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

Re: Avian flight stroke origin



Dr Ronald Orenstein <ron.orenstein@rogers.com> wrote:

> It can be objected that these are much better fliers than maniraptorids, and 
> that getting into a tree does not require special climbing adaptations if you 
> can fly there. In that case, what about (say)
> squirrels, which range similarly from the ground to the treetops and include 
> gliding species. If a marmot and a flying squirrel can be close relatives, 
> why can't dromaeosaurids (say) have included a
> similar mixture?


In general, small terrestrial mammals have an intrinsic capability for
climbing (scansoriality), because of the requirement to negotiate
uneven terrain on the ground.  The shift to fully arboreal habits
often involves improved mobility at the joints (especially rotative
mobility) and changes to the extremities, such as (but not limited to)
the proportions and shapes of the phalanges associated with grasping
or clinging, and (in many cases) a prehensile tail.  All these changes
represent adaptations to life in the tree-tops, to allow better
grasping of branches and movement within the canopy.  For small
quadrupedal mammals the shift from terrestriality to arboreality does
not require an overhaul of the animal's bauplan, because existing
osteological features can be 'tweaked'.


However, as cursorial bipeds in which motion at the joints was highly
constrained, theropods appear poorly adapted for arboreality.  In
birds, the pes became specialized for perching.  The inability of the
ankle to rotate, combined with the general lack of
branch-grasping/prehensile ability of the manus, meant that the pes
became the sole instrument for gripping branches (notwithstanding that
some birds use their beaks, and in the case of juvenile hoatzins,
wing-claws - but these are rare exceptions).  The development of
powered flight and (more importantly) ground-level take-offs obviated
the need for trunk-climbing.  Because the feet of non-avialan
theropods (and most basal avialans) lack specialized grasping/perching
adaptations, Dececchi and Larsson (2011) argue that these taxa were
non-arboreal.  I agree.  However, just because theropods were
maladapted for living in the canopies does not automatically mean they
could not occasionally venture into trees.  _Jeholornis_ has the
proportions of a ground-foraging biped - but it was found with seeds
in its stomach.  Assuming the seeds came from a tree that was taller
than _Jeholornis_, did it wait for the seeds to fall to the ground; or
did it climb trees to get them?  I would argue the latter, and that
this opportunistic behavior was typical of small paravians - including
incisivosaurs (like _Protarchaeopteryx_), archaeopterygids, and small
deinonychosaurs.


> Further, as I have said here before, a number of birds, including 
> particularly the cracids, can shift back and forth between trees and the 
> ground with ease, and others feed on the ground and nest in trees
> (including such unlikely tree-dwellers as ducks). Could there have been tree- 
> or cavity-nesting dinosaurs?  We'd be unlikely to find fossil evidence on the 
> point.


As I understand it (and correct me if I'm wrong here) all such birds
have a perching pes.  Even the cracids have a reversed and fully
descended hallux.


I tend to agree with Dececchi and Larsson (2011) that no dinosaur
(aside from derived avialans!) roosted or nested in trees.


> Also, even flightless or near-flightless species of birds can get up into a 
> tree without having climbing-adapted front limbs. It's quite amazing to see 
> the ability some birds have of getting around in trees by
> simply jumping from branch to branch, and if the trees have limbs near the 
> ground they can reach the canopy in this way too. A good example might be the 
> Kokako (Callaeas cinerea) of New Zealand,
> which is a fairly weak flyer (and, oddly for a passerine, a folivore), but 
> which is almost if not entirely arboreal; it tends to leap upwards and 
> fly/glide downwards.


The kakapo (_Strigops habroptila_), a flightless parrot, climbs trees
with its feet, and glides down to the ground.  The zygodactyl feet are
superb for climbing.






Cheers

Tim