Re: Dilophosaurus Forelimb Bone Maladies

[Jura <pristichampsus@yahoo.com>]

----- Original Message -----

From: Tim Williams <tijawi@gmail.com>
To: dinosaur <dinosaur@usc.edu>
Sent: Monday, February 29, 2016 1:44 AM
Subject: Re: Dilophosaurus Forelimb Bone Maladies

I know it sounds counterintuitive, but theropods like _Dilophosaurus_
might not have used their forelimbs much in predation.  In general,
the forelimb had limited reach, highly proscribed mobility, and was

incapable of one-handed (unimanual) prehension. 


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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 taxon.


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I'm not arguing that the forelimbs were doing nothing at all.  Small prey could 
be seized
with the jaws, and dispatched with the hand claws - although this
would be a bit difficult when the hands were so far from the mouth.
For larger prey, the forelimbs could be used to help secure or
position the prey, again after the prey was seized by the jaws (this
has been proposed for _Tyrannosaurus_, for example).  Forelimbs might
also have been used to hold and tear carcasses underneath the body

during scavenging.

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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. 


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. Again, I am left a bit confused by your argument. All of these actions 
should count as acts of predation. Are you limiting the definition of forelimb 
predation to something more along the lines of primary form of contact (say, 
swiping the claws, or reaching out and grabbing first)?


___________________________________________________________________
Mammalian predators certainly use their forelimbs during prey capture.
But birds of prey manage quite well using only their head and feet.
This ability might long precede the origin of flight.


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I don't think birds are a good example of theropod forelimb function. We know 
that birds evolved/exapted their forelimbs for a very specific function that 
basically made them useless for everything else. Birds have done a fantastic 
job of employing other body parts to do the work that their forelimbs could 
have done, but to use them as a template for theropod forelimb motions is a bit 
like arguing the function of extinct lizards based solely on the known 
abilities of extant snakes. 


Outside of Mammalia, forelimb use during predation/eating (it's not always 
clear where one ends and the other begins) has been observed in varanids and 
snapping turtles (actually, most turtles). These are both animals that use 
their forelimbs for locomotion. It seems pretty unlikely that theropods, which 
decoupled their forelimbs from locomotion, were not using their forelimbs for 
something (with notable exceptions for Tyrannosaurs, Abelisaurs and a couple 
others). Predation seems like the most likely use.

Again, I'm not arguing for their role as a primary means of attack, but rather 
as supplemental tools during prey dispatch and ingestion.


Jason

References

Burch, S.H. 2014. Complete Forelimb Myology of the Basal Theropod Dinosaur Tawa 
hallae Based on a Novel Robust Muscle Reconstruction Method. J. Anat. 
225(3):271–297.

Hutson, J.D., Hutson, K.N. 2012. A Test of the Validity of Range of Motion 
Studies of Fossil Archosaur Elbow Mobility using Repeated-Measures Analysis and 
the Extant Phylogenetic Bracket. J. Exp. Biol. 215:2030–2038.

Tsai, H.P., Holliday, C.M. 2014. Soft Tissue Anatomy of the Archosaur Hip 
Joint: Structural Homology and Functional Implications. J. Morph. 00:1–30.