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I've been trying to understand alvarezsaurid arms from an evo-devo perspective.
I looked into anchondropalsia and dwarf breeds of dogs to see what happens to
pectoral morphology when genetic changes shorten the limbs.
Guess what I found? The long bones tend to retain their ancestral diameter, but
they get much shorter, making them proportionately more robust. Much more
importantly, the processes appear to hypertrophy, because they also stay large
while the overall size of the bone reduces. . Below I have photographs of a
bulldog skeleton and a coyote skeleton for comparison. I am not an expert on
mammal osteology, but please tell me if you see what I see. The humeri have
expanded proximal tuberosities, a hypertrophied deltoid tuberosity that has
moved proximally, and enlarged epicondyles.
The olecranon also looks longer and more rugose to me, do you agree? This is
also true in the achondroplastic human skeleton I've linked to on the bottom
These details are strikingly similar to alvarezaurs. This seems, to me, to
suggest that the characteristic morphology of alvarezsaur pectoral anatomy may
be a consequence of the genetic process that reduced it.
Now, it may appear that the processes for muscular attachment have enlarged
from the ancestral state but they haven't. Not at all. They have only become
RELATIVELY larger as the rest of the bone became smaller at a faster rate. I
think that this has confused many workers, who may say that the limbs have
become MORE robust when, in fact, they have only become wider relative to
length as, overall, they reduced to a tenth of their ancestral length.
I also read the Linhenykus paper this morning and checked the measurements on
the forelimb. Linhenykus weighed the same as a large pigeon (C. livia), 450
grams. It's hand was 17mm long, the same as your thumbnail, and the rest of the
arm was about equal in length, so the whole pectoral limb was the same length
as the last segment of your thumb. Wouldn't it take several kilograms of force
to break the concrete - hard matrix of a termite mound? If so, if those tiny
arms could somehow produce that much force, wouldn't the arm of Linhenykus push
it strongly away from the termite mound before the mound cracked? The little
fellow would be doing pushups!
Human achondroplastic olecranon:
Dachshund humerus with enlarged proximal tuberosities:
Comparison of human humeri and femora with and without achnondroplasia:
Senior Principal Preparator
American Museum of Natural History
(212) 496 3544