[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

RE: dinosaur synapomorphies? (posttemporal opening)



> From: owner-dinosaur@usc.edu [mailto:owner-dinosaur@usc.edu]On Behalf Of
> Ken Kinman
>
>      Anybody can take two taxa and use them to define a
> holophyletic clade.
> The question is whether they should.

Agreed, on both counts.

> Clades should be named for
> groups that
> appear to have strong synapomorphies,

Disagreed, to a degree.

Brochu has already dealt with the issue of the "strong vs. weak
synapomorphies".

For a different aspect, what might be considered "strong synapomorphies" (or
rather, larger morphological discontinuities between the primitive and
derived state) actually means that one has a sampling gap.  When there are
very marked contrasts in morphological conditions between two supposed
sister taxa, it means that you are missing a lot of the taxa and the
incremental changes along the lineages in between.

For some classic examples:
If one looked at living taxa only, there is a vast morphological gulf
between Aves and Crocodylia.  If you just added in Compsognathus and
Archaeopteryx the gulf would be greatly reduced (between non-neornithines
and crocs), but the clade comprised of Archaeopteryx and Neornithes and
Neornithes itself would both be supported by very large numbers of
synapomorphies with almost no character distribution amibiguity.

Then throw in Scleromochlus and Lagerpeton and Lagosuchus and
Pseudolagosuchus and Pisanosaurus and Eoraptor and Saturnalia and
Herrerasaurus and Syntarsus and Ceratosaurus and Torvosaurus and
Eustreptospondylus and Piatnitzkysaurus and Afrovenator and Monolophosaurus
and Sinraptor and Proceratosaurus and Ornitholestes and Scipionyx and and
Sinosauroptery and Eotyrannus and Pelecanimimus and Coelurus and Caudipteryx
and Beipiaosaurus and Sinornithoides and Alvarezsaurus and Shuvuuia and
Deinonychus and Sinornithosaurus and Microraptor and Archaeopteryx and
Rahonavis and Confuciusornis and Sinornis and Iberomesornis and Patagopteryx
and Ichthyornis and Asparavis and Lithornis (and critters up the line to
crocs, as well), and you find that there AREN'T big morphological jumps
anymore and large numbers of unambiguous characters anymore.  And that's
with the crappy record we DO have: that's not counting the vastly larger
number of taxa which weren't lucky enough to make it into the fossil record
and be discovered and described yet.

In any case, back to the suggestion you made (i.e., that clades should be
named for groups that appear to have strong synapomorphies).  While a very
stable group with a big morphological gap from its closest currently known
relatives is a good spot on the Tree for a name (in fact, I would suggest a
node-based definition for such a group), there are other reasons to put
labels at particular parts of the tree.  These include taxonomic historical
usage, developing node-stem triplets, and others.  In fact, Sereno has
published a couple of papers on this subject (in Syst. Biol. 48: 329-351 and
N. Jb. Geol. Paeont. Abh. 210: 41-83).  Although I don't always agree with
his particular decisions in these papers, they both discuss interesting and
useful criteria for assigning clade names.

>       What about posttemporal opening reduced to small foramen?

Actually, you are converging on here is an important point about studying
evolution (rather than trying to do taxonomic book-keeping): ultimately we
are going to find that practically EVERY character will have a gradational
series involved with it.  That's how evolution works.  Basal reptiles had
posttemporal fenestrae; dinosaurs and some other taxa lost it.  When skulls
of the lineage that includes taxa closer to dinosaurs than to other
archosaurs are known, the prediction is that some of them will have
progressively smaller posttemporal fenestrae.

The same case would go for the "essential property" of Tetrapoda (or
Stegocephalia or whatever), the limb with digits.  When does a digit fully
become a digit?  Prior to the discovery of complete limbs of Acanthostega,
Ichthyostega, and other Devonian stegocephalians, there was always a
morphological jump between a definite lobe-fin and a definite hand/foot.
That isn't the case anymore.

> Is there anyone who knows more about it who wants to defend
> this synapomorphy?

CLADO-MAN! Defender of Synapomorphies!! :-)

But seriously, the ball is in your court.  It is your turn to falsify this
character as a potential synapmorphy of Dinosauria.  There are several
potential ways of doing so, but let's see if you can do it.

> I see no reason to trust such a character.

Yeah, with those beady little eyes and low hairline...  (Sorry, just
thinking that the sentence "I see no reason to trust such a character" would
have a very different meaning in a different context).

                Thomas R. Holtz, Jr.
                Vertebrate Paleontologist
Department of Geology           Director, Earth, Life & Time Program
University of Maryland          College Park Scholars
                College Park, MD  20742
http://www.geol.umd.edu/~tholtz/tholtz.htm
http://www.geol.umd.edu/~jmerck/eltsite
Phone:  301-405-4084    Email:  tholtz@geol.umd.edu
Fax (Geol):  301-314-9661       Fax (CPS-ELT): 301-405-0796