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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
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
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
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
(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
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.
Timothy J. Williams
Iowa State University
Ames IA 50014
Phone: 515 294 9233
Fax: 515 294 3163
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