[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Subject Index][Author Index]

Megalosaurus and Pickering's nomina nuda

With the recent publication of Day and Barrett (2004) and Pickering's posts
to his Yahoo group about his unofficial Metriacanthosaurus species,
something interesting has turned up.  If you recall, Day and Barrett
separated supposed Megalosaurus bucklandii femora into two groups.
Morphotype A is straight in anterior view, slightly anteriorly convex in
lateral view, has a sharp longitudinal ridge anteriorly on the proximal
third of the shaft, and lacks an extensor groove.  Morphotype B is strongly
sigmoidal in anterior view, slightly sigmoidal in lateral view, lacks the
ridge and has an extensor groove.  One of the paralectotype specimens of M.
bucklandii is morphotype B, so that morphology is provisionally assigned to
that taxon.  But what taxon is morphotype A, and what other supposed M.
bucklandii remains belong to it?  The authors also examined scapulacoracoids
and ilia, but found no evidence for multiple taxa from these elements.

This weekend Pickering posted which specimens belong to his nomina nuda
Metriacanthosaurus "brevis" and M. "reynoldsi".  The desired holotype of M.
"brevis" is an ilium illustrated by Day and Barrett, which they couldn't
distinguish from M. bucklandii.  Pickering says of M. "brevis" and
Metriacanthosaurus parkeri, "their great height and shortness -- diagnostic
for Metriacanthosaurus -- separates them from ilia of Megalosaurus
bucklandii and Allosaurus."  However, Day and Barrett found this was an
illusion caused by a broken postaceuabular process and a plaster
reconstructed preacetabular process.  By using Photoshop to resize and
rotate the "M." brevis ilium and overlay it on a M. bucklandii ilium, this
is quite apparent.  It is not necessarily shorter, and in fact only differs
from M. bucklandii in a few very minor proportions easily caused by
individual variation and/or distortion (see
http://students.washington.edu/eoraptor/Megalosaurus%20ilia.jpg ; the lower
right ilium is M. "brevis", while the two upper ones are M. bucklandii, the
left superimposed with M. "brevis").  Indeed, the characters Pickering lists
as distinguishing M. "brevis" from Metriacanthosaurus parkeri are largely
also those that distinguish M. bucklandii from M. parkeri (nearly straight
upper margin; taller preacetabular process), or don't distinguish M.
"brevis" from M. bucklandii (narrower notch between preacetabular process
and pubic peduncle; longer, lower acetabulum; 250 mm long).  So I see no
reason to support M. "brevis" and provisionally synonymize it with M.
bucklandii.  The referred remains of M. "brevis" include a pectoral girdle
Day and Barrett couldn't distinguish from M. bucklandii, two tibiae I have
no information on, and one of the morphotype A femora.  If this femur (or
another diagnostic element) could be shown to be associated with the ilium,
my opinion would be revised, of course.  But even in this case, I see no
reason to assign the morphotype A femora to a Metriacanthosaurus, since the
latter has a strongly sigmoidal femur and sinraptorids have deep extensor
grooves, medially oriented heads, and no groove on the lateral condyle.

The desired holotype of M. "reynoldsi" is also an ilium, though I lack an
illustration of it.  Pickering refers a lot of material to this taxon,
including another morphotype A femur.  However, I'm uncertain which (if any)
of these elements are associated, nor which might indicate referral to
Metriacanthosaurus.  Pickering's diagnosis for Metriacanthosaurus is oddly
"As for Metriacanthosaurus parkeri", despite him placing multiple species in
the genus.  And though earlier he references "great height and shortness" as
being Metriacanthosaurus characters, M. "reynoldsi" is partly diagnosed by
low preacetabular and postacetabular processes, and M. parkeri lacks most of
either process.  M. "reynoldsi" is also partly diagnosed by the "more
horizontal crest".  If this is in reference to a difference from the angled
posterodorsal edge on M. parkeri's ilium, it's just a plesiomorphy also
shared with Megalosaurus.  The other supposedly diagnostic characters are
the longer peduncles, the ischial peduncle being more ventrally directed,
the postacetabular process "with a much greater area exposed below the
spine" (= brevis shelf with more lateral exposure?), and the fourth sacral
being longer ventrally than dorsally.  The latter is unknown in M. parkeri
or M. "brevis", and the brevis shelf of M. parkeri seems very poorly
preserved as well.  Much as with M. "brevis" above, I don't see why the
femur in the M. "reynoldsi" hypodigm should be assigned to
Metriacanthosaurus.  I'll pick up Reynolds (1939) tomorrow at the library
before I form an opinion on M. "reynoldsi", but right now it's not looking
very diagnosable based on the intended holotype, nor very referrable to

So what is femoral morphotype A?  The anteromedially directed head
eliminates derived eustreptospondylines and avetheropods.  The aliform
anterior trochanter eliminates coelophysoids, non-abelisaur ceratosaurs, and
non-megalosauroid+avetheropod tetanurines.  The proximal extent of the
trochanter eliminates the first two groups as well.  The absent extensor
groove eliminates tetanurines.  A groove on the lateral condyle is only
known in coelophysoids and ceratosaurs.  So my best guess is that this taxon
is an abelisaur.  Interestingly, Masiakasaurus is described as having a
triangular femoral cross section proximally much like in morphotype A.

Mickey Mortimer
Undergraduate, Earth and Space Sciences
University of Washington
The Theropod Database - http://students.washington.edu/eoraptor/Home.html