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Re: DIGIT LOSS INQUIRY



George Olshevsky (Dinogeorge@aol.com) wrote:

<This hasn't bothered taxonomists in the past. E.g., it was once found that 
dinosaurs had
perforate acetabula and that "thecodontians" had imperforate acetabula. So, 
suddenly, any
non-pterosaurian archosaur with a perforate acetabulum was a dinosaur, and any 
non-pterosaurian
archosaur with an imperforate acetabulum was a "thecodontian.">

  Not so at the time that Gauthier and then later Sereno and Novas and Arcucci 
were working on
dinosaur origins, in such a manner as to disprove the perforate acetabula as an 
exclusive,
synautapomorphy ... it is only an autapomorphy, and one of many hip/hindlimb 
features of which the
group is diagnosed. This circular argument can be disregarded as an historical 
anecdote, and not
something on which to validate a newer argument.

  Anyways ... one way to consider this, even though it reverses in a later 
group, is to note the
primitive members of a selective membership as to having the condition, whereas 
later forms
reverse it. I see this enough times to make me cringe: an analyst will see a 
later reversal as a
suggestion that the character is "bad" because every single descendant lacks 
this feature, or
reverses it to the condition prior to the autapomorphic condition. No direct 
ancestor to
Dinosauromorpha possesses a perforate acetabulum, and the feature only appears 
in Archosauria in a
group outside of Crocodylomoprha, but well on the pseudosuchian stem 
(Rauisuchia). Otherwise, this
feature is synapomorphic, even if it is known elsewhere in Archosauria. 
Secondarily, features of
the pes in a relatively derived group do not affect in retrograde the 
expression in an ancestor.
In this way, the condition in sauropods is not exemplary of the condition in 
prosauropods, like as
not, as the pes is secondarily shortened in the earliest sauropod for which 
such a pes is known
(*Shunosaurus*) and the form of the lower limb in more primitive sauropods 
(such as *Isanosaurus*
& *Vulcanodon*) appears to indicate a primarily graviportal limb, and thus a 
shortened pes. The
fifth metatarsal is expanded broadly, to reflect bearing weight, but this is 
not so in more
primitive dinosaurs or any descendant thereof except sauropods, thus the 
condition in sauropods is
unique to dinosaurs and cannot be used to gague any other dinosaur condition, 
even prosauropods.


=====
Jaime A. Headden

  Aaaaaaaaaaaaaaaaaaaahhhhhhhhhhhhhhhhhhr-gen-ti-na
  Where the Wind Comes Sweeping Down the Pampas!!!!

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