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Re: Vestigial Arms (was: Theropod limbs - how mobile?)



 
Michael Skrepnick
Paleo Artist
Visit my Website:
http://www.dinosaursinart.com
 
Okay, I'm slowly getting a picture here. This isn't grasping the way I normally think of grasping (opposable thumb etc), it's more like a pair of pliers or mole-grips.  Does the immobility of the hand and it's size tell us anything about the prey it was going after then?
 
*** Although there may be others on this list who won't agree, I think the association between Deinonychus / Tenontosaurus material speaks for itself.  I think they hunted Tenontosaurs in groups and scavenged carcasses when the opportunity presented itself. If you want to draw an analogy ( I know many won't) with modern African cats hunting a variety of larger herbivorous quadrupeds, in spite of differences is size differential between predator and prey yadda yadda yadda. . . in either case the smaller predators are more than willing to risk potential injury in favor of securing a meal.  Whether they selected injured animals, juveniles or just went with a pack hunting, bloodlust mentality, Tenontosaurs were likely choice victims on the dromaeosaur menu (pers. comm. Bob Bakker). Maybe they rushed in, inflicting deep wounds / disembowling the target animal and then followed at a safe distance until the victim died of shock / bled to death. Dinosaurs are not mammals obviously, nor were they brittle skeletal remains that have a propensity to fracture into a thousand pieces when dropped on hard museum collections floors ( and no, I haven't done that either).  In life, I'm sure they were capable of handling the stress factors involved in predation, scavenging, clepto-parasitic backhanded (now you see it, now I steal it ) parlour tricks (ala Tom Holtz DML archives )or any other strategy necessary to play on the "Survival of the Fittest" gameboard ( action figures and batteries not included ) as attested to by their lengthy proliferation recorded within fossilbearing strata.
 
Seems to me our poor, much abused cat would be better grasped with a smaller, bendier hand whereas the dienonychus' manus would be better suited to larger prey?
 
*** Alternate scenario. . . In order to capture smaller, swift and agile prey, Deinonychus thrusts its hands forward temporarily enclosing the prey, pins it to the ground with pedal digit 2 ungual and dispatches victim with snapping jaws.
 
Also, is it possible to tell if these adaptations were evolved for prey capture or if they were later exaptions (is that the right word?) from Archie-type wings?
 
*** 10 paleontologists - 10 hypotheses with subjective variables.
 
David Marjanovic wrote: . As I cited, "As with most extant predators, the mouth was used to grasp the prey. Then the short, powerful arms were used to grasp or clutch the prey against the body to prevent its escape while the teeth were disengaged and repeated bites made to kill the prey."
 
*** Or. . . Tyrannosaurs may NOT have used their forelimbs in securing live prey, period. and evidence of trauma in postcrania including arms, hindlimbs and ribcage could simply be a result of thrashing prey animals responding to crushing jaws. OR by intraspecific ritualistic bump and grind between two rival adult theropods. OR by clumsy utilization of basic motor skills ( too busy eyeing up the blue plate special, accidently stumbles into large innocuous tree trunk / humans manage to do it all the time, what's their excuse).  I have a hard time imagining a tyrannosaur being able to grasp with any degree of leverage, the bulky girth of any struggling adult ornithopod!
 
Sorry if I seem to be labouring this point, but I'm still having trouble with it. While I'm not disputing that it is physically possible, why is it better to do this rather than deliver a big bite and back off? As I understand it, theropods can't take falls and knocks the way for example big cats can. So why risk grappling with prey about as heavy as they were?
 
*** That would SEEM to be a more practical means of dispatching prey when you consider large strong jaws / reinforced skull, puncturing teeth, sizable neck musculature all support a bite, twist and tear scenario, leaving the victim momentarily disoriented, traumatized, mangled and eventually weakened from blood loss, awaiting the inevitable end.  PROVE IT? Don't have to. . . that's the province of paleontologists.  I just get to paint it ;o) !
 
Mike Skrepnick