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Re: Dilophosaurus Forelimb Bone Maladies
----- Original Message -----
From: Tim Williams <email@example.com>
To: dinosaur <firstname.lastname@example.org>
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.
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.
I'm not arguing that the forelimbs were doing nothing at all. Small prey could
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
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
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.
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.
Burch, S.H. 2014. Complete Forelimb Myology of the Basal Theropod Dinosaur Tawa
hallae Based on a Novel Robust Muscle Reconstruction Method. J. Anat.
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.