Re: Alvarezsaur Polarity and Features

["Jaime A. Headden" <qilongia@yahoo.com>]

I wrote:

<<There are two choices for the evolution of the [meta?]tarsus>>

  No, I'm afraid I referred to the tarsus. As in, the functional mechanics of 
the ankle, or
tarsus, which includes the [proximal and distal] tarsal bones and the 
metatarsal bones. Appologies
for confusion.

David Marjanovic (david.marjanovic@gmx.at) wrote:

<IMHO, while both are equally parsimonious, 2) is much less plausible because 
it requires
*Alvarezsaurus* to lose the arctomet while obviously staying cursorial (the 
metatarsus is long and
compressed like in *Archaeopteryx*, more than in *Coelophysis* or IIRC 
*Compsognathus*). Judging
from the line drawing in Glut's Encyclopedia, it looks a lot like that of 
*Archaeopteryx* in
general.>

  This is a simple state of loss, and is one transformation. The 
hyper-arctometatarsaly is
another. In hypothesis 1, there is a reversal (from arctomet to non-arctomet) 
earlier than
*Alvarezsarus*, and there are two acquisitions (arctomet and hyper-arctomet) 
after it. This makes
hypothesis 2 more parsimonious, as I alluded to previously.

  Please also note that the arctomet does not confer a special phylogenetic 
inferrence compared to
the abscence of it. Functionally considering, the structure is a natural 
adaptation for
hyper-cursoriality, seen convergently in various ungulate groups, and also in 
theropod dinosaurs.
Having it does not make an essential coelurosaurian clade unless backed up by 
other features. The
distinctions between the various arctomet theropods (Holtz, 1994) has been made 
before and should
be referred to. One would expect the arctomet in various cursorial theropods. 
The more cursorial
the limbs, the more likely the arctomet appears. Tyrannosaurids are apparently 
more dervied, and
basal forms, like *Alectrosaurus*, have slender limbs nearly identical to those 
of ornithomimids.
Phylogenetically, they retain the structure, and I believe Holtz (1994) 
discusses a plausible
reason why: Tyrannosaurids are larger and become primary carnivores, based on 
size-weighting
hunting ... they still chase their prey, and rely on the feet to get the mouth 
where it needs to
be. There have been other suggestions that are in keeping with this, such as 
*Allosaurus* being a
grappling predator, and not have required such cursorial features to hunt with.

  I would attach no real significance to the arctomet phylogenetically without 
corroborating
support.


=====
Jaime A. Headden

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

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