i’m watching Jurassic Park with my dad and talking about how birds are dinosaurs, but it made me wonder: where is the split between dinosaurs and mammals? did it split back in amphibians? earlier than that? i’m pretty sure i read there’s a greater gap between crocodiles and dinosaurs than dinosaurs and mammals, and we’re both warmblooded, so i’m curious about how close we really are.
Dinosaurs and mammals are actually not very closely related at all! And neither evolved from amphibians!
Amphibians, Dinosaurs, and Mammals all share a common ancestor, and are grouped together in a clade called "Tetrapoda"
Amphibians split off from Amniotes (the group that includes Dinosaurs and Mammals) long before dinosaurs were even close to showing up
The first amniote, upon evolving, immediately split into two groups: Sauropsids and Synapsids. Synapsids include mammals and all their extinct relatives. Sauropsids includes reptiles (and birds) and all of their extinct relatives. This split happened around 320 million years or so ago.
Dinosaurs are a specific group of reptiles, their closest cousins being Pterosaurs ("flying reptiles"). The group that includes Dinosaurs and Pterosaurs is actually "sister" to Crocodiles and their relatives - so the closest *living* relatives to dinosaurs are crocodiles!
Dinosaurs are defined as "the most recent common ancestor of Megalosaurus and Iguanodon and all of that ancestor's descendants" (those are the first two non-avian dinosaurs we named). Since all birds descend from that ancestor, all birds are dinosaurs.
So, Crocodiles are the closest living relatives to dinosaurs, and birds are the only living group of dinosaurs. Crocs are closer to Birds/Dinosaurs than they are to lizards, snakes, or turtles. Mammals are equally closely related to all kinds of reptiles, including birds. Amphibians are equally closely related to mammals and reptiles!
I hope that helps!