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Re: Dilophosaurus Forelimb Bone Maladies
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- Subject: Re: Dilophosaurus Forelimb Bone Maladies
- From: Mike Habib <firstname.lastname@example.org>
- Date: Wed, 2 Mar 2016 21:50:45 -0800
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> On Mar 2, 2016, at 8:34 PM, Tim Williams <firstname.lastname@example.org> wrote:
> Jason <email@example.com> wrote:
>> I am very skeptical of statements about limb mobility based largely on
>> joint-focused, bone-to-bone range of motion (ROM) comparisons. As Tsai and
>> Holliday (2014), Hutson & Hutson (2012) and Burch (2014) have indicated,
>> we can tell very little about limb mobility when we don't take into account
>> soft tissues that can both add (via cartilaginous extensions, and muscular
>> action) or subtract (via muscle attachments and cartilaginous padding)
>> from ROM. No such study has been done on _Dilophosaurus_ yet (AFAIK),
>> so I am leery of any absolute statements about forelimb mobility for this
> Excellent point, and I understand your leeriness. Holliday &c have
> observed that certain rather sophisticated functional/biomechanical
> analyses of dinosaurs give little or no attention to cartilage. So
> 'bone-on-bone' models likely underestimate the range of motion that a
> limb was capable of in life by a substantial margin.
> However, with many theropods the major constraints are the size of the
> forelimb and the lack of prehension in the manus. No amount of
> cartilage can make a manus opposable (or even semi-opposable) if the
> bones aren't adapted for this…
I understand the leeriness, as well, but even if we were to lay out the
forelimb elements of, for example, an Allosaurus (supposedly a more “robust”
forelimb taxon) end to end, they still would not get anywhere near the mouth.
No amount of cartilage or liberal reconstruction of the ligaments can get the
hands of the vast majority of theropods into any space that is near the mouth
or even where the animal can see them. Trying to validate specific functional
hypotheses without a good soft tissue reconstruction is quite risky, as
Holliday, Hutson, and Burch have noted. But in this case the proportions are so
extreme that I propose some options can be excluded.
>> I agree that the jaws would have been the most likely first point of
>> contact with prey (delicacy of crests notwithstanding). However, I don't
>> see how using the forelimbs to aid in prey capture doesn't count as using
>> the forelimbs in predation.
> I didn't mean to imply this; my apologies for not being clearer.
> Certainly, the forelimbs are still used for predation even if they are
> not used for initial prey capture (apprehension), but for securing or
> manipulating prey thereafter. This is still 'predation’.
He may be responding to me on that one, because I *did* mean to imply that the
forelimbs of most theropods would not be used even as an aid in prey capture.
Without really impressive gymnastics or incredibly perfectly positioned large
prey, most theropods couldn’t use the forelimbs to engage something they were
biting. And if we dial back the classic “theropod death match” model of
predation (i.e. attacking things larger than themselves, of which we should
probably be extremely skeptical), then the mouth basically has to miss before
the forelimbs could get anywhere useful (at which point the predator is ramming
prey it can’t see with its chest and flailing at it with forelimbs that
probably have limited prehension).
> My point was that the forelimbs of many theropods were really only
> useful for engaging large prey - such as those theropods that had
> extremely robust forelimbs that could be used to grapple with large
> prey that the predator already held in close contact. For theropods
> that targeted small prey (and I'd provisionally place _Dilophosaurus_
> in this category), the forelimbs would have been of limited use. Here
> the major issue was the size of the forelimbs - for many theropods
> (both ceratosaurs and tetanurans).
And I would go further to say that engaging large prey is something that only
very few, if any, theropods probably did with any regularity. The classic image
of a theropod grappling with a dangerous prey animal needs serious overhauling.
Predators overwhelmingly eat juveniles, and only specialist cases hunt things
even their own size. Given that the Mesozoic must have been basically full of
small juveniles, I don’t see any reason to posit that theropods regularly
hunted adult or large prey. Hone and Rauhut (2009) cover this well:
> Jason <firstname.lastname@example.org> wrote:
>> Also, why limit carcass holding and tearing to just scavenging? Holding
>> and tearing a carcass would be a useful skill for both a scavenger and an
>> active hunter.
This strikes me as much more reasonable, but even in this scenario, I wonder if
the forelimbs are long enough in most taxa. I actually checked the best LACM
Allosaurus this morning, and even just laying out the forelimb bones end to end
(completely dislocating everything), they don’t even reach the back of the
skull. If you place the head in a flexed position (munching on a carcass), the
hands just can’t get very close to where the animal is pulling.
Michael Habib, MS, PhD
Assistant Professor, Cell and Neurobiology
Keck School of Medicine of USC
University of Southern California
Bishop Research Building; Room 403
1333 San Pablo Street, Los Angeles 90089-9112
Research Associate, Dinosaur Institute
Natural History Museum of Los Angeles County
900 Exposition Blvd, Los Angeles, CA 90007