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re: tiny theropod on a tree trunk question



David Peters (davidrpeters@earthlink.net) wrote:

<Try [functional] analog, Jaime. Not homolog.>

  Functional analogs have little basis for assessing evolutionary constancy
between two similar, if epistemologically different structures. Should a
lineage lose a sacrum and develop a new one from a dorsal or a caudal, and then
incorporate a synsacrum from fused caudals or dorsals, this morphology will be
entirely incapable of telling us anything about the origins and history of a
reptilian sacrum. This is about convergences, which cloud phylogenetic origins
and descent, not reveal them.

  BTW, flexibility at the wrist, or the forming of large swiveling blocks in
squirrels does not automatically reveal a pre-flight structure, but a
prehensile adaptation. This relieves stresses through pulley-like wrist bones
and allows "hanging" behaviors, and is seen also in the digits of brachiating
primates and extant sloths, both which developed their morphologies independant
of one another. Squirrels use their ability to twist their wrists to walk up
and down trucks or steep surfaces, which is unlike any other quadrupedal
climber. Indeed, the ability to pronate and swivel the wrist in squirrels seems
singularly adapted for climbing DOWN trunks, while the fingers and toes both
project upwards, allowing the claws greatest purchase with the weight, however
minor, of the animal. Colugos, on the other hand, tend to be suspensorial,
rather than scansorial, and climb UP trunks, hang from branches, and glide DOWN
(rinse and repeat). Indeed, colugo motion even when climbing is different from
that of squirrels, or bats for that matter, as they move the arms as a pair,
then the legs as a pair, while squirrels and bats alternate gait. This gives
colugos the greatest purchase while moving unsteadily on a surface, such as a
trunk, as they cannot exactly clambor. This may even be ancestral, as at least
most small rodents and rabbits I've observed move on the ground or in branches
do so with a fore-then-hind gait, which rabbits/hares have adapted into
ricocheting (though, I have seen  squirrel use an alternating gait ONCE, on a
horizontal fence post).

  Once again, we have convergences (wing membranes) and a priori observations
(gliding must lead to flight, or that a flighted animal had a gliding ancestor)
that are precluding alternate theories and the potential to exapt different
features for homologous purposes. Even should colugos be allied to, say,
pteropid bats (flying foxes) within Euarchonta or outside of them, they may
still arrive at their physical adaptations entirely independantly, just as FOUR
separate clades of sciurognath rodents have developed nearly the same physical
adaptations (Storrs et al., 1996): Eomyidae, Gliridae, Sciuridae, and
Anomaluridae. Basal taxa lack these features, and it can disappear in groups,
and reappear. There are THREE separate marsupial glider groups, though their
homogeneity is likely, along with bats and colugos. Gliding, it appears is not
uncommon for small "rodenty" mammals to evolve. Flight [and by which I mean
_powered_ flight], however, seems to have arisen only once (but possibly twice
if bats are paraphyletic).

  Storrs, G., B. Engesser, & M. Wuttke. 1996. Oldest flying record of gliding
in
   rodents. _Nature_ 379:439-441.

  Cheers,

Jaime A. Headden
http://bitestuff.blogspot.com/

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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