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The taxonomic identity of two well-preserved polycotylid plesiosaur skeletons from the Pierre Shale of far northern Wyoming and southern South Dakota has been controversial since their discovery. Originally referred to Dolichorhynchops osborni, the material was almost immediately christened Trinacromerum bonneri Adams 1997; more recently the material has been referred toPolycotylus. Recent preparation of the well-preserved skull of one specimen permits detailed examination of the cranial morphology of this animal for the first time, and allows for its inclusion in a cladistic analysis of the Polycotylidae. This analysis reveals a stable sister-taxon relationship with Dolichorhynchops osborni. However, the taxon possesses a bewildering mosaic of character states, superficially resembling Polycotylus in overall size and tooth morphology and Trinacromerumin details of the palate and lower jaw, while sharing several critical synapomorphies withDolichorhynchops osborni. This wide-ranging homoplasy among characters previously diagnostic among polycotylid genera challenges the alpha taxonomy of forms from the western interior seaway. The conservative taxonomic course of referring the species T. bonneri to Dolichorhynchopsis followed here, but a species-level review is necessary. Lastly, material described here reveals important new information concerning the polycotylid orbital region and lower jaw, allowing confident reconstructions of these controversial regions for the first time.


This is an Author's Accepted Manuscript of an article published in O'Keefe, F. R. (2008). Cranial anatomy and taxonomy of Dolichorhynchops bonneri new combination, a polycotylid (Sauropterygia: Plesiosauria) from the Pierre Shale of Wyoming and South Dakota. Journal of Vertebrate Paleontology, 28(3), 664-676, as published in the JOURNAL OF VERTEBRATE PALEONTOLOGY, 2008, copyright Taylor & Francis, available online at: