xkcd #3204: Dinosaurs And Non-Dinosaurs
Title text:
Staplers are actually in Pseudosuchia, making them more closely related to crocodiles than to dinosaurs.
Transcript:
Transcript will show once it’s been added to explainxkcd.com
Source: https://xkcd.com/3204/
I'm a bit thick. Not certain if this is a joke that went over my head or not.
Im still in a rabbit hole trying to understand why some aren't dinosaurs.
If you take the tree of life showing the evolutionary branch, you won’t be able to pick off a branch that includes all of the things you would think of as “fish” but excludes all of the things you think of as “not fish”.
The reason for that is that all land animals with a four limb body plan, including reptiles, mammals, and ~~birds~~ dinosaurs evolved from a family of fish called tetrapods. But there are still tetrapods in the ocean that you would think of as “fish”, and I don’t mean whales.
Hank Green has a much more entertaining and complete discussion at https://youtu.be/-C3lR3pczjo
Basically all life on Earth that's not some bacteria or fungi originally developed in the oceans, and only later adapted to life on land. If you go back far enough, the common ancestor of anything that walks, crawls, or flies ultimately originally swam.
At the same time, all mammals are evolved from a common ancestor on land, which means that whales, seals, dolphins, etc. are the evolutionary result of land-based mammals "re-entering" the ocean.