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Re: Parts & Non-Recreation (was RE: dino-lice)
2011/4/27 Jaime Headden <email@example.com>:
> My assumption is that this was progressive, with development of the manus
> prior to limb size reduction, which seems to coincide with body size
> reduction. This corresponds with phylogeny. Why? I have no clue.
This may be true, and that perhaps the alvarezsaur forelimb gave some
task strenght advantage at a relatively larger size, but then reduced
with possible loss/lack of use into the Late Cretaceous forms. Such a
hypothetical stage is unknown to me.
> Tim and Jason had an interesting exchange talking about what the "reduced"
> limbs of alvarezsaurs might mean, but I would argue that regardless of
> reduction of the limb _size_, comparably "reduced" limbs in other animals
> (the kiwi has been brought up) lack the robusticity of the bones of
> alvarezsaurs. Jason mentions stout-limbed dachshunds, which have bowed
> diaphyses and robust epiphyses; Jason shows other animals (and humans) with
> various limb deformities, but there is an issue involved: dachshunds and
> bulldogs are the product of bred deformity, most significantly
> chondrodysplasia, which is congenital and abnormal for developing dog
> fetuses. We have this information because we have the critical context we
> need to use it. To use an old analogy, aliens coming to the earth may not
> have this information, can look at the morphology of dogs found as fossils,
> or genetics from subfossils, and use this information to infer a collective
> species concept distinguishing these organisms wholly; but we'd know they'd
> be wrong, due to the critical context of lacking the developmental issues
> involved. Dachshunds are the product of a genetic breeding program selecting
> for a sometimes-critical disease. I cannot find any contextual reason why
> you'd want a maloccluded bulldog other than some sense of its appearance, or
> the nearly nonexistent nasal sinuses in various breeds, also in some cat
> breeds. We humans can be horrible at pretending breeding equals evolution,
> and this shows in many, many ways.
True, but limb shortening while maintaining a similar capacity for
power (a relatively shorter limb with more robust aspect) may be also
result of non-human evolutionary processes. For example, Speothos have
relatively shorter legs than in other canids, but as it has to move
the same mass that other canids in its size league, their limbs may
have still to maintain similarly thick muscles and probably lever
arms. The limbs of otters, relatively short-legged for carnivores,
also show robustness relative to their lenght when compared with the
limbs of other long-legged carnivores in their size range.
> The only problem that I think is present, and the one that dominates this
> discussion, is one of size, and this seems incomparable to the issue of
> deformation above. This is primarily due to the length of the neck and to the
> length of the limbs, which point habits for the animal in different
> directions. I have NO idea why we should be trying to constrain limb
> functionality in the fore to the hind in a bipedal animal, and this puzzles
> me in the discussion involved.
Relatively small size in a limb in principle suggests decreased
strenght (even if the alvarezsaur forearm is strong for its size, it
is not likely so for the animal size), and in a tooth or nail,
suggests reduced capability to modify a bitten or grasped element
(although both may modify similarly a relatively smaller element).
However, I actually do not know if we can say alvarezsaur had
relatively small claws for their body size (compared with Haplocheirus
or some hypothetical basal alvarezsaur whose morphology is inferred
from ancestral character states in a phylogenetic analysis, and
discarding size differences and thus allometric effects), or if we can
say that its forelimb muscles were capable of smaller force output
than in Haplocheirus or an hypothetical alvarezsaur ancestor (this
would require a comparative biomechanical analysis of inferred
muscular size, shape, and cross sections, as well as lever arm
lenghts, of mucles and reaction forces).