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Tyrannosaurid hindlimb from Campanian of Utah



From: Ben Creisler
bcreisler@gmail.com

A new online paper:


Tracy J. Thomson, Randall B. Irmis & Mark A. Loewen (2013)
First occurrence of a tyrannosaurid dinosaur from the Mesaverde Group
(Neslen Formation) of Utah: Implications for upper Campanian
Laramidian biogeography.
Cretaceous Research (advance online publication)
doi: http://dx.doi.org/10.1016/j.cretres.2013.02.006,
http://www.sciencedirect.com/science/article/pii/S0195667113000335



Although upper Campanian dinosaur assemblages are well-known from
Alberta, Montana, southern Utah, and New Mexico, specimens from
Wyoming and central and eastern Utah are very rare. This area
constitutes a biogeographic break between northern and southern
biogeographic provinces, so any specimens from this region are
critical to understanding the origin, evolution, and limits of upper
Campanian biogeographic zones on the west margin of the Western
Interior Seaway. We report the discovery of a theropod dinosaur
partial hindlimb from the Book Cliffs area northeast of Green River,
Utah. The specimen was recovered from the Palisade coal zone in the
Neslen Formation (Mesaverde Group), which is dated to the
mid-Campanian based on ammonite biostratigraphy and radioisotopic age
constraints. The specimen, comprising a partial fibula, the distal
half of metatarsal II, a complete metatarsal IV, and a partial
metatarsal V, can be assigned to Tyrannosauridae based on a number of
synapomorphies, including a bipartite iliofibularis tubercle on the
fibula and a teardrop shaped articular surface for metatarsal III on
the medial surface of the distal portion of metatarsal IV. This is the
first unambiguous tyrannosaurid dinosaur reported from the Mesaverde
Group and represents an important biogeographic record situated
between northern and southern upper Campanian vertebrate assemblages.
Specifically, we identify morphological evidence on the pes that
separates northern (Montana and Alberta) and southern (southern Utah
and New Mexico) tyrannosaurid dinosaurs, and suggests that the Book
Cliffs specimen belongs to the northern group. This implies that
either the biogeographic boundary between the northern and southern
Campanian assemblages lies somewhere between central and southern Utah
or that the Book Cliffs taxon represents a northern emigrant in the
southern assemblage.