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Re: 11th specimen of Archaeopteryx‏



Scott Selberg <ssselberg@hotmail.com>

> Isn't it possible that the sickle claw could have allowed basal 
> deinonychosaurs to perch?


I doubt it.  There is no evidence that this digit (or its claw)
allowed the pes to better grip branches.


> If so, wouldn't that have made a reversed hallux redundant, and also allowed 
> the third and fourth
> toes to retain their more corsorial proportions?


As Jaime says, a reversed hallux (if long enough and low enough on the
foot) allows this digit to oppose the other three digits.  It is true
that many modern arboreal vertebrates grip branches *without* an
opposable manus or pes - like sloths.  But these animals have other
adaptations for holding onto branches - which are also absent from
theropods.


> Or perhaps these animals were more in the habit of
> clinging to tree trunks than perching?


I rate this as a real possibility, although it requires supporting
ecomorphological data.


> I think that the reason so many people put some maniraptors in trees (besides 
> the "if it looks like a duck..." intuitive sense of it) is that
> it's hard to believe that animals as adaptable they appear to be would NOT 
> have invaded the canopy when so many others have. If
> snakes and frogs existed only as fossils, would anyone be postulating 
> tree-snakes and tree-frogs?


With the caveat that I don't know a heckuvalot about snake or frog
biology, it's my understanding that the skeletons of these animals
lend themselves to either an arboreal or terrestrial lifestyle.  I do
know that frogs have switches between arboreal and terrestrial
lifestyles multiple times in their evolution, without any profound
changes to the anuran skeleton.  On the other hand, for a non-avian
theropod to be specialized for arboreality would require profound
changes to its skeleton, particularly to the limbs.  Birds can fly, so
arboreal birds can fly between branches or trees within the canopy,
thereby obviating the need for branch-climbing.  But if
branch-climbing abilities preceded the origin of powered flight in
birds, then we just don't see any evidence of this is in the skeletons
of any theropods.  The same goes for perching or roosting - aside from
birds.


> And even if I'm wrong about the sickle claw and perching, aren't fossils of 
> arboreal creatures very rare, making it possible that most of
> the fossils we have found were the more terrestrial species of what may have 
> been more arboreal groups?


Nevertheless, we do find evidence of other vertebrates from the
Mesozoic that are inferred to be specialized for arboreality,
including arboreal mammals, arboreal pterosaurs (like
_Nemicolopterus_), gliding reptiles (_Longisquama_, _Sharovipteryx_,
kuehneosaurids, etc), and true perching birds.  So I think it's a
tough sell to say that there were many specialized arboreal non-avian
dinosaurs around, but the fossil record just hasn't yielded these
forms yet.  Currently, the most parsimonious explanation is that the
reason we don't find specialized arboreal dinosaurs (aside from
perching birds) is that they never existed.


IMHO, it is not wholly unexpected that no non-avian theropod became
specialized for arboreality.  No known non-avian theropod became
specialized for an aquatic/marine lifestyle either - although it's
clear that some groups dabbled with amphibious behavior (such as
spinosaurids).  For theropods, a specialized aquatic or marine
lifestyle might have awaited the evolution of birds.  (I would dearly
love to see dinosaurian analogs of manatees or desmostylians, perhaps
evolving from something like _Lurdusaurus_... but I'm resigned to
disappointment on this front.)




Cheers

Tim