Colours of Wildlife: Cotylorhynchus

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Willem is a wildlife artist based in South Africa. He says "My aim is simply to express the beauty and wonder that is in Nature, and to heighten people's appreciation of plants, animals and the wilderness. Not everything I paint is African! Though I've never been there, I'm also fascinated by Asia and I've done paintings of Asian rhinos and birds as well. I may in future do some of European, Australian and American species too. I'm fascinated by wild things from all over the world! I mainly paint in watercolours. . . but actually many media including 'digital' paintings with the computer!"

Cotylorhynchus by Willem

The Pinheadest of them All

Here you have the amazing Cotylorhynchus hancocki. This one has a number of claims to fame. First, it was huge for its time – the Early Permian, about 280-272 million years ago. Vertebrates had only seriously started colonizing the land in the period before, the Carboniferous. That was the time of the huge swamp forests that turned into most of the coal we use today. The first forests were initially devoid of large living things; their first inhabitants were insects like giant dragonflies and other invertebrates which included huge millipede-like things. Once the insects were there, however, insect-eaters followed suit; these were at first small, lizard-like things. They had learnt the trick that made reptiles so successful from then on: they were able to lay eggs with tough, leathery shells to keep the embryos inside safe and moist, even if laid in dry places. They also were able to fertilize these eggs internally. This allowed them to venture much further inland than their contemporary amphibian-like compatriots, which still needed to return to open water to breed. These first reptiles and reptile-like things diversified rapidly and in the Carboniferous the main groups of them had already formed: the group leading to modern reptiles and snakes, and also to crocodiles, dinosaurs and birds; a group once thought to have included the ancestors of modern tortoises and turtles but which it now appears may have been a different and now quite extinct assemblage; and lastly, the group that eventually led to the mammals. Initially they were all small and eating insects or other small things. By the onset of the Permian, some of these had grown big and were now eating fish, amphibians and other reptiles.

But already by the late Carboniferous, some of the land-livers had turned to vegans. Plant growth in the forest was plentiful and less risky to obtain. From the first lizard-like plant eaters, larger types evolved. We have already seen Casea broilii, one of these. But Casea was not the biggest. Its relatives soon became enormous, the largest land-living things this planet had yet seen. Such was Cotylorhynchus. It lived in what is now the USA. Similar critters were found in Europe and likely most of the rest of the world at the time.
This animal reached 6 m/20' in length. Some of that was made up by its long tail, but it also had a deep, wide rib cage, and massive front and back legs and feet. Such a big body could contain and digest a huge amount of plant food.

But the question about Cotylorhynchus was, how did it ingest all this food? For here we now have to look at its second remarkable feature: its tiny head! For its size, it had one of the tiniest heads ever. It rivals in tininess relative to body size the heads of Stegosaurus, considered the most brainless of the dinosaurs, as well as the long-necked small-headed giant dinosaurs, the sauropods. And what is funniest about it, is that its tiny head sat directly upon its huge body, with hardly any neck. It had to turn its body all the time as it ate because it could only munch on what was directly in front of it. And how did it drink? Its head could not reach ground level; it would have needed to wade into the water until it reached the height of its upper chest, in order to drink.

One possible solution to this riddle is that it might have been living in the water all the time. It might have ate soft water plants or maybe even pond scum drifting on the water surface. Its big, flat feet would have served well as paddles. But its normally-shaped toes and strong limb girdles show that it would still have been able to move around on land as well, so it likely was semi-aquatic at best. Once can imagine it lazily floating down a river shovelling pond scum into its mouth, later crawling out onto the bank to bathe in the sun and digest its meal.

Without more, and better, fossil specimens, as well as more information about the times and places it lived in, we are unlikely to know exactly how this strange old thing lived. Sadly, it and its close kin soon died out. Perhaps they were too vulnerable to the new, big, meat-eating animals that evolved to prey on the plant-eaters. But they constituted a vital early experiment in herbivory. Cotylorhynchus, like its close relative Casea, was a synapsid, the group from which mammals evolved much later. As you've seen in these columns, this group proved amazingly successful and diverse, even prior to evolving into mammals. After Cotylorhynchus, new groups of synapsid herbivores evolved, such as the plant-eating dinocephalians and the dicynodonts. These were more agile and, especially, in possession of much larger heads!

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