Selmasaurus

Selmasaurus is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Plioplatecarpinae subfamily alongside genera like Angolasaurus and Platecarpus. Two species are known, S. russelli and S. johnsoni, both are exclusively known from Santonian deposits in the United States.

Selmasaurus Temporal range: Late Cretaceous, Campanian 85.8–70.6 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Selmasaurus johnsoni mounted skull in the Rocky Mountain Dinosaur Resource Center in Woodland Park, Colorado.
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Order:	Squamata
Superfamily:	†Mosasauroidea
Family:	†Mosasauridae
Subfamily:	†Mosasaurinae
Genus:	†Selmasaurus Wright & Shannon, 1988
Species
†S. russelli Wright & Shannon, 1988 (type) †S. johnsoni Polcyn & Everhart, 2008

Selmasaurus is unique among the mosasaurs in that its skull is unusually akinetic, meaning that it is incapable of widening to swallow larger prey. Most mosasaurs have skulls which possess "coupled kinesis" (mesokinesis and streptostyly), that is, parts of the jaw can open widely to accommodate large prey.

Description

Selmasaurus was a small predatory mosasaur at approximately 3–5 meters in length. It possesses a relatively low number of teeth for a mosasaur, the lowest of any known species at the time of its discovery.[1] Originally classified as a plioplatecarpine mosasaur, it differs from all other plioplatecarpine mosasaurs in several respects, listed below as stated by Polcyn and Everhart (2008):[2]

"Suprastapedial process descends to about half the quadrate height, and is in contact with but not fused to the peg-like infrastapedial process; elongate medially constricted, subrectangular parietal table, broadening anteriorally and constricting posteriorly forming narrow parasagittal ridges that diverge at their terminus; posteroventral median process of parietal meets supraoccipital in narrow elongate contact; anteromedial process of supratemporal clasped by medial excavations of the parietal rami for nearly entire length of rami; canals for the basilar arteries enter the basioccipital as two small foramina separated by a median septum, diverge within the basioccipital and cross the basisphenoid suture in a lateral and deeply ventral position".

History of discovery

Skull roof of the S. russelli holotype specimen GSATC 221, showing frontal, parietal, postorbitofrontals, and left jugal. Dorsal view.

First recognized by geologist Samuel Wayne Shannon in his 1975 Master's thesis, "Selected Alabama Mosasaurs", the taxon remained a nomen nudum until it was officially described in 1988 in an article coauthored by Wright.[3] The type specimen, formerly reposited at the Geological Survey of Alabama and cataloged as GSATC 221, was transferred in 2005 to the Alabama Museum of Natural History (Tuscaloosa). The holotype of this genus consists of a very well preserved but incomplete and disarticulated skull, the left atlantal neural arch, atlas centrum, and a single neural arch from a cervical vertebra. Preserved skull elements include the frontal, parietal, left ectopterygoid, left jugal, supratemporals, basioccipital and basisphenoid, and quadrates. The species was named in honor of paleontologist Dale A. Russell, for his extensive work on mosasaurs. The holotype and only known specimen of S. russelli was collected from an unknown location in western Alabama, and for decades, uncertainty surrounded the precise stratigraphic horizon from which the specimen had been recovered. Then in 1998, Caitlín R. Kiernan extracted chalky matrix from the basilar canal of the basiocciptal and identified calcareous nanoplankton that indicated GSATC 221 had originated from basal Campanian beds within the lower unnamed member of the Mooreville Chalk Formation (Selma Group). In her study of Alabama mosasaur biostratigraphy, Kiernan placed S. russelli within the Clidastes Acme Zone, though it was the rarest element in the fauna, accounting for only 0.3% of the biozone's assemblage (one specimen).[4]

A remarkably well preserved and nearly complete Selmasaurus skull and partial postcranial skeleton was discovered by Steve Johnson and family in 1996, from the Santonian or Campanian marine horizon in the Niobrara Formation of Niobrara Chalk, western Kansas. Recovered in 1997 and donated to the Sternberg Museum of Natural History in Hays, Kansas, in 2001, the remains were determined to be a new species of Selmasaurus in 2008 after over a decade of study by Polcyn and Everhart.[2] Named S. johnsoni after its discoverer, the skull is one of the most complete mosasaur skulls recovered and thus provides new anatomical information for Selmasaurus, a better understanding of plioplatecarpine ingroup relationships, extends the geographic and temporal range of the genus, and documents further diversity within Plioplatecarpinae. The holotype and the only known specimen is housed at the Sternberg Museum of Natural History under catalog number FHSM-VP-13910.

Classification

Wright and Shannon classified Selmasaurus as a member of the mosasaur subfamily Plioplatecarpinae, which also includes the genera Platecarpus, Plioplatecarpus, and Ectenosaurus, largely on the "basis of the mode of circulation through the basicarnium." The genus may be most closely related to Ectenosaurus, though it possesses a much shorter, stouter skull. Additional specimens would greatly expand our understanding of Selmasaurus russelli.

The cladogram below follows the most resolved topology from a 2011 phylogenetic analysis of the Plioplatecarpinae by paleontologists Takuya Konishi and Michael W. Caldwell.[5]

Life restoration of a Selmasaurus hunting a Tarpon (Megalops)

Clidastes prophyton       Kourisodon puntledgensis   Russellosaurina     Yaguarasaurus columbianus       Russellosaurus coheni     Tethysaurus nopcsai             Tylosaurus kansasensis     Tylosaurus proriger     Plioplatecarpinae   Ectenosaurus clidastoides       Angolasaurus bocagei         Selmasaurus johnsoni     Selmasaurus russelli         Plesioplatecarpus planifrons       Platecarpus tympaniticus       Latoplatecarpus willistoni         Latoplatecarpus nichollsae     "Platecarpus somenensis"         Plioplatecarpus primaevus       Plioplatecarpus houzeaui     Plioplatecarpus marshi

References

1. "Selmasaurus johnsoni2". oceansofkansas.com. Retrieved 2017-09-29.
2. 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: In: Proceedings of the Second Mosasaur Meeting, edited by Everhart, M. J, Fort Hays Studies, Special Issue number 3, p. 13-28.
3. Kenneth R. Wright; Samuel Wayne Shannon (March 31, 1988), "A new plioplatecarpine mosasaur (Squamata, Mosasauridae) from Alabama", Journal of Vertebrate Paleontology, 8 (1): 102–107, doi:10.1080/02724634.1988.10011686, JSTOR 4523177
4. Kiernan, Caitlin R. 2002. Stratigraphic distribution and habitat segregation of mosasaurs in the Upper Cretaceous of western and central Alabama, with an historical review of Alabama mosasaur discoveries. Journal of Vertebrate Paleontology 22 (1): 91–103.
5. Konishi, Takuya; Michael W. Caldwell (2011). "Two new plioplatecarpine (Squamata, Mosasauridae) genera from the Upper Cretaceous of North America, and a global phylogenetic analysis of plioplatecarpines". Journal of Vertebrate Paleontology. 31 (4): 754–783. doi:10.1080/02724634.2011.579023.