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Re: Triassic archosaur bipedality and cursoriality

The comments about the forelimbs of many non-avian theropods being of not much use might just be a wee bit premature. Many of them have stout arms, fingers, and claws with marked muscle attachment scars and well developed deltopectoral crests. Yes, carnotaurines have weird forelimbs, but they are not sunk into oblivion. I suspect they had a specialized function we can't now, and maybe never will, be able to discern.

If one only had the skeleton of a emu or ostrich to study, their splint like forelimbs might appear to be quite useless, but they carry large wings that have important thermoregulatory and behavioral functions. If that is the case in a form, such as a ratite, with very reduced and simplified limbs, I suspect that the arms of non-avian theropods did many things "wondrous to behold" if we could just watch them in action.


On 6/28/2012 2:40 AM, Tim Williams wrote:
This is something I've been saying for a long time: That the shift
from quadrupedality to bipedality in various archosaurs (including
dinosaurs) was associated mostly with a locomotory function
(cursoriality).  It had little or nothing to do with freeing up the
forelimbs for non-locomotory functions (like grasping prey or plants).


Once the forelimbs became free from their use in terrestrial
locomotion, *then* they could be exploited for other functions.  Such
as predation; or as defensive weapons; or as wings.  However, by and
large, for many non-avian theropods the forelimbs don't appear to have
been terribly useful.  Velociraptorines are an exception, in which the
forelimbs were apparently used to grasp hefty prey.  But most other
predatory theropods seem to have used their forelimbs to *help*
position or secure prey already captured by the jaws or feet.  Not the
carnotaurines though, which shrunk their forelimbs into oblivion.

I suspect one of the reasons that forelimbs transformed into wings in
the ancestors of birds is that the forelimbs were not much use for
anything.  The semilunate carpal joint was there to help fold the
hands, and help keep them out of the way.

On Thu, Jun 28, 2012 at 1:01 PM, Ben Creisler <bcreisler@gmail.com> wrote:
From: Ben Creisler

New in Paleobiology:

Tai Kubo  and Mugino O. Kubo (2012)
Associated evolution of bipedality and cursoriality among Triassic
archosaurs: a phylogenetically controlled evaluation.
Paleobiology 38(3):474-485
doi: http://dx.doi.org/10.1666/11015.1

Supplementary materials deposited at Dryad: doi: 10.5061/dryad.t0d447t7

Bipedalism evolved more than twice among archosaurs, and it is a
characteristic of basal dinosaurs and a prerequisite for avian flight.
Nevertheless, the reasons for the evolution of bipedalism among
archosaurs have barely been investigated. Comparative analysis using
phylogenetically independent contrasts showed a significant
correlation between bipedality (relative length of forelimb) and
cursoriality (relative length of metatarsal III) among Triassic
archosaurs. This result indicates that, among Triassic archosaurs,
bipeds could run faster than quadrupeds. Bipedalism is probably an
adaptation for cursoriality among archosaurs, which may explain why
bipedalism evolved convergently in the crocodilian and bird lineages.
This result also indicates that the means of acquiring cursoriality
may differ between archosaurs and mammals.

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