[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Re: Akidolestes (Cretaceous symmetrodont mammal) postcranial skeleton



Ben Creisler <bcreisler@gmail.com> wrote:

> Meng Chen and Zhe-Xi Luo (2012)
> Postcranial Skeleton of the Cretaceous Mammal Akidolestes cifellii and
> Its Locomotor Adaptations.
> Journal of Mammalian Evolution (advance online publication)
> 2012, DOI: 10.1007/s10914-012-9199-9
> http://www.springerlink.com/content/23252m78k072p834/


This paper illustrates why, as a general rule, it's valuable to post
papers to the DML that are not directly dinosaur-related.  The
_Akidolestes_ paper has a nice section on determining arboreality vs
scansoriality vs terrestriality in Mesozoic mammals.  Not only does it
present conflicting views on the definitions of
arboreal/scansorial/arboreal, but outlines how the use of
morphological criteria to distinguish these lifestyles is not at all
straightforward.


The paper actually dovetails quite well with the recent re-description
of _Yixianosaurus_ by Dececchi, Larsson and Hone.  That paper looked
at various forelimb proportions in theropods, including (as the
_Akidolestes_ paper did) phalangeal indices of the manus.  Dececchi et
al. (2012) concluded that the hands of the small theropods most often
regarded as possibly arboreal were not especially well-adapted for
grip-based climbing.


Using quantitative measures such as the relative length of the
penultimate phalanx, and the overall length of the non-ungual
phalanges, Dececchi &c found that taxa such as _Archaeopteryx_,
_Microraptor_, _Anchiornis_ and _Bambiraptor_ were actually fairly
average (or worse) graspers by theropod standards.  On the basis of
the manus, there is nothing to suggest that they were particularly
adept at gripping branches.  On the other hand (so to speak),
_Epidendrosaurus_ has a very high phalangeal index.


The manus of basal ornithomimosaurs also had long fingers (and high
phalangeal indices) that were excellent at encircling small objects;
but other features (reduced ligament pits, poorly curved claws, weakly
developed flexor tubercles) suggest the hands were quite weak.  The
manus of _Yixianosaurus_ had the best of both worlds: grasping
proportions and features suggesting a strong grip.


Nevertheless, even though _Archaeopteryx_ and _Microraptor_ were
rubbish at grasping branches, I don't think this refutes the
hypothesis that these and other small paravians could venture into
trees.  It may be they were opportunistic trunk-climbers, which
avoided the need for branch-grasping (i.e.,which required encircling
small objects with a firm grip).  But, following on from Dececchi &
Larsson's 2011 paper, it certainly throws cold water on the hypothesis
advocated by Feduccia and friends that _Archaeopteryx_ and
_Microraptor_ were specialized for arboreality.  These paravians
clearly did NOT spend most of their lives in trees, and therefore
likely did NOT use their wings to glide or fly between trees.






Cheers

Tim