Wild Neighbors: What, if Anything, is a Reptile?

By Joe Eaton
Monday July 26, 2010 - 06:24:00 PM

I don’t know how I got on the Center for North American Herpetology’s mailing list, but the traffic has been interesting so far. You get announcements from researchers with surplus boas to give away, notices of Florida sand skink post-doc opportunities, abstracts of new journal articles on euthanizing lizards, that kind of thing. Last week there was something a bit startling: a press release for a new book on the amphibians, turtles, and reptiles of Kansas.

Turtles and reptiles? Turtles have always been considered reptiles in good standing. If they weren’t reptiles, what would they be? What’s going on here? 

From further exploration of the CNAH site, it seems that Joseph Collins, a herpetologist at the University of Kansas, has decided to shake up the taxonomy of what used to be the Class Reptilia. Under his proposed revision, turtles are in a class of their own, the Chelonia. Crocodiles and alligators form another separate class, the Eusuchia. Reptilia in a strict sense is limited to lizards, snakes, amphisbaenids (odd legless creatures), and the lizardlike tuataras of New Zealand.  

Counterintuitive as this may be, it makes sense from a cladistic perspective. Cladistic taxonomy was developed by a German entomologist named Willi Hennig in the 1950s. The working vocabulary is not user-friendly, but it comes down to recognizing only named groups that have a common evolutionary origin and that include all the descendants of the ancestral form. 

Hominids are a good example of a valid clade: you’ve got a presumed ancestral ape, orangs, gorillas, chimps, bonobos, and us, 

along with Ardi, Lucy, and the Neanderthals. 

Reptiles used to be simpler to deal with. They all had scaly skin. They were all poikilothermic, with body temperatures governed by their external environment. (Remember the Gary Larson cartoon with the alligator in court: “Of course I did it in cold blood, you fool! I’m a reptile!”) They all laid amniotic eggs, or gave birth to live young. They did not have hair or feathers. They were where vertebrate evolution went after the amphibians. 

But there’s a problem. It’s well established that mammals evolved from a group of primitive reptiles, and that birds evolved from another reptilian lineage (although a few diehards still dispute the  

descent of birds from dinosaurs.) If you’re a cladist, there’s no way to define reptiles without including mammals and birds. Reptilia, Collins says, is “an unnatural grouping.” 

There’s a similar situation with fish, which ichthyologists have split up into several classes. The former class Pisces is long gone. Cladists like to point out that we are much more closely related to lungfish than we are to, say, trout. But us lay folk can still use the word “fish” to collectively denote lampreys, sharks, coelacanths and tuna if we want to. 

So far, Collins’ treatment of the reptiles, or former reptiles, is only a proposal that may or may not catch on. There’s no international group that makes the rules on animal nomenclature, as there is for plants. I can foresee some problems with its application, though. 

The turtle part is the easiest to swallow. Turtles have been evolving on their own for a long time. They’ve been separated out from the rest of the reptiles as anapsids, based on the absence of holes in their skulls (other than the standard eye sockets.) And, at least in terms of living species, there’s no ambiguity as to what is a turtle and what is not. Snakes, lizards, and tuataras also form a coherent lineage, although the fact that snakes evolved from lizard ancestors makes “lizards” another non-natural grouping. 

Alligators, though? The last I looked, crocodilians were considered to be a sister group of the archosaurs, the creatures that gave rise to dinosaurs and ultimately to birds. So there’s a closer connection between birds and alligators than between birds and turtles. Shouldn’t that be recognized somehow? And what do you do with all the extinct forms: the dinosaurs themselves, the pterosaurs, all the seagoing lineages? Are they left in taxonomic limbo? Do the dinosaurs get promoted into Aves, with the birds? Where do you redraw the lines? 

This must all be so much neater if you’re a creationist: God created turtlekind, end of story. Once you acknowledge that life has its own history, things tend to get messy.