Colours of Wildlife: On To Mammals!
Created | Updated Feb 24, 2019
On To Mammals!
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!"
In this series of articles I've now comprehensively discussed the proto-mammals, starting with the very lizard-like types such as Casea broilii and showing a series of progressively more mammalian types. I didn't quite write the articles in chronological order as the critters themselves showed up over time; nor did I put them in order from least mammal-like to most-mammal-like. Indeed, this would not really be feasible, since there were so many proto-mammals with so many different kinds of features and adaptations. Certain mammal-like features were apparently independently evolved in different groups, and many lines evolved specialisations not necessarily taking them closer to mammalian status. But we did generally move from less mammal-like to more mammal-like. The sequence goes more or less from least to most mammalian: Pelycosaurs – Biarmosuchians – Dinocephalians – Therocephalians/Gorgonopsians – Cynodonts (omitting side-branches like the Anomodonts and the Dicynodonts). The cynodonts in turn went from the 'primitive' Procynosuchus, Charassognathus and Dvinia of the late Permian, to the extremely mammal-like Tritylodonts we saw in the last instalment, which lived from the late Triassic to the early Cretaceous. But the Tritylodonts were not on the direct line to the mammals, instead being a very successful side branch which existed for a long time alongside the early true mammals. But let us now, at last, look at the line which directly led to the first mammals! What kind of things were they, where and when and how did they live?
The most direct ancestors of the true mammals were unusual particularly in one respect: they were absolutely tiny! You'll remember, if you've read at least some of the previous prehistoric articles, that the proto-mammals in general weren't very big. The biggest ones were those living in the Permian, which included hippo- and rhino-sized animals. In the Triassic, they were generally smaller, and in the middle of that period, the dinosaurs arrived on the scene, and soon started to dominate all the ecological niches for big animals, whether carnivorous or herbivorous. Most of the proto-mammals of these later periods were consequently modest-sized; either to escape competition with the dinosaurs, or to escape the notice of large predators. So many of the advanced cynodonts were the size of an average dog or cat.
But the most direct ancestors of mammals took it farther still. The critters you see here, were the ones most close to the direct line of mammalian evolution, and they were the size of rats or mice! Not only that, but they looked very much like rats and mice (and shrews) as well. Here you see a full-body reconstruction of Brasilodon tetragonus ('four-cornered tooth from Brazil'). If you saw it today, you'd call it a mouse or a shrew. This little critter, as you might gather from its name, lived in South America, in the Late Triassic period, about 228-209 million years ago. It differed from mammals in but a few respects. It did not yet have the fully mammalian jaw joint/inner ear structure; it still did not have the single tooth replacement event that mammals have – milk teeth and then permanent teeth – instead having multiple tooth replacements continuing late into life.
Here you have a selection of faces of other very-almost-mammals. They basically fall in two groups, the Brasilodonts (related to Brasilodon) and the Trithelodonts (related to Diarthrognathus), but all quite close to mammalian ancestry. They all were of a similar size and build to Brasilodon. Protheriodon estudianti (' student pre-beast-tooth') and Brasilitherium riograndensis ('Beast from Brazil from the Rio Grande' – that is the state Rio Grande do Sul) were Brasilodonts, existing in South America from the Middle to the Late Triassic, about 240 to 201 million years ago. They had rather long-snouted faces. Brasilitherium also showed advancing development in the middle ear, and a proportionally big brain.
The other four critters are all Trithelodonts (not to be confused with Tritylodonts). Diarthrognathus broomi ('Broom's critter with the two-jointed jaw' after palaeontologist Robert Broom) was found in South Africa. It lived from the Late Triassic to the Early Jurassic, that is, from about 201 to 189 million years ago. The name refers to the jaw hinge, which was marvelous for being transitional between the old pre-mammalian or so-called reptilian form, where the joint was between the articular bone of the lower jaw and the quadrate of the skull, and the mammalian joint, which is where the dentary becomes the sole jawbone, and hinges against the squamosal bone of the skull. This little critter had the jaw utilising both of these joints! It is therefore the kind of transitional fossil that is often demanded by Creationists who deny that evolution between highly different critters can happen. Of course, even if you show them the jaw joint of Diarthrognathus, they only come up with new demands. But back to our little critter. With the inception of the new jaw joint, the bones making up the old joint became free to fulfil other functions. As I've mentioned before, two of the old jawbones, the angular and articular, shrank greatly in size, and joined another small bone of the skull, the stapes, in an entirely new structure which ended up inside the middle ears of true mammals. These tiny bones now serve to conduct sounds to the inner ear, where the vibrations are processed into nerve impulses, allowing us to hear sounds.
Pachygenelus was very similar to Diarthrognathus, and also lived in South Africa. It was too late to be a direct mammal ancestor, since it lived in the Early Jurassic, while the first true mammals were already present in the Late Triassic. Riograndia and Chaliminia are both also Trithelodonts from the late Triassic of South America, both with rather short, round faces. Their teeth differ a lot: Riograndia has especially large lower front teeth, while Chaliminia as lots of fine, peg-shaped teeth. The latter might have been more of an insectivore, while the former likely ate a lot of plant foods.
It is difficult to say with precision which critter was the actual ancestor of mammals. Whatever it was, though, it was something similar to these ones. But by the Late Triassic, the real thing was finally on the scene! And this is what it looked like; this is what all of that evolution finally culminated in:
This is Megazostrodon rudnerae, '(Ione) Rudner's large-girdle tooth', its name relating to a ridge found on its molar teeth. This thing was only about 12 cm/5" in length! But it was finally a real mammal. Its teeth were of four types: incisors, canines, premolars and molars. It had a single large lower jaw bone, the dentary, which hinged with the skull – the small middle-ear bones were still in contact with this big bone, but no longer serving as a hinge, instead being free to shrink even more to conduct soft and high sounds better. It had a bigger brain for its size than the proto-mammals preceding it. It was certainly warm-blooded and furry. It had a shortened ribcage with a diaphragm separating its chest from its abdomen so that it could breathe more efficiently. It had a secondary palate so it could breathe while chewing. It had a nasal cavity that in life likely had membranes and mucus to help it detect smells more efficiently and to warm, cool and moisten air it breathed in. Its limbs were set below its body, only its elbows sticking out a bit. It likely did not yet have external ears or pinnae, and it likely still laid eggs – but those two are missing in two kinds of modern mammal as well, the duck-billed platypuses and the echidnas, so lacking them is not enough to exclude it from mammal-hood. There are only some very fine technicalities, that place Megazostrodon just outside of the crown-group, that is to say, the group that unites all modern mammals while excluding everything else. For instance, it still had unfused vertebrae in its neck, and fewer sacral vertebrae than modern mammals do.
Fossils of Megazostrodon have been found in Lesotho and South Africa, and date from the Late Triassic to the Early Jurassic. Some other very early mammals include Sinoconodon ('Chinese cone-tooth') from China and Morganucodon ('Glamorgan tooth') from Wales. From here on forward, a whole slew of tiny shrew-like mammals appear in the fossil record. I'll see whether to do an article about them … these articles of mine focus very much on diversity, and those very, very early mammals do not appear to be very diverse just to look at them. They're basically ancient equivalents of rodents and shrews, pretty much all adapted to creeping about at night. But … there was already evolution and diversification happening. In the Jurassic, the mammals, while remaining small and keeping a low profile, diversified into new types such as diggers, climbers, swimmers and even gliders, and by the Cretaceous there were many orders of mammal types active and successful, already giving a hint what would happen if only they were allowed out in the sun. Which happened finally when the non-avian dinosaurs died out at the end of the Cretaceous, kick-starting the explosion of mammalian evolution which yielded ourselves and all other modern mammals such as rats, cats, bats, whales, elephants, giraffes, rhinos, monkeys, kangaroos and more. But this and all later mammalian successes were made possible by the evolutionary potential somehow already contained in the tiny bodies of these very first true mammals.
