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Feet, bellies, and biogeography (was Re: Reevolving bones?)



Well, I wanted to stay out of the great therizinosauroid wars this time
(I've done my time on that battlefield often enough, as the archives will
attest) but I guess I ought to comment on at least one thing...

At 02:50 AM 5/7/97 -0400, Dinogeorge wrote:

><< >The theropod foot is remarkably conservative, being present virtually
> >unchanged, except for the fusion of certain elements, in all groups from
> >ceratosaurians to birds.

Well, within birds themselves, there is a mighty massive amount of variation
in foot structure: witness grebes, storks, hummers, woodpeckers, ostriches,
falconiforms, penguins, etc.

>         Tell that to Tom "The Arctometatarsalian Pes" Holtz!
>         So what if it had been conservative? If the animals were
> experiencing selection pressure for a four-toed foot, I wouldn't be
> surprized if the foot changed drastically...>>
>
>Tom Holtz knows this as well as anybody on the planet. Now tell me what kind
>of selection pressure would create a four-toed foot from a three-toed foot.
>Is this the same kind of selection pressure that would cause the pelvis to
>spread laterally? The claws to narrow and elongate?

Well, the first two might be causally linked.  It wouldn't be to hard to
envision a scenario in which a lineage of coelurosaur evolved an herbivorous
diet and a sloth-like habit, in which the belly became broader and more
capacious (supported dorsally by a laterally expanded ilium and increased
distally by opsithopuby).  Without selection pressure for greater speed
(indeed, the previous adaptations would be contradictory to greater speed),
a shorter, broader pes would not be selected against, and could be favored
for support (as in the origin of larger armored dinosaurs, in large
quadrupedal sauropodomorphs, etc.).

As Dale Russell has suggested, digit I on the foot of therizinosauroids
might not have been elongated as much as digits II-IV have been shortened.
It is a shame that the proximal part of the metatarsus of Alxasaurus is not
known, but from Late K therizinosauroids, the long axis of the shaft of mtI
is not parallel to mtII (as in herrerasaurids, prosauropods, etc.), but is
at an angle to the others.  In the alledged Therizinosaurus pes, on a thin
section of mt I contacts the ankle, although a much broader section contacts
the ankle in the other forms for which feet are known (Erlicosaurus,
Segnosaurus).

As for the narrow and elongate pedal claws: good question.  The narrow,
elongate claws of the manus in Therizinosaurus are probably associated with
its feeding habits and/or defense.

><< >preacetabular processes of the ilia flare way out laterally, 
>         Featuring a hooked process also seen in some other maniraptoform
>groups.>>
>
>So are segnosaurs maniraptorans? Maniraptoriforms?

Most recent analyses put them within Maniraptoriformes.  My work puts them
in Maniraptora (closer to birds than to Ornithomimus), Sereno's and Clark et
al.'s studies (as presented in 1995, at least) put them within
Arctometatarsalia (closer to Ornithomimus than to birds), and Hans Sues'
puts them (plus the oviraptorids and caenagnathids) in a polytomy with
Arctometatarsalia and Maniraptora. 
 
><< >the pubis is opisthopubic to the point of being fused to the ischium,
>         With lower pubic poduncle, as in dromaeosaurs, I believe. Also, the
> ischium bears a triangular obturator process, which has migrated distally.>>
>
>So are segnosaurs dromaeosaurs?

No, they don't seem to be dromaeosaurs, but these features are potential
synapomorphies between therizinosauroids and dromaeosaurids (and some other
forms).
 
><< >there is a postacetabular lump on the ilium,
>         Who cares?>>
>
>This is a clear segnosaur apomorphy. Cladists should care.

It is an autapomorphy.  It is important for the recognition of
Therizinosauroidea as a group, but characters found in only a single taxon
in an analysis do not help elucidate the relationships among the taxa in an
analysis.  They are still important for the understanding of that taxon, of
course.

><< >And I find "cladistic infallibility" repugnant and dogmatic.
>         Give it a rest George. No one says it's infallable.>>
>
>Cladists insist that their methodology must for philosophical reasons take
>precedence over biogeography and functional analysis.

With regards to analysing the distribution of shared derived characters:
yes, cladistic analyses have a lot more analytical power.  With regards to
functional studies: cladistics may shed some light on a structure, but
biomechanics and morphometrics are still prefered by those of us who do such
studies.

As someone with more than a passing interest in biogeography, consider the
following aspect of the fossil record with regards to the falsifying power
of biogeography: we can confidentaly code a "0" for an observed absence of a
derived character on a bone.  How confidentaly can we code a "0" for the
observed absence of a taxon at a specific region or fossil locality
(particularly for the terrestrial record of rare animals)?

How many man-hours were spent in the Morrison over the past dozen decades
before the discovery of Mymoorapelta and Gar..., er, Morrison ankylosaurian
number 2?

Thomas R. Holtz, Jr.
Vertebrate Paleontologist     Webpage: http://www.geol.umd.edu
Dept. of Geology              Email:th81@umail.umd.edu
University of Maryland        Phone:301-405-4084
College Park, MD  20742       Fax:  301-314-9661