FHSM VP-13910-1.jpg (76014 bytes)

Selmasaurus johnsoni

Reconstruction of the skull

Copyright 2001 -2009 by Mike Everhart

Last revised 03/26/2009

 

UNDER CONSTRUCTION

 

IMPORTANT NOTE:  The recent publication of an article in the JVP by Takuya Konishi and Michael Caldwell clarifying the identification and relationships of the various species of Platecarpus will necessitate some major changes in some of my web pages. Please note that Platecarpus planifrons Cope (1874) is now identified as the most common species of Platecarpus in the lower chalk (late Coniacian to middle Santonian), and P. ictericus (Cope, 1871) is the most common species of this genera in upper chalk (middle Santonian through early Campanian). P. coryphaeus (Cope, 1872) is a junior synonym of P. ictericus.  The name Platecarpus tympaniticus (Cope, 1869) is now limited to a single specimen (holotype) from Mississippi. The species that I had previously identified as Platecarpus planifrons (above) is now "unidentified" and possibly a new genus / species which we are currently working to identify / describe.  I consider this paper to be a major improvement in mosasaur phylogeny. The citation is:

Konishi, T. and Caldwell, M. W. 2007. New specimens of Platecarpus planifrons (Cope, 1874) (Squamata: Mosasauridae) and a revised taxonomy of the genus: Journal of Vertebrate Paleontology 27(1): 59-72. 

In January, 2001, Steve Johnson and I talked to Mike Triebold of Triebold Paleontology about creating a 3-D skull from the bones of Steve's discovery (now Selmasaurus johnsoni). Steve was about to donate the specimen to the Sternberg Museum and wanted to have a cast made so that he would have something to keep.   Mike was enthusiastic about the project and the opportunity to work with a species that so little was known about. The bones were shipped off for additional  preparation, repair, and casting.  In late September, 2001, the process was complete and the assembled skull was returned to Steve along with the original specimen.  Meanwhile, Steve and I had submitted an abstract to the Society of Vertebrate Paleontology for the 2001 meeting in Bozeman, Montana, and it was accepted into the Poster Session.  So, in early October, I loaded up a model of the skull, and the poster materials and headed off for the SVP annual meeting. A overall view of the poster is shown, along with detailed images and the abstract of the presentation. The skull was donated to the Sternberg Museum in November 2001 by Steve Johnson and was curated under catalog number FHSM 13910.


!postera.jpg (11690 bytes) Systematics History:

Clidastes planifrons Cope 1874

Platecarpus planifrons (Cope 1874)

Platecarpus planifrons (Cope 1874) nomen vanum per Russell (1967)

Platecarpus planifrons (Cope 1874) per Schumacher (1993), Sheldon (1996) and Bell (1997)

Konishi, T. and Caldwell, M. W. 2007. New specimens of Platecarpus planifrons (Cope, 1874) (Squamata: Mosasauridae) and a revised taxonomy of the genus: Journal of Vertebrate Paleontology 27(1): 59-72. 

!cope1a.jpg (18011 bytes)

EXPLANATION OF PLATE XXII: Clidastes planifrons, Cope, one-half natural size.

Fig. 1. Superior view of parts of cranium preserved, including parts of the parietal (a), postfrontal (b),prefrontal (d), and fronto-nasal (c) elements

2. The same, seen from below.

3. Posterior part of left mandibular ramus, without angle.

4. Median portion of right mandibular ramus, showing hinge. In both figures, a is the coronoid, b the surangular, c angular, d articular, and e splenial.

5. Portion of the right quadrate bone : 5, proximal part, inner view ; 5 a, proximal view of the same ; 5 b, external view of the same.

6. Distal portion of left quadrate, external view ; 6 a, internal view ; 6 b, posterior view.

7. Right palatine bone, from below ; 7 a, right palatine bone, from outer side.

 SOURCE: Cope, E. D., 1874. Review of the Vertebrata of the Cretaceous period found west of the Mississippi River. U. S. Geol. Surv. Terr. Bull. 1(2):3-48.

!plan05a.jpg (5362 bytes) Left lateral view of skull and lower jaws.  It is interesting, to me at least, the the teeth are being replaced in pairs... (i.e. the 2nd, 4th, 6th, 8th and 10th teeth in both lower jaws had fallen out and were being replaced by new teeth at the time of death of this mosasaur)
!plan06a.jpg (4010 bytes) Left upper view of the back of the skull
!plan07a.jpg (4423 bytes) Left upper view of the muzzle
!plan08a.jpg (4520 bytes) Left side view of upper and lower jaws.  Note relatively large size and low number of teeth. This species has the smallest number of teeth of known mosasaur.
!plan09a.jpg (5716 bytes) Dorsal view of reconstructed skull. Note the position of the parietal foremen and the relatively large size of the eyes in comparison to the length of the skull.

Everhart, M. J. and S. E. Johnson, 2001. The occurrence of the mosasaur, Platecarpus planifrons, in the Smoky Hill Chalk (Upper Cretaceous) of western Kansas. Jour. Vert. Paleon. 21(suppl. to 3):48A. Abstract

Two recently collected specimens of Platecarpus planifrons extend the range of this uncommon mosasaur upward to the early Santonian level of the Smoky Hill Chalk Member (second quarter of the member, below Marker Unit 7, biostratigraphic zone of Cladoceramus undulatoplicatus) of the Niobrara Chalk. Early records of the species (more than a dozen specimens, including the type, collected by the Cope, Marsh, and Williston parties), although imprecise about locality and stratigraphy, appear to have been collected primarily from Trego County, Kansas, and mostly from lower chalk exposures along Hackberry Creek. These localities would place the species in the late Coniacian level of the Smoky Hill Chalk Member (lowermost quarter of the chalk, below Marker Unit 5, biostratigraphic zone of Protosphyraena perniciosa).

The type specimen in the American Museum of Natural History was originally described as 'Clidastes planifrons'. The name stood for twenty-five years until the material was re-examined and moved into the genus Platecarpus on the basis of characters observed in the frontal, parietal and vertebrae. The relative rarity of the species and a lack of well preserved, complete skeletons has contributed to a continuation of its uncertain taxonomic position. Until resurrected in 1993 as a valid species, P. planifrons was long considered a nomen vanum. It is far less common in the Smoky Hill Chalk Member than P. tympaniticus (including P. ictericus and P. coryphaeus). The early Santonian material reported here (FHSM VP-13907 and 13910) includes two partial skeletons from Gove and Lane Counties, one with a nearly complete skull. These specimens represent the latest known occurrence of this species and further document the temporal distribution and diversity of the fauna in the Western Interior Sea during the late Cretaceous.


References:

Bell, G. L. Jr., 1997. A phylogenetic revision of North American and Adriatic Mosasauroidea. pp. 293-332 In Callaway J. M. and E. L Nicholls, (eds.), Ancient Marine Reptiles, Academic Press, 501 pages.

Cope, E. D., 1874. Review of the Vertebrata of the Cretaceous period found west of the Mississippi River. U. S. Geol. Surv. Terr. Bull.
1(2):3-48.

Everhart, M. J., 2001. Revisions to the Biostratigraphy of the Mosasauridae (Squamata) in the Smoky Hill Chalk Member of the Niobrara Chalk (Late Cretaceous) of Kansas. Kansas Academy of Science, Transactions 104(1-2):56-75.

Konishi, T. and Caldwell, M. W. 2007. New specimens of Platecarpus planifrons (Cope, 1874) (Squamata: Mosasauridae) and a revised taxonomy of the genus: Journal of Vertebrate Paleontology 27(1): 59-72. 

Polcyn, M.J. and Everhart, M.J. 2008. Description and phylogenetic analysis of a new species of Selmasaurus (Mosasauridae: Plioplatecarpinae) from the Niobrara Chalk of western Kansas. Proceedings of the Second Mosasaur Meeting, Fort Hays Studies Special Issue 3, Fort Hays State University, Hays, Kansas, pp. 13-28.

Russell, D. A., 1967. Systematics and morphology of American mosasaurs. Peabody Museum of Natural History, Yale University, Bulletin 23.

Schumacher, B. A., 1993. Biostratigraphy of Mosasauridae (Squamata, Varanoidea) from the Smoky Hill Chalk Member, Niobrara Chalk
(Upper Cretaceous) of Western Kansas, Unpub. Masters Thesis, Fort Hays State University, 68 pp.

Sheldon, M. A., 1996. Stratigraphic distribution of mosasaurs in the Niobrara Formation of Kansas, Paludicola 1:21-31. (The Rochester Institute of Vertebrate Paleontology)

Williston, S. W., 1898. Mosasaurs. The University Geological Survey of Kansas, 4:81-347, pls. 10-72.