Re: No more paedomorphosis

["Matthew Troutman" <m_troutman@hotmail.com>]

<<As you know, I consider the hallux reversed in practically all 
theropods. It articulates loosely at the back of the second metatarsal 
and was probably as laterally mobile in life as the human thumb--which 
can align parallel with the other digits or oppose them. So this 
character is not absent in dromaeosaurids, but is secondarily modified 
so that the reversed hallux was kept off the ground during walking or 
running (a cursorial adaptation).>>

     Norell and Makovicky have discussed this problem in their recent 
dromaeosaur paper. The perfectly articulated pes shows the hallu 
non-reversed in both feet. And the arrangement that you propose is not 
seen in any birds where cursorial birds just lose the hallux ( the 
exception is the roadrunner, where the zygodactyl foot has been turned 
into an extraordinary running adaptation ). 

<<The other character is probably taxonomically ambiguous. Perhaps you 
could be a bit more specific? Tell me, for example, how presence or 
absence of one character would exclude a species from a particular 
family, if it possesses numerous other characters of that family.>>

     In birds the ischium has many proximodorsal processes. These are 
not seen in dromaeosaurs as shown by the dromaeosaur skeleton described 
by Norell and Makovicky.

<<The _Rahonavis_ specimen gives me the idea that the enlarged claw on 
the second digit of dromaeosaurids and other theropods might have been 
the most specifically opposed ungual to the retroverted hallux, so that 
perching was accomplished in these forms primarily by the claws of 
digits 1 and 2. In the ground-dwelling descendants, digit 1 shrank, but 
digit 2 was exapted into thecelebrated weapon.>>

That would be a _heterodactyl_ foot ( the ancestral bird foot is 
_ansiodactyl_, where the digits I and IV are reversed is a zygodactyl 
foot, and where all the digits are anterior [ but digit I and IV are 
still reversible at will ] is pamprodactyl ). 

MattTroutman

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