Tylosaurus kansasensis

A new species of mosasaur from the Smoky Hill Chalk

Written and Illustrated by Mike Everhart; Copyright © 2000-2008

Page created 01/30/1999 Updated 04/21/2008

ABSTRACT: Tylosaurus kansasensis sp. nov. is described herein on the basis of thirteen specimens collected from the Smoky Hill Chalk (upper Coniacian) of western Kansas, USA. The new species, originally designated Tylosaurus n. sp., co-occurred with T. nepaeolicus and exhibits a number of primitive characters that place it in a basal position in the mosasaur phylogeny. Among the key differences separating this species from other tylosaurines are a shortened, more rounded pre-dental process of the premaxilla, a distinctive quadrate lacking an infrastapedial process, and a parietal foramen located adjacent to the frontal-parietal suture.

RIGHT: The four articulated cervicle vertebrae that were the first portion of the remains of VP-2295 to be discovered. Note that these are preserved in almost a straight line compared to the 45 degree bend in the vertebral column closer to the skull. This suggests to me that the mosasaur's neck was broken by the larger mosasaur at the time of the attack.

RIGHT: The premaxilla, anterior left maxilla and anterior portions of both dentaries in left lateral view. Note the relatively small size of the rostum compared with Tylosaurus proriger (FHSM VP-3)

RIGHT: The right quadrate in lateral view. The quadrates of Tylosaurus kansasensis are different from those of other tylosaurs because they lack an infrastapedial process... Go HERE for a picture of larger and more typical tylosaurine quadrate.

Everhart, M. J. 2004. Tylosaurus novum sp. – An update on an unnamed species of basal tylosaurine. Abstract book and field guide of the First Mosasaur Meeting, Schulp, A. S. and John W. M. Jagt (eds.), Natuurhistorisch Museum Maastricht, the Netherlands, pp. 35-39.

Tylosaurus novum sp. – An update on an unnamed species of basal tylosaurine.

Michael J. Everhart

Sternberg Museum of Natural History, Fort Hays State University, Hays, KS.  meverhar@fhsu.edu.

ABSTRACT

            Tylosaurines were among the earliest of the mosasaurs which flourished in the oceans of the earth during the last 25 million years of the Late Cretaceous. Tylosaurus (“Macrosaurus”) proriger was the first mosasaur described from the Niobrara Chalk of the Western Interior Sea by Cope in 1869. Since then, the remains of tylosaurine mosasaurs have been recovered from many localities around the world, including Europe, Japan, New Zealand and Antarctica. The remains of the earliest known tylosaurine species, T. nepaeolicus (Cope 1874), are found in the Late Coniacian in the western Kansas along with Platecarpus tympaniticus Cope 1869, and Clidastes liodontus Cope 1874. Another early species of tylosaurine, originally identified as “Tylosaurus n. sp.” by Stewart (1990), apparently co-existed with T. nepaeolicus and exhibited a number of primitive characters which place it in a basal position in the mosasaur phylogeny of Bell (1997). Differences between it and all other tylosaurs include: quadrates that lack an infrastapedial process; a shortened, more rounded premaxilla; and an anteriorly located parietal foramen that often invades the frontal-parietal suture. Except for a few fragmentary and problematic remains found in Texas, the third species of Tylosaurus is known only from the lower Smoky Hill Chalk Member of the Niobrara Chalk of western Kansas. The species apparently became extinct by the Early Santonian. Since 1990, other than brief, unpublished descriptions by Bell (1993) and Schumacher (1993), and inclusion within Bell’s (1997) phylogenetic revision of the Mosasauroidea, the new species has only been mentioned in the literature without further comment. For a variety of reasons, and somewhat without precedent, this species still does not have a formal name, even though there are a number of well preserved, articulated skulls and several reasonably complete skeletons in museum collections.  The purpose of this paper is to update the information regarding this species and to begin the process of describing and naming Tylosaurus novum sp.

INTRODUCTION

            While the place of origin of mosasaurs is yet to be determined, it is reasonably certain that at least three genera flourished first in the Western Interior Sea. The isolated remains of early mosasaurs, cf. Clidastes, have been found in the Turonian Carlile Formation of Kansas (Martin and Stewart, 1977), and mosasauroids have been documented from the Turonian of Texas (Bell, 1995) and South Dakota (Bell and VonLoh, 1998).  By the Late Coniacian, Clidastes, Platecarpus and Tylosaurus were well established in the shallow epicontinental sea covering the middle of North America (Russell, 1967). Following the end of the Coniacian (circa 85 mya), it appears that mosasaurs evolved rapidly and spread quickly around the world during the last 20 million years of the Late Cretaceous (Lingham-Soliar, 1999). Their remains are known from marine sediments on all continents, and are especially abundant and complete in the Smoky Hill Chalk Member (Late Coniacian through Early Campanian) of the Niobrara Formation in western Kansas and the overlying Pierre Shale (Campanian – Maastrichtian) of Kansas and South Dakota.

            During the five-year period between 1869 and 1874, most of the mosasaur species currently considered to be valid from the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas were named by E. D. Cope (Everhart, 2001). These include: the genus Clidastes Cope 1868 and its type species, C. propython Cope 1869 (originally described from the Alabama chalk); the genus Platecarpus Cope 1869 and its type species, P. tympaniticus Cope 1869, and P. planifrons (Cope 1874); and the type species of Tylosaurus.

            The first mosasaur described from Kansas, “Macrosaurus” proriger (Cope 1869) was found in the upper (Santonian or Early Campanian) Smoky Hill Chalk near Monument Rocks in western Kansas in 1868, and was acquired by Louis Agassiz for the museum at Harvard. The genus Tylosaurus (Rhinosaurus) is only mosasaur genus named by Marsh (1872a; 1872b) to survive the test of time, but it was Leidy (1873, p. 271) who formally placed Cope’s “Macrosaurus” into Tylosaurus.  While T. proriger had been originally reported only in the upper chalk (Williston, 1898; Russell, 1967; Schumacher, 1993; Sheldon, 1996), somewhat older remains (FFHM 1997-10) have recently been reported from the middle chalk (early Middle Santonian) by Everhart and Everhart (1997). A second species, T. nepaeolicus, was described from the Smoky Hill Chalk on the basis of fragmentary remains discovered by B. F. Mudge along the Solomon River (Cope, 1874). T. nepaeolicus is an early, somewhat smaller species that has only been collected from the Late Coniacian Smoky Hill Chalk (Everhart, 2002).

            A third species of tylosaurine was recognized but not described by Stewart (1990) from his biostratigraphic zone of Protosphyraena perniciosa (Late Coniacian) and designated “Tylosaurus n. sp.”  Bell (1993) reviewed museum collections and designated seven specimens as Tylosaurus novum sp.   About the same time, three specimens in the collection of the Sternberg Museum were assigned to an “undescribed species” of Tylosaurus by Schumacher (1993).  Tylosaurus novum sp. was included a revision of mosasaur phylogeny by Bell (1997).  Schumacher (1993), Sheldon (1996) and Everhart (2001) revised the stratigraphic occurrence of mosasaurs in the Smoky Hill Chalk, including Tylosaurus n. sp. However, Carpenter (2003) did not mention the third species in his report on the vertebrate stratigraphy of the Smoky Hill Chalk and Sharon Springs member of the Pierre Shale.

ABBREVIATIONS

AMNH, American Museum of Natural History, New York; FHSM, Fort Hays Sternberg Museum, Hays, Kansas; FFHM, Fick Fossil and History Museum, Oakley, Kansas; FM, Fryxell Geology Museum, Augustana College, Rock Island, Illinois; LACMNH, Los Angeles County Museum of Natural History, Los Angeles, California; MCZ, Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts; TMM, Texas Memorial Museum, University of Texas at Austin, Texas; and YPM, Peabody Museum of Natural History, Yale University, New Haven, Connecticut.

MATERIAL

            Although the occurrence of the third species was first reported by Stewart (1990), he did not refer to any specimens in museum collections. A list of currently identified specimens is shown in Table 1. In his phylogenic revision of the Mosasauroidea, Bell (1993, p. 254; 1997, p. 328) listed seven specimens as Tylosaurus novum sp.: FHSM VP-2295, VP-78 and VP-2495; LACMNH 127815; MCZ 1589; YPM 3392 and 40796. Schumacher (1993) reported three specimens of an “undescribed tylosaur” in the Sternberg Museum collection: FHSM VP-2295, VP-78 and VP-3366. Two more recent additions to the Sternberg collection (VP-9350 and VP-13742) are here identified as belonging to the third species of Tylosaurus. An articulated skull and lower jaws of the third species of Tylosaurus is currently on exhibit in the Fryxell Museum of Geology. In addition, the author’s collection includes two incomplete specimens that are attributable to the third species. Two unreported, fragmentary specimens from the Late Cretaceous of Texas in the collection of the Texas Memorial Museum exhibit characteristics similar to those of the Kansas specimens of Tylosaurus novum sp.

DISCUSSION

            In 1979, a Tylosaurus specimen consisting of a nearly complete skull, articulated vertebrae and front limbs was collected by the author from near the base of the Smoky Hill Chalk (Late Coniacian). It was noted to have an unusually short anterior extension of the premaxilla and a parietal foramen that was adjacent to the fronto-parietal suture, and could not be identified to species until recently. Another specimen (FHSM VP-13742) which included a disarticulated skull and limb elements was collected by the author from the lower chalk in 1990. While neither the parietal or the quadrates were recovered with these remains, the medial posterior edge of the frontal is notched in a way that would suggest that it had been invaded by the parietal foremen. The anterior extension of the premaxilla is short and rounded, and does not resemble that of T. nepaeolicus or T. proriger. A portion of the anterior end of the premaxilla also appears to have been removed by the bite of a large shark and the tip of a Cretoxyrhina mantelli tooth is embedded in the dorsal surface of the bone.

            Later in 1990, a juvenile Tylosaurus skull and cervical vertebrae were collected by the author from the same general area and stratigraphic level as VP-13742. The specimen preserves both quadrates, the premaxilla, parietal and frontal. The anterior of the premaxilla is shortened and rounded compared to other species of Tylosaurus, the parietal foremen invades the suture between the frontal and the parietal, and neither of the well preserved quadrates exhibits an infrastapedial process. The remains are very similar in size and preservation to those of FHSM VP-2495 which was identified as Tylosaurus novum sp. by Bell (1993; 1997).

            In his discussion of the lowest biostratigraphic zone in the Smoky Hill Chalk, Stewart (1990, p. 22) indicated that “two tylosaurine species have been found in this horizon, Tylosaurus nepaeolicus and Tylosaurus n. sp.” He also noted (ibid., p. 25, 29) that “Tylosaurus n. sp. is limited to the zone of Protosphyraena perniciosa” and included the species in his taxonomic listing.  Stewart’s (1990) biostratigraphic zone of Protosphyraena perniciosa is Late Coniacian in age and includes Hattin’s (1982) marker units 1 through 5.

            Bell (1990,  pers. comm.) noted that one of the exhibit specimens at the Sternberg Museum (FHSM VP-78) was mis-labeled as Platecarpus and was, instead, the undescribed species of Tylosaurus. Bell (1993, p. 188) cited the LACMNH 127815 specimen as an example of Tylosaurus novum sp., and noted, among other characters (Table 2), that the infrastapedial process was absent and the “parietal foramen was close to the fronto-parietal suture.” Bell (ibid., p. 254) lists seven specimens in museum collections that he attributes to the third species. Schumacher (1993) noted independently (pers. comm., 2004) that the infrastapedial process was greatly reduced or lacking on the quadrates of three specimens of an undescribed species of Tylosaurus that he examined in the Sternberg Museum collection. He (1993) also reported that the rostrum of VP-78 and VP-2295 were “unusually pointed and rounded” compared to the blunt, cylindrical rostra Tylosaurus proriger and T. nepaeolicus, and that in VP-78, the parietal foremen invades the suture between the frontal and parietal. The parietal foramen is not visible in VP-2295 due to crushing. Another, more recently collected juvenile specimen in the Sternberg Museum collection (FHSM VP-9350) exhibits a shortened, rounded premaxilla and a quadrate without an infrastapedial process.

            An articulated Tylosaurus skull (FM V-43) collected from the Smoky Hill Chalk by George Sternberg and acquired by the Fryxell Geology Museum is another good example of the undescribed species. Although the parietal foramen is obscured by plaster in the exhibit mounting, the right quadrate and the premaxilla are typical of Tylosaurus novum sp.   An isolated premaxilla (TMM 40092-27) and an associated premaxilla and right quadrate (TMM 31051-64) in the Texas Memorial Museum collection have similar characteristics to the Kansas specimens but have not been examined in detail.

            The basal tylosaurine mosasaur, Tylosaurus novum sp., is represented by a number of well preserved and documented remains recovered from the Late Coniacian Smoky Hill Chalk of western Kansas.  As such, the species should be formally described and named.

LITERATURE CITED

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Bell, G. L., Jr. 1995. Middle Turonian (Cretaceous) mosasauroids from Big Bend National Park, Texas. Santucci V. L. and McClelland, L. (eds.), National Park Service Paleontological Research; U. S. Department of the Interior Technical Report NPS/NRPO/NRTR-95/16, p.34-39.

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Carpenter, K. 2003. Vertebrate Biostratigraphy of the Smoky Hill Chalk (Niobrara Formation) and the Sharon Springs Member (Pierre Shale). pp. 421-437 In Harries, P. J. (Ed.), Approaches in High-Resolution Stratigraphic Paleontology, Kluwer Academic Publishers, The Netherlands.

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Everhart, M. J. 2002. New data on cranial measurements and body length of the mosasaur, Tylosaurus nepaeolicus (Squamata; Mosasauridae), from the Niobrara Formation of western Kansas. Kansas Academy of Science, Transactions 105(1-2):33-43.

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