another very interesting form is mesosaurus, which as far as I know was the first amniote, first amniote grouping to return to a fully aquatic lifestyle. So we've sort of just moved out of the water, of the Devonian seas and evolved into tetrapos and now we've moved away from the water into amniotes. What do we go straight back to the water? So mesosaurus was a fairly small organism and a couple of meters in length and it had a very long tail. Quite scary actually. It looks a little bit like an eel with legs and a very sort of long, narrow jaws. Quite a disturbing

organism if you want to Google that, mesosaurus. It sounds like a dinosaur, but it's not a dinosaur. In fact, it's not even closely related to dinosaurs. So there were a number of these different forms of these parereptiles that existed, and most dominant in the Permian period. So many of the parareptile forms went extinct at the end of the Permian. The mesosaurs, for example, were already extinct by the end of the permian, as well as the periosauromorphs. Procolophonids, the sort of salamander-like ones, continued during the Triassic,

but were extinct by the end of that. So the parareptiles have not been very widely studied, And to be honest, I never even heard of them before beginning research for this series. So they don't get a lot of attention, but they're particularly important during the Permian period. And so I think should include quite a lot of interesting forms there. So it should get a bit more love than I think they do. But that being said, we have now concluded discussion of the parereptiles, or the anapsids. And now we're moving to the diapsids.

So that's the animals with two openings in the temporal fenestray, the back of the skull. And this is the reptilian lineage. So the reptilian lineage diversified substantially at the end of the Permian and then even more at the end of the Triassic. And it was much more diverse during the Mesozoic era than any of the other lineages, as I said, which the parareptiles went extinct completely. And then the synapses almost went extinct with just a small synodont lineage living on and becoming developing into mammals. So to help us to understand this somewhat complicated reptilian lineage, we're going to break it down into several main groups. And all of these groups diversified either at the end of the Permian or just at the beginning of the Triassic. You'll see slightly different reconstructions here, and this is still a bit controversial in terms of working out exactly what order they branched in.

The source that I'm following lists the first branching in the late Permian as the snakes and lizards. So they branched off first. And in some ways, they retained more of the ancestral characteristics of the early reptiles. It seems that the ancestral form for really any of these groups, but including the reptiles, is looked morphologically something like a small lizard. And then they, you know, diversified off from that. I'm not really going to talk about them much in this episode because they're still around, but they were less prominent during the Mesozoic era.

So we're going to focus on the other group things. The other group that I won't too much talk about are the turtles. So they were probably the next to branch off. And as I said before, they are anapsids in the sense that they secondarily lost their temporal fenestray. But they're not grouped with the parareptiles. In fact, I think that there were some earlier hypotheses that they may have been parereptiles, but it's now thought that they are more closely related to the other reptiles and just secondarily lost their temporal fenestray.

So there are some quite interesting Mesozoic forms of turtles as well, like very heavily armored ones and giant ones and things. but we're not going to talk about them too much today just to focus on the forms that don't have any living descendants. We get the reptiles first branching off with the snakes of lizards and then the turtles branch off. We're going to focus on the other lineages that mostly were dominant during the Mesozoic era of the remaining reptiles, which technically fall under a group called Arcalisoria, are split into two groupings of these. So these two groupings are the Uriapsids and the Arcasors. I'm simplifying a little bit there, and you will see slight differences in

different classifications, but the one that I'm using follows this grouping. There are some people who don't think that the Uriapsids, that's the marine reptiles who lost one of their temporal finestre, if you recall. There are some people who don't think that those are a single clade. I'm not too worried about that here, but just bear that in mind, but the source that I'm following does treat them as monophyletic. So there's the split of Uriapsids on the one hand, so that's your pleasiosaurs, Icteasors, and placodons.

And then on the other hand, the Arcasors. So, Archasaur is potentially not a term you've heard before, but Arcosaur is actually a very interesting group. The Arcosa has really dominated the terrestrial landscape throughout the Mesozoic. As I said, by the end of the Triassic, there weren't really many synapsids left and no para-reptiles. There were turtles, but they're mostly marine. And there were snakes and lizards, but they weren't as dominant. There were Uriapsids, but they were also marine, at least most, I think all of them were. So what was dominating the terrestrial landscape?

Well, it was pretty much all archosaurs. Archosaurs still exist today. The living representatives of this clade are birds, as well as crocodiles, including alligators, like the crocodilian life. So you might not have thought that crocodiles and birds are closely related, but relatively speaking they are. They shared a common ancestor approximately at the start of the Triassic, so about 250 million years ago.

Birds, of course, being the only living representatives of dinosaurs. So dinosaurs were archosaurs, and crocodiles branched off. off from dinosaurs around that time about 250 million years ago. So most of what we're going to be talking about for the rest of this episode will be these archosaurs. But first I will talk about the uriapsids, the marine reptiles. So let's go through them and then we'll talk a bit more about the archaurs. The uriapsids, as I said, have a single temporal opening located higher than in the synapsids. It's thought that they secondarily lost the lower one, so they fit still within the diapsi grouping,

even though they only have one opening. most of them were marine reptiles. The most commonly known examples are ichthyosaurs. So these lived in the Lake Triassic through to Lake Cretaceous, so a very long time. These are sort of dolphin-like in body shape with the dorsal fin and the tail, which they beat from side to side in order to propylum cells forward, and the long kind of nose and teeth. So they look quite a bit like a dolphin superficially,

but dolphins are mammals, whereas ichthosaurs are reptiles. So the body shape seems to be convergent evolution, just to adapt to their niche. Then there's also the pleasosaurs, probably the most well-known example of the ancient marine reptiles. So these also lived in the late Triassic to Lake Cretaceous. And these have a broad flat body with a very long neck and a sort of a relatively smaller head. And the Loch Ness monster, quote, the Nessie, is often depicted as looking like a pleasaur, even though, well, first of all, there is no Loch Ness monster. And second of all, even if there was, it couldn't possibly be a pleasiosa or they went extinct over 60 million years ago.

Another form of the marine reptiles are the placodonts. So these lived in the Triassic. These are heavily armored animals that look a bit like turtles, but they have... So a turtle, you can really only see the legs sticking out either side and then the head and a bit of the neck. But the placodons also had sort of a tail that extended out the back of the light shell. And the shell was sort of broader and flatter than you see in many contemporary turtles, at least. So these are heavily armoured and they lived mostly in shallow waters because they were actually, their armor was so heavy that they wouldn't be able to modulate their buoyancy in deeper waters. So they had to stay fairly close to the surface. Their armor helped protect them from predation, but probably meant that it was not as easy for them to maneuver around the seas. And so they went extinct in the end of Triassic extinction. Another group called the Thalatosaws, who also lived during the Triassic. They were quite varied. Many of them had a very long nose, long tail and sort of paddle-like limbs.

but there was quite a bit of variation there. The last group of marine reptiles that I want to talk about, which really dominated in the late Cretaceous, they kind of took over as the primary predator in the Cretaceous oceans from the Pleasiosaurs and the Ectheusaurus, which still existed in the late Cretaceous but were on the decline. So the Mososaurs sort of took over from them. And I'm putting Mosasaws here because all of the other organisms

that we've talked about are marine reptiles. but try not to be confused because mosasaws are not actually eurapsids they're actually squamates so they are closely related to lizards and snakes the groups that i said that i wasn't going to talk about but i will mention them here because mosasaws became extinct at the end of the cretaceous they're quite interesting organisms because they look fairly different to the these souls or the pleasiosaurs they have very large bodies a bit like monitor lizards but much more streamlined nose and jaws unlike the pleasiosaurs who have a long neck and then a a head on the end of that, the mosasors have a head that streamlined as part of the body,

but much more so than even the ichthyosaurs who have that kind of pointed, almost beak as part of their nose. The mosasaws, yeah, they look kind of like a monitor lizard, except their limbs have become flippers and they have big tails. So very sort of scary-looking creatures, and they became the dominant predators at the end of the Cretaceous until they were wiped out by the asteroid, as many other organisms were. So those are the sort of four, there were some others as well, but these are the four sort of biggest groupings of marine reptiles during the Mesozoic. Ictheusaws, Pleasios, Placodons, the armored ones, the Thalotosaurs, then the Mosasors.

Remember that Mososaurs are technically not Uriapsids, I just talk about them here because they're marine reptiles. They're actually more closely related to lizards. But that concludes our discussion of the Uriapsids, and now we're going to talk about the dominant terrestrial grouping during the Mesozoic era, especially the Jurassic and Cretaceous. and that is the archosaurs. So, in my opinion, it is unfortunate that the public consciousness has become most focused on dinosaurs, because that tends to lead to presentations that focus on dinosaurs, even though from a classificatory point of view, like big lizards from the past that are really cool, I think what people are really interested in, isn't the same thing as dinosaurs.

Probably one of the clearest examples is what I mentioned before, Dimetrododon, which is not even a reptile. It looks kind of like a reptile, but it's actually one of these pelicosaurs, these sort of early synapsids, but is often sort of showcased along exhibitions with dinosaurs. So, you know, we can't have everything in one category, but the point is, if we're going to pick a clodistic,

like a monophylletic single clade to focus on, I think it should be the archaurs and not the dinosaurs. Because archaosaurs includes the dinosaurs, it's a broader grouping, but it also includes some really other interesting animals that I think should get more attention as well. So you may be familiar. with the pterosaurs, the big flying reptiles, they're often called flying dinosaurs,

but technically they're not dinosaurs. They are relatively closely related to dinosaurs, and they fall under this grouping of archosaurs. So if we think about archaurs, that includes the dinosaurs and the pterosaurs. In addition, it also includes a third major grouping called pseudosukea. Sudoysukia is a term that's used to refer to crocodiles, like modern-day crocodiles, alligators, plus their ancestors. And there are actually lots of very interesting early forms that existed, mostly during the Triassic. But so these pseudosukians were, they overlapped with the dinosaurs and had quite a lot of interesting morphological forms. So we'll talk about them as well. So I'm kind of a big fan of archosaurs now, and I think that we should

sort of replace dinosaurs of popular culture with archaurs, because we get more interesting organisms than to examine. So first of all, let's talk about the origins of archaurs and some of the early forms. So there are some very interesting organisms here. It seems like the most basal forms of archaurs, so the sort of common ancestor, was what we would today recognize as looking something like a small lizard, though they had longer legs and a longer neck than many lizards do, and somewhat more of an upright gait, it seems. They were still quadrupedal, but like the early therapsids, they seem to have been moving towards a more erect postures. So one of the earliest forms is called Protorosaurus, it's a little bit hard to say, and they grew up to two meters in length.

It was a slender lizard-like animal. It looks a bit like a lizard, but as I said, with longer legs and longer neck. And they lived in the Lake Permian and likely resembled what the earlier archosaurs looked like. Another interesting form called Proterosukids also lived around this time, so late Permian, early Triassic. And they looked a fair bit like modern crocodiles. So you had these early forms which looked something like lizard slash crocodiles, which existed around the late Permian and they survived in the early Triassic. It's thought that one of the reasons that the archosaurs became so dominant during the Triassic is because the climate became relatively drier during the triassic. And the archosaurs were particularly well suited to a dry climate.

Archosaurs have scaly skin. Think about particularly a crocodile. Although many crocodiles today are marine, the fact that they're able to, that they're answer. ancestors who had the capabilities of living outside of water for long periods of time and were able to prevent themselves from losing moisture. Another factor that was relevant was the fact that they had uric acid-based urine system, so they excreted a lot less water with their urine than we do, which than we as mammals do, we have a urea-based system. So these factors helped them to be more resilient to dry climates and a thought to have contributed to why they become so dominant during

the Triassic. So all of these early Arcosaur forms were quadruly. quadrupedal and carnivorous. The morphological forms that they had were generally retained in the crocodile line. So these early forms probably, as I said, they looked somewhat across between like a modern crocodile and a modern lizard. But we did see substantial variation on that theme within groups that spawned off from this, so particularly the pterosaurs, dinosaurs, and also within the pseudosukins as well. So terrosaurs are extinct. There are no surviving terosaurs. They are flying reptiles, which are all warm-blooded. That's another important trait that many of these archosaurs developed was being warm-blooded or endothermic is the term. So they lived during the

Mesozoic, so they emerged in the Lake Triassic and became extinct at the end of the Cretaceous. Terosaurs are the earliest known vertebrates to have evolved powered flight. So their wings were formed from a thick membrane of skin and muscle and other tissues. It stretched from their ankles up to a lengthened fourth finger. So if you look at depictions of their skeleton, It's a little bit disturbing, actually, because it looks like that they have their arms outstretched and that the wing membrane extends from their ankles out to the sort of the end of their arms, which it does, but the thing is most of the length of their arms is actually an extremely elongated fourth finger. So it's not like that they have their arms stretched out like a person would.

It's mostly that they just have their fingers extended. And so most of that wing membrane is actually just extending out. to the finger. It's thought that they on land walked quadrupedally, but in a sort of erect posture, it's a little bit hard to describe, but they had fairly short and stubby hind limbs, which it's thought they could walk on. And then they walked on, they walked essentially on their hands as the forelimbs, but because of the way that their forelims were much longer than their hind limbs, and so they had a sort of an erect posture, a little bit like a giraffe. The pterosaurs also had very long necks and very long beaks.

And so when they were walking, they looked a little bit like a giraffe, except that their ribcage of a giraffe is roughly parallel with the ground, whereas it's thought that in many of the pterosaurs, the ribcage was sort of angled. So it wasn't fully upright, like a human ribcage is perpendicular to the ground when we stand upright, which is quite unusual. I think the only other extant animals that have that fully erect posture are macropos, like the kangaroos when they're hopping they stand with their ribcages essentially perpendicular to the ground. Most other animals, including birds, which although bipedals, still have their ribcages like roughly parallel to the ground or slightly angled.

So it seems that the pterosaurs were sort of in between. So they didn't have a fully erect posture, but they were much more erect than most other animals. The thing is that they were still quadrupedal because they used their forelims to walk. but because their fourth finger was so long, their finger was sort of tucked back and behind them, which meant that their wings tucked back sort of under their forearm and then backwards. So a very interesting morphology there. It's not known how flight was originally evolved or exactly why. The two main hypotheses are ground up or tree down.

According to the ground up hypothesis, they would have started by, with a running start, at least evolutionarily, the earliest forms had a running start and then developed the, developed wings as a way to sort of glide for progressively longer distances. Initially, sort of helping them with their running and then they could glide for short distances. The tree-down hypothesis is that they originally had a vertical vantage point, like they climbed up a tree or a cliff or something, and then jumped off that and used it for gliding down to the ground, and then gradually they could glide for longer, and then they developed powered flight from that. So it's sort of not clear which of these was the evolution, was the origin of flight. I think there's a similar debate for birds as well. But it's important to remember that pterosaurs and birds are not closely related. They developed powered flight completely separately from each other.

Same for bats as well. Terosaurs varied significantly in their size, so there were some quite small ones, but many of them, particularly the ones that get a lot of attention, were truly enormous. So the very largest known terosaur is Ketzelchorticalis, which could attain estimated wingspans of about 10 meters across, and probably weighed about 200 kilograms. When walking, they would have been about as tall as a giraffe. I think that's about seven meters or so.

So absolutely enormous organisms. And there's still some controversy I understand about exactly how they flew because it seems that they probably weren't quite as efficient in some ways as birds. But they certainly were capable of power flight and spent a lot of their time flying. Many pterosaurs were thought to be surface feeding fish eaters, so that flying along the surface of the ocean or a lake, and then gulping at fish as they see them. However, it's also known that there were other terrestrial carnivorous forms of

pterosaurs that would have eaten probably small lizards or other organisms on the land. And there were also insectivore varieties as well. There were many different species of pterosaurs. I'm not going to discuss the different forms here, partly because morphologically they're all relatively similar, at least as far as they are very distinctive from the other archaurs that we're talking about. internally they didn't differ as much as they different from others.

So I thought I'd just focus on their morphological distinctiveness from other types of archaurs. So that's all I have to say about pterosaurs. Now I'm going to move on to talk about the third main branch of the archaurs, which is the pseudosukia. And this is a branch that gets very little attention. So pseudosukea means false. Soosukea means false Sobeck. And sobeck is an Egyptian god that was depicted with a crocodile face. So it sort of means false crocodile.

And although there's other terms that mean that as well. But anyway, the point is that these are the crocodiles or the crocodilian lineage plus their immediate ancestors up to the point of divergence with the dinosaurs. So we can call these the crocodilian-line archosaurs, as distinct from the bird-line archaurs, which are the dinosaurs and then the pterosaurs. It's thought that the basal form of, so the earliest forms of the pseudosukea were probably endothermic, just like the terosaurs, and as we'll see later, just like the dinosaurs. This endothermia was then subsequently lost in crocodilians, which today we know are ectothermic, so cold-blooded, when the crocodilians moved into the oceans.

Maybe we would tend to think of the pseudocene as more crocodile-like, but in many ways they seem to have been more dinosaur-like. Modern-day crocodiles almost seem to represent kind of a reversion to some of the more ancestral traits of the earliest form of archosaurs, so more of the sprawling gait, ecothermy, less active lifestyle compared to, say, the dinosaurs. So we think that many of the pseudosukeans were indeed endothermic, and many of them had more erect posture. So the posture and gait of the pseudosukins was actually quite varied. So some of them were quadrupeds, so walking on all fours, although in many cases more erect than a crocodile. So sort of imagine a crocodile, but walking, instead of in a sprawling fashion, walking with its legs sort of upright underneath the body. So crocodiles are actually capable of walking like that for short periods of time, but often they don't. I don't know if it uses more energy.

I guess it probably does. But the point is that many of these early Pseukians seem to have mostly walked like that. So for some of these forms, imagine somewhat like a contemporary crocodile, but more upright and more active. Crocodiles tend to walk fairly slowly. They spend a lot of their time basking, and they only move for very short periods when they're just grabbing prey or running after them. But for very short sprints, they're not capable of sustained exertion. So many of these early pseudo-sukin forms probably were much more active animals. So, as I said, many of them were quadrupedal, but some of them were facultative bipeds, such as the Ornitha Succas, which were capable of walking on their hind legs, but also capable of walking on all fours.

And they had very distinctive downturned snouts. So they look a bit funny, if you look at their image of them. They look a bit like a crocodile that's sort of standing up on its hind legs and looking a bit puzzled, at least to my eye. So those are the Ornithasukisids. Another interesting form of the Etosaurus forms, which again looks somewhat like a crocodile, but very heavily armored on the back, and they

have a lot of plates along their back and also tail, and very robust hind limbs. There's the Epetesukids, which were fairly lightly armored and a little bit sort of slimmer in build, almost more lizard-like than crocodile-like. But they also did have some armored plates along the tail

and along their back. So another important clade of the pseudosukians are the Popasaurids, or Popasaurids, I'm not sure how to pronounce it, and they were very morphologically diverse and also lasted a very long time. They became extinct, as I think all of the pseudosukians, other than the crocodilian line, became extinct at the end of the Triassic,

which is possibly one reason they don't get as much attention because they only overlapped with the dinosaurs for a short amount of time in the late Triassic, and they didn't live during the time when most of the most famous dinosaurs lived during the Jurassic and Cretaceous. But these poposaurus were particularly interesting. So they're very morphologically diverse. I'll just mention a few interesting forms here that showcases the diversity of these pseudosukians.

Arizona Soros had a large back sale, so a bit similar to Dometrodon, not quite as large. But that was also thought to be used for thermoregulation and or for mate signaling. Pupposaurus was an obligate biped biped that looks a lot like a theropod dinosaur. In fact, if you showed nearly anyone an image of the Proposaurus, they would assume it was a dinosaur. It looks like a theropod dinosaur, that's your sort of T-Rex, your carnivorous line. It's almost indistinguishable to a layperson, and it's sort of uncanny because it evolved entirely independently. And then you have Lhotosaurus, which was a very heavily built herbivore with a very sort of long, thin sail along its back that looks a little bit like a pelicosaur, but the sail is not nearly as tall. It also has a very long and fat tail and quite a small head.

So it looks more like one of these early pelicosaurs or even a bit like a parareptile. And then there was a form called effigia or ephigia. Not sure about all these pronunciations. It was also a biped but had a very long neck and sort of a toothless beaked skull and looked a bit like some of the ornithopod dinosaurs, which we'll talk about later. So it's very interesting to me that within this one clade of the superiors, pseudosukians, you have bipeds, you have quadrupeds, you have animals that are looking like crocodiles, you have animals that are looking like pelicasaurs, you have animals that are looking

like theropod dinosaurs, animals that are looking like an or nethician dinosaurs. And so there's huge diversity even within the pseudusukians. And so many of the forms that we would subsequently see within the dinosaurs, as well as some others, are already presaged within the pseudosukians. So I think it's a shame that they don't get more attention, especially because they seem to have been endothermic, and many of them had bipedal or other more advanced forms of like erect gates. And so they were probably quite active animals and had a lot of interesting behaviors. But they haven't received nearly as much as attention as there are dinosaur cousins. So, you know, just like the parropyls, I think we need to give a bit more love to the pseudosukians.

And that's another reason why I think that we should shift our focus from dinosaurs specifically to the broader group of archa-sores, because then we can, in addition to already including the pterosaurs, which is nice because a lot of people think they're dinosaurs anyway. we can also then look at all of these cool pseudosukyin forms and include them in our popular coverage. But all right, so it's time to come to the group that obviously is most well known and sort of gives the episode its name, the dinosaurs. I'm sorry if you feel a bit short change that we're about an hour in to an episode allegedly about dinosaurs and I'm only just now talking about them. But it's important to give the context as to how dinosaurs are distinct from other groups that existed at the time because there were many other reptiles. Kilean forms that superficially resemble dinosaurs, especially some of the pseudosukians,

and that are often confused for dinosaurs, like pterosaurs, for example. And so we need to distinguish those and explain their evolutionary origins and what makes something a dinosaur. So dinosaur are one of the three major groupings that split off from the early archaurs during the Triassic, so the other two being the pterosaurs and the pseudosukians. So dinosaurs are a very diverse group. They diversified massively, especially after the Triassic in Triassic extinction. and they were the dominant form of archosaurs on land during the Jurassic and the Cretaceous periods. It depends if you count the pterosaurs as terrestrial.

I mean, they were mostly flying, but they did obviously walk on land as well. But the pseudosukians, apart from the small crocodilian lineage, all went extinct by the end of the Triassic, and so as well as the earlier Arcosaw form. So it was mostly the dinosaurs that were carrying on the Arcosaur tradition, although there was still the terrorsorsesars too, and the surviving crocodilian line. But in terms of the specific definition, so dinosaurs, dinosaurs are defined as being the group consisting of the most recent common ancestor of modern birds and triceratops as well as all the descendants of that common ancestor. This might seem

like a bit of a strange definition, like why triceratops? It's just that it sort of turns out that triceratops is one of the most distantly related forms of animals that were traditionally regarded as dinosaurs based on the early studies, based on some of the very earliest paleontologists like Richard Owen, he's the one who coined the word dinosaur, but specifically the organisms that he studied. If you then try to construct a clade from those, you get something like a dinosaur. And there's a little bit of debate about exactly which organisms fit into that in terms of some of the very earliest forms. But pretty much, it's why they agreed upon that if you take modern birds and triceratops and then go to their common ancestor, everything descended from that is a dinosaur, well, that's defined as what a dinosaur is.

and that encompasses sort of all of the typical things you think of as dinosaurs, you know, like the sagasaurus and the sauropause and the T-Rex and all those other things. So that's the technical definition of dinosaurs that's used today. There have been some proposals to modify that, but I'm going to stick with that for our purposes here. So there are a number of traits that distinguishes dinosaurs from their arcosaur cousins. So dinosaurs, as I said, the basal form of dinosaurs was bipedal. So this is similar to pterosaurs, but differentiates them from pseudosukians, where it seems the basal form of those was quadrupedal forms, although there were bipedal forms that subsequently developed.

But the earliest dinosaurs were all bipeds. Subsequently, a number of dinosaur groupings returned to quadrupedalism, but the earliest forms were bipeds. They also had a distinct neck, which is sort of an S-shaped. So, again, not all dinosaurs retained this form, but at least the earliest dinosaur forms had. So their rib cage was roughly parallel to the ground, but their neck extends somewhat upright or maybe like a 45 degree angle. And then their head is sort of relatively elevated. And their tail then extends. They have a relatively long tail that extends backwards, which sort of helps balance them.

So this is relatively similar to the posture of birds. Obviously birds are dinosaurs, so that's not surprising. But quite different to the posture of mammalian bipeds, like macropods, like kangaroos, like kangaroos as well. well as humans and other primates that have a biped posture. We don't have, so when we stand erect, our ribcage is roughly perpendicular to the ground, and our neck is extends straight upwards from our torso. Whereas for a dinosaur, it's very different. As I said, their ribcage is parallel, roughly parallel to the ground.

Their face is elevated by the fact that their neck bends backwards, is this kind of S-shaped that then elevates their head above the rest of their body. So this sort of general posture of the reduced four limbs, the bipedal posture, the horizontal stance, but with the S-shaped neck is very characteristic of dinosaurs. Although, as I said, not exclusively because there were some pseudosukian forms like the Popasaurus, which also had this similar morphology. But that's apparently convergent evolution. They're not actually dinosaurs. Now, what I've been saying applies to the primarily to the bipedal dinosaurs, including the earliest ancestral forms, but also the theropod dinosaurs, so like the T-Rex. There are also many quadrupedal dinosaurs which still have the kind of S-shaped neck,

although sometimes in a reduced form, depending on the exact morphology. But in that case, the four limbs are relatively elongated so that they are now standing on four limbs instead of just two. But interestingly, in some of the, in some dinosaurs like Stegosaurus, I think that this is most clear. Their front limbs are still noticeably shorter than their hind limbs, even though they are quadrupedal organisms. And because of that, they sort of have this, they sort of have this, they sort of of look like they're angled forwards, like their head is angled downwards relative to their body, and they carry their tail pointing relatively upwards. This seems to be partly a reminence of the fact that they descended from bipedal organisms, and in some cases their forelimbs didn't

grow as long as their high limbs, and so they sort of retained this sort of asymmetric posture. I mentioned before that there was some controversy about this in decades past, but it's now firmly established that dinosaurs were endothermic, so they're like birds, they are warm, or were warm blooded, which means they regulated their, they had a relatively active metabolism, they regulated their body temperature to be above ambient levels, and they would have been quite active animals. So not like crocodiles, which are fairly slow and don't move very much, they would have been much more like birds. And so in some ways more behaviorally interesting. One of the earliest dinosaurs is called platyosaurus. It probably resembles some of the basal forms

of the dinosaur form. So it was a biped, but had relatively, longer forelimbs, certainly than a T-rex. The T-rex, the four limbs are very, very reduced. Plataosaurus is sort of like in between. So if you look at a diagram, it's four-lims weren't long enough to touch the ground, but they were like three-quarters of the way there, and then they have this characteristic S-shape of their head and very long tail that balances them out. So that's probably what some of the earliest dinosaurs looked like. When dinosaurs first appeared in the mid-Triassic, they were not the dominant terrestrial animals. The most dominant terrestrial animals at the time were some of the residual therapsids, some of the residual

parereptiles, as well as pseudosukians, which were dominant during the Triassic. However, all of these forms, the non-cococodalian pseudoscians, all of the parareptiles, and all of the essentially non-mammalian therapses all went extinct at the end of the Triassic. And so although dinosaurs did exist during the late Triassic, they weren't that dominant, and they really took off and diversified and kind of took over the

terrestrial landscape during the early during the early Jurassic period. It was during this time around the late Triassic that dinosaurs diverged into two primary branches, and this still forms the primary organizing feature of understanding the different types of dinosaurs. These are the Soricians and the Ornificians. This distinction was identified very early in studying dinosaurs, and it's based on the shape of their pelvis. So Soricians have a very differently shaped pelvis to the Ornificians. But as luck would have it, it's different in a very consistent.

confusing way. Ornithician actually means like bird-shaped or bird-like. And it's because the ornithician dinosaurs have a pelvis, which is shaped similarly to a bird. Specifically, the pubis bone is directed forwards, rather than backwards, which the Sorrishian dinosaurs have. Now, the confusing thing about that is that birds are Sorishian dinosaurs, not Ornithician dinosaurs. So the dinosaurs that are named after having a bird-like pelvis are not the dinosaurs that gave rise to the birds it's actually the other branch of dinosaurs it would be so convenient if it was if it was sort of fit neatly but unfortunately evolution has this weird sense of humor it has to make things confusing so sorrition dinosaurs have a non-bird-like pelvis even though birds did come from

surrition dinosaurs and then subsequently evolved a pelvis that resembled the other type of dinosaurs which have a pelvis that's named after birds so this these names were decided long before we knew that birds were dinosaurs, so perhaps in retrospect not the wisest choices, but we're stuck with it. So these are the two main branches of dinosaurs, Sorician and Ornithician. Although there are many sub-brances of both of these, there are only two that I'm going to focus on here. Sooricians in turn divided into two further main groups, sauropods and theropods. And this is an important group because morphologically they're quite different. So sauropods are your big quadrupedal herbivores.

As far as I know all sauropods were herbivores, maybe some of them were omnivores, but but as far as I know, they were all plant eaters. And this includes all of the sort of giant, very long-necked, enormous animals that I'm sure you've seen depicted a million times. So well-known genre include Apatosaurus, brachiosaurus, brontosaurus, diplodocus, and a newer discovered form called Argentinosaurus, which is thought to have possibly been the largest,

at least it's the largest well-documented land animal to ever live. these larger sauropods achieved lengths of about 30 meters long and weighed up to 80 tons. Argentinosaurus is part of a broader group called Titanosaurs, and they were the very largest forms of these sauropod dinosaurs, which lived at the very end of the Cretaceous. They sort of got progressively larger and larger over the Cretaceous, until by the very end we had these just titanically large forms, like the Argentinosaurus, which were up to 80 tons.

And by comparison, an African elephant, which is the largest extantrestrial life form, is about three meters long and seven tons. So these sauropods were 10 times longer and 10 times heavier than an African elephant, which are already very large. So that gives you a sense of just how enormous they were. The legs of these fully grown sauropods, the larger forms, the leg alone was much taller than a human. So we wouldn't have, if the humans stood underneath and reached their hands upwards, they may have just been able to touch the belly of these large sauropods. Again, I'm talking about the full-grown forms, obviously the juveniles are smaller. And one of the main reasons that they became so large, it thought, was because there was an advantage to reduce the risk or avoid theropod predation. Because the sauropods are these huge plant-eating dinosaurs.

at the same time also developed the theropods, which are your carnivorous bipedal forms. So most well-known forms of that are your T-Rex, but there were many other forms that were sort of similar as well. So you had these two groupings, your sauropods and your theropods, and they were evolving alongside each other. The theropods were trying to evolve to be, well, I say trying to. The theropods were evolving to better predate on the sauropods, and the sorropods were evolving to better avoid theropod predation. So becoming enormously large is helpful.

it's just incredibly difficult for a much smaller animal to take on something that's so large as these, you know, huge sauropods, especially when they had these long, powerful necks and tails that they could use to bat and hit away the theropods that came after them. The sauropods eat plants, and they have to spend pretty much their entire day just eating in order to get enough food in order to sustain their huge body weight. Remember, they're endothermic, so they have to maintain that high temperature as well, and that takes a lot of energy. there were a lot of adaptations that they developed in order to maintain this large size because basically the point of the large size is to avoid predation. But the cost of the large size and the reason that other organisms don't use the strategy as well is that it's just so hard to eat enough in order to maintain that body size.

So they developed many mechanisms to offset this challenge, including very long necks that could move up and down. And so they could just strip all of the vegetation from an area at any height. So they can reach the tops of the trees and right down to the, like bushes and low-lying branches as well because their necks were so long and flexible. So like a giraffe, it's maybe a little bit harder for them to reach low down, but these very long necks sauropods had these necks that allowed them to move up and down vertically and also side to side.

And another advantage to that is that it meant that the animal didn't have to move around very much. It just moved its neck. It didn't have to move its enormous body so much. So they basically just like, imagine a bus just parking next to a bunch of vegetation. And then the neck just sort of moves around. and just like sucks down all the vegetation that it could possibly reach before needing to move on somewhere else. And that's something like how these things fed. And I say suck down because that's almost how they fed.

So they had teeth that they used to rip off like the leaves of or other vegetation that they're eating. But it seems like that they didn't chew their food or chewed it very little. The evidence is that they swallowed most of their food pretty much whole, and then their complex digestive systems progressively essentially fermented it and digested it. So that was another thing that saved energy rather than having to chew. I think it's not just an energy thing, but chewing takes time. And I know that many modern, like mammalian grazing animals just take a lot of time chewing their food. I mean, they have multiple stomachs as well that help with this.

But it seems that these large sauropods just sort of gave up on the chewing process. Probably this allowed them just to eat much more quickly because if they had to chew all their food, it wouldn't have been enough time for them to get enough calories in. So they just swallowed it whole. and then it was digested in their stomachs where they could digest more at once and therefore sort of be more efficient. So these long necks and large stomachs that could digest all of their food without having to chew it were some of the adaptations that allowed them to be so enormous. Another advantage of being so large is that it's actually help, it's actually easier to maintain

a higher body temperature. Basically, the smaller you are, the smaller your volume is relative to surface area. And so you lose heat much more quickly to the environment. That's why some of the highest metabolic rates are very small birds and very small mammals, because they lose so much heat. They just need to replenish it with a very high metabolic rate. The bigger you get, you can maintain that same temperature with less caloric expenditure because you lose less energy proportional to your volume, because your surface area increases with the square of your size, whereas the volume increases with the cube of your size. So that's another benefit that they had from being very large, is that they were actually more efficient metabolically. These sauropods are almost certainly the largest land animals to have ever lived. It's even possible that some of them could have been the largest animals to ever live, period.

And the main competition there are contemporary whales, particularly blue whales. So blue whales get up to about 200 tons. If you look at claims of the larger sauropods, there are claims of sauropods well over 100 tons, even I've seen 150 tons. The problem with this is that sauropod skeletons fossilize very poorly. I'm not entirely sure why this is, possibly just because they're so huge. So in many of these cases, all we have is like a single vertebrae or a single femur, for example. And we have to extrapolate from that the size and mass of the whole organism. And that's very difficult to do.

And there have actually been some cases, at least one or two fossil findings from India that were described, and then the subsequent, the fossil itself was lost. So all we have is some descriptions and photographs of them of a single bone, and from that we have to reconstruct the size of the organism. And unfortunately, there is a tendency to exaggerate the size when you're making these extrapolations, because obviously you're going to get more attention if you claim to have the largest sauropod to ever have existed. So you will see some claims of sizes of sauropods of like the 100 to 150 ton range. But those are disputed and not well described. From what I can find, the Argentinosaurus up to 80 tons, metric tons, is the larger sort of relatively well-documented sauropod. However, the fact that they didn't, that soropods tend not to fossilize very well, and fossils are rare to begin with, means that it's almost certain that there were larger animals, not just larger species, but also larger individuals within that species.

Obviously there's variation within a species. So I suspect that there probably were soropods that were over 100 tons, maybe even up to 150 tons, but we don't really know. Hopefully there'll be further finds in the future, which will clarify this point. But so it's even possible that the very largest individual sauropods at the very largest species might have rivaled blue whales in terms of mass. But that is a bit of speculation. We don't know if they got quite that large. So that's the sauropods. Now I mentioned a bit about the theropods, and I'll just talk a little bit more about them briefly here. here. The theropods are probably some of the best known dinosaurs because these are the biped

carnivorous form, which includes the T-Rex, Megalosaurus, and Allosaurus. But also within this broad grouping are a somewhat less a known group called Comsignathids. And these are quite small carnivores, I think roughly the size of a chicken. They were bipedal, but much, much smaller than the like the T-Rex or the Allosaurus forms. Another important grouping within the Theropods, are the maniraptors, mani raptora. So this is the group that gave rise to the birds, and also includes some other well-known dinosaurs, such as Phyllociraptor and Dynonicas.

On this point, I want to say a little bit about the Mani Raptor lineage. So as I said, this gave rise to birds. Birds evolved, it seems, at the end of the Jurassic early Cretaceous. So there were some interesting transitional forms, like Archaeopteryx is fairly well known, but there were quite a few others as well, which have progressively more bird-like features, but birds proper only originated sometime around the early to mid-Cretaceous, early to mid-Cretaceous.

And there were some of these other sort of more transitional forms then became extinct, and all of the other non-avian dinosaurs became extinct at the end of the Cretaceous. So by the mid-Cretaceous period, birds were already fairly distinct, like morphologically from the other dinosaurs. You know, they looked fairly different, and they had different niches, and that's probably why they're able to survive when all of the other dinosaurs became extinct. But the other forms of the Mani Raptor, the lineage that gave rise to birds, still had many bird-like characteristics, and it's now thought that, it was now known that definitely all of the Mani Raptor forms

had feathers, and many of them had quite a lot of feathers, and in some ways looked like a cross between a dinosaur and a bird, which is in a sense what they were. And in fact, that leads me to talk about the velociraptor, which possibly is the best known dinosaur behind T-Rex, obviously. The main reason that velociraptors are so well-known is because of the Jurassic Park movies, where they are prominently depicted. So I wanted to comment on that. Velociraptors, as they depicted in that movie, it was already a little bit outdated. I think it was 94, the movie came out. It was already a bit outdated then, but it's definitely outdated now. First of all, in the film the velociraptors are depicted as very scaly and no feathers at all, whereas we now

know that velociraptors had quite a lot of feathers and almost looked bird-like. The other thing is that the velociraptors had feathered tails, which they extended back behind them, and they held more erect than I think is generally shown in the movies. They show them as sort of like flopping their tails around more, but whereas the form that you see depicted in the modern illustrations of the velociraptors. It's more of a sort of a stiffer feathered tail. The other thing is that the velociraptors in the movie are shown as being sort of human, like adult human size. In fact, velociraptors were much smaller than that. They're about the size of a turkey. And actually kind of looked a bit like a turkey as well. So if you look up some illustrations of velociraptors,

they look very little like how they're shown in the movie. That being said, there were dinosaurs that do look more like how the velociraptors are depicted in the movies. There is a form called Dynonicus, which is related to the raptors. They were much larger. And another form called the Uttar Raptor, which was larger still. And although they all had feathers, neither the Dynonicus nor the Uttar Raptor, or at least the depictions that I've seen, were quite as feathered as the Velociraptor. So there were dinosaurs that did look more or less like how the velociraptors are depicted in the films,

although they should still have some feathers. But it's just that the name doesn't match what they show. Although, interestingly, the premise of that film is. is that the animals have been genetically engineered. So you could just argue that the forms were genetically engineered to not have feathers because people didn't expect the dinosaurs to have feathers. So there's an interesting sort of in-universe work around there. But anyway, so that is the Pheropod lineage.

The third lineage of dinosaurs are the ornificians, which I mentioned before. These are the bird-hipped dinosaurs, even though they're not actually closely related to birds, that they're the other lineage. So the ornificians are, in some ways, the most morphologically diverse of the dinosaurs. So there's many different specialized anatomical adaptations. And they include, many of them were herbivorous, but some of them were also omnivores and, I think, some carnivorous forms, but mostly herbivorous. And there's a mixture of bipedal and quadrupedal postures that happened multiple times. So they should have shifted between them.

So that's distinct from the sauropods, which are all quadruped and the theropods, which as far as they know were all biped. and they retained the basal form, which was bipedal, whereas in the ornithicians, they sort of swapped around bipedals and came and went, and there's a wide variation. So I'll just talk about some of the more well-known and interesting forms of the ornithonnesian dinosaurs. So I'll start with the Stegosaurus, so they're very well known. Obviously, they have their armoured plates along their back and the spikes in the end of the tail.

Armoured plates were used for a combination of thermoregulation and defense, and possibly for mating signals as well. I'm not sure how well that's known. There's the ankylosaurus, which had armor over their back of their torso, as well as a very large heavy club at the end of their tail, which was used to batter predators with. There are the serotopsians, the most well known of which is the triceratops, with their crest and the famous three horns. Triceratops did live alongside Tyrannosaurus rex or T-Rex in the Lake Cretaceous, and as far as you know, they did in fact fight. So it's thought that the horns and crest were used in mating clashes, where the bulls or other animals will sort of butt heads or antlers on deer, for example.

It's also thought that they were used to defend against predators. And so the sort of classic image of the Tyrannosaurus rex attacking the Tricerotopsus defending itself with its horn and crest, that probably did happen. Another form of the Pachycephalosaurs, so they are the boneheads. They had a very pronounced bony protuberance from their skull, which they also used to ram each other in mating displays. I don't know if they used that actually to fend off against predators. Another important form of the duck-billed hadrosaws. So these were grazers, and they traveled in very large herds, as many of these other ornithician dinosaurs did as well.

and they have the, as they're so named, the sort of duck bill form around it. It's kind of like a beak at the front of their mouth. Let's conclude by talking about some other general facts about dinosaurs. So one of the interesting things is that most dinosaurs are much larger than comparable mammals today. We've already talked about sauropod gigantism, but even there are other forms. They're still much larger than comparable animals. So studies have looked at this. Most of the terrestrial carnivorous dinosaurs, so here we're looking at theropods,

mostly, were in the range of hundreds of kilograms up to a thousand kilograms. If you compare that to contemporary carnivore mammals, most of those are in the range of tens of kilograms up to 100 kilograms. So about a factor of 10 larger compared to contemporary carnivores in similar ecological niches today. Likewise, if we compare dinosaurs as a whole, the modal body mass was around 1 to 10 tons compared to Cenozoic mammals where estimates are of the animals. the most common body masses are a few kilograms. So they were talking a factor of about a thousand difference from a few kilograms to a few tons. That's probably a large part because of the huge sauropods and the fact that there's many, many small mammals today.

Whereas there were some smaller dinosaurs, but just not that many. And many of them were actually very large. So it's very interesting how that sort of changed, that the dinosaurs were predominantly very big, whereas a lot of mammals are quite small. I've already talked about how dinosaurs were endothermic. There's quite a few lines of evidence for that, including the structure of their bones and the fact that many of them were feathered, which is indicative of high metabolic rates as well. This allowed them to be more active, sustained locomotion over longer distances.

All dinosaurs laid amniotic eggs, so many dinosaurs constructed nests, and at least some of them are known to have nursed their young after hatching. It's still contested as to exactly how early feathers developed in dinosaurs. there are some accounts that essentially all dinosaurs had feathers. We definitely know that all of the Manny Raptor lineage, so that's the ones that are closer to birds. Definitely all of those had feathers. Probably all pheropods had at least some kind of feathers as well. But it's important to understand that there are different forms of feathers. So earlier, simpler forms of feathers are much more broadly distributed among pheropods as well as the sort of fish and dinosaurs as well.

However, true veined feathers, like similar to we see in modern birds, those are only fans. in the manoporaptor lineage that includes modern birds. So that would have included organisms like the velociraptor, for example. However, it's also not clear whether all of these feathers share a common origin. So it's possible that feathers develops multiple times during the evolutionary sequence of dinosaurs. Feathers predated powered flight by a very long time, like at least tens of millions of years. And the initial purpose of feathers seems to have been primarily insulation, then as the the organism acquires more feathers, then it may have been able to start using those for brief

gliding or even just helping them run faster, which can then let a brief gliding when you're jumping off a small cliff or a tree or something, and then that can lead to selection for more more powered fly. We'll discuss this in more detail when I talk about birds. At some point, I'll do an episode on birds, and we'll talk about their evolution. But the important point here is that many dinosaurs had some form of feathers, particularly the manoraptors, which had fully veined feathers, and this then gave rise to the modern bird lineage and that they originally evolved for insulation to maintain those body temperatures because dinosaurs were endothermic. Now by this time everyone knows what happened to the dinosaurs they went extinct because of a giant asteroid collision. One thing that I didn't realize

when researching this episode was just how recent this hypothesis was. So it was proposed by a paper in 1980 which attributed the KPG extinction, the late Cretaceous extinction to an asteroid impact. It's called a bolide hypothesis because it's a bollight is something that an extraterrestrial projectile that impacts the planet. And they pointed to a number of ranges of evidence, including eridium, a sudden increasing eridium levels at centimeter layers around the world, as well as other phenomena like shocked quartz, which is a particular type of mineral form of quartz, which was shown to emanate from an impact event, particularly it was found in North America. However, when this hypothesis was first suggested, the actual impact. site was not known. It was only discovered and announced in 1991 that there was an impact site found in the Yucatan Peninsula, just off the coast of the Yucatan Peninsula, in southeastern Mexico. So since then, it's now widely accepted that the dinosaurs, well, not just the dinosaurs, but also the pterosaurs

and the surviving uriapsids as well, the marine marine animals, they all became extinct shortly after this impact event. The impact would have created massive tsunamis, which covered much of North America, and this would have triggered huge wildfires, which would have released massive amounts of carbon dioxide and soot into the atmosphere. The soot and other aerosols would have covered the entire Earth within weeks and lasted for years. This would have reflected a lot more sunlight, causing the planet to cool by a significant amount, disrupting photosynthesis because of the blocked sunlight, thereby reducing the food available for these huge sauropods. The sauropods need to eat pretty much all the time just to stay alive, so they would have died.

This then eliminated most of the food source for the theropods, so they died off as well. And so particularly these very large organisms that were well adapted to the conditions of the Jurassic and Cretaceous, just couldn't survive this huge disruption, and so it all became extinct. In terms of terrestrial life forms, the snakes and lizards carried on, more better adapted to drier conditions and probably not requiring as much food as the big sauropods and the birds survived and as well as a small number of mammals and the small mammalian lineage so typically smaller animals seem to have survived better than the very large ones that's probably why the dinosaurs and like the huge terrorsors were the worst hit now you will see i mean there's still a

fairly steady stream of these articles debating about whether the dinosaurs were really did become extinct as a result of the asteroid so i just want to clarify here what the controversy actually is and is not. There is no controversy as to whether there was an asteroid strike. There was. We know that it hit the Yucatan Peninsula and there's evidence that this had effects across the world, like the iridium layer and the shock quartz and things like that. It's also very clear that this was the most direct cause of the extinction of the dinosaurs. What is debated is whether there were other causes that pre-existed, like that developed before the asteroid strike, such that

Is it the case that the asteroid hit was the final nail in the coffin for the dinosaurs that were already declining? Or was it the sort of sole cause or the primary cause of the dinosaur extinction? So how important were other factors in addition to or alongside in the lead-up to the asteroid collision? That's what the dispute is about. And so there are still papers that are arguing that there are signs that dinosaurs were in decline in the 5 to 10 million years before the the asteroid collision. There are also those who argue that the few hundred thousand years or maybe a million years prior to the asteroid strike that there was an increasing volcanic activity in the deck and traps in India. Well, I mean, we know that that was the case, but the question is how severe was

it and to what extent did it contribute to the extinction of the dinosaurs? It does seem to me that one of the challenges in discerning whether the dinosaurs were already in decline is you have to have some way of sampling in a representative way across the world to see how diverse they were, whether there were already extinctions of some species leading up to the Asteroid's track. And there are some papers that have argued that this is the case. However, they've been challenged to how representative they were, particularly because it seems like there may have been a decline of some dinosaur species in North America, but that may not necessarily be representative of dinosaurs elsewhere in the world,

and a lot of the research has focused on North America. It's also the case that some dinosaur species or subgroups of dinosaurs may have been in decline, but that doesn't mean that other dinosaur groups weren't doing fine. So I think it's a very hard question that may never really be fully answered because it's too poorly specified as to what would it exactly mean for the dinosaurs to be in decline. I mean, it could be some dinosaurs in decline in some places, but for how long a period of time and what exactly is like the baseline. Species always come and go at some rates. And so the other important thing that I think is to realize when talking about this is that it may have been the case that some dinosaur lineages had been struggling beforehand, maybe due to the decontraps or other factors. climate change has also been proposed. But that doesn't mean that without the asteroid, they would

still have gone extinct. The world could still be dominated by reptiles and like dinosaur, non-avian dinosaur-like forms to this day if there had been no asteroid strike. So I think that even if there were other factors that were involved, we can't extrapolate from that and just assume that dinosaurs would have still gone extinct in a similar way without the asteroid strike, because we actually don't have any idea. What we do know is that the immediate direct cause of the extinction was definitely the asteroid. So I think that some of these coverage that emphasizes these other factors is sort of missing the key point there because even if there was some decline or some other challenges, that doesn't mean that they were on the way to being

extinct. You can't read it like that. What most clearly and directly led to their extinction was definitely the asteroid, which took out all non-avian dinosaurs. They were doing, generally they were doing very well in terms of very highly diversified across all of the continents at that time. So all of them went extinct as well as all the pterosaurs and all of the remaining uriapsids as well. Mosasaws were also became extinct at that time. So really all of these large reptiles that existed. And it seems to have just been the smaller organisms, snakes and lizards, birds, and some of the small mammals that survived. And then went to, went on to diversify in very interesting ways in the beginning of the

Sinozoic, which then sets us in the story for what we'll talk about in the next episode, which is the mammalian evolution with a focus on human evolution. So that was quite a long episode, but I wanted to put all that together because I think that it fits best in a single episode. So thanks very much for listening. Hope you found this interesting. If you would like to support the show, you can make a financial contribution via PayPal,

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