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Morrison Allosaurus (was RE: GSA Abstracts Posted)
This brings up a total of at least three Morrison allosaur species: _A.
fragilis_, _A._ sp. 2 (including "Big Al"/MOR 693), and _Saurophaganax
maximus_ (= _Allosaurus maximus_).
VARIATION AND STRATIGRAPHIC DISTRIBUTION OF ALLOSAURUS WITHIN THE LATE
JURASSIC MORRISON FORMATION.
LOEWEN, Mark A.
The taxonomy of the Late Jurassic theropod dinosaur Allosaurus has long been
a contentious subject of debate, with 16 species assigned over the past 125
years. A recent study suggests that there are only two valid species of
Allosaurus within the Morrison Formation. New specimens and recent work
refining the stratigraphic relationships of the Morrison Formation allow an
assessment of the variation and distribution of Allosaurus in the western
U.S.Variation within the species Allosaurus fragilis was assessed using
thousands of specimens from the Cleveland-Lloyd Dinosaur Quarry (CLDQ) of
central Utah. Morphometric analysis of the cranial and postcranial elements
within this large sample document changes related to ontogeny as well as
individual variation within the species. Analysis of Allosaurus specimens
housed in institutions throughout North America combined with CLDQ variation
data support the view of only two distinct species of Allosaurus. The two
species are distinguishable on the basis of several characters, including
the morphology of the ventral jugal margin. A. fragilis (i.e. UUVP 6000 and
CLDQ specimens) possesses a strongly sigmoidal ventral margin of the jugal,
while in A. sp. 2 (i.e. DINO 11541 and MOR 693) the ventral jugal margin is
relatively straight. In addition, the nasals of the two species are
distinct. This variation in cranial elements between the two species is well
beyond the range of ontogenetic variation, and can be documented in
different sized animals from juveniles through adults.The stratigraphic
distribution of Allosaurus specimens further supports the distinction
between these two species. A. sp. 2 has only been recovered from the Salt
Wash Member in Utah and its lateral equivalents in the lower part of the
Morrison Formation in northern and southern Wyoming. Specimens of A.
fragilis are restricted to the overlying Brushy Basin Member of the
Morrison. Furthermore, where the two species co-occur in the same section,
they are stratigraphically separated. This study documents the range of
variation (ontogenetic, individual and interspecific) within Allosaurus and
suggests that there are two temporally and morphologically distinct species
within the Morrison Formation.
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