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Re: Theropod limbs - how mobile?



 
----- Original Message -----
Sent: Sunday, May 19, 2002 1:56 AM
 
Mike Skrepnick wrote in reply to my cat / hands experiment:
 
*** Now, lets modify your first experiment. ..(snip).... Suddenly your struggling cat has been reduced to "puppy chow" in less time than it took you to read the words "puppy chow".***
 
My question is not whether Dienonychus could kill a cat!
Then take a triconodont :o)
What I'm asking is why the immobile hands would be better at killing the cat than a Dienonychus with more mobile hands.
Try holding said cat. When you have Deinonychus arms (or those of at least any tetanuran, it seems), it can't escape laterally. I have to do that with great muscular effort because my hands can move through almost 180° in that direction. In Deinonychus, this arc was apparently 0°. -- In contrast, Deinonychus can move its hands dorsoventrally through 190°, while mine alllow something like 45° or less (it's done between the proximal carpals and radius + ulna, not between the distal carpals -- 2 of which form the semilunate -- and the proximal ones). This allows to fold the arms up like wings to get them out of the way during running, and to extend them very fast if needed. Tyrannosaurus doesn't have a semilunate (anymore?). Apparently it has totally stiff wrists, with disc-shaped carpals. Throws away all velocity, but makes it even harder for the prey to escape. -- "In support of this interpretation, we note the pathology along the medial side of the humerus in FMNH PR 2081. The site of damage corresponds to the medial head of the M. triceps humeralis, which serves to adduct and extend the lower arm. As noted above, the pathology is characteristic of partial avulsion caused by abnormally high stress loads. Such loads might occur while clutching a large, struggling animal, such as an adult hadrosaur (see Carpenter in press)[.] Indeed, the straight shaft of the humerus, as compared with that of Allosaurus [...], is precisely what is expected for maximum strength per unit mass [...]. Such conditions occur where the bone must resist axial compression, as it would do in this case with the powerful M. biceps (see fig. 9.12). Furthermore, the very low [...] values for the humerus, ulna, and radius indicate bones selected for ultimate strength or impact loading. Finally, to ensure that the struggling prey not escape while the mouth is attempting to kill it, the two ungual claws [sic] point somewhat inward (fig. 9.13C) so that they do not slip out of the prey easily." The fingers are not immobile but can be flexed strongly.
David Marjanovic wrote:
"Who mentioned PDW?"
I did!
Ah.
"While many parts of PDW do still hold, 1988 was 14 years ago. "
Is one of the parts that still holds the bit that says tyrannosaurs killed by delivering the big powerful big they were designed for, then backing off, to avoid being hurt?
Maybe not :-)
Seriously, tyrannosaurs evolved from creatures with longer forelimbs. (Does anyone doubt this?)
No. And then they threw away dexterity (finger III) and velocity (length) to concentrate only on brute force. And the Force was with them :-)
By the time advanced tyrannosaurs appeared the heads were big and the arms were small. If the arms were used in prey capture, why would they be small?
Because by simple lever mechanics this increases force (at the expense of unnecessary speed). Really get the book. (Maybe they'll print more next time and make them cheaper if more people buy it...) University libraries should already have it, no?
How would they draw the prey towards them in the first place? They have serrated, slicing teeth and "D"-cross section gouging teeth, not conical croc types so the jaws would be no good.
The jaws, with thick, strong teeth and exactly the D-cross-section pmx teeth, should be very good. 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."
All extant carnivores that use forelimbs to hold / grapple prey have long, mobile forelimbs.
All extant carnivores that use forelimbs are quadrupeds that very simply can't afford to reduce forelimb length. :-) Plus, they can't afford to lose forearm rotation. Closest that might come among more or less recent carnivores might be Titanis, but the known forelimb material is not enough to compare AFAIK.