Re: the definition of Reptilia

["T. Mike Keesey" <tmk@dinosauricon.com>]

On Sat, 1 Dec 2001, David Elliott wrote:

> Have reptiles been formally defined, and if so, what are they?

> I've heard them referred to as an unnatural group (diapsids excluding
> dinosaurs), and - of course - as synonymous with "diapsid",

I've never heard of either of those usages.

> but the thing that inspired me to ask was the fact that non-mammal
> synapsids are referred to as "mammal-like reptiles".

Incorrectly, at least if you're using phylogenetic taxonomy.


> So if you include all descendants of the common ancestor of those
> animals commonly referred to as reptiles, we should be they (so
> Reptilia = Amniota). Aesthetically, i like this (because the presence
> of the term "mammal-like reptile", and also: in my little brain
> "reptile" conjures up the concept "tetrapod adapted to dryness" more
> strongly than it does "diapsid" (and even more strongly than "outdated
> paraphyletic group"), but i thought i'd better ask to see how things
> really are.

Interesting concept, but _Amniota_ is a better name for the clade, anyway.
(_Reptilia_ means "creepers")

> So has it been formally defined, or is it one of those common words whose
> appropriateness everybody has a different idea about?

There are a lot of different ideas about it (it originally included
sharks, apparently). Here are the major ones:

traditional: any amniote that is not avian or mammalian
phylogenetic: the most recent common ancestor of _Crocodylus niloticus_,
_Lacerta agilis_, and _Chelonia mydas_*, plus all of its descendants

(* I think that's the type species...)

The former includes non-mammalian synapsids (the traditional _Synapsida_
did not include mammals, BTW) but not avians. The latter includes avians
but not synapsids.

Personally, I'd like to get rid of the term, but it doesn;t seem to be
going anywhere anytime soon....

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