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Re: segnosaurs



In a message dated 97-07-31 11:45:55 EDT, longrich@phoenix.Princeton.EDU
(Nick Longrich) writes:

<< George is using his old rhetorical device again: if somebody
 repeats something often enough and forcefully enough and talks with a
 tone of authority, it *must be true*. I think that's my major objection to
 the post- that emphasis, rather than substance, forms its core.>>

I do nothing more here than what cladists have tried and continue to try to
do to us all. But be that as it may, I'm certainly not about to write a
treatise on segnosaur anatomy and post it as an e-mail every time I have a
cavil about some posted comment on segnosaurs. If you want more details on
why I don't accept segnosaurs as theropods, you can visit the archives and
check my previous posts on the subject.

<< There are some gross excesses of other kinds, of course. Does the most
parsimonious explanation have to be the correct one? No. Do those employing a
cladistic methodology say that it is? Of course not. >>

Of course they do, character versus character! Otherwise, why are they using
parsimony in their algorithms? PAUP: Phylogenetic Analysis >Using Parsimony<.
Unless they're engaged in the pursuit of meaningless cladogram generation.

<<      As for the bit about postcranial characters being the most important-
that's out and out ludicrous. Darwin even explains why. The little
unimportant details that
 George berates are less likely to be adaptive or highly adaptive. That's why
they are so important, because highly adaptive things- like proportions of
limbs, or size, or arm length, or whatever- are more subject to changes,
convergencies and reversals. If we took this advice, we would be placing
Hesperornis next to the loons and grebes because in overall proportion and
functional detail they look like each other, but
 the reason Hesperonis looks like a grebe or loon is simply because it
functions like one. Segnosaurs may vaguely look like prosauropods simply
because they lived like them. >>

Nonsense. The "little unimportant details" are not the >only< characters that
must be considered when doing phylogeny. Would you group acanthopterygian
fish and catfish because they happen to have pectoral spines? I don't know
where you got the idea that I'd consider grouping Hesperornis with loons just
because they look a bit alike. I don't group segnosaurs with prosauropods
just because they >look like< prosauropods (although they do!); I group them
there because they share a number of character suites with prosauropods,
including palatal complex, teeth and jaws, hind feet, vertebral column, and,
yes, even the manus, such as carpal elements and the fact that the first
digit is longest and strongest in _Alxasaurus_.

Let me go over this one more time. >Cranial< characters are most useful in
sorting genera within more inclusive clades; >postcranial< characters, being
more conservative and less variable, are most useful in defining the larger
clades. This is not to say that cranial characters should never be used to
define larger clades, or that postcranial characters should never be used to
sort genera within clades; I am simply outlining the way things seem to work
among archosaurs. For example, among hadrosaurs the genera are virtually
indistinguishable without good skull material, and likewise among
ceratopians. But the postcranial elements of these groups are certainly
identifiable to family and usually identifiable to subfamily.

<<      Recall, of course, that when talking about why Allosaurus had
 ancestors flapping around in the trees, the fact that it has three digits
 is all-important and overriding, but now when we're talking about
 segnosaurs, we've conveniently forgotten that little character ;).  >>

I certainly haven't. But remember that it is much easier to >lose< digits
through vestigialization than it is to >regain< lost digits. How many mammals
have six-digit hands these days, besides pathological cases? Not even
cetaceans. But how many have four, three, two, and one-digit hands? Most
ferungulates, for starters. So: digital loss is like an evolutionary one-way
sign. Happens easily one way, but seldom if ever the other way. Consequently,
the fact that segnosaurs and theropods have tridactyl hands is not nearly as
important (since it could easily have happened twice by convergence) as the
fact that segnosaurs have functionally four-digit feet whereas theropods have
functionally three-digit feet.

Dinosaur feet in general and theropod hind feet in particular are
astoundingly conservative. Theropods from ceratosaurians to birds have
basically the same foot design, and no matter how modified it becomes--for
diving in diving birds, for perching in perching birds, for grasping prey in
raptors, for running in ratite birds, or for disembowelment in
dromaeosaurs--it retains a suite of distinctive theropod-foot characters,
among which is a reduced, distally placed metatarsal I. Yet among segnosaurs,
this feature does not exist. Why not? Why should it have changed in just that
one lineage, when it doesn't change in thousands of others? More
specifically, why should the segnosaur foot have changed--in this character
>and< in others-->to resemble the prosauropod foot<? I submit that it
>didn't< change; that it retained its prosauropod features from its
prosauropod ancestors; and that in the light of this notion many other
puzzling features of segnosaur skeletons can be seen as derived from a
prosauropod ancestry.

Please note that I'm not suggesting that the >large< prosauropods, such as
_Lufengosaurus_ or _Plateosaurus_, were close to ancestral segnosaurs. Their
postcranial skeletons do not suggest segnosaur ancestry without a great deal
of trouble. For example, they've essentially lost their pneumaticity. It is
the smaller, less-derived prosauropods, exemplified by _Anchisaurus_ and
_Ammosaurus_ (which retain pneumatic skeletons, or at least, have hollow
bones), that are closer to segnosaur ancestry. And ornithischian ancestry,
for that matter.

Indeed, it is positively amazing that segnosaurs have >so many< non-theropod
characters, considering that they're supposed to be theropods. It's also
positively amazing that segnosaurs have >so many< prosauropod characters,
considering that they're not supposed to be related to prosauropods by
descent.