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Mosasaurus hoffmanni from Fox Hills Formation of South Dakota,

Ben Creisler

A new paper:

T.L. Harrell and J.E. Martin (2014)
A mosasaur from the Maastrichtian Fox Hills Formation of the northern
Western Interior Seaway of the United States and the synonymy of
Mosasaurus maximus with Mosasaurus hoffmanni (Reptilia: Mosasauridae).
Netherlands Journal of Geosciences (advance online publication)
DOI: http://dx.doi.org/10.1017/njg.2014.27

We report here a large mosasaur skull, preserved three-dimensionally
in a concretion recovered from Ziebach County, South Dakota, USA. This
fossil represents the first articulated mosasaur skull from the Trail
City Member of the Fox Hills Formation and the first definitive
occurrence of Mosasaurus hoffmanni Mantell, 1829 from that area and
the northernmost occurrence in the Western Interior Seaway, greatly
extending the paleobiogeographic range of this taxon. The age of this
specimen is determined to be between 68.3 and 67.6 Ma based on the
associated invertebrate fauna. Although previous authors have
suggested synonomy of the North American Mosasaurus maximus Cope, 1869
and the European M. hoffmanni, this is the most comprehensive analysis
to date and is based on comparisons with Mosasaurus specimens
recovered across the Northern Hemisphere, allowing an emended
diagnosis of the species M. hoffmanni. Minor differences are
considered individual variation or to reflect ontogenetic stage,
including slender dentaries in some individuals, range of development
of the C-shaped notch of the coronoid and differences in the shape of
the supratemporal fenestra.