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Re: tiny-armed theropods
On Thu, Oct 6, 2011 at 2:19 AM, Mike Taylor <email@example.com> wrote:
> Earlier you said "I believe the prevailing theory is that limb
> reduction occurred as the head become enlarged (in tyrannosaurs), such
> that the head is the primary tool for acquiring prey."
> If that is the theory then it plainly doesn't work for taxa whose
> heads didn't become enlarged.
Based on a whole lot of evidence (especially biomechanical), I would
say that the head was the primary tool for acquiring prey in most
theropods, even the long-armed ones. For example, the anterior reach
of _Acrocanthosaurus_ was actually fairly limited, and contact with
the prey was probably initiated by the jaws (Senter and Robins, 2005).
This is undoubtedly true of derived tyrannosaurids - the major
difference being the relative contribution of the forelimbs in holding
the prey in place. Many eudromaeosaurs (such as _Velociraptor_) seem
to have used their forelimbs (and probably the hindlimbs too) to grab
large prey - but this method of engagement might have been unusual. A
number of deinonychosaurs have short forelimbs (_Mahakala_,
_Austroraptor_, non-basal troodontids). Turner et al. (2011) raise
the prospect of short arms being the primitive condition for
Deinonychosauria. However, the fact that archaeopterygids, basal
troodontids (like _Jingengopteryx_), and most dromaeosaurids have long
forelimbs probably weighs against this being the most parsimonious
There a great many small-armed theropods, big and small. In
spinosauroids the arms were short and stocky - again this suggests a
role in securing prey seized by the jaws. Compsognathids also had
short and strong arms, each equipped with an enlarged claw on the long
hand - though because compies presumably targeted small prey (with the
jaws or feet), it's not clear to me what the forelimbs were useful
for. The short and highly derived forelimbs of alvarezsaurids have
been tied to a specialized purpose (i.e., digging, such as ant nests);
but we don't know if forelimb truncation preceded the development of
the extremely robust, coossified and functionally monodactyl
morphology in this group.
To me, the truncation of the forelimb in so many theropods reflects my
hunch that forelimbs really weren't as useful in predation as
traditionally believed, and were therefore sometimes expendable.
Thus, they could be reduced in size, or re-directed toward
non-predatory functions (digging; climbing; gliding/flight; extracting
grubs from crevices; defense; hooking branches). The non-carnivorous
ornithomimosaurs and therizinosaurs actually had quite long forelimbs.
I'm not saying that theropod forelimbs were useless for predation -
although they likely were in carnotaurines, in which the forelimbs
were vestigial. But as bipedal primates for which the forelimbs are
so integral to our survival, I think we humans tend to overstate the
importance of forelimbs to theropods.