Geology Reference
In-Depth Information
Some common saurischian dinosaurs are illustrated in Figure 20-8.
Figure 20-8:
Saur-
ischian dinosaurs.
Laying the Groundwork for Later Domin-
ance: Early Mammal Evolution
Throughout the Mesozoic, reptiles dominated the land and the seas. But it was during
this time that early mammals, or at least their predecessors, began to appear. A group of
animals called the
therapsids
appear to have bridged the gap between reptiles and mam-
mals prior to 251 million years ago. One group of the therapsids led the way to mam-
mals.
The
cynodonts
were a therapsid animal that had many characteristics in com-
mon with modern mammals. These mammalian traits include some very specific
skeletal features of the head, jaw, and ear. Other traits exhibited by cynodonts
that are linked to mammals are an outer covering of skin instead of scales and in-
