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The Turtle Ankylosaur: Minmi, and Problems of Ankylosaur Systematics



*Minmi* is a problematic taxon known from two postcranial skeletons and a
skull.

  The postcrania is unique for the possession of neomorph paravertebral
bones (os paravertebralis), and the possession of a plesiomorphic pubic
shaft lying parallel to the that of the ischium. The latter is known in
*Gastonia* as well, to some degree, and non-Ankylosauria. Otherwise, the
ilia are flat horizontally, short caudally, and tapering cranially in the
preacetabular ala. Furthermore, the acetabulum is not completely closed
off. The limbs are fairly short and stocky, but the femur is slender,
partially recurved, and possess a proximal fourth trochanter.

  The skull possesses the most unique features: maxillary tooth rows do
not converge strongly rostrally, the basisphenoid is horizontal and one
can see the basisphenoid tubera in occipital view; the occiput is more
than twice as wide as deep, and the skull is rostrocaudally longer than
wide; the nuchal crest is well developed dorsally, but does not obscure
the occipital condyle in dorsal view. The quadrates are vertical, and the
lateral condyle extends further ventrally than does the medial condyle.
The snout is short as preserved and the orbits are visible in rostral view
and one can easily say that the animal had the best stereoscopic vision of
any thyreophore. Cranial bones are well-defined and separated, though they
are strongly reworked, and the postorbital is fused to the squamosal.
There is closure of the supratemporal fenestrae, and probably closure of
the infratemporal fenestrae, rather than caudal orientation of the latter
(bones of the cheek are disarticulated and it is obvious from sagittal
rotation of the palatal bones and maxillae that much distortion of this
area has taken place); the frontals are longer than wide; the skull is
about as wide above the orbits and at the paroccipital region. There are
dorsal supranarial ossifications, and squamosal and quadratojugal horns,
and a prominent space between quadratojugal and quadrate that suggest a
foramen. The quadrate is preserved oriented diagonal to the sagittal
plane, but dislocation and distortion of the basicranial region and palate
indicates the quadrate is splayed laterally, and this artificially widens
the cheek; the occiput is measured between the exoccipitals and thus
quadrate placement does not effect the width, and depth:width was
estimated from a vertical quadrate anyway. One could easily describe the
skull as "turtle-like" and a probable nickname could be the Turtle
Ankylosaur.

  Teeth are distinctively similar to those of basal ankylosaurids and
nodosaurids, but more advanced than *Scelidosaurus*. The lower jaw bears a
tall acromion process, and the dorsal dentigerous region is not sigmoid in
lateral view, but gently convex.

 ---

  *Minmi* is a distinctive taxon with a mixture of both basal and derived
features relative to *Scelidosaurus* and ankylosaurids. Recent
phylogenetic hypotheses have suggested an affinity with ankylosaurids or
ankylosauroids. Many features of the skull are distinctively
scelidosaur-like, and the pelvis is an odd example of both basal (long
pubis, open acetabulum) and derived (horizontal ilia, multiple rib
contacts in the preacetabular alae, short postacetabular alae) features.
Therefore, I would invite discussion on this interesting ankylosaur, and
would greatly enjoy seeing what may possibly derived from an analysis
specifically delineating this taxon, rather than just including it....

=====
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.

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