Early Mammal Evolution

This week on Mesozoic Mondays, we're going to talk about early mammal evolution. Those of you who have been reading this blog since the beginning may remember our post on Dimetrodon, the early mammal relative. You may remember even further back to our post on What is a dinosaur, and about the importance of temporal fenestrae, or the holes in the back of the skull, throughout  the evolution of animals. Mammals are what we call synapsids, which are the group of animals that have just one temporal fenestra. As previously mentioned, Dimetrodon was also a synapsid, one of the earliest relatives of modern mammals. 

The very beginning of mammal evolution takes us back to the Carboniferous, when the first amniotes (animals with an amniotic egg) evolved into two separate lineages: the synapsids, which eventually give rise to mammals; and the sauropsid branch, which gave rise to lizards, snakes, dinosaurs and eventually birds. The first big group of synapsids were the pelycosaurs, like Dimetrodon. They were the largest land animals to live during the Early Permian, and gave rise to therapsids. Therapsids had larger temporal fenestrae, and included some very strange animals like dinocephalians, and the carnivorous gorgonopsids.

Struthiocephalus, a dinocephalian from the Permian

The gorgonopsid Arctops. Image by Nobumichi Tamura

Remembering back to the blog post on mass extinctions, you may remember that the largest mass extinction to have ever happened was at the Permian-Triassic boundary. Therapsids, like all groups, were very hard hit, but the cynodonts survived, and eventually gave rise to modern mammals. Cynodonts first evolved in the Late Permian, and have several mammal-like features including a reduction in the number of lower jaw bones (modern mammals have one bone while reptiles have many). They continued to evolve into the Triassic by further evolving precise occlusion of their teeth, the mammalian ear started to evolve, and their olfactory lobes increased in size to provide better sense of smell. 

Like all evolutionary transitions, the line between mammalian ancestors and true mammals can be hard to distinguish. Early mammals were small, typically the size of a rat or mouse. Their size, the environment they lived in, and delicate bones mean that they were not commonly preserved and are rarely found in the fossil record. Some of the earliest mammals, or mammaliaforms as some scientists call them, include the morganucodontids. These mouse-sized animals like Morganucodon were transitional between cynodonts and true mammals, seen by the lower jaws with two bones, and bony venomous spurs like modern monotremes (platypus and echidna). 

Morganucodon, a Late Triassic mammaliaform. Image by FunkMonk

Early mammals were evolving alongside dinosaurs, and therefore were restricted to ecological niches not occupied. They were generally small, and required little food and sustenance. Although mammals were typically thought of being "in the shadow" of dinosaurs throughout the Mesozoic, there is evidence that they were capable of defending themselves, and feeding on dinosaurs. 

That's all for the initial evolution of mammals, and we hope you've enjoyed it. Stay tuned for additional posts on further evolution of mammals, including the evidence we have of mammals defending themselves and feeding on dinosaurs!