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Re: Dilophosaurus Forelimb Bone Maladies

Jason <pristichampsus@yahoo.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 
> taxon.

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.  Also, no amount of cartilage is going
to alter the range of motion at joints such as the semilunate carpal
or the mesotarsal ankle, where motion was essentially limited to a
single plane (arcuate or hinge).  Further, certain contacts between
elements restricted motion - such as the proximal radius fitting
snugly into a concave surface of the proximal ulna, thereby preventing
movement of the radius relative to the ulna, precluding pronation or
supination.  It should be noted that the study that found limited
anterior reach in _Acrocanthosaurus_ actually simulated the presence
of soft tissue at the articulation between the glenoid and humeral
head using foam rubber.  I know this may not quite do justice to the
original soft tissue, but there is a recognition that articulations
were not just bone-on-bone.

> 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'.

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).

> 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)?

No, these all count as predation (except scavenging on things already
dead).  The exact role of the forelimbs was likely very different
according to predatory strategy.  A theropod that targeted small prey
would have little need for holding and tearing large carcasses (unless
during scavenging).  Since most carnivorous theropods likely targeted
prey smaller than themselves, this probably did not arise too often.
Theropod forelimbs were not adapted for securing or handling small
prey.  This is especially acute for short forelimbs.

> 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.

I'm not so sure about this (that wings rendered the forelimbs useless
for anything else).  Basal birds retained fairly mobile digits and big
claws alongside their wings (e.g., archaeopterygids, jeholornithids,
confuciusornithids, sapeornithids).  One hypothesis is that the clawed
hands were used for climbing up or clambering around in trees.  I'm
skeptical of this (insofar as although I think basal birds might have
spent time in trees, I doubt the hands were of much use in grasping
branches).  The forelimb of basal birds could have retained non-flight
functions as it transitioned into a wing.  In Ornithothoraces, the
manus skeleton became fully absorbed and committed to a flight
function.  Nevertheless, some extant flighted birds do use their
forelimbs in agonistic behaviors (and are often equipped with spurs
etc for combat).

> 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.

I don't think this analogy works.  In many theropods (including
pennaraptorans) the forelimbs may have become specialized for display
and/or intraspecific combat.  Large feathers would interfere with the
forelimb's ability to catch small prey (incidentally, this has nothing
to do with forelimb mobility - merely the very presence of large
feathers, which would have impeded the hand from contacting the
ground, and prevent the two hands from being brought closely
together).  Large forelimb feathers would not have an obstruction when
dealing with large prey, due to the orientation of the feathers.  In
either case, if display feathers became co-opted for aerodynamic
purposes, then little would change vis-a-vis predation as far as the
forelimbs were concerned.

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

I don't disagree - especially when contending with large prey.  But
this role was very much constrained when dealing with small prey.