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/
Non-biologist here. Is this a taxonomy thing? So, if it’s under /animalia/chordata/reptilia/dinosauria/, it’s in that dino box, right?
What about penguins then? According to Wikipedia, they’re under /animalia/chordata/aves/etc. I don’t see …/reptilia/dinosauria/ anywhere in that classification. Likewise, seagulls are under /animalia/chordata/aves/… etc. so nowhere near dinosauria. What am I missing here?
That untangling evolutionary history is really messy :D It’s not part of Kingdom/Phylum/Class/etc classification, but biologists also use clades to understand evolutionary history. Birds and theropod dinosaurs belong to the clade Theropoda, and to the clade Saurischia which includes some more dinosaurs, and they all belong to the Dinosauria clade which includes all dinosaurs.
The division (as interpreted by evolutionary biologists) between birds and the rest of Saurischia is smaller than the division between Saurischia and the rest of Dinosauria. So, if everything in Dinosauria is a dinosaur, so are birds.
Ok, so what’s the deal with this clade thing? Why don’t we use that for classifying birds?
Then again, maximizing the number of inconsistent exceptions seem to be a running theme in biology, so I guess it’s on brand…
Clades are used for classifying birds and everything else, it’s just a little messier than the neat Kingdom/Phylum/Order/etc way of laying out evolutionary history. If you think of a species as being a specific pinpoint on the tree of life, clades are more like drawing a circle around a lot of pinpoints and branches.
But yeah, biology is nothing but ‘the last thing we taught you was an oversimplification!’ all the way down.
Yeah, that’s biology, all right. 😃
Anyway, thanks for the explanation. Never heard of clade grouping before. Is it handy for dealing with long extinct species, or why does such a grouping even exist?
We do, a clade is just a group consisting of an ancestor and all its descendants. So basically, the evidence indicates that birds are all the descendants of a more traditional dinosaur and are more closely related to more classic dinosaurs like t Rex and velociraptors than those dinos are to ones like brontosaurus.
In part this is done by examining physical characteristics in fossil specimens and seeing where traits seem to appear and how they seem to evolve over time. Like it’s obvious that marsupials are more closely related to us placental mammals than either are to birds or reptiles. After all we give live birth and have hair and milk. And both groups are more closely related to each other than egg laying mammals (monotremes) like platypuses, but that all three are more related to each other than birds, alligators, and lizards. And you can keep going looking at less and less obvious traits. And eventually you see that a weird division in how jaws work happened in the Paleolithic that separated these two clades, and that actually dimetrodon is more closely related to us than to dinosaurs.
And since you can do stuff like that you can see that in the jurassic a group of theropod dinosaurs started evolving feathers, and some even evolved beaks and wings. And by the cretaceous some of these dinosaurs were what we’d call birds today. And even better for them, many were small generalists, which we suspect is the best thing to have been at the end of the cretaceous as species fitting that description seemed to survive the best during the extinction event.