ExtinctZoo - When Earth Turned Into A Giant Jungle

Episode Date: June 20, 2026

You probably think the age of mammals is all cold and grey right? I mean we are in an ice age and all. Yet, this belief would be FOOLISH, cuz guess what, it was actually during the age of mammals, that earth experienced once of its hottest moments ever.

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With ServiceNow, you can do the parts of your job you're best at and delegate the rest. To put AI to work for people, visit servicenow.com. When we think about prehistoric times, especially the age of dinosaurs and before, we usually imagine the earth as being hot and muggy, whereas in the other hand, we often view the Cenozoic, or age of mammals, as being the opposite, in the sense that we consider it colder and more dreary. And this perception has largely been created by the Ice Age, which brought worldwide temperatures down to their lowest points in hundreds of millions of years, potentially.

And even when looking at the Cenozoic in general, it does tend to have been colder than previous eras. However, what is also true is that it is just a myth to believe that the whole thing has been one giant, cold spell, when in reality there have been hot periods sprinkled throughout. And ironically, one of these periods ended up being one of the hottest moments in Earth's history, in an event that turned the world into essentially an oversized microwave jungle, which shared some resemblance to the bizarre Carboniferous period, which transpired 300 million years before. And this was the Paleocene-Eocene Thermal Maximum, or P-E-T-M, for short. Now, historically, the dinosaur-ending asteroid

has often been depicted as having ushered in a nuclear-like winter in a permanent colder temperatures. And while did certainly dampen the temps for a bit, the world did ultimately return to a pretty warm climate just as the Cretaceous had been, with the first stage preceding the Mesozoic, having been a relatively warm time for Earth. In fact, even at that time, there was no permanent polar ice caps on the planet, while carbon dioxide levels were far higher than today. And this heat remained relatively stable for some time, and indirectly eventually led to extensive tropical and subtropical forests developing worldwide, with maingrove, cloud, rain, and swamp forests all becoming fairly abundant.

Though that said, most of the trees you'd see in these ancient forests were the same as each other, in other words, not much diversity, being dominated by just a few species. Meanwhile, animals, on the other hand, were still recovering from the extinction and were in general fairly small. But they were getting bigger, especially as the Paleocene, the epoch to which the Danian fell under, was coming to a close. And by then, you did see animals that could be half-tonned in weight, so size was increasing fast. However, this recovery, in somewhat relatively stable climate, was about to be thrown straight into the depths of cats. chaos. As about 55.8 million years ago, the temperature was suddenly cranked to 100. And I don't

mean that things started to slowly get warmer. No, no, no. It was more like one day warm and the next day you're in an oven. And we see this reflected by graphs that have examined past temperatures. They've got a lot of smooth uphill and then suddenly this giant ugly spike that sticks out like a sore thumb, literally. Now, this is not something we've never seen before. Such rapid global warming events are referred to as hyperthermals. But this one is by far the most intense of the entire Cenozoic age, with temperatures rising up to 8 degrees Celsius or 14 degrees Fahrenheit worldwide, which just for comparison, with all of humanity's activities, the world is only increased by about 1.4 degrees Celsius since the Industrial Revolution, and that has already

had a massive impact. And also, just to reiterate, the world was already hot, with the average being up to 28.3 degrees Celsius or 82.9 degrees Fahrenheit, so about 40% hotter than currently. And again, this was before the Paleocene Eocene Thermal Maximum. And so during it, it's not that the average might have skyrocketed up to a blistering 34.5 degrees Celsius or 94.1 degrees Fahrenheit, which for perspective, is hotter than even the current hottest country on Earth. And it's believed that even the high latitude seas of the Arctic were as warm as subtropical waters now, meaning that if you went to Antarctica, you better abroad some swim trunks and sunscreen. And so this all begs the question of, why? Well, we know this

unusual event had something to do with an astronomical increase in carbon dioxide, as the PPM, or parts per million, went up from about 1,000 to over 2,000, which is more than four times the present levels. In such a massive rise in carbon dioxide isn't just a normal part of the carbon cycle. It clearly indicates that something transpired, something which happened to inject a massive amount of carbon dioxide straight into the atmosphere. Now, we do not know exactly what the culprit behind this event was, but there are a lot of contenders. eruptions, massive underground peat fires, changes in the ocean circulation, and even common

impacts have also been cited, and more. I mean, even terrestrial animals themselves have been blamed by some, who believe that an increase in respiration efficiency led to an increased output of carbon dioxide, since that is exhaled when an animal breathes out, and this therefore might have pushed carbon dioxide levels even higher. But again, for now, any agreement on what really happened remains elusive. What is very clear, though, and very agreed upon, is that this heat wave had major impacts on the entire planet and seems to have lasted for about 200,000 years, or about 20 times longer than the last glacial maximum lasted during the Ice Age. And if the latter, i.e., the peak of the Ice Age, caused so many changes in just 7,000 years, imagine how impactful

all this heat was. And so unsurprisingly, virtually every aspect of the planet underwent dramatic shifts. And one thing that also had its own impacts was that the climate not only got hotter, but also much wetter as well, with evaporation rates increasing, especially in the tropics, and rainfall getting way amped up. This supercharged the already abundant forests, leading to tropical and subtropical forests expanding even further and increasing their range, leading to them growing in places that we don't usually associate warm tropical forests with. For example, Texas, obviously much drier area, was at the time covered in tropical rainforests and seasonal tropical vegetation, while in the arctics, palm trees were plenty, and even azola ferns could be found.

two flora which today are completely void from these areas. And so basically, this growth in subtropical and tropical vegetation basically turned the planet into one giant jungle and swamp. The swamps too became far more prominent at this time, especially as sea levels rose and flooding occurred. This totally transformed the face of the planet and turned it into an environment reminiscent of the extensive forests and swamps Earth experienced during the mid-Cretaceous, and even the very far-back Carboniferous, which happened so long ago. And just like those two previous times, the changes of environments didn't just make things

look different, it also forever altered the timeline of evolution, and is actually very relevant to our own story. You may have thought that it was the dinosaur's demise that led to the mammals you see today, including us. And while it did open up the door from mammalian rule, it was really the P-T-Met that had a far more direct involvement in the specifics, as it led to a large increase in mammalian abundance, which included the emergence of many important mammal clades that are a massive part of the biosphere today.

Among them were the Arteodactyla, or even toadungulets, which includes things like giraffes, bison, and even whales, then there was the parasodactyla, or the odd tododunulates, who were composed of horses, rhinos, tapirs, and the extinct hyanodontidae, and then finally, the world even saw the rise of the first true protoprimates, such as the Purgatorius. What's odd is that this explosion in diversity is thought to have stemmed from something that may come as a surprise, and that's dwarfism. You see, virtually all mammals are warm-blooded, meaning that we could. can regulate our own internal temperatures, which is good, but it can also make us more prone to

overheating. And therefore, when it got as hot as it did during the Paleocene-Eocene thermal maximum, mammals started to shrink in order to stay cool. And this dwarfing effect might have led to more speciation, with the larger mammals like equids, mesonikids, and the oxyonids, being especially affected. On the flip side, though, already very small mammals, such as the Aranesceids, were left pretty much untouched. Then, there were also animals that experienced somewhat of the exact opposite effect. And this was, of course, our cold-blooded friends, the reptiles. As since they rely externally for heat, the increased warm supplied them with more spare energy. Therefore, with the concern of staying warm, no longer being as much of an issue, many reptiles grew

in size and became highly diversified, with some formations from this period harboring over 100 species of reptiles. Based on fossil records, it seems that worldwide, turtles became very large, and many snakes balloon in size as well, especially those that were aquatic, leading to literally the largest sea snakes of all time, such as the Paleophyllus, a potentially 40 feet or 12.3 meter long monster that lived in the marine waters surrounding Europe, North America, and North Africa. Crocodillomorphs also got a helping hand from this event, with multiple groups of predatorial land crocs becoming quite successful, such as the Stebekids and Mechusukene, both of which

would produce more than a handful of effective killing machines that frankly make Komodo dragons seem rather tame. And so this a rise of large and abundant reptiles, as well as the speciation of dwarf mammals, led to a pretty interesting world, where in many environments, especially in the Western hemisphere, you'd find an extensive amount of mammalian families, but then they would still have a few reptiles being at the top of the food chain, despite the Sinozoic commonly being nicknamed The Age of Mammals. And one of the best examples of this is the Wasatch Formation, a massive sprawling area that

covers numerous states in the United States, and is well known for being highly fossiliferous, giving us a clear look into what life was like during this hot pot era. And one thing we see is that the forests were not just dense and plentiful, but they had also become diverse as well, as large amounts of magnolias, birches, redwoods, nettles, laurels, and more, were present. And within these ancient, rich forests, the earth was teeming with all walks of life, and this included our very own kind, not mammals, well, yes mammals, but specifically primates, who were still quite primitive and didn't look like how you might typically imagine primates to be.

as things like apes and monkeys were still a distant twinkle in evolution's eye. Instead, most primates here were members of the extinct plesiodapiform group. In general, these guys tended to be very small and grassile, while sporting longer tails and skulls, giving them an appearance most comparable to that of lemurs. There were also those that looked arguably more like squirrels than anything else. Well, I guess more like if a squirrel had a baby with a monkey. And thus far, dozens of primates have been unearthed from this time,

showing that they were thriving and benefiting off of the abundance of trees. And the dense coverage the forest provided also supercharged other small groups like the Lepticta, Lipotifla, Macroskelete, multituburculata, Fullydota, and McGliars, which consists of rodents and lagomorphs. Many of these groups did appear archaic and ancient, but they were still very clearly mammalian in overall looks. And we do see this pattern in the slightly, and I do mean slightly, larger mammals like the early are deodactyls, who were dwarfs compared to those that live today. Many of them, in fact, you can likely pick up with just one arm, and they were at this time a pretty adorable bunch, as some that we know of, like the Dichodexus and Dico-Bunidae are often compared to still extant dwarf artiodactyls,

such as the Chevrolete and Duikr, who almost seemed to have been designed for peak cuteness. I mean, just look at them, absolutely adorable. Alongside this group, you would have also found the first parasidactyla, who were actually more abundant, and again slightly larger, as many reached the size of medium-to-farist dogs. So, not giant by any means, but still a step in the right direction. At this time, most of them tended to look a bit like what you may imagine a primitive horse to look like, regardless of group, as the rhinoceratoids, equids, brontotheres, and tapyroids, all in a bit of the same build in a superficial sense, with them being compact, four-legged,

and without the visible structures that we often use to distinguish them visually. And what I mean by this is that the rhinocerotidia had any yet devolved the horns, the tapyroids were snoutless, and the brontotheres lacked their built-in-rams. And compared to their modern or later selves, I had argued that horses look the most unchanged, with the biggest visual difference at this point being basically their many sizes. And though each group would eventually balloon in size, their small stature at this time they'd wonders for them through the PETM, as it not only helped keep them cool, but it also allowed them to use a thick brush as cover.

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Namely the Pandadonta and Fanadontana. day, two extinct groups that were essentially the first mammals become relatively large in the absence of dinosaurs, with the genera like the Barry Lambda, even managing to reach the size as a pony, weighing about 650 kilos, which basically made it the Godzilla of its time, albeit a herbivorous one. And this relatively giant size would have protected it from almost all the predators within its environment. And trust me when I say, there were a lot of predators, with about five different groups being

the shock collars so you could say. do add the myocidae, a family of small weasel and ferret-like carnivores that were in some cases arboreal, and often fed upon tiny prey like bugs, birds, and small mammals. Then there was the oxi-a-day, meaning the sharp hyenas, who remembers the fairy order, and despite what their name may have you believe, were not tyenas, and did not really resemble them either, actually being more similar in appearance to otters and badgers that walked on flat feet, intended to be larger than the myasids, with some of the species being about the same size as a large fox.

With that being said, they would get much bigger in the distant future. In the same situation was also seen in another group that would one day become one of the Earth's most dominant predators, the hyena-dons. Now again, despite the hyena portion of their name, these guys were not really hyenas, only being named hyanodons due to their similarly shared teeth. Though with that said, while the two groups did share some dental features, the teeth of the hyena-dons were actually better designed for sharing flesh instead of crushing bones, as some hyenas teeth are.

And given that they were wolf size at the time, they had quite a few options for flesh to choose from. Some evidence also suggests that this group was relatively cursorial for the time, meaning there were efficient runners. And that might not sound so special nowadays, but back then it was pretty revolutionary, as many mammals were quite slow compared to their modern day counterparts. Moving down the list, our third shot collars were the Viveravidae, and once again falling in line with confusing names, this extinct group were not the Viverid's earth still has.

again you may ask why the name? Well essentially it was once thought that this ancient group were the ancestors of the Viverids given the almost identical looks and presumably had similar lifestyles. Yet we now know that they actually were not related but the name has stuck and unsurprisingly said name means the ancestors of Viverids and like I said there were some that were indeed Viverid like but there were also others that broke the blend with a few being more like to Mongooses than the former and ultimately this was a very diverse group who were extremely successful predators during this time. But with that said, as far as time-traveling humans go, they were not the worst group to run into. That title instead likely went to the wolf-like

mesonikids, who somehow against all odds had resisted the heat and had become remarkably big. With the Pakiayina, for example, sometimes weighing up to 396 kilos were 873 pounds, an absolute anomaly amongst the landscape, and who was no doubt a lethal apex predator, who utilized massive robust teeth to demolish various animals. And based off of wear patterns, it's also thought that bone made up a large part of its diet, thus increasing its versatility. And then finally, last but not least, the archdecyanids, who were like the mesoninids fairly large for the time, as well some were just the size of a cat, domestic cat, that is. Others like the archdeciant Mumach are believed to have rivaled small bears in size, making them again amongst the largest

predators alive at the time. Albeit, truthfully, calling them shock collars is, relatively generous, as they were not doing so well in the heat. And actually, it may have been directly, or at least indirectly, the cause of their demise, as they had first evolved during the late Cretaceous, but were now finding themselves in a catastrophic decline that did lead to their extinction before the PETM had even ended. But as you can see, even with the imminent doom of the Arctosyanids, there were still plenty of mammalian carnivores to choose from, along with, of course, the plenty of herbivores that we just talked about. But remember, these were in many ways not the true rulers, as that of course would again be the reptiles, who had rode the heat wave all the way

to glory. And this list included some of the earth's first bow constrictors, a multitude of turtles, both hard and soft-shelled, plus a few with extremely long tails, iguanas, worm lizards, Anguaday, potential beaded lizards, and a trove of other unidentified lizards and snakes. And like I mentioned, there was also a plethora of crocodilians thriving as well, many looking superficially similar to modern ones. But because of the increased diversification amongst them, you did have a lot of these species specializing in niches that you might not usually associate with crocs. For instance, one species had turned it into a shelled animal's worst nightmare, whereas fish, on the other hand, probably had no qualms with it. That said, there were, of course, crocs that were pretty standard, too, being of the larger stature side,

and sporting opportunistic lifestyles where anything that fit in their mouths was good enough for them. But there's one caveat that is very important to remember, which is, do not think that staying away from the water would keep you out of a crox reach, As during the Paleocene Eocene Thermal Maximum, the dangers were not just limited to fresh water, as one group walking around were the hoofed crocodiles. NDS you heard that correctly. Hoofed. Although truthfully, this group, scientifically known as the Plano Cranidae, didn't actually have any hooves. Rather, they were equipped with blunt clod feet that simply resembled hooves.

Yet, regardless, this was still a very clear sign that these guys were highly adapted to life on land and running, also sporting long athletic legs. Of course, though, in typical crock fashion, they also kept what makes crocs, well, scary. In other words, extensive body armor and jaws that would take a nice chunk out of you, making them among the last animals you'd want to encounter. And though not in the Wasatch Formation, you cannot forget that in this time, if you were traveling the globe, you could have also came into contact with the Stebikids and Mechusukene, two additional groups of crocodilomorphs that also had, of course, wicked teeth, and in many cases were even larger than the hoofed crocodiles.

Oh, and uh, these were land crocs. too. So, clearly the lands of Earth were teeming with all sorts of life at this time, and I guess you could say, has it ultimately benefited mammals. Yet, with that said, the situation in the water was a bit different. It seems that the excessive heat had made things a bit more uncomfortable underwater than above, as the PETM was a bit of an aquatic extinction event in reality. So much so, in fact, that certain groups like Benthic for Manifera experienced dioffs that was more severe than what they experienced during the KT extinction event. You know, when that giant asteroids slammed it to Earth.

And it seems that shallow marine ecosystems in particular were devastated by this event, with many of them having simply collapsed under the heat. Although it wasn't necessarily the heat itself that it was the killer, rather other changes that occurred as a result of this event. And one major change that took place was anoxia, where an oxygen is depleted from the waters. You see, elevated seawater temps, water stratification, inoxidized methane released from undersea structures,

all led to this depletion in oxygen that seemed to affect it most oceans and sea. seas, especially the North Atlantic, turning all of them into death traps for many. Then on top of that, the PETM also seemed to have radically changed the planet's circulation patterns, essentially reversing currents worldwide in transporting warm water to the deeper parts of the ocean, which didn't help either. And, uh, the problems would only continue, as the ocean started to experience acidification as well, with a pH dropping about 0.46 units, which is no bueno as it reduces the availability of carbonate ions, which are essential building blocks. that marine organisms need to build and maintain their shells and skeletons.

And so with all these factors combined, there were some major hurting in many marine environments and even freshwater ones too. Yet I will caution you that to those with Thalasophobia would still need to be wary, as numerous dangers lived beneath the waves. Sharks, particular, were thriving despite the challenges, having filled in the gaps left behind by the KT extinction event, while the warmer waters resulted in their ranges becoming far greater, meaning that it even in place is like Scandinavia, the UK, or the Poles for that matter, sharks of many kinds were commonplace. A lot of them are also the same kind of sharks you still see today. In other words, goblin sharks, Requiem sharks, sand sharks, bullheads, cow sharks, etc. And of these species, a lot of them

were surprisingly similar to the extant ones as well. But then there were also a few species that, uh, let's just say we should be glad remain in the past. And some prime examples of this are seen in the mackerel sharks. Specifically, Cretelamina, Pendicolata and the Otis Obelicus. Two very large predators, with the former being comparable to a great white, while then the Otis Obelicus was able to reach nine meters or 30 feet in length, making it not only the largest predator on earth at this time, but likely the largest animal period during the entire PETM. And just as a cherry on top, you also had giant ghost sharks, along with various predatory fish like early barricudas. And then, of course, you can't forget

by those giant aquatic snakes I mentioned earlier, who were definitely lurking around these waters as well. In addition to these guys, fully marine crocodiles were a reality too, namely being the Dyrosaurus who ruled the waters as far as Crocs go. Now this group was defined by having very long and thin and garial-like snouts

that took up large parts of their bodies. Yet don't get it twisted because fish weren't the only thing on the menu, regardless of what their skull shapes may have you believe. As across the board, they held various niches and also sported different dentitions, suggesting that each was specialized in something a little bit different. And like the sharks, they too had been around during the Cretaceous,

but in the absence of the Mesozoic's aquatic threats and the presence of warmer waters, had made quite the name for themselves and held quite the global range. Thankfully, they didn't all tend to be supersized, but some of the species did hover around 20 feet or 6 meters in length, so swimming into one of them would definitely not have been very fun, or good for life. And on top of aquatic marine environments, this family is pretty much found in every other kind of water too, both brackish and fresh water, meaning that on top of the blistering heat, crazy mammals, and numerous reptilians, you pretty much had to watch after giant crocs no matter where you were, including the possibility of them running you down on land. And oh, uh, did I mention there are bugs too?

Because there are lots and lots of them, as the heat was very helpful for them too, leading to massively expanded ranges, and to a bloom in diversification and a surge in incenter-repellant prices. So, with all this said, it doesn't need to be, well, said, that of all the times you could choose to visit the Sinozoic, the Paleocene-Eocene Thermal Maximum would definitely be one of the more questionable and uncomfortable times to choose. But the good news is that this whole strange event wasn't that long, at least in a geological sense, seeing that as mentioned, it only lasted for about 200,000 years. In other words, shorter than the time that even we have been around, and we're pretty fresh. Now this leads to the logical question of,

why did it end? And the answer to that is, it's believed that a mixture of silicate rock weathering which traps carbon and stable molecules and the organic burial of carbon through essentially dead things getting stuck in sediment is what brought the carbon dioxide levels down to a bit more reasonable levels but overall the climate would remain pretty toasty even after this and actually it would not be the last time real heat reared its head either as for assumingly similar reasons another event like the ptm would happen not so long after that has now been dubbed very creatively the eocene thermal maximum two were in temperatures spiked once again and ultimately Finally, both of these hyperthermal events are part of the broader early Eocene Climatic Optimum,

a sustained interval of a few million years that is one of the hottest periods of sustained heat the Earth has ever experienced. But still, of all the heat that generated, it was the PETM that turned the dial to the highest, and since then, nothing has been able to top it, probably for the better. Thanks for watching, and until next time, on Extinct Zoo.