Meet some of the Mystery Mosasaurs

Most of the mosasaurs in the North Dakota State Fossil Collection were collected from the eastern portion of North Dakota, where gorges and stream valleys cut down into the underlying rocks of the Pierre Shale. However, rocks from the Pierre Shale are also exposed in the southwestern corner of the state that have produced a few mosasaur specimens as well. One of these is in the state fossil collection, but is still in its field jackets waiting to be cleaned up (which it will be if this project is funded!). Another specimen, which we were previously unaware of, was collected by the Pioneer Trails Regional Museum in Bowman, Montana. That specimen, which includes a partial skull and some of the postcranial skeleton, belongs to the species Plioplatecarpus primaevus, and was described in a short paper by Nancy Schaefer and colleagues in 1995.

 

Figure from the short paper published by Nancy Schaefer and colleagues in 1995 on a mosasaur specimen from southwestern North Dakota (Bowman County).

One of the most important features used to refer that specimen to Plioplatecarpus was the presence of a large parietal foramen in the skull roof (labeled paf in the figure above). One of the better mosasaur specimens collected by the North Dakota Geological Survey from the eastern portion of North Dakota also shows this feature (see figure below), and likely also represents the genus Plioplatecarpus, though we aren't sure exactly what species yet.

 

Part of the skull of the "Cooperstown Mosasaur." Note the large parietal foramen in the skull roof, just like in the figure above. Collected in eastern North Dakota (Griggs County).

These two specimens suggest that Plioplatecarpus was present throughout North Dakota during the Late Cretaceous. While that is an important piece of the puzzle for understanding the diversity and distribution of mosasaurs in North Dakota, there are many more pieces yet to work out. Do both these specimens belong to the same species, or were there different species of Plioplatecarpus living in opposite sides of the state? What other mosasaur species shared the seas with Plioplatecarpus? Those are some of the questions we will address during this study!

 

Cast of the "Cooperstown Mosasaur" on display in the Geologic Time Gallery at the North Dakota Heritage Center and State Museum, with its shadow projected on the wall.

NOTE: We would like to thank Nancy Schaefer for bringing her prior work on North Dakota mosasaurs to our attention!