September 23. Early mammals?

You remember the therapsids – that’s the group that includes all mammals, together with their pre-mammalian ancestors. They got their start in the Permian, and only a few pre-mammals, cynodonts and dicynodonts, survived the extinction at the end of the Permian. And most of them were gone by the Late Triassic. The only group that survived that long is called the eucynodonts, meaning “true dog teeth.” They were still probably not true mammals, but they were becoming more and more like modern mammals. One variety, called Tritylodonts, lived in what is now South Africa in late Triassic time. They were small warm-blooded rodent-like critters and were initially considered to be the first mammals, but since the 1980s they’ve been seen as close relatives to the mammals. There is no good consensus as to whether that group, or one of several others, was the direct ancestor to modern mammals. 

All of the descendents of the cynodonts and dicynodonts that survived into the late Triassic were probably insectivores or herbivores, an interpretation based on their teeth and jaw structures. Some were burrowing animals and some may have been arboreal and nocturnal. They were only a few inches long, so the traditional comparison of early mammals and their ancestors to modern shrews is a good one. 

Eozostrodon was one of these animals, found in the very late Triassic rocks of England. It dates to about 201 to 205 million years ago, the last 4 million years of the Triassic. It may be a true mammal, but there is ongoing discussion about that question. Its teeth were very much mammalian. Bones of some of its close relatives, called Morganucodon, have been found around the world, from China to South Africa and in North America. Whether Morganucodon is a mammal or a pre-mammal almost seems to me to be a matter of semantics, and exactly how one defines a true mammal versus a close ancestor that wasn’t quite a mammal. It has some teeth that are absolutely mammalian, as well as hair, and some skeletal features that are not so mammalian. Its ear bones are closer to those of reptiles than to mammals, but are clearly evolved from the original reptilian anatomy. So it’s a little subjective, but I guess maybe I’d call it one of the earliest mammals. But I’m not an evolutionary paleontologist, so take that with a grain of salt.

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.
Drawing by FunkMonk (Michael B. H.) used under Creative Commons license.

In any case, we were getting very, very close to having true mammals on the scene by the very late Triassic. But the Triassic mammals or pre-mammals were certainly not as dominant as their ancestors, the cynodonts and their kin, had been several million years earlier. The Triassic favored the development of the archosaurs and their descendants, the dinosaurs, perhaps because they had better ways of managing water – mammals excrete urea, which includes a lot of water, while archosaurs excreted a paste containing little water. But the dominance of archosaurs might have been a blessing in disguise. If the only surviving pre-mammals in the late Triassic were small insectivores and herbivores, their small size might have pushed them into better modes of regulating temperature, and their likely nocturnal lifestyles might have pushed the development of senses like hearing, sight, and smell, as I suggested in the post on the Antarctic a couple days ago. Those developments in turn might have helped these animals develop larger brains.

So although the ancestors to mammals barely survived the Permian extinction, and then barely survived the rigors of the Triassic world, enough did survive to fill some specific ecological niches so that they could expand when the time was ripe – even if that time wasn’t for another 135 million years.

—Richard I. Gibson

New blog post about Morganucodon

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.
Drawing by FunkMonk (Michael B. H.) used under Creative Commons license.