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

Re: 11th specimen of Archaeopteryx



> Scott Selberg wrote:
>
>> Isn't it possible that the sickle claw could have allowed basal
>> deinonychosaurs to perch?
>
> Tim Williams wrote:
>
> I doubt it.  There is no evidence that this digit (or its claw)
> allowed the pes to better grip branches.

Manning et al. (2006, "Dinosaur killer claws or climbing crampons?", does
conclude that the second toe claw of Deinonychus has a curvature
measurement consistent with climbing animals. Xu found similar results for
Microraptor.

Briefly, though, we don't need to picture small paravians grasping
branches of small diameter in order to achieve elevated positions in
trees. Large boughs and crotches in trees, such as are found today in
older ginkgos and redwoods, can provide broad surfaces for roosting
without any need of perching adaptations. Redwood canopies develop
crotches so large that the canopies hold tons of soil and even secondary
communities of plants that are not specialized epiphytes, like huckleberry
bushes.

But to your precise point - gripping branches, one reason there may not be
any such evidence is that I can find little investigation of this question
in the literature. How strongly could non avian paravians, let's say
dromaeosaurs, flex the toes? Much has been made of the hyperextensiblity
of toe II but, taking Deinonychus as a well - preserved but improbably
large example, there is also a deep arc of the trochlea of Mt II ventral
to the shaft, suggesting a great range of flexion as well. In Ostrom's
figure 76 in his 1969 monograph on Deninonychus he diagrams the range of
flexion of the toe, but keeps the 1st joint extended. If we move the
flexed toe to a flexed position relative to the Mt we find that the point
of ungual II points toward the heel of the foot. This could allow a branch
to be pinched between the heel of the foot and the point of the ungual.
The strong flexor processes of the ungual and second phalanx are already
well documented, and provide evidence for a strong grasping capability
which likely exceeded that in a  hallux.

My observation would have to be tested with actual 3d fossil specimens,
the role of soft tissues would have to be estimated,  and limits on the
ranges of motion would have to be considered, for this to be valid as
"evidence", but the possibility certainly has not been excluded.