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!"
Finally I come to the last group of proto-mammals that I want to discuss here (although perhaps I will revisit some of the earlier groups; I haven't actually treated the pelycosaurs and caseasaurs in as much detail as I now would like to), namely the most 'advanced' group, the cynodonts. The name means 'dog-toothed'. The early cynodonts indeed include many very dog-like forms. But the later cynodonts included many more kinds, and indeed, the cynodonts evolved into the true mammals. Which means you and I, cats and dogs, whales and bats and horses and mice and otters and giraffes and kangaroos and duck-billed platypuses and every other kind of mammal you can think of, are also cynodonts. For this group of articles I will limit myself to the non-mammalian cynodonts, the 'last' group of proto-mammals. The first ones evolved in the late Permian, but the group diversified greatly in the Triassic. At the end of the Triassic, all proto-mammals other than the cynodonts (and very perhaps, some dicynodonts) died out. True mammals indeed evolved just about right at the end of the Triassic, but non-mammalian cynodonts still remained well into the early Cretaceous (and that doesn't even include the Mammaliaforms – see below).
The non-mammalian cynodonts have long been known to science; Richard Owen named the group in 1861. The earliest members were already very mammalian: almost certainly they were furry; they had teeth diversified into incisors, canines and molars; they had secondary palates, which means they could breathe while their mouths were full, and therefore could chew their food properly – which means they could digest their food better and get more energy from it; they had respiratory turbinates, or shelves of bone and cartilage that cooled/heated and moistened the air they breathed; even early forms appear to have had diaphragms, meaning they could breathe more efficiently; in the cynodonts, the dentary became almost the sole remaining bone of the lower jaw, the other bones, the articular and the angular, becoming small and detaching from the jaw joint so that they could become, together with the stapes, the tiny sound-conducting bones in the middle ears of the true mammals. Late cynodonts evolved such mammalian features of the skull (nevertheless not actually present in all modern mammals) as losing the bony bar behind the orbit of the eye, as well as losing the bones at the tip of the snout, meaning the snout could become fully muscular and free-moving. Cynodonts had an advanced 'carriage' in that they walked and ran with their bodies lifted well off the ground; their hind limbs were mainly upright, but the forelimbs in the earlier kinds still splayed a bit to the side. Cynodonts likely included day-active as well as night-active types. Sometime likely during the late Triassic, some of them started producing milk and suckling their young. It is indeed not really possible to say exactly when they became true mammals. Even today, scientists are still arguing whether to consider a whole slew of species as true mammals, or whether to consider them highly mammalian cynodonts that are nevertheless outside of the group of true mammals – Mammaliaforms. Those were small, furry critters very much like mice, shrews, rats, moles, squirrels, flying squirrels, otters and badgers, and if you saw them, you wouldn't doubt that they were mammals. But these as well as very early types of true mammals likely all laid eggs. True live-bearingness evolved only in the therian mammals, which today include the placentals (us) as well as the marsupials.
The earliest cynodonts were not very brainy, though. They had big skulls, but the braincase itself took up only a small part of it; much of the skull volume was taken up by bulging jaw muscles. But in later, true mammals, the brain started enlarging significantly. I still wonder if those early mammal-ancestors were actually stupid, or whether they made more efficient use of the brainpower they had, or simply had less complex environments or were skilful at de-complicating their lives.
Let's look at a few typical early cynodonts. First we have Charassognathus gracilis ('slender notched jaw'), the earliest and most 'primitive' known true cynodont. It lived in the late Permian, about 260 million years ago, in South Africa. It had fully differentiated teeth and a skull about 5 cm in length, meaning the animal as a whole was roughly the size of a big rat (unless the skull remains we have came from youngsters, which grew larger as they matured). It might have eaten insects and other small critters, as well as some plant foods.
From Russia, another very early cynodont was Dvinia prima ('Prime/first critter of the Dvina River'). It was somewhat larger than Charassognathus, about the size of a cat. It had a skull both high and wide at the back, but as I said, this was more for housing jaw muscles than cerebral matter. It was likely an omnivorous animal as well.
A rather specialized very early cynodont was Procynosuchus delaharpeae (De La Harpe's before-dog-crocodile'). From the late Permian of South Africa, Zambia and Germany, it was fairly small, reaching 60 cm in overall length. It is believed to have been semi-aquatic, living much like an otter. It had a flexible spine and a deepened tail, by which means it could have swam with undulating movements, and it had flat, broad fore- and hindfeet, good for paddling. It also had attachments for strong muscles to draw the thigh backwards, which could have been for a powerful leg-stroke during swimming. Its typical diversified cynodont teeth could have dealt with fish of various sizes, and perhaps other foodstuffs also.
The next one I've already featured here, it's good old Thrinaxodon liorhinus. Briefly: this small cynodont lived in the early Triassic in South Africa, and shows a very dog-like shape and carriage. It lived in burrows. Its reinforced ribs were mainly in the forward part of its ribcage, leaving a short-ribbed hind area, suggesting that there was a division between the lung region and the belly/gut region, with a diaphragm separating the two.
Very dog-like indeed, was Cynognathus crateronotus, 'dog jaw marked by a hollow(?)', the most famous of the pre-mammalian cynodonts. With a skull 30 cm long, it was the size of a large dog or small wolf. Its body was rather heavy-set and short-limbed, and it had a short but thick tail. It lived in the middle Triassic, 247-237 million years ago, in Southern Africa, South America and Antarctica (not so amazing, since those areas were all welded together back in the Triassic). Like Thrinaxodon, its skull shows pits which might have been conduits for nerves linked to sensitive whiskers on its snout. With its long, sharp canine teeth, it was likely an efficient predator.
In my drawing, you see that it caught a small reptile-like critter. That is actually an early Archosaur, which lived in the same time and place. The archosaurs soon diversified massively, into crocodiles and their kin, the pterosaurs or flying 'reptiles', and into the dinosaurs – indeed, the very first of the dinosaurs (though still small) were contemporary with Cynognathus. Soon, archosaurs became the dominant predators on land, and turned the tables on poor Cynognathus and its kin. It was one of the last large predators among the proto-mammals. First the crocodile-like rauisuchians, and then the true dinosaurs (one of the earliest large types of which was the double-crested Dilophosaurus) became the new critters to fear.
Lastly we have good old Diademodon grossarthi ('Grossarth's diadem-tooth'). I am fortunate to have a little book (published by the old South African Department of Mining) with lovingly detailed illustrations of a large variety of skulls of these found in South Africa, Namibia and Zimbabwe. They show an amazing diversity in the different individuals, such as proportions of the faces and the shapes of the bones at the sides and backs of the skulls. Diademodon was close to Cynognathus and very dog-like, but differs in that its cheek teeth were broad and flat. This would have made them better for grinding. In addition, the cheekbones and those at the back of the skull flared very wide (widest of all in some kinds that are assigned to D. grossarthii in my book) which means absolutely massive cheek muscles. Such strong, grinding jaws today are associated with plant eaters – it is not necessary to chew meat so very thoroughly. So, Diademodon was either a plant-eater or an omnivore. Since it had sharp front and canine teeth, it still would have been able to kill other animals. But maybe those teeth were more used for defense. I like to think of these as relatively peaceful herbivores. Diademodon varied much in size. Some skulls clearly came from juveniles, but even adult skulls range from about 15 cm to about 40 cm in length. A close relative, Titanogomphodon crassus, had a skull 50 cm long, belonging to an animal that might have been 2.5-3 m in overall length, making it one of the largest if not the largest of the pre-mammalian cynodonts. Diademodon fossils, like those of Cynognathus, were found in southern Africa, South America, and Antarctica.
These are all rather typical early cynodonts. Next, we'll be looking at some more advanced cynognathans. These, though extremely mammal-like, were not yet the direct forebears of the true mammals.