Amniote

This week I tried to find out which came first, the dinosaur or the mammal. I fell short. 

The easy answer to the question is that no one knows which came first; Dinosaurs and tiny mammals developed at about the same time.  But new questions developed.

Along the way I learned that dinosaurs, warm blooded creatures, are classified as reptiles, which are cold blooded by definition. (The several different ways in which we sort animals into groups are "by definition.")

Apparently biologists have a rule, which is that an animal is the same thing as its ancestors. The ancestors of dinosaurs are reptiles, so dinosaurs are reptiles. 

Mammals are not descended from reptiles but from an earlier life form called amniotes. (Reptiles also developed from amniotes.) 

Birds, as you know, descended from dinosaurs, so birds are dinosaurs.

Dinosaurs are reptiles, so birds are reptiles, according to some but not all biologists. 

You and I, by this logic, are amniotes, parts of the amniote glade. 

Humans invent categories that we find useful. That can take us down many forking paths.