Colours of Wildlife: Therocephalians
Created | Updated Jun 24, 2018
Therocephalians
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'm currently working on a huge project, an illustrated guide to prehistoric life. I'd like to share some of it with you folks on h2g2, so I will feature a Colours of Wildlife prehistory special every now and then. Today, let's look at some Therocephalians!
It is rather shocking that so few people have the faintest idea what therocephalians are. Do you have a clue? It has to do with the origin of mammals, among which we count ourselves. Mammals have an amazing prehistory. The first true mammals are known from about 200 million years ago, in the Late Triassic period. Modern mammals divide into three big groups: First, the placentals, which we are as are all our pets and domestic animals. Placental mammals carry their young for a long time in their uterus, feeding them through the placenta, and giving birth to them at a fairly advanced state of development, but still suckling them for a while longer. Marsupial mammals carry their young internally only for a very short period, then giving birth, keeping and suckling their young for a long time in a pouch or marsupium. The smallest group is the Monotremes, consisting of the Platypus and the Echidnas of Australia and New Guinea. Monotremes lay eggs, from which the young hatch, who then lap up milk that secretes from glands on their mother's belly.
It would seem that these three groups only separated from each other over the last 200 million years. The first mammals were likely very much like the monotremes, hatching their young from eggs; the marsupials and placentals separated from this stock later on, in the Jurassic and Cretaceous periods. There were also a few mammal groups apart from the main three, that went extinct without leaving modern descendants, primarily the Multituberculates, a group that was amazingly diverse and long-lived.
But there is also an amazingly long process of evolution that went on prior to the emergence of the proper mammals. I've already covered some of it, mostly in the article about Casea broilii. Pre-mammals go back over 300 million years. The first kinds were very lizard-like but as they diversified they became weirder and/or more mammal-like. As far as we know, Casea and its closest kin were without hair or fur, but not long after it lived, the first furry, fuzzy and fluffy critters must have turned up. Fossil coprolites (petrified dung) from the Late Permian period, about 260 million years ago, contain things that very much look like hairs, so something that lived at the time, must have sported it. We don't know which, though, only that it was something that got ate by something else.
There are a few candidates for what might have been furry critters in the Late Permian: Dicynodonts and other Anomodonts; Gorgonopsians; the first Cynodonts; and the Therocephalians. All of these belong to the main pre-mammal line called the Therapsids. Now I hope to soon cover all of these but for now let's look at one of the main suspects, the Therocephalians. The name 'therocephalian' is a mouthful, but it merely means 'beast-headed'. This is because the therocephalian skulls were among the first to show several highly mammalian characteristics. These include: a bony palate separating the airways from the mouth and throat, to allow breathing while feeding; ridges inside the nose cavity that likely supported sheets of cartilage called turbinates, to heat, cool and moisten inhaled and exhaled air; indications of whiskers and sensitive facial tissues; diversified dentition – in the most advanced forms, there were clearly distinguishable incisor, canine and grinding teeth. Some forms also show the loss of the post-orbital bar of the skull, which is a strange feature the practical significance of which I haven't heard explained yet, but it occurs in the earliest and in many ensuing kinds of mammal also.
For all these highly mammalian features, though, the therocephalians weren't on the main branch of the evolution of the pre-mammals that led to the true mammals. Instead, they were a fertile and for a time highly successful offshoot; it branched and re-branched as the therocephalians repeatedly re-invented themselves and weathered the many crises that decimated other groups, only to finally, and tragically peter out and go extinct themselves in the mid-Triassic. Most therocephalian finds are from South Africa, but some wonderful fossils of theirs also come from Russia.
The earliest therocephalians included Lycosuchus, Pristerognathus and Glanosuchus. They were dog- to wolf-sized predators. Their teeth weren't much differentiated but they typically had at least one pair of huge, sabre-like canine teeth in the upper jaws. Lycosuchus has been found with two canines on each side, but what happened here is that likely a couple of replacement canines emerged while the old pair was still functioning. Apart from their first set of functional teeth, Therocephalians mostly had a single set of replacement teeth which emerged as needed. This, too, was a step towards the modern mammalian way of having a set of milk teeth that is then replaced by the set of permanent teeth. Reptiles like dinosaurs replace their teeth one by one, and continuously throughout their lifetimes.
In the Late Permian, therocephalians were mostly displaced as big predators by the Gorgonopsians. Most of them became rather small, and diversified their diets to include insect food. The unusual Choerosaurus had strange bumps on its face and lower jaw, that might have been used in pushing-and-thumping matches between adult males for dominance, territory or access to females.
Larger predatory therocephalians persisted, like dog-sized Theriognathus. The end of the Permian saw a massive extinction of almost everything that lived – something like 85%-95% of all species living in the late Permian were extinct by the early Triassic. But some therocephalians made it through. Moschorhinus in the late Triassic was a wolf-sized predator with a short and strong skull and sabre-teeth; it survived the extinction event, but those found into the early Triassic had experienced a shrinkage of their dimensions, to merely dog-sized. This was likely because much less suitably-sized prey animals were around, as well as lasting unfavourable environmental conditions made it hard for everything to thrive and grow.
In the Triassic, the therocephalians also had to contend with the new kids on the block, the cynodonts, amongst which counts the amazing little Thrinaxodon. These diversified and rapidly produced a host of new, sophisticated features bringing them closer to the level of the first true mammals. The therocephalians did respond with evolution of their own. Small species like Ictidosuchoides (another survivor from the Permian) and Regisaurus might have been insectivorous, although they could also have hunted critters close to their own size. Scaloposaurus is known from very small skulls that might have been juveniles, but even so, the adult might not have been much larger than a rat. Intriguingly, these late therocephalians produced some of the first small, furry, herbivorous types. Bauria was exemplary of these. A small, fox-sized animal with short legs, it had a beautiful skull with teeth fully differentiated into cutting incisors at the front, a pair of stabbing canines, and cheeks full of grinding molars which fit nicely onto each other. These allowed the efficient chewing of leaves and other plant food. It also had a fully developed bony palate. A relative of Bauria was the somewhat smaller Microgomphodon, the latest-surviving therocephalian, which made it into the mid-Triassic. Sadly, when this little critter died, there were no new ones to replace it. The therocephalians passed the torch on to the cynodonts, which carried it from there to the first true mammals, who carried it on all the way up until today!
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I've only shown you a few therocephalians for now. Many more than this have been discovered, some now being known quite well. I'll soon devote an entire article to one that is suspected of having been one of the first truly venomous predators on the planet. Many therocephalians are known only from sparse remains and there are still a great many unanswered questions regarding them. But I hope this article gave you an inkling of this strange and diverse group of distant cousins to us proper mammals and their lives so many millions of years ago.
