Oceans of Kansas by Michael J. Everhart

Author:Michael J. Everhart
Language: eng
Format: epub
Publisher: Indiana University Press
Published: 2017-11-17T16:00:00+00:00

9.8. A cast of the reconstructed skull (length 38 cm; 15 in) of Selmasaurus johnsoni (FHSM VP-13910) in the collection of the Sternberg Museum of Natural History. This specimen was discovered in Gove County by Steve Johnson of Wichita, Kansas, in 1996, and is the only known complete skull of this rare species.

The foregoing story highlights the difficulty in identifying mosasaur remains based on descriptions from more than a century ago. Although many mosasaur workers have agreed that Cope’s original descriptions are inadequate for separating the various species of Platecarpus, there had been little effort until recently to straighten out the problems. Fortunately, there are now many excellent specimens available for study.

Konishi and Caldwell (2007) examined new material from various collections and determined that Platecarpus planifrons was a valid species in the Western Interior Sea along with Platecarpus tympaniticus. In a follow-up paper, Konishi and Caldwell (2011) named a new species of plioplatecarpine mosasaur, Latoplatecarpus willistoni, from the Bearpaw Shale (Campanian) of Montana, and moved P. planifrons into a new genus, Plesioplatecarpus planifrons. Platecarpus tympaniticus Cope was redescribed by Konishi, Caldwell, and Bell (2010), and then discussed in greater detail by Konishi et al. (2012) on the basis of their examination of one of the best-preserved Kansas specimens (LACMNH 128319).

Bottom line for those of us working in the Smoky Hill Chalk is that P. planifrons occurs in the lower chalk (Upper Coniacian–Middle Santonian) and is replaced by P. tympaniticus in the upper chalk (Middle Santonian–Lower Campanian). P. tympaniticus appears to be then replaced by Plioplatecarpus primaevus in the Sharon Springs Member of the Pierre Shale.

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