In this video Owl, The Best Pet Reptile? you can hear Clint say,

Crocodiles are more closely related to birds than they are to lizards and snakes.

Is this claim true?

  • 2
    The claim in the video is, "Crocodiles are more closely related to birds than they are to lizards and snakes." – Alpha Draconis Apr 22 '20 at 19:43
  • @user34258: Well done for checking with the source. Updated question to correct claim. – Oddthinking Apr 22 '20 at 19:59
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    Downvoted because this is a well-established fact that is easily found even with minimal google-fu skills. – David Hammen Apr 22 '20 at 21:06

While it is certainly not obvious to the eye, the fact that the closest relatives of crocodiles are birds appears to be well-established. These two clades are the only living members of a group known as archosaurs (Green et al. 2014):

Crocodilians, birds, dinosaurs, and pterosaurs are a monophyletic group known as the archosaurs. Crocodilians and birds are the only extant members; thus, crocodilians (alligators, caimans, crocodiles, and gharials) are the closest living relatives of all birds (1, 2). . . . [C]rocodilians diverged from birds more than 240 million years ago.

(I should note here that Janke & Arnason 1997 discuss various other times for the split; they cite a mean value of 254 million years, but different techniques give varying results, from 242 million years to 276 million years. The authors also state explicitly that crocodiles are only "distantly related to lizards".)

There are of a number of morphological features which distinguish archosaurs from early diapsids and lepidosaurs, some of which are not readily apparent. In Vertebrate Paleontology and Evolution, Robert Carroll notes that

As a group, the archosaurs are characterized by a host of skeletal specializations, many of which are associated with a more upright posture and more effective fore-and-aft movements of the limbs than is evident among the primitive diapsids and lepidosaurs. However, the most primitive archosaurs can be distinguished from other early diapsids only by a single, clearly definable skeletal character, the presence of a large opening anterior to the eye, the antorbital fenestra.

In addition, Carroll notes, among other characteristics:

  • An extension of the premaxilla, a small bone in the jaw
  • Teeth set in sockets instead of grooves
  • An elongate neck with seven or eight vertebra
  • A unique foot and ankle structure not seen in primitive diapsids

All of these are distinguishing features of archosaurs.

Though I'm trying to find exact figures, the archosaur/lepidosaur split appears to have occurred in the Permian at the latest, not too long before the crocodile/bird divergence. Ezcurra et al. 2014 argue for that it must have occurred at least 254.7 million years ago. While this clearly falls into the range discussed by Janke & Arnason, it is still widely assumed that the archosaur/lepidosaur split happened before the crocodile/bird split.

It's also important to note that genomic evolution did not occur at the same rate for all species. As Green et al say:

The most striking of our results is the remarkably low rate of genome-wide molecular evolution among all major crocodilian lineages. although they note that the reason is not completely clear.

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