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.
|The gorgonopsid Arctops. Image by Nobumichi Tamura|
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!