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

Re: Avian flight stroke origin



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

> OK, granted, but I keep coming back to guans (or, among mammals, tree
> kangaroos or, for that matter, people).  Are their feet well-adapted for an
> arboreal life?


Modern tree-kangaroos (_Dendrolagus_) are adapted in many ways for an
arboreal life, including their feet - although the feet are certainly
not as specialized as those of other arboreal marsupials.  Dececchi &
Larsson (2011) explicitly singled out the tree-kangaroos as being poor
analogs for hypothetical arboreal theropods: "Tree kangaroos have a
modified calcaneocuboid joint to accommodate
increased mediolateral rotation of the ankle [116–118], reduced curial
indices (between 80%–55% those of terrestrial kangaroos) and hindlimb
lengths [119], and increased forelimb and axial
column flexibility, most notably via highly mobile shoulder and wrist
joints [120,121]."  Earlier works agree that tree-kangaroos have quite
a number of arboreal characters that are lacking in fully terrestrial
macropods.  For example, take Flannery et al. (1995): "Their
morphology reflects this [arboreal] lifestyle, for all possess large
and powerful forelimbs, and adaptations in the hindlimb which allow
for extensive rotation of the foot."  The latter is also apparent in
the giant fossil tree-kangaroo _Bohra_ (shortened tibia-fibular
contact; open calcaneal-cuboid  articulation).  Added to this,
tree-kangaroos also tend to have wider hindfeet (pedes), and the claws
on the manus and pes are very robust and recurved.  The same or
similar arboreal features evolved independently in _Nambaroo_, a basal
macropod that still retained an opposable toe, which was lost in the
ancestors of tree-kangaroos.


> And birds today, even terrestrial birds, enter trees for
> other reasons than food-finding - roosting, for example, in order to stay
> out of the way of predators.  I can well imagine that a small maniraptorian
> might spend the day on the ground, but ascend for the night as high into a
> tree as it could get.  It wouldn't have had to be as agile as a squirrel to
> do that - it could have used bipedal leaps to get up to its roost, and,
> perhaps, done a bit of volplaning on the way down the next morning.


This must be an intuitive thing, because I find a roosting lifestyle
difficult to imagine for these maniraptorans.  The sticking point for
me is the lack of an enlarged, fully descended or opposable hallux.
It is not until we get to confuciusornithids or _Sapeornis_ that there
is evidence of some kind of perching pes, consistent with spending
long intervals of time in the trees.  The modern secretary bird
(_Sagittarius serpentarius_) spends most of its time on the ground,
but roosts and nests in trees.  It has a perching pes.  So small
maniraptorans could similarly have employed the hallux for perching
without interfering with terrestrial progression  But it was not until
long after they evolved wings that the first perching pes evolved.
Even therizinosaurs recruited the hallux for contact with the
substratum.





Cheers

Tim