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2011/4/22 Jason Brougham <email@example.com>:
> I suppose that I should restrict my question to limbs, because there are, of
> course, birds that have reduced bills that are used for gleaning. But really
> almost any adaptation in the mouthparts will function in feeding.
You got a point here... I may say that my beloved egg-eating
hypothesis does not imply a trophic similarity with any Recent
vertebrate, and so may imply a unique adaptation, but the fact that it
seems to apply to all vertebrates and all diets disables this
argument. I cannot mind any vertebrate with small limbs using they for
trophic reasons... well, perhaps some shrimp or crab with small claws
who catch relatively small food particles, but not vertebrates...
Although, most non-mammalian vertebrates I can mind do not use they
limbs in food manipulation (except some turtles and anurans), so you
likely have a point. I wonder if this may even be extended to all
dinosaurs, and to a lot of dinosaur forelimb food gathering/processing
hypotheses, overall among deinonychosaurs and other maniraptorans, in
addition to alvarezsaurs. Forelimbs seem to relate more to locomotion
than to food processing/acquisition in most non-mammals.
I think phylogenetic bracketing may give you another point, in favour
of display or intraspecific fighting over manipulation or capture of
food: the Recent animals which more related to alvarezsaurs are,
namely birds, use their limbs principally for locomotion (flying,
swimming, even climbing in the hoatzin), but also for display while
mating, fighting (often with spikes), but, strangely, as far as I
know, not for food acquisition or processing. The other Recent group
bracketing alvarezsaurs, the crocodiles, do not make use of forelimbs
for food manipulation either, just use them for quadrupedal locomotion
or nest excavation.
The forelimb lenght reduction hypothesis with decrease in use
(occurring in a way different from that more common coelurosaurian
forelimb reduction which also implies a reduction in transverse
proportions) merits great respect, to me. It may be that alvarezsaur
forelimbs faced a conflict between a necessity to reduce limb lenght,
and another to retain use as long as forelimbs were reduced. Speothos
dogs reduce limb lenght while these are still functional. Viewed in an
evolutive context, perhaps known Cretaceous alvarezsaurs represent
just the moment at which the forelimb length reduction resulted in
lack of function, in a clade where previous stages with a lesser
degree of lenght reduction did not show a degree of lack of use
similar to that in other coelurosaurs with similarly reduced
forelimbs... As locomotion may be mostly impaired, and we can lost
favour for trophic hypotheses given your point, they may have used
mostly in mating or display. Once this leg reduction mechanism
appeared in these early alvarezsaur types, perhaps it was the leg
reduction mechanism more likely to account for further forearm
reduction which ultimately implied lack of use.
By the way, very funny stuff about human butts extracting snails and
breaking termiteria, even if mocking my proposal.