|Origin of Mammals
Triassic Period is often remembered as the time when dinosaurs
and other archosaurs rose to dominance,
and their ancestors also played an important role. In fact, these
synapsids (once called “mammal-like reptiles”) were
very abundant in the early Triassic world. They came in a wide variety
of sizes and shapes, from large, cow-like dicynodonts
(such as Lystrosaurus), to fanged, carnivorous gorgonopsians,
to tiny insectivorous tritylodonts. A Triassic landscape
in South Africa would have been filled with great numbers of these
early synapsids. By the Middle Triassic (about the same time as the
first dinosaurs) one of these synapsid groups had evolved into the
very first mammals. Although many different types of mammals evolved later in the Mesozoic, for the most part they remained small, probably because of the dinosaurs'
success. However, later in the Mesozoic some mammals achieved the size of a 30 lb. (13 kg) dog despite the dominant success of the dinosaurs. Many
types of mammals appeared in the Mesozoic, and evidence from Early Cretaceous outcrops (130 million-years ago) in China shows that at least one carnivorous mammal,
Repenomamus robustus, feasted on small dinosaurs.