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Re: semilunate carpal



Tracy Ford wrote:

Why is that so hard to grasp? Just because Velociraptor is a carnivore does
not mean, in anyway, that Protoceratops was not the aggressor!
I've said that before on the list (should be lost somewhere in the
archives).

OK, maybe the _Protoceratops_ was launching a pre-emptive attack against a roving _Velociraptor_, perhaps to defend its nest or its young. But, you know, I wonder what possible advantage an unprovoked attack would serve to a _Protoceratops_.


Has anyone seriously suggested that the reason why a _Tenontosaurus_ skeleton was found associated with _Deinonychus_ material is because the _Tenontosaurus_ one day decided to rush headlong into a pack of slashing-clawed predators? To me it looks like a recipe for suicide.

Anything's possible, I suppose. But when a predatory theropod (like _Velociraptor_) is found locked in an embrace with a herbivorous ornithischian (like _Protoceratops_), I guess I jump to the most logical conclusion that the predator picked a fight with the herbivore, not the other way round.


Patrick Norton said:

I meant exactly what I said, which has nothing to do with BCF.

Only insofar as the semilunate carpal was evolved explicity for the animal's movement through the air, rather than for predation.


I sense this thread is getting a bit tired, [snip] The "predatory stroke", at >least as far as I'm aware, is no more or less than a >hypothetical< explanation of the function of the semilunate carpal in the maniraptoran wrist. Another >hypothesis is that it was an adaptation for better orientational control among >cursorial bipeds with aerodynamic surfaces on their arms.

To role up this debate then...

The semilunate carpal is an element which constrained the movement of the hand (manus) to a mediolateral arc (the "swivel wrist"). There are two major exaptive hypotheses for the presence of the semilunate carpal in terrestrial predatory theropods and flighted birds.

(1) Predatory stroke: The "swivel wrist" allowed the hand to spread out toward the prey as the arms moved forward and downward, and (if necessary) then fold in on the prey - anticipating the downstroke and upstroke, respectively, of flighted birds. According to this hypothesis, the function of the semilunate carpal in the flight stroke of birds is the derived condition. The long arms and hands of these predatory theropods was also oriented toward the same function - seizing prey.

(2) Aerial locomotion: The "swivel wrist" was evolved for orientational control (steering and stability) during aerial descents. Thus, the aerodynamic function in performs in modern birds in executing the flight stroke is the primitive function of the semilunate carpal. The relatively immobile hand in terrestrial theropods (such as velociraptorines) is a consequence of the semilunate carpal's original function in aerial locomotion. The long arms and hands of maniraptoran theropods can be attributed to their elongation for achieving maximal leverage during aerial descents.

(1) holds that the semilunate carpal and the predatory stroke evolved among terrestrial theropods and was coopted for aerial locomotion (culminating in powered flight). (2) holds that secondarily non-aerolocomotive ground-dwelling theropods were "stuck" with the semilunate carpal, which reduced the predatory potential of the manus. Hence, the raptorial abilities of the jaws and pedal claws had to compensate for the relatively ineffectual manus.

One of the things that I've tried to draw attention to is the distinction between terms such as "manipulating" and "grasping" and "grappling". The former implies some degree of dexterity and fine coordination in handling an object; I don't think this was too important to maniraptoran predators.

According to (1), the relative inflexibility of the manus was *specialized* as a grappling device to seize prey. The long arms and medially-directed grasping hands acted in opposition to hold onto the prey - and if the manual claws ripped through the prey's hide in the process, so much the better (for the predator). As such, the hands were principally grasping or grappling devices; there was no selective pressure to enhance the *dexterity* of the manus. The exclusively mediolateral movement of the hand was integral to the process by which the forelimbs and hands became devoted to prey capture and prey holding. Manual dexterity was lost, but the strength of the predator's hold was increased.

Tyrannosaurids, I would guess, adhered to this theme. The forelimbs were dedicated to securing (or to help in securing) the predator's hold on the prey. But the forelimbs (and manus) played no part in bringing the food directly to the mouth. The latter might require some degree of manual dexterity (and, in tyrannosaurids, the ability of the manus to actually reach the mouth). The reduced forelimbs indicate (very strongly) that, unlike dromaeosaurids and most smaller maniraptoriforms, the arms and hands were not used to *catch* the prey, just to secure and subdue it.

As for oviraptorids, if they did feed on eggs, they could use both hands to grasp the egg, crack it open with the jaws, and perhaps scoop the contents into the mouth with one hand - or the jaws could simply dive straight into the egg. Whatever works

The individual manus of all maniraptorans had limited prehensile ability as the consequence of a strategy to dedicate the forelimbs to prey capture. The evolution of the predatory stroke and semilunate carpal were intimately tied to this strategy.


Tim



---------------------------------------------------------------------

Timothy J. Williams

USDA-ARS Researcher
Agronomy Hall
Iowa State University
Ames IA 50014

Phone: 515 294 9233
Fax:   515 294 3163



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