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Re: More reptile stuff



At 03:05 PM 12/17/98 -0600, Chris Campbell wrote:

>And since it's arbitrary, why not choose groupings which have some
>significance beyond mere convenience?  Turtles are so distinct from
>lepidosaurs or archosaurs that lumping them together makes little sense
>to me.

But there are a lot of derived features unique to turtles PLUS lepidosaurs
PLUS archosaurs.

That doesn't mean that lepidosaurs plus archosaurs don't deserve their own
group.  They do.  They have one.  It's called Diapsida.

>No, what I mean is "do turtles have more in common with archosaurs than
>they do with the ancestors of reptilia, thus justifying the use of such
>a broad taxonomic category?

Yes.  See almost any paper on amniote phylogenetics.  Among these features are:
Uricotely
Maxilla separated from the quadratojugal by jugal
Single coronoid
Femoral shaft long and slender
Single centrale in ankle
Tabular small or absent
Suborbital foramen in palate
Parasphenoid wings absent
Surpaoccipital narrow in posterior view
Large post-temporal fenestra

(Note that modern turtles themselves have lost some of these features
through transformations, but their ancestors and their relatives had them).

See also the Tree of Life webpages on Amniotes (www.dinosauria.com and Mike
Keesey's pages will have links).

>  Do they have enough features in common with
>this group we're calling reptiles that they ought to be lumped in with
>it, rather than being given their own category a la mammals?" 

Yes, BUT they *do* have their own category.  They have Testudines, and they
have their larger group Anapsida.

>Testudines seem to have diverged from the lepidosaur and archosaur lines
>so early on that grouping them with these lines seems a bit pointless to
>me.  You can, using their common ancestor as a point of reference, but
>then you could do the same thing with these groups and mammals.

And we do.

As per my biannual statement, just because some systematic grouping seems
pointless "to you" doesn't mean it is pointless (or more to the point,
useless) to people interested in, or even actively researching, basal
amniote relationships.  These groupings DO become useful to such individuals.

>> As Gauthier has said, there are no animals alive which closely resemble a
>> Paleozoic amniote.  All living amniotes (mammals, turtles, tuataras,
>> squamates, crocodilians, and birds) have highly specialized anatomies.  The
>> idea that Permian or earlier amniotes were somehow "lizard-like" or
>> "turtle-like" comes from peoples unfamiliarity with how sophisticated the
>> anatomies of lizards or turtles actually are.
>
>Which is why it seems odd to me to even retain the notion of Reptilia at
>all.  There don't seem to be a whole lot of things uniting the groups
>other than a general tendency toward egg-laying and some common
>ancestry.

And a whole lot of shared detailed skeletal features.  There is a lot more
to vertebrate anatomy than 100-level college courses (or their textbooks) go
into.  (This isn't a put-down: I helped write a 100-level college course
textbook chapter on dinosaur osteology).

>My argument here is that the term doesn't really mean anything anymore.

On the contrary, it has a very specific and precise meaning, if by "meaning"
we mean "definition".  If by "meaning" you are talking about something more
metaphysical, then ask a philosopher, not me.
 
>We basically lump all extant vertebrates who aren't fish or mammals into
>one group, even though they don't really have much in common beyond
>common ancestry.  Yes, lumping lepidosaurs and archosaurs and testudines
>together is a valid clade, but what's the point? I'm just not very
>clear on what this really tells us about the groups involved.

What is it you want it tell us?  No, really, I am intrigued by this
question.  You (and you are not the only one this list: the same questions
rise up every six to nine months, it seems) seem to be expecting something
from taxonomy other than labelling particular branches of the tree of life.

What is the "point" of labelling siliciclastic sedimentary rocks with less
than 10% mud matrix and more than 90% of the sand-sized particles a quartz
arenite?  Not much point beyond a reference term.  Giving such rocks a name
allows us to talk about such things in comparison to lithic arenites, or
felspathic arenites, or quartz wackes, or what have you.

>The thing is, I'm not arguing that basal amniotes should be included in
>turtles + lepidosaurs + archosaurs; I'm just arguing for more of a
>lumper mentality, wherein amniota is the main group and pelycosaurs and
>other synapsids, testudines, arcosaurs, and lepidosaurs are branches
>therein.

Yes.  And this differs from what I was discussing how?

Amniota is the "main" (or at least most inclusive) group.  Synapsida and
Sauropsida are the two main divisions within it.  Within Sauropsida we have
Mesosauria and Reptilia; within Reptilia with have Anapsida and Diapsida;
within Diapsida we have Lepidosauromorpha and Archosauromorpha and
Euryapsida (which might belong in one or the other, who can tell).

You might not care if turtles are closer to diapsids than to synapsids, but
I do.  It is useful to me to have labels to refer to this group.

I personally don't care if horseflies are closer to deerflies or to
houseflies, but I wouldn't argue that entomologists shouldn't name any
groups within Diptera.

>I don't think I'd object to the notion of reptilia so much if
>the turtles were at least kept out of things.

As a loyal Terrapin, I find your comments bigoted and outragous.  Turtle
Power!! :-)

(For those who don't know, the U Maryland mascot is Testudo the Terrapin,
and so I would object to those who object to keeping turtles out of things).

More importantly, there is a group which excludes turtles (probably: unless
the "turtles as really derived lepidosauromorphs" hypothesis turns out to be
true!): Diapsida.   A name well loved in paleontological circles.

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:tholtz@geol.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661