this post was submitted on 15 Dec 2025
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They're separate biological classes.
So they're about as far apart as you are from a reptile, bird or fish.
That would mean...not very. Reptiles are an extremely broad and diverse group, containing everything from penguins and crocodiles to tuataras and pythons. Mammals are the most closely-related extant clade that is generally not considered "reptile", to reptiles.
Arachnids, on the other hand, are more distantly related to insects. Crustaceans form their closest relatives, followed by myriapods (centipedes & millipedes). Only then do arachnids appear.
Hold on...
Yup. Birds are reptiles! If you want to define a monophyletic clade that includes crocodiles and lizards, there is no way to do that without also including birds. To define a clade, you take the evolutionary tree and make a "cut" somewhere on it. Everything below that cut is part of the same clade, you can't selectively remove some branches but not others, unless it's by changing where you make your single cut.
So in this diagram:
collapsed inline media
The green circle notwithstanding, you would usually define reptile as a cut at the "C" on the diagram. You could put the cut at Lepidosauria, but that would mean crocodiles and turtles are no longer considered reptiles either.
A more zoomed-in look would show that after crocodiles and birds branched apart, you also get another branch where pterosaurs branch away from dinosaurs, and that birds are one of many branches and subbranches of dinosaur.
I really appreciate the info and the way you laid it out. Just curious, is that knowledge part of a hobby and/or career? Or was that like just one of the random tidbits you picked up somewhere?
Yeah just a casual interest. Not enough that I'd really call it a hobby, and certainly not a career. But a little more than a few random titbits. This video's not a bad quick intro to the reasoning I used in my previous comment.