This thing.

This thing. When I was a kid I would get the plastic dinosaurs.
The set made it seem like woolly mammoths and T rex lived together; I knew better.
My favorite was this Dimetrodon.

The funny thing is that they are closer related to us than they are to dinosaurs.

They are as ancient to dinos as dinos are to us.

Synapsids like Dimetrodon had a nose like a dog, legs under the body, differentiated teeth like us, and that hole in the side of the skull like we have.

A branch of synapsids evolved into Therapsids and eventually those critters shrank down into the little proto-mammals running around under TRex's feet.

These are just some of hundreds of missing links found since I was a kid.

Humans are Synapsids but the Dimetrodon were doing it way before it was cool (well, before several ice ages anyway). Our head holes (temporal fenestra),eventually closed to protect our brain. We no longer have the trait that classifies us taxonomically as synapsid.