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Theropod mani (was Re: Triassic dinosaur evolution)
Augusto Haro wrote:
"we should not infer that the mani of non-avian theropods were used for
grasping......we should infer they were used for quadrupedalism or flight in Theropods,
although both possibilities seem to be unlikey."
I guess that depends on how you define grasping. In Allosaurus, and many other non-maniraptoran theropods, the manus would certainly seem to be used for grasping. The offset and slightly "opposable" thumb in Allosaurus would seem to be used for grabbing or some limited sort. I have not seen an illustration of the manus in the Hoatzin, but I doubt it has much resemblance to that of Allosaurus and related ilk. The hands seem to be used in quite different ways.
Does anyone know of a functional study of the manus of the Hoatzin?
I was thinking that, according to the "Extant Phylogenetic Bracket"
approach of Witmer (1995) we should not infer that the mani of non-avian
theropods were used for grasping, in as much as the Recent Archosauria do
not grasp with their mani, and accepting they do is a less parsimonious
reconstruction. But with the same approach, we should not say they were
useless for locomotion, as they are not useless for locomotion in Recent
Archosauria (either used for flight or walking). Based on it, we should
infer they were used for quadrupedalism or flight in Theropods,
although both possibilities seem to be unlikey.
Mani grasp in the Hoatzin, but as they do only in this avian taxon as far as
I know, it seems to be more parsimonious to think grasping is an
autapomorphy of this taxon (and perhaps other related yet unknown taxa).
This suggests, against the arboreal theory, that many should not have
primitively grasped in birds. I think, departing from the bracket, that the
arms were largely useless, but there are few evidences of grasping except
from the curved claws, which may be just the result of they not being in
contact with the soil (for example, in cats the fore claws are more curved
and contact the soil less than the hindlimb claws).
But, not contradicting the bracket, they may have been used in fighting, as
there seems that goose-like forelimb-beating the mating adversary is
relatively common and may be primitive for birds (present in passerines and
ratites, as far as I remember). Now imagine at least in Paraves, that the
medioventrally pointing claws would tend to hit the adversary with their
points when the extremity is strongly extended (in the plane defined by the
elbow, which is approximately the same on which the wrist articulation
moves). A relatively more anterior direction of the tips of the unguals,
along with the extensive range of flexion-extension in the plane of the
elbow of the semilunate carpal may represent a improvement in hitting. With
the paravian pedal "sickle claw" perhaps something similar applies: the
basal state for Neornithes and Crocodylia does not seem to be hitting prey
with pedal talons, but are more coincident with kicking mating adversaries,
which seems to be widespread in birds. These intraspecific fights may have
been much lethal, but plumage should have defendended them a little.
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