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So what is a reptile? A better question to ask might be: What isn't a reptile?

The colloquial definition of a reptile is essentially any scaly, "cold-blooded", sprawling animal that makes most people go *ick*. This isn't a very useful definition. The traditional Linnean definition for these animals wasn't very helpful either. It was essentially reserved for all non-avian/non-mammalian animals. In this definition, sharks were considered reptiles.

In an attempt to clear this mess up and make Reptilia a stable taxonomic group, systematists redefined reptilia using cladistic methods. The result was:
Any creature descended from the closest relative to chelonians, crocodilians and lepidosaurs.
The result of this redefining is a very stable meaning, but one that lacks any substance whatsoever. Furthermore since cladistics doesn't separate creatures based on differences, birds wind up being a group of reptiles as well. Compounding this problem is the use of turtles and tortoises (chelonia) as anchors. No one is sure exactly where chelonians came into existence and their placement effects the taxonomic standing of many early reptiles (e.g pareiasaurs & captorhinids).

"I am impressed by the fact that we know less about many modern types than we do of many fossil groups." - Alfred S. Romer
Osteology of the Reptiles 1956
Okay then, so let's dump the most recent common ancestor (MRCA) approach and go back to diagnostic characters. Checking The Vertebrate Notes website, one finds that the diagnostic characters that make up Reptilia are:

  • Tabular small or absent
  • large post-temporal fenestra
  • suborbital foramen (small hole near the lateral edge of palate, between the pterygoid, palatine, and ectopterygoid or jugal, when ectopterygoid absent)
  • supraoccipital plate narrow.
As one can see, defining reptilia is not an easy thing to do. On the bright side, defining living taxa is a little easier than figuring out the who's who of extinct forms. All extant reptiles are characterized by one important trait: Scales.

The scales of reptiles are more diagnostic then they might first appear. Though they come in a variety of colors, shapes and sizes, they originate the same way in all living reptiles. Scales form by a folding of the epidermis during embryonic development. The result is skin with a scaley extension. Thus scales are a form of integument. Skin can be seen between the scales in certain species, while it is readily visible in snakes that are swallowing something large.

So why is this important? Don't fish also have scales?

The answer, of course, is yes. Fish do have scales. In fact, certain amphibian species are also known to possess scales (caecilians and prehistoric taxa), but the scales of fish and amphibians are dermal in origin. If one were to descale a reptile one would wind up with a bloody, but still skin covered animal. If one were to do the same thing to a fish or caecilian one would find organs and muscle instead. So, where the scales play a role as skin in fish and amphibians, they form a type of integument in reptiles. Other unique traits to reptile scales involve their chemical make up, which also differs from that of fish and amphibians. The end result though, is that the scales of living reptiles are a trait unique to this class only.

Whether or not this was true back in the Permian when the first reptile appeared, is unknown. It is also unknown as to whether or not early mammal ancestors, pelycosaurs and therapsids (no longer considered to be a branch of Reptilia), had this integument as well. Lack of fossil impressions makes using scales as a character useless when viewing extinct forms.

Ok, so what about everything else? What about egg laying, or sprawling?

Even though there are lots of oviparious (egg laying) reptiles, there are also quite a few reptiles that give birth to live young just like mammals. Speaking of mammals, the monotremes, which are a very archaic form of mammal, are egg layers. So no, egg laying doesn't make a reptile.

As for the sprawling posture, dinosaurs, crocodylians and other extinct reptiles lack the sprawling posture that has long characterized the reptilia.

Is it confusing enough yet? It would appear that the only easily definable, ubiquitous trait is scale formation, and even then it only works for living forms. More detailed work is required to figure out reptilian ancestors.

More on reptilian classification will follow for the extinct taxa. For now just click below to learn more about the fascinating and diverse creatures that make up this class of animals.