Re: Triassic dinosaur evolution

[Mike Habib <habib@jhmi.edu>]

 don ohmes wrote:

> As a tall biped, seizing an animal that weighs as much as you is a  
> good recipe for getting pulled off your feet, and having some hooked  
> levers to push off/hold on the flank of the 'grippee' would be very  
> handy for the 'gripper'. Could also increase wounding power,  
> especially in pullback/worry phase, and help to maintain position/ 
> balance for another bite (or even flight).

This is a good point, and I have generally assumed about the same sort  
of thing.  However, I am beginning to doubt the mechanical feasibility  
of a grip and bite dynamic, at least for most mid to large sized non- 
maniraptoran theropods.  It's actually not a simple task to get, say,  
your average allosaurid into a position in which the jaws can engage  
the flank of hypothetical large target while the arms are also hooked  
in - the required cervical position appears to violate articulation  
limits.  Even in something with a longer, more flexible neck (like  
Dilophosaurus, for example), the range of positions that allow both a  
bite and a grip seem awfully limited (though, granted, I haven't had a  
chance to sit down with any Dilo material and actually articulate it -  
thoughts from those with more expertise would be appreciated).


Mark Witton wrote:
> I really, really don't have time to write this, but the heads of  
> allosaurs, dromaeosaurs and a bunch of other 'saurs ain't build for  
> holding struggling animals. They may get there first, but theropod  
> arms are surely going to be holding things in place while sickle  
> claws, raking jaws or whatever do the dirty work. Noteable  
> exceptions to this, of course, as tyrannosaurs, which appear to have  
> jaws that both hold and dispatch prey. Maybe that's why their arms  
> were comparatively diminutive.

Again, seems reasonable so long as it actually works in terms of  
articulation and range of motion.  Okay, so it certainly works in  
dromaeosaurs, no worries there.  But I'm not sure if the same can be  
said of allosaurs, for example.  That's not to say that I disagree  
with the issues of holding struggling animals - the skull architecture  
is indeed not particularly well adapted for such a task.  If I'm  
wrong, and your typical allosaur or ceratosaur can actually bite into  
something that is held by the arms, then problem solved.  Otherwise,  
it's a bit of a conundrum.  And, of course, there is the arm reduction  
weirdness in several abelisaurids...

Cheers,

--Mike




Michael Habib, M.S.
PhD. Candidate
Center for Functional Anatomy and Evolution
Johns Hopkins School of Medicine
1830 E. Monument Street
Baltimore, MD 21205
(443) 280-0181
habib@jhmi.edu