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Re: Avian flight stroke origin



2011/8/11 Tim Williams <tijawi@gmail.com>:

> Nevertheless, it's pleasing to see Dececchi & Larsson (2011) refute
> the hypothesis that the ancestors of birds were specialized for
> arboreality. That's always good to see, because this is the central
> plank of the BANDit's scenario-driven approach to the evolution of
> avian flight.

It seems to me that the possibility that theropods were arboreal would
harm more the BANDits, because they would lost the argument that
theropods, as cursorial animals, were not ancestral to birds, for the
ancestors were required (in their view) to be arboreal.

> It is possible
> to reconcile the cursorial/terrestrial morphology of paravians with a
> "trees-down" origin of flight.
>
> In fact, a possible answer to this paradox is provided by Dececchi &
> Larsson (2011) themselves:
>
>   "In general, nonavian theropods clustered with scansorial and
>    grip-based climbing taxa..."

Then, IS there morphometric evidence supporting scansorial habits in
theropods? This would mean trees down has support (for, as you say
with other words, trees down does not require one to be a gibbon).

> But what if the ancestors of birds were *not* predators?  The
> "scansorial and grip-based climbing" features might then actually have
> been real scansorial and grip-based climbing features.  The ancestral
> predatory adaptations would therefore be exapted by paravians for
> grip-based climbing.

Here the possibility of scansorial habits would not require basal
birds being not predators (better said, diet would not count), as
predatory cats can also get advantage of curved claws in climbing,
while except for some particular cases as the margay, not having other
specifically climbing adaptations, as far as I know. Besides, if
ancestral birds were not predatorial, the predator/scansorial features
may be alternatively seen as simple inheritance. A good exploratory
study may be imagined searching for differences between adaptations
for scansoriality and predation in the limbs.

> Dececchi & Larsson's emphasis on grasping branches and negotiating
> tree crowns to assess "arboreality" seems slightly misplaced for
> paravians, given the dominance of large cycad (and cycad-like) plants
> throughout most of the Mesozoic, including the time that the ancestors
> of birds were getting their wings.  A reversed and enlarged hallux is
> needed only for perching.  If paravians were scaling trunks, grabbing
> food items, then returning to the ground, a perching pes would not
> have been necessary at all.

Great point! If the trunk of these Jurassic-Cretaceous cycads resemble
that of Recent ones in their more irregular, step-like surface
(produced by the bases of fallen leaves), they may have been easier to
climb than most modern trees.

Cheers,

Augusto