Re: Broader than Dinosauria [Additional Info]

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

Alessandro Marisa (amaris@tin.it) wrote:

<I want to remember that in his paper on SVP 16 (4) 1996 Dr. Novas argued that 
the presence of a
perforate acetabulum in Marasuchus is equivocal, the problem merges from an 
erroneous
reconstruction of the acetabulum based on PVL 3870 in which the ilium forms a 
triangular ventral
projection, as in others Archosaurs with imperforate acetabulum (es. 
Saurosuchus, and
Gracilisuchus). The ischium and pubis do not contact the ilium because the 
acetabular extremity
was broken, resulting in an artefact semicircular gap, erroneously interpreted 
as an acetabular
perforation. In the right pelvis of PVL 3871 the acetabular portion of both 
pubes and ischium is
completely preserved and lacks the semicircular gap of PVL 3870, Novas 
concluded that the combined
information of pelvis morphology of PVL 3870 and 3871 suggests that Marasuchus 
has an imperforate
acetabulum.>

  It is a possibility that *Marasuchus* possesses a complete medial acetabular 
wall. A specimen of
*Marasuchus*, as pointed out by Sereno and Arcucci, 1994 (JVP 14(1)) PVL 3871, 
has only the short
triangular apse, and as I described for *Lagerpeton*, a ventral process of the 
iliac portion of
the acetabular wall thatinserts between the pubis and ischium.

  At the behest of Ken Kinman, I looked at the pelvis of *Guaibasaurus* from 
the publication,
thanks to Mickey Mortimer (my copy is buried, he photocopied the pelvis for 
me). The ilium and
ischium have only partial regression of the medial acetabular wall, but the 
iliac portion is
level, not projecting ventrally or concave along that margin. The pelvis 
otherwise resembles
*Staurikosaurus*, especially that of the ilium and pubis. I never agreed with 
the position between
herrerasaurs and more basal dinosaurs, and I back this up now: the pubes are 
nearly
characteristically "herrerasaurian" with a broad, ventrally narrowing apron, 
large lateral _m.
pubo-ischio-femoralis internus_^3 tubercle, notch between distal ends of the 
pubes. The ischium is
a dead-ringer for *Staurikosaurus*, and the ilium is characteristic for all of 
basal Dinosauria
(*Lesothosaurus*, *Marasuchus*, *Chindesaurus*, *Eoraptor*) suggesting 
herrerasaurid affinities.
The ilium is not as modified as *Chindesaurus* or *Camposaurus* toward an 
elongate postacetabular
process, which suggests it is, in fact, a herrerasaur-grade theropod.

  Mickey has the *Saturnalia* paper, and indicates the text says that the 
pelvis has a partially
open acetabulum: I lack details, and the only pelvic figures are part of a 
reconstruction. I need
to see the pelvis before I can comment.

  Until then, a fully open acetabulum, which I define by the regression of the 
medial acetabular
wall of the pubis and ischium from the ilium and no ventral projection of that 
wall from the
ilium, to be diagnostic of a group including *Pisanosaurus*, *Lesothosaurus*, 
*Thecodontosaurus*,
*Herrerasaurus*, *Eoraptor*, and *Coelophysis*, and all descendants of the most 
recent common
ancestor of these taxa exclusive of *Marasuchus*.

  By dint of this definition, I am able to use the cranially and 
laterally-positioned astragalar
ascending process higher than 50% of the astragalar body height as a 
supplementary diagnostic
character, as both *Marasuchus* and *Lagerpeton* lack this feature. The 
condition in *Lagerpeton*
is caudally positioned on the astragalar body, and is received by a notch in 
the tibia, so there
is no indication of its being switched (it was also found _in situ_ in this 
position), and
*Marasuchus* possesses a cruciate, pyramidal process that projects a caudal 
ridge between tibia
and fibula, the latter which progresses onto the astragalar body. This is 
primarily unique, but
present in crocodylomorphs as well (I lack the resources for an in depth 
discussion). In
*Lesothosaurus*, *Eoraptor*, *Herrerasaurus*, *Coelophysis*, there is an 
anterior (cranial) notch
in the tibia that receives the astragalus. In *Staurkiosaurus*, like 
*Marasuchus, there isn't.
There is a lateral notch, and this makes me question the form of the tibia. In 
the likelyhood that
*Staurikosaurus* is nested within herrerasaurs (very likely), this is a 
reversal. In the chance
that it is not, two possibilities exist: 1) *Staurikosaurus* is not a member of 
a group diagnosed
by the ascending process of the astragalus; or 2) the process does not diagnose 
a monophyletic
group, and is convergent depending on *Staurikosaurus*' position with regards 
to Dinosauria [note
that hypotheses of this position consider it as a basal theropod, saurischian, 
or outside of
Saurischia but within Dinosauria, or outside of Dinosauria (these have all been 
published)].

  And finally, there is yet a third option: there is one monophyletic condition 
for the ascending
process of the astragalus, including *Marasuchus* and *Staurikosaurus*, which 
is convergently
modified into a craniocaudally narrow yet triangular blade, positioned on the 
cranial face of the
tibia instead of laterally. This is easy to imagine as the ornithischian 
process is still short
and broadly triangular, while that saurischian one until the Sauropoda, is thin 
and broad. An
increase in breadth and height occurs only in Theropoda. This alternative 
suggests that, indeed,
the ascending process is not a synapomorphy of Dinosauria, but rather of 
{*Marasuchus* +
*Pisanosaurus*, *Lesothosaurus*, *Thecodontosaurus*, *Herrerasaurus*, 
*Eoraptor*, *Coelophysis*,
and *Staurikosaurus*}. This clade might be better left unnamed; however it 
includes Dinosauria,
and the group may not correspond to *Marasuchus* + Dinosauria. I have my doubts 
about the position
of *Eoraptor* and *Staurikosaurus*, and this may indicate that the "true" 
theropods begin at
*Segisaurus* and coelophysids _sensu stricto_. No analysis on my part yet backs 
this up. I am not
wholly familiar with the form of the ascending process of the astragalus in 
*Eoraptor*.


=====
Jaime A. Headden

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

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