xkcd #3204: Dinosaurs And Non-Dinosaurs
Title text:
Staplers are actually in Pseudosuchia, making them more closely related to crocodiles than to dinosaurs.
Transcript:
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Source: https://xkcd.com/3204/
TIL dimetrodon is not a dinosaur, based on a silhouette in a cartoon.
I haven't really thought about dimetrodons since I was a kid.
What is funny is that you probably struggled to say words like "shoulder", but "dimetrodon" posed no problem to you.
The name (di metro don ≈ two teeth sizes) is a clue, as teeth specialization is very much a synapsid (i.e., mammal and proto-mammal) thing.
How about snakes?
True, (some) snakes have also evolved specialized fangs.
Several times independently with significantly different designs, it seems.
I'm a little disappointed, I was hoping for some very specific explanation why it's technically different :-) It's cool that they are independent, thanks!
Well, reptiles seem to have split pretty early on between the ancestors of lizards and snakes (and the lonely tuatara)), and the ancestors of turtles, crocodiles, and dinosaurs, the main differences seemingly being in the bones of the skull, and specifically for the group with the snakes and lizards in, the ability to self-amputate the tail (though that's lost in many of their descendants), and the keratinized scales; you won't see a turtle, crocodile, or dinosaur melting its whole skin in one go like lizards and snakes do, they'll molt their scales (or feathers, or scutes) one at a time.
That's pretty awesome! I may have gotten old, but I never got tired of learning stuff about ~~dinosaurs~~ prehistoric beasts.
So what's the deal with the pterosaurs, what makes them "not dinosaurs"? I believe the really weird looking silhouette is a Quetzalcoatlus, that's just fun to say.
That one I'm not entirely sure about, but it seems that, in the same way being a mammal (from a bone perspective) is all about the teeth and inner ear, being a dinosaur is all about the hips (dinosaurs have an upright stance, with the legs under their bodies; even with the quadruped ones you can see how they're really something evolved to walk on its hind legs walking on its hands and feet), and pterosaurs and their non-dinosaur ancestors just don't have the right kind of hip.
It's a bit muddy, though. Once you get into archosaurs and before you get into more specialised things like crocodiles, dinosaurs, or pterosaurs it's mostly “this thing seems to be more closely related to this group than to this other group, so we'll throw it in with them even if it doesn't really look anything like them”.
There's a small bipedal reptile, for instance, scleromuchlus, that's been bundled up with pterosaurs because it apparently seems more related to them, even though if you look at an artist's representation you'd assume it must be a dinosaur, but might in fact not fit in either group and be instead just a basal avemetatarsalian (or maybe even lower in the tree) with no other identified close relatives.
They are actually closer related to mammals than to any modern reptile