Re: semilunate carpal

["Tim Williams" <twilliams_alpha@hotmail.com>]

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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