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Re: tiny-armed theropods

2011/10/8 Jaime Headden <qi_leong@hotmail.com>:
>   Understand that I am making an assumption here that the oversized skull and 
> neck of a tyrannosaur is not as "massive" as may be assumed. That is, the 
> skull and neck are very pneumatic, and in many cases, the additional volume 
> of the skull from, say, a similar-sized carnosaur is likely to be filled with 
> air and muscle, on top of the wide throat and vaulted palate. One may then 
> consider than the "massiveness" of the skull in some taxa is over-exaggerated 
> and that the balancing effect made up for by the reduced limbs is not 
> substantial at all.

By more massive I was thinking of a structure with more mass. Large
skulled theropods are pneumatic yes, but theropods with relatively
smaller skulls are also pneumatic, so an increase in volume may imply
an increase in mass (even if taxa with large heads are proportionally
more pneumatic than the others). Of course, a similar mass or even a
decrease in mass if pneumatization is enough are also possible, but I
do not see it much likely. My point is that whether or not the head
increases in mass (and thus weight), in response to some functional
selective pressure, that does not contradict selection of weight
decrease in less useful organs. For these are hypothesized as two
different selective pressures, regardless of the possibility that one
can influence on the strenght of the other by oppossing it.
Contradiction between selective pressures may be explain, for example,
that although pelagic animals generally have lighter bone, and turtle
Archelon much lightens its carapace with fontanelles, it has yet a
proportionally enormous head, probably out of some selective reason,
if not just an allometric one. Not advocating weight reduction is due
to weight problems, just want to point out that is a logical
possibility. Other is that the limbs, if useless, consume resources,
during development and maintenance, which would be directed to more
useful structures.