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Re: Dilophosaurus Forelimb Bone Maladies



Jura <pristichampsus@yahoo.com> wrote:

> I have to disagree. The authors of the paper pointed out that theropod 
> forelimb injuries are remarkably common. The Dilophosaurus specimen that they
> looked at showed multiple injuries to the left forelimb that all showed signs 
> of healing prior to death (so it did manage to survive the injuries, although 
> I
> suspect that the wording of the last post might have been a bit muddled, 
> since I think that is what Mike was arguing). The right arm showed pathologies
> that agreed well with compensatory focus of the right limb over the left, 
> which we wouldn't expect if the forelimbs were doing nothing.


I know it sounds counterintuitive, but theropods like _Dilophosaurus_
might not have used their forelimbs much in predation.  In general,
the forelimb had limited reach, highly proscribed mobility, and was
incapable of one-handed (unimanual) prehension.  I'm not arguing that
the forelimbs were doing nothing at all.  Small prey could be seized
with the jaws, and dispatched with the hand claws - although this
would be a bit difficult when the hands were so far from the mouth.
For larger prey, the forelimbs could be used to help secure or
position the prey, again after the prey was seized by the jaws (this
has been proposed for _Tyrannosaurus_, for example).  Forelimbs might
also have been used to hold and tear carcasses underneath the body
during scavenging.

The clawed forelimbs might have been used for agonistic behavior,
similar to extant macropodines (who also use them for slow/pentapodal
locomotion).   Senter & Juengst actually mention intraspecific combat
as a possible cause of _Dilophosaurus_'s injuries.   In theropod
evolution, this agonistic function could segue into a display function
via the development of elaborate feathers along the forelimbs
(including the hands).


> The evidence presented in the paper supports theropods (or at least this one 
> Dilophosaurus) using their forelimbs a lot. Although we can't say for certain
> that the forelimbs were engaged in prey capture, that is a major activity for 
> most extant predators.


Mammalian predators certainly use their forelimbs during prey capture.
But birds of prey manage quite well using only their head and feet.
This ability might long precede the origin of flight.

As primates, it does seem strange that theropod forelimbs may have
been so under-utilized for prey capture.  But thanks to our arboreal
ancestors, our forelimbs are endowed with a wide range of motion and
superb opposable grasping abilities.  These do not apply to theropods.
It's no surprise to me that the forelimbs of so many theropods were
turned to non-predatory functions (even in animalivorous lineages), or
were drastically reduced.