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Re: 11th specimen of Archaeopteryx



Jason Brougham <jaseb@amnh.org> wrote:

> 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.


Yes, that's an excellent point.  But as you know the claw curvature
isn't necessarily a tree-climbing adaptation - as Manning et al.
(2006) make clear w.r.t. _Deinonychus_.  In predators like
_Deinonychus_ and _Velociraptor_,  the second toe claw was likely used
to grip large prey.  But for microraptorines, it's an open question as
to what the second toe claw was used for.  It was likely either for
tree-climbing or prey capture (or both)  - but claw curvature alone
may not be able to discriminate between the two, because both involved
the use of the sickle-claw as a "climbing crampon".


The other thing about archaeopterygids, microraptorines etc is that
the manus was poorly adapted for grasping small prey.  So the hands
were either used to grasp large items (as inferred for _Deinonychus_
and _Velociraptor_, which appear to have targeted large prey), or the
hands were not used for grasping at all (and I doubt this very much,
based on the size and shape of the unguals).


> 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.


The lack of arboreal specializations in archaeopterygids,
microraptorines etc certainly makes trunk-climbing an intuitively
attractive option.  But then you need a rationale for *why* these
paravians went up tree trunks, but didn't venture out onto branches.
For me, if these small paravians were climbing trunks in search of
fruit or small easy-to-catch prey, then there was no reason to stay in
trees.  Once they had obtained the food item, they could return to the
ground.


Based on their dentition, archaeopterygids and microraptorines seem to
have been insectivorous or fed on other small prey.  So trunk-climbing
to snatch small prey might make sense - although I doubt a paravian
could catch a small arboreal animal, which would have been too quick
and agile (unlike paravians, they could have scampered around in
narrow branches).  Alternatively, these paravians might have climbed
large animals (like sauropods) to feed on parasites.... but here we're
heading into sci-fi territory.







Cheers

Tim
> 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.
>
>