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Re: A couple of questions

Ivan Kwan (dino_rampage@hotmail.com) wrote:

<1) Pterosaurs: Ornithodira (& thus the sister group to Dinosauria), or 
Prolacertiformes? If they are prolacertiformes, would that exclude them
from the Archosauria?>

  Yes. They are basal archosauromorphs in that scheme. This data has been
reached at through various analyses, but there has not been ONE major
systematic revision of the Archosauromorpha in a while.

<2) Scleromochlus: What is it? Prolacertiform or a basal ornithodiran? (Or
a likely pterosaur ancestor)>

  Sereno (1991) proposed that the following 4 features are indicative of a
*Scleromochlus* + Pterosaurs clade:

  1) skull more than 50% presacral column length;
  2) length of scapula less than 75% humeral length;
  3) fourth trochanter absent;
  4) metatarsal I at least 85% length of metatarsal 3.

  Benton (1999) criticizes that the first three are likely related to the
small size of *Scleromochlus* (it is at maximum length 180mm or so, give
or take 5mm, and that's about 7in.); 1) occurs in phytosaurs and nearly
all crocodylomorphans (primarily due to snout elongation), 2) is
criticized because the scapula form in *Scleromochlus* is genuinely short,
having nothing to do with relative humeral elongation, and its morphology
recalls that of other short-scapular animals, crossing only 1/5 of the
presacral length, given a short neck as well, 4) is also present in some
basal crocodylomorphans. Sereno also listed 5 features that were found
between *Scleromochlus* and Dinosauromorpha:

  1) forelimb less than 50% hindlimb length;
  2) femoral head projects perpendicular to femoral shaft (projects
  3) distal tibia is expanded at the contact for the astragalus;
  4) astragalus bears ascending process;
  5) metatarsal V short and bound to the lateral surface of metatarsal IV.

  Benton (1999) criticized Sereno by pointing out that *Scleromochlus*
lacks (2) and (4) [I disagree about (2) as the dorsal margin of the femur
is level and perpendicular to the shaft, even if the femoral condyle is
more conical in aspect than spherical, and would imply the ilium/femur
relationship was not sprawling but "erect"]; he noted that (1) is true of
all Ornithodira, and (3) and (5) were poorly defined. Sereno noted
precisely that (5) includes a fifth metatarsal that is not laterally
deflected, and one may infer this means the metatarsal was not joined to
the tarsus only at its proximal end, as in nearly all non-dinosaurian
archosauromorphans, as I have done. The appression of the fifth metatarsal
to the fourth along its length is a "dinosaurian" characteristic.

  Benton (1999) found a clade comprising of *Scleromochlus*, Pterosauria,
and Dinosauria excluding Prolacertiformes and Crocodylomorpha as well as
all other Archosauromorpha; this clade is named Avemetatarsalia, and is
diagnosed by:

  1) forelimb/hindlimb ratio is 0.55;
  2) pubis longer than ischium;
  3) tibia longer than femur;
  4) distal tarsal IV is subequal in width relative to dtIII;
  5) compact metatarsus with metatarsals I-IV appressed;
  6) metatarsals II-IV more than 50% tibial length;
  7) absence of dorsal osteoderms.

  There are several problems with these characters, though: any cursorial
form will have (3) and (6), and arguably (2), which relate to dynamics of
speed and walking in a bipedal manner; current argument over pterosaur
metatarsal positions in basal forms appear to argue (5) is not basal to
them, but occurs in some derived forms only. (7) is false, as BMNH R3146A
(larger specimen) preserves dorsal osteoderms over the upper half of the
precaudal series, including the hips, as several rows of strap-like
transverse banding. Benton criticized Sereno (1991) in using (1), and the
same argument plays here: all pterosaurs have a forelimb more than 110%
the length of the hindlimb, whereas no dinosaur has this condition; the
form of the scapula with a broad end, contra other posited pterosaur
ancestors, would relate well to the condition known in *Marasuchus* and
other avesuchians (crown archosaurs).

  Benton's tree is arranged thusly:


<3) Protarchaeopteryx robusta: primitive oviraptorosaur like Caudipteryx,
or still "incertae sedis"?>

  Should still be _incertae sedis_. I know Mickey Mortimer finds a basal
oviraptorosaur position, but this is based on scanty data that occurs in
Maniraptoriformes generally. Many features of *Protarchaeopteryx* are
unique, including the iliac shape, and premaxillary shape and tooth
morphology (non-recurved with serrations on both edges); others are, as
previously stated, very general in nature to other maniraptoriforms and
maniraptorans. It is likely a maniraptoran, but the absence of well
preserved cranial, vertebral, ischiadic, proximal femoral, and scapular
anatomy clouds the issue very strongly.


Jaime A. Headden

  Little steps are often the hardest to take.  We are too used to making leaps 
in the face of adversity, that a simple skip is so hard to do.  We should all 
learn to walk soft, walk small, see the world around us rather than zoom by it.

"Innocent, unbiased observation is a myth." --- P.B. Medawar (1969)

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