Re: Two new FAQs: Everything You Wanted To Know About Cladistics

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

On Thu, 23 Aug 2001, Mike Taylor wrote:

> > And these fellows are still in academics? *shakes head in wonder* I
> > thought Romer and Cox got rid of all that nonsense decades ago? I'm
> > glad to see people are keeping mammals from Reptilia [...]
>                                          ^^^^
>            Did you mean "keeping mammals _in_ Reptilia"?

Who ever put mammals in Reptilia?

> Which is what I originally used in my examples.  Then I checked the
> classification in the dinosauricon, spead over the two pages:
> http://dinosauricon.com/taxa/tetrapoda.html
> http://dinosauricon.com/taxa/sauropsida.html
> which has Synapsida _outside_ Reptilia.  So I changed my example to
> how it is now, which is a shame, because I was previously using
> "traditional reptiles" as an example of a doubly paraphyletic group,
> {Reptilia-Aves,Mammalia} and I no longer have any such example.
>
> Now reading Jamie's comments, I am getting the impression that my
> first idea was right, and Mammalia _is_ inside Reptilia after all.  Am
> I right?  What's the story with the dinosauricon?

A brief history of Reptilia (please correct me if any of this is wrong):

Linnaeus originally named the taxon (which means "creepers") to include a
puzzling variety of vertebrates. For example, I believe it included sharks
in addition to the usual scaly terrestrial beasts.

Eventually it was whittled down to stand for what we usually think of as
"reptiles": crocodylians, lizards & snakes (squamates), tuataras, and
turtles. Basically, any amniote that isn't a bird or a mammal. They were
seen as a "lower" class of life, not have reached the higher grade of
"organization" of the warm-blooded amniote classes, Mammalia and Aves.

When people started finding fossils of extinct amniotes dissimilar to
modern forms, they, too, were placed in this "lower grade", the Reptilia.
In some instances this is because they were thought to be allied to
certain modern reptilian forms. For example, the classic dinosaurs
(_Iguanodon_, _Megalosaurus_, etc.) were originally thought to be enormous
lizards. Even when this was corrected, they were retained in Class
Reptilia.

In this manner, a huge variety of animals came to be classified in Class
Reptilia, from marine ichthyosaurs to flying pterosaurs to gigantic
sauropods to mammal-like synapsids to bird-like coelurosaurs.

Some researchers began to question the utility of this, arguing that
certain forms were more like birds and mammals than like modern reptiles.
Specifically, synapsids (or at least therapsid synapsids) were more like
mammals, and pterosaurs and dinosaurs were more like birds. Thus, they
should be separated from the "cold-blooded" Class Reptilia.

While progressive, this still reflected the older "gradistic" approach
wherein animals are classified, in part, according to their "level of
complexity". Under this approach, warm-blooded animals (including
ourselves, of course), are more "complex" than cold-blooded animals, and
thus deserve ranks separate from more "lowly" forms of life.

Today, there is an increasingly popular approach which eschews this older,
subjective style of thinking, for it obscures such facts as crocodylians
being more closely related to birds than to lizards, or turtles being
extremely derived in terms of skeletal anatomy. Under this "cladistic"
paradigm, taxonomy is based only on common ancestry. The only differences
between cladistic taxonomies must be based on phylogenetic hypotheses, and
not on subjective opininons as to whether one group is or is not "complex"
enough to deserve a new "rank".

Under this system, Reptilia was re-purposed as a clade. Specifically, the
most recent common ancestor of the classic modern reptiles (crocodylians,
squamates, turtles, and tuataras), plus all of its descendants. Animals
like the non-mammalian synapsids, traditionally called "mammal-like
reptiles", were hence excluded. Birds, on the other hand, were included.

To sum up, there have been four major versions of Reptilia (at least):

Classis Reptilia _sensu_ Linnaeus: sharks, scaly beasts, and other "creepers"

Classis Reptilia (traditional): all non-avian, non-mammalian amniotes
(especially crocodylians, lepidosaurs, and turtles)

Classis Reptilia (revised): all non-dinosaurian, non-therapsid,
non-pterosaurian amniotes

Clade _Reptilia_: Clade(_Crocodylus niloticus_ + _Lacerta agilis_ +
_Testudo graeca_) (or equivalent formulation)

Added Personal Note:

The wildly disparate usages of "Reptilia" make me want to just abandon it
as a formal name, keeping it just as a convenient label for those modern
animals which it has always included, with the same level of formality as
"herptile" or "raptor". But, hey, I'm no scientist, and I'll go along with
whatever the community eventually (more or less) agrees on.

_____________________________________________________________________________
T. MICHAEL KEESEY
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