Descending through the therapsid/theriodont line, the first true mammals appeared in the Late Triassic (although some scientists place them a bit earlier). They were small, about the size of a large mouse, and covered in fur. One example of these early mammals was Morganucodon whose features (although primitive) bear resemblance to modern mammals (see image). Marganucodon possessed the distinguishing features of a true mammal1 such as specialized teeth and a mamal-like jaw (although their jaws still bore some reptile characterisitics).
There are other interesting early mammals (such as eozostrodon), but they engender endless debate and untangling all such arguments must be left to the experts. The mammals share their ancestry in the synapsid line with the mamal-like reptiles: dicynodonts and cynodonts. It is important to note that there is no definitive fossil evidence illustrating the final transition from the mammal-like reptiles to the true mammals.
Mammals remained small and insignificant throughout most of the Mesozoic unable to compete with the dynamic developments in dinosaur physiology and size. This size difference between the dinosaurs and the mammals would remain in place until the dinosaurs were exterminated at the end of the Cretaceous. Once the dinosaurs were gone, and the oxygen levels increased, there would be a dramatic jump in the size of all mammals.2
Mammals were most likely nocturnal and fed primarily on insects and other small beasties. It is easy to imagine these clever little creatures scurrying in the underbrush trying to keep out of the way of the dinosaurs, living in burrows in the ground or in the hollows of trees. Since their brains (in relation to their body size) were larger than any other animal of the time, they would have been able to outsmart many of their predators; yet it wouldn’t be until after the dinosaurs were destroyed that mammals would have their opportunity to flourish.
1. Prothero and Dott, Evolution of the Earth,
2. Ward, Out of Thin Air, 226.