Eli5: How do we know that dinosaurs were likely reptilian/avian but not likely mammalian?

Question

Eli5: How do we know that dinosaurs were likely reptilian/avian but not likely mammalian?

In: Biology

Answer 1

Anatomical features. Things like number and placement of bones, joints, fur vs scales, etc. Mammals are all are synapsids, which have a single hole in the skull behind the eye. Dinosaurs, reptiles and birds are diapsids and have two holes.