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Re: HYPSILOPHODONTIDS



Rutger Jansma (fam.jansma@worldonline.nl) wrote:

<Hmmmmm.... Recently I did this skeletal reconstruction of Agilisaurus
louderbacki (available on the new and very improved Dinosauricon!) based
on the information given in the paper from the Polygot and when finished
all that I could say was: Heterodontosaurid! This is at least the
conclusion I can make based of this reconstruction and the information
presented in the paper, as well as some very valuable discussing this with
HP Alessandro Marisa. And because this is no gut feeling or just something
based on the impression given by the skeletal reconstruction, below is
given a short character list that puts it at least as a sister group to
the Heterodontosaurids, if not within:>

  Let me first caution about use of Peng's work, considering his
description follows from a belief that *Lesothosaurus* and *Agilisaurus*
were sister taxa; much of his description in fact rotates around this. I
have the original Peng reference, and the figures aren't all that good. 

<1) somewhat fang-like premaxillary teeth;>

  They are big and blade like, quite unlike the single premaxillary "fang"
in *Heterodontosaurus*.

<2) three _functional_ premaxillary teeth (five are present, but only two
as some small remnants), like Heterodontosaurids but are present also in
Pachycephalosaurs within Marginocephalians, Neoceratopsids have a
reduction to two premaxillary teeth, all other Ornithschia, when present,
the premaxillary teeth are in the number of 5, 6 or 7.>

  The theory goes that the reduction from 5 to 3 teeth occured from front
to back; in *Agilisaurus*, the tiny teeth as preserved are erupting
crowns, not diminished teeth, and are placed between the larger crowns:
large, small, large, small, and large again, in that order mesial to
distal (front to back). This is unlike the condition in
*Heterodontosaurus*, where the teeth progress in size caudally with the
third the largest tooth in the entire jaw.

<3) the chisel-like cheek teeth;>

  This is not actually true, as the teeth of *Agilisaurus* are nearly
rhomboid in lateral view and show distinct interlocking of the crowns,
rather that wear of the tips of crowns down to blades, as occurs in
*Heterodontosaurus* and Marginocephalia. If it were true, it would only
suggest that, rather than a fabrosaurid relationship, *Agilisaurus* is an
ornithopod or basal cerapodan (whatever that is, technically there cannot
_be_ a basal cerapodan); that is, a neornithischian, in Sereno's preferred
terminology. All basal ornithopods (err, hypsilophodontids) have this type
of tooth, just that heterodontosaurids (*Abrictosaurus*, *Lycorhinus*,
*Heterodontosaurs*) have a more developed form that is seen in Ceratopsia
as well, where the cutting edge is continuous along the crowns; in
non-heterodontosaurids or non-ceratopsians, the cutting edges are not
continuous but often "canted" as in a saw's edge. As it is, the crowns are
a symplesiomoprhy for neornithischians in their shape.

<4) diastema between the premaxilla and maxilla (the premaxilla obscures
it somewhat in Peng's 1992 description, but there is a small piece of it
left to be seen in the figure;>

  A lateral fossa on the maxilla for the insertion of a dentary tusk? Even
*Abrictosaurus* lacks one, despite being a heterodontosaurid, and it is
not
determinable whether the figure offers a fossa or a scarf joint for a
broken and distorted skull (twisted on two axes).

<5) 24 presacral vertebrae, like all other Ornithopods, but since
Heterodontosaurids preserve fewer presacral verts, this could be the basal
condition;>

  Yes, but this doesn't tell us anything about the relationships of
*Agilisaurus* as it is a [sym]plesiomorphy.

<6) absence of an obturator process on the ischium;>

  Purported in *Lesothosaurus* (new material indicates its there), and
absent in *Heterodontosaurus*, but present, albeit small, in *Agilisaurus*
and all neornithischians.

<7) relatively long caudal series;>

  This is ture, both *Heterodontosaurus* and *Agilisaurus* have a
relatively long caudal series, but quite frankly, the latter has a much
longer tail than the former, whereas the former has a tail that might be
expected, for a basal neornithischian, but is still shorter than that of
*Homalocephale*. A long tail, however, in the light of other skeletal
features, does not affirm or deny any relationship.

  ...

As for the monotypy of MOR 756, referred to *Zephyrosaurus*, I doubt a
museum that tends to catalogue different parts of a single bonebed, even
when they belong to the same animal, would catalogue different animals
under a single specimen accession number. The material I've seen of it,
though small, appears to show they are all part of a single, similar-sized
animal.

  Cheers,

=====
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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