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Re: Fwd: Aw: RE: Arboreal Theropods: The prize at the bottom of the cracker jack box



Jason Brougham wrote:

>> Some on this thread have mentioned that, if an animal is using trees in its 
>> biology, it should show specializations.

>> Recent work has found an ingenious way to test the time between a behavioral 
>> shift and a clear morphological specialization.

>> In fossil proboscideans that switched from a tree leaf diet to a grass diet 
>> (as marked by carbon isotope ratios) the answer is 3 million years.

>> I bet that is a long interval but, then again, tooth crown height should 
>> increase faster (by shifting to the taller end of the pre-existing 
>> distribution of tooth crown heights in the population) than complete descent 
>> and reversal of a toe.

Mike Keesey wrote:

>That's really cool! But consider that basal eumaniraptors probably had
>much shorter generation times (not to mention more offspring) than
>proboscideans.

Another consideration relative to the rate of morphic change is the situational 
profile -- if you chew grass 1% more efficiently than your neighbor, you may 
prosper, but your neighbor is unlikely to starve prior to reproducing -- 
whereas a fleet herbivore that can only run at 99% of average herd speed may 
well be below the survival threshold. It follows that genes are removed at 
varying rates, per situation. 

The feet of birds and their ancestors are shaped by daily foraging/escape 
locomotor activities, not sleep habits -- and the implied argument that the 
perching foot is a convenience conveyed to it's owner by sleeping on a branch 
needs to be put to rest (pun intended).