So not long ago, I pushed back against the notion that life in the past was always bigger by describing how whales only became truly enormous in the briefest blink of geologic time, as recently as our ancestors left the trees. Today I would like to go much further back and ask; if whales are the last giants, what were the first? Life took several billion years to even become visible to the naked eye, and even when animals emerged they were pretty tiny for a long time. Anamalocaris was a massive, terrifying super-predator at slightly over a foot long. Still, the oceans are an excellent place to grow large, and soon we had potential 30-foot shelled cephalopods. But what about the land? It took several million more years for anything to gain a foothold on the shore, and even the biggest arthropods can only get so big. So let’s turn to the tetrapods, and their first titans; the caseids.
A Cotylorhynchus romeria mount from the Sam Noble Oklahoma Museum of Natural History. This is the nonsense we’re dealing with here.
Tetrapods are earth’s land-dwelling vertebrates (and aquatic descendants of those land-lubbers), including amphibians, reptiles, and mammals. Amphibians are their own odd branch that continued to straddle the boundary of land and sea, while the rest are what we call amniotes. The key feature that distinguishes amniotes from amphibians and their extinct relatives is that their developing young, whether they be in a womb or an egg, are surrounded by a membrane that holds fluids. Amphibians and fish have to lay eggs directly in water to keep their babies moist; the amnion retains fluid, and with a shell over the top, they don’t need to worry about their babies drying out. This freed amniotes entirely from the water, allowing them to live entirely on land. So towards the end of the Carboniferous, over 310 million years ago, we finally get our first fully terrestrial animals, creatures that could go their entire lives without immersing themselves in the water.
Another Cotylorhynchus romeri fossil, this one seen from above. Notice the pineal eye opening on the top of the head. Photo by Ryan Somma.
The amniotes split early into two lineages; the sauropsids, containing the modern reptiles and birds, and the synapsids, of which mammals are the only living representative. Non-mammalian synapsids used to be called “mammal-like reptiles”, but they aren’t considered reptiles at all anymore, although they were somewhat mammal-like. It’s the latter that spawned the caseids, but you wouldn’t know that they were our distant cousins to look at them. They were primitive synapsids, primitive in the scientific sense of being more like their ancestors. As such, they were quite like lizards in their basic structure, that being the basal form of amniotes at large (although true lizards wouldn’t emerge for another 100 million years). They walked on sprawled legs and had a surprisingly massive pineal eye, a weak third eye on the top of their head. Of course, their most iconic feature was that head. That ridiculous, tiny, muppet-like head on top of a massively bulky body. They’re glorious, aren’t they?
The caseids emerged sometime in the very late Carboniferous period, although their earliest members, like Eocasea martini, were small and carnivorous like other basal synapsids. The only plant eaters on land at that point were still invertebrates, like the giant millipede Arthropleura, which Eocasea was contemporary with. These 8-foot long arthropods were another contender for the title of the first giants, but soon Eocasea‘s relatives would put it to shame. At some point in the early Permian, the next era of the planet, they turned to herbivory, and that was the key to gigantism. Herbivores don’t need to be sleek enough to chase down their prey and can simply rely upon sheer mass to deter predation, and a more massive gut is necessary to digest tough plant matter.
25 million years later, all known caseids are herbivores, a staple of their environment, with several species living together. This shouldn’t be surprising; multiple large herbivores very commonly live alongside one another, specializing in different food or otherwise finding ways to avoid competition. Thus, between 280 and 270 million years ago, several genus of caseids shared the warm floodplains of what is now Texas. Caseiodes had the traditional barrel body of its family and was perhaps 10 feet long, while the similar in length Caseopsis was surprisingly gracile in build, perhaps a speedier member of the family. Much more massive were Angelosaurus and especially Cotylorhynchus, the best known genus of caseid. Both genuses had members that reached a total of 20 feet, likely too large for the local Dimetrodon to bother with.
While the animal depictions are outdated, the plants capture the general foliage the caseids would have lived among. By Dr. Fr. Rolle.
How these creatures actually lived has undergone a lot of revision over the years. If you can tear your attention away from the caseid’s absurd head for a moment, their feet are also quite notable. Their toes are quite long and tipped with broad, flat claws. Shovel-like claws, some might say. From this, the old point of view is that caseids did a lot of digging for their food, looking for roots and tubers like a modern-day pig. However, in just the last few years, this idea has been challenged, with another reason behind their long fingers being proposed.
Caseids have surprisingly spongy bones, which were likely relatively fragile for such massive creatures. This and their short necks, potentially making it difficult to reach the ground with their mouths, depending on the position of their shoulders, led several researchers to propose a different role, one more like a hippo than a pig. Those long fingers were for swimming rather than digging. This goes mostly for the particularly massive members of the family, such as the potentially 20-foot Cotylorhynchus hancocki. If they were indeed mostly aquatic, this also suggested that they had a more mammalian diaphragm. Mammals use their muscular diaphragm to draw more air into their lungs, gaining much more oxygen than reptiles can. It was thought that this feature only evolved in synapsids closer to mammals, with higher metabolic needs (more “warm blooded”), but the primitive caseids would have benefitted greatly from the ability to breathe deeply, staying underwater for longer periods and recovering quickly when they surface. If this take is accurate, the smaller caseids might have been the dominant land herbivores, while the larger ones mostly stuck to the rivers.
Images showing the pineal eye of a caseid (scaly integument speculative), a Cotylorhunchus swimming, and a member of the same genus facing down a Dimetrodon. Copyright me.
The caseids were only successful for a few million years. In the middle of the Permian, there was event known as Olson’s Extinction. The exact cause is unknown, but over a very short period, there was a taxonomic turnover. It effected all forms of life on the planet, but on land it’s most known for what happened to syanpsids. Primitive synapsids like the caseids and Dimetrodon vanished, replaced by a group known as therapsids, more similar to mammals than their cousins were. Multiple waves of new herbivores followed, creatures like Estemmenosuchus reaching sizes similar to the caseids. The last known member of the family was Ennatosaurus, living in the Wordian era about 265-255 million years ago. After this, the first giants are gone, replaced by their distant cousins.
The caseids are long gone, but they paved the way for increasingly massive waves of terrestrial giants until the sauropod dinosaurs took the crown in the Jurassic period. They were bizarre creatures, like nothing before or since, and strangest of all, they were our distant cousins.
Want more on the prehistory of mammals? Check it out here:
