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!"
We're getting closer and closer to the true mammals now! If you've read the previous articles about the proto-mammals, you'll (I hope) remember that the first true mammals evolved at the end of the Triassic period, and that they were tiny little things. The other non-mammalian synapsids, or proto-mammals, mostly died out, leaving only the real mammals to continue into the Jurassic and beyond – except for one group, and that's the one we're looking at in this instalment. The tritylodonts were a very advanced and successful group of proto-mammals that held their ground alongside the true mammals, surviving through the Jurassic and even into the Cretaceous period. And also, these tritylodonts were, while not huge, typically quite a bit bigger than the true mammals that lived at the same time. While extremely mammal-like, indeed, originally having been classified as mammals, the details of their jaws and skulls show that they were actually just short of true mammals. They were an evolutionary side road, which went on for quite some distance before tragically coming to a dead end at last.
The name 'tritylodonts' means '(critters with) three-knobbed teeth'. This signifies their cheek teeth. These were grinding teeth, with three rows of knobs in the upper jaw, and two rows in the lower jaw, to mash their food. The two rows of the bottom teeth fit the two grooves that lay between the cusps of the upper teeth. These teeth were very similar to true mammalian molar teeth. Tritylodonts also had rodent-like front teeth (called incisors), followed by a long gap where the canine teeth ought to have been. Such a gap is called a 'diastema'. This arrangement is similar to that of rodents, and of many other plant-eating present-day mammals that, like the tritylodonts, have lost their canine teeth. From their teeth, we reckon tritylodonts to have been mostly herbivorous. Their jaw hinges were still between the 'old' reptilian bones the articular in the lower jaw, and the quadrate in the upper jaw. They had a tall 'crest' of bone on the upper and back portions of their skulls, and wide-flaring cheek bones, all for anchoring large and powerful jaw muscles. They chewed by moving their jaws by biting down and moving backward in one stroke, which is the opposite of rodents, which chew with a 'back-to-front' jaw motion. They had complete bony palates, meaning they could chew and breathe at the same time.
Another very mammalian feature in the skulls of tritylodonts is that they lacked a post-orbital bar. This is a usually thin bar or sheet of bone closing off the back of the eye socket. Some modern mammals lack this bar, while many still retain it. But very few of the non-mammalian synapsids lacked the post-orbital bar, the others that do also being very close to true mammals.
In the rest of their skeleton, tritylodonts also resembled mammals in having an erect gait (even though most had very short legs) with the feet brought right below the body. This makes supporting the body easier, for quicker and more nimble movements. In their hip girdles, they had bone extensions called epipubic bones. While these strengthened the hip girdle, they also constricted the
abdominal region so that they couldn't give birth to large, well-developed babies. They either laid eggs, or gave birth to tiny larva-like babies, like those of modern marsupials. Because of the very undeveloped state of their babies, they very likely gave them intensive parental care, and may even have suckled them. We lack direct evidence for that, though.
It is almost totally certain that tritylodonts were warm-blooded and covered in fur. Their skeleton shows so many mammal-like features and they clearly were quick and alert mammals with a fast metabolism. Because they were also quite small, a furry coat would have been a virtual necessity for keeping their little bodies constantly warm.
Let's look at three representative tritylodont species. First, there's Tritylodon longaevus itself (its species name means long-aged). This is one of the first ones to be found, having been named by the famous palaeontologist Richard Owen in 1884. Its fossils were found in the Orange Free State province of South Africa. They come from the early Jurassic period, about 190 million years ago. That period in the history of South Africa was not as fully represented in fossils as the earlier Triassic and Permian periods were, but they did feature some of the first early dinosaurs. Tritylodon existed at the feet of these. It was about the size of a cat or small dog, reaching an overall length of 50-60 cm/20"-24". It likely lived in burrows, and its short but stout forelimbs would have helped it dig.
The next one is Oligokyphus triserialis ('small, curved critter with three series') from America. The scientific name again refers to its groups of three cusps on the cheek teeth, and to its small size and curved backbone. Oligokyphus is one of the first proto-mammals I read about, seeing as it coming from America meant it got much more press than the earlier proto-mammals from South Africa did. It was quite a small animal, about the size of a ferret. It likely would have been a cute little thing, with very short limbs but a comparatively large skull. Its high-domed head was due to the skull crest that anchored its jaw muscles, rather than braininess.
Like other tritylodonts, Oligokyphus likely dug using its short front legs. It might have unearthed small bulbs and tubers to eat. Its teeth also could have dealt efficiently with seeds and nuts. It lived in a woodland habitat with seasonal rain. It was not likely to be a good climber. Its fossils were found in the Kayenta formation of Arizona, representing the early Jurassic period, with very similar fossils being found in Europe, Asia and maybe even Antarctica. It occurred with early dinosaurs but also with other tritylodonts, at least three species being known from the Kayenta formation.
Our last tritylodont indeed also comes from the same formation. It is called Kayentatherium ('Kayenta Beast'). It was much larger than Oligokyphus, which means it fulfilled a different ecological role and didn't compete with it directly. Kayentatherium wellesi, the only species known, is named for palaeontologist Samuel Welles, who did research on the fossil critters of the Kayenta formation. It (Kayentatherium, that is, not Samuel Welles) reached a skull length of 30 cm and an overall length of 1.5 m, making it the size of a big dog. It was long and low in shape, though, with a barrel-like body supported on fairly short limbs.
By its build, Kayentatherium was thought to have been a good digger, but recently has been interpreted as perhaps being semi-aquatic. The same broad forelimbs that might have made it adept at digging, would also have been serviceable paddles for swimming. Maybe it's not an either/or proposition, maybe it was good at both! Even today we have animals that both dig and swim. The duck-billed platypus, for example, both swims superbly well and digs its own burrows. Seeing as this mammal is perhaps the closest thing we have left compared to the old tritylodont proto-mammals, it wouldn't be a surprise if Kayentatherium lived similarly. We also today have a few swimming mole species, which I hope to feature here soon.
But certainly, the powerful forelimbs and strong, curved claws of Kayentatherium would have helped it dig out succulent roots, rhizomes, tubers and bulbs. It could also have used its strong front teeth to tear them out, as I illustrate here.
The most amazing discovery concerning Kayentatherium pertains to its mode of reproduction. Recent examination of the fossil of an adult individual disclosed the tiny bones of babies all around it. When all fragments were counted and accounted for, no less than 38 separate babies were found! This is an enormous number for a mammalian litter. Presently the only mammals that get close to this number, are the Tenrecs of Madagascar. These are often considered 'primitive' mammals but despite this, they are still actually fully modern placental mammals. Kayentatherium either laid a large number of small eggs, or bore these babies live but in their undeveloped state, nursing them until they were bigger and able to fend for themselves. Since the babies were proportionally not that different from the adults, it is possible that they became self-sufficient while still physically very small. The baby Kayentatheriums all had proportionally small brains, compared to those of baby true mammals. Again, as I've said earlier in this series, the smaller brains of the proto-mammals do not necessarily mean they were stupid. Their environments might have been somewhat less complex, so they didn't need to deal with as many different challenges in order to survive. But we don't know that for sure; there's very much about them that we still don't know.
The tritylodonts, like I said, persisted into the early Cretaceous period. Just to recap: the first synapsids, the group to which the proto-mammals and true mammals belong, appeared in the Carboniferous, the age of great forests that became our coal reserves, about 300 million years ago; they diversified greatly in the next period, the Permian, with the first signs of warmbloodedness and fur appearing, and the groundwork being laid for them to become true mammals. The Permian was the last period of the Paleozoic or the age of ancient life; it ended in a mass extinction which very few species survived. But proto-mammals of at least three lineages did survive into the next period, the Triassic, and diversified again. The Triassic, the first period of the Mesozoic or age of middle life, lasted from about 250 to about 200 million years ago. In this period, the archosaurs or ruling reptiles diversified and the first dinosaurs appeared. These gave the proto-mammals some very stiff competition. By the end of the Triassic, the first true mammals appeared, and almost all other lineages of proto-mammals died out. The next period was the Jurassic, and the dinosaurs took over almost completely. The tritylodonts remained, as did the tiny true mammals. The Jurassic also saw the first true birds. The Jurassic was followed by the Cretaceous, during which the dinosaurs became extremely advanced, while the birds also flourished. The tritylodonts hung on at least through the early parts of this period, but then died out. But the true mammals were by this time already diversifying into several lineages and even a few reasonably-sized ones existed, like Repenomamus, which was similar to a badger. So, when the Cretaceous ended, and the non-avian dinosaurs with it, all these mammals, tiny or not-so-tiny, inherited a whole world to do with as they please. Our own age, the Cenozoic, or age of modern life, ensued and the whole lineage of the synapsids, started so long ago in those ancient coal forests, finally came to its full flowering.
We might wonder how much longer the so-called 'age of mammals' will last, and if mammals themselves finally go extinct (if they ever will), what new kind of critters (if any!) will follow them?
The next palaeo-article will look at how the proto-mammals finally became the true mammals.