South American Advanced Cynodonts
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!"
I started this series of articles about prehistoric critters mainly to talk about the wonderful critters whose fossils have been discovered in South Africa. My country indeed has a wonderful treasure trove of especially these proto-mammals, and if you've been keeping up, you'll have an idea of the diversity that has been found down here. And I haven't even told you about all of them! But other countries have their wonderful fossils also. As I've told you, in South Africa, after the middle Triassic the fossils somehow get rare; there are some, but not nearly as many as from the Permian and the early Triassic. This likely has something to do with how fossils form and the environmental conditions at the time, rather than showing an actual absence of proto-mammals around then. In other places, the fossil record continues. From the middle to the late Triassic, South America takes over to provide us some glimpses of what these old mammal-ancestors were up to back then.
Trials of the Triassic
You may remember that the period preceding the Triassic, the Permian, ended in a mass extinction. Those critters that survived, re-diversified in the Triassic. Towards the end of the Triassic, there was another extinction, but this one was spread out a bit over time, and not quite as sudden and severe as the Permian one. It seems to have been caused by the massive volcanic eruptions of the time. Once again you may remember that, during the Permian, the continents of Earth were joined into a single supercontinent, Pangaea. Towards the end of the Triassic, this landmass started to break up. The rifts that separated the continents that we know today, were caused by volcanic activity and accompanied not only by vast outpourings of lava, but also spewed huge volumes of gases like carbon dioxide and sulphur dioxide into the atmosphere. This caused global warming and acidification of the oceans, leading to extinctions of marine species. On land, changed climatic zones turned life unfavourable for species that couldn't move or adapt fast enough. (Note that in addition to volcanic activity, other events that we still don't know about might also have contributed to the Triassic extinction.)
Now in general the proto-mammals were hardy and adaptable, showing themselves able to evolve rapidly. But these perturbations would still have taken a toll on them. But what was worse, was that they also had to contend with new kinds of competition. Also from the Permian into the Triassic, a new group of animals started to become very successful: the archosaurs (which means 'ruling reptiles'). In Triassic times, these included a big variety of sorts, many of them crocodile-like, as well as kinds like nothing that survives today. Archosaurs even took to the air, as the very successful pterosaurs. Most successful of all were the dinosaurs, which started their evolution and rise to power in the mid-Triassic. Early dinosaurs were small animals that ran on their hind legs and were likely hunters of even smaller animals. They soon diversified into a greater range of types. Some evolved into herbivorous form; the so-called bird-hipped dinosaurs in the Triassic were still fairly insignificant, mainly being small running kinds. The so-called lizard-hipped dinosaurs included the first large plant-eating types, starting out bipedal but going back down onto all fours as they became bigger still. These kinds, the sauropods (or sauropodomorphs, in the case of the earlier ones) became the dominant herbivores, and there were already several species all over the Earth by the end of the Triassic. They were stiff competition for the earlier herbivorous proto-mammals, like the dicynodonts.
But worse still for the proto-mammals were the other lizard-hipped dinosaurs (just a note– some recent research appears to dispute the lizard-hipped/bird-hipped division of the dinosaurs, which I'll go into greater lengths discussing once I get to the dinosaurs proper in this series) who fed on flesh rather than leaves: the theropods. These were initially small but very swift, running on their hind legs with long tails stretched behind them. It is likely that even these very early ones were metabolically sophisticated, with high and constant body temperatures, efficient digestion and breathing, and rapid growth. They had air sacs as extensions of their lungs, as modern birds do; they likely had a covering of feathers or at least of a downy kind of fluff, to keep in their body heat. Some kinds lived in packs and likely hunted cooperatively; by the end of the Triassic, a few had evolved to huge size. Even though they were not yet as huge as they became during the later Jurassic and Cretaceous periods, they were larger than any of the proto-mammals, and likely much faster. Against such predators, the herbivorous proto-mammals had little defense, while the carnivorous ones were outcompeted.
Still, this was not the ultimate end for the proto-mammals. They went underground, so to speak: they became smaller, and retreated into ecological niches where they had an advantage over the dinosaurs, or at least, where the dinosaurs didn't have a significant advantage over them. They became smaller, mainly; many likely became nocturnal. I'm not exactly sure why, but it appears there was always a dearth of nocturnal dinosaurs; even today, the descendants of the dinosaurs, the birds, are mostly day-active. So back in the Triassic, the mammalian ancestors started to rule the night, leaving the day to their dinosaur rivals.
So let us now look at three kinds of advanced cynodont proto-mammals that lived in South America in the mid- to late Triassic. First we have Chiniquodon brasilensis. It was the size of a small dog or a fox, to which it had some resemblance, except for likely not having had external ears. It was likely carnivorous, having had sharp canine teeth as well as cheek teeth with sharp extensions forming shearing ridges, like the carnassials of modern predatory mammals. It had large eye sockets and might have been able to see in the dark. Though its legs were rather short, they were almost upright, lifting its body well off the ground and allowing it to run fast. It is very likely that it was covered in fur. Its small size made it less of a direct competitor for the first predatory dinosaurs which were already active in that time and place.
Our next critter is an Exaeretodon argentinus. Though I don't like criticising the work of other artists, one model of Exaeretodon (I won't link it here) is one of the ugliest things I've ever seen! Though it's possible that it was ugly, there's no need for insulting it like that. I hope I reconstructed it more flatteringly. Exaeretodon was one of the largest remaining proto-mammals of the late Triassic, reaching an overall length of about 1.8 m/6'. It was long and low on the ground, with a comparatively large head. Its grinding cheek teeth show that it was mainly or fully herbivorous. In its bones it was extremely advanced, very close to fully mammalian. Bones of youngsters are known, though we're not sure if it gave live birth or laid eggs; these had somewhat different proportions from the adults, a typical situation in the proto-mammals.
Last but not least, here is Trucidocynodon riograndensis. It lived in Brazil, in the Late Triassic, about 220 million years ago. What is interesting about it, is that it was not only very, very close to the first true mammals, but also that it was by far the largest of these very-very-almost-mammals. Its other close relatives, on the way to full mammalhood, were quite small, a few of them being mere mouse- or shrew-sized. Trucidocynodon, instead, grew to the size of a small leopard! This means that even as late as this, some proto-mammalian carnivores were still 'contenders' against the larger dinosaurs and other predatory archosaurs.
Trucidocynodon's skull reminds one of that of a large dog or cat; it had sharp, recurved canine teeth, and sharp-edged shearing cheek teeth. Its body looks quite flexible, with an unusually long and thin tail, quite similar to a leopard's. Its limbs were almost directly below its body. In addition, it appears to have started to walk and run more on the front part of the foot rather than flat on the entire sole. The toe-walking style is called 'digitigrade' and is used by many modern mammals (not us, though, we humans are still sole-walkers, that is plantigrade) especially ones that can run fast. Consequently, Trucidocynodon was likely one of the fastest runners among the proto-mammals. This highly adapted and successful predator might have been a victim of the life-changing, extinction-producing events of the later Triassic; if it wasn't for these, it might very well have hung on into the Jurassic and perhaps evolved into sophisticated dog-like and cat-like predators almost a hundred and fifty million years ahead of time!
Trucidocynodon's far smaller relatives, however, were the ones who would win the big prize of ongoing existence. These very insignificant-looking critters were the ones to make the news. By the end of the Triassic, they had evolved into the very first – and very tiny – true mammals! But even then, it still wasn't a sure thing that they would survive and thrive. There was one group of proto-mammals that never made it to full mammals, but came very, very close, and in the event was so close, that they remained successful alongside the true mammals, throughout the whole Jurassic and most of the Cretaceous. These were the tritylodonts, and we'll look at them next.