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Re: Triassic dinosaur evolution



 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