Tylosaurus kansasensis

A new species of mosasaur from the Smoky Hill Chalk

Written and Illustrated by Mike Everhart; Copyright © 2000-2010

Page created 01/30/1999 - Updated 07/12/2012

FHSM VP-2495 (paratype), a disarticulated, near-complete skull (missing the premaxilla) of a juvenile Tylosaurus and the anteriormost portion of the vertebral column (83 vertebrae). There was no limb material associated with the specimen. This is a common condition for the remains of many mosasaurs collected from the chalk and is most likely the result of scavenging by sharks. The remains were collected by R. Zakrzewski in November 1969 from the lower Smoky Hill Chalk of Trego County. The length of the left lower jaw is 50 cm, and represents an individual that would have been about 3.6 m long.  Both quadrates were present.

LEFT: The frontal and parietal of VP-2495 in dorsal view. Note the location of the parietal (pineal) foramen immediately adjacent to the frontal-parietal suture. It is unclear what function that this opening served in mosasaurs but in some lizards such as the Tuatara, the pineal gland has some light sensing function.

FHSM VP-3366, an unmounted, partial skull (including premaxilla and a right quadrate) with twenty-four cervical and dorsal vertebrae, rib fragments (two with healed fractures), and preserved thoracic cartilage. Scavenging by Squalicorax falcatus is indicated by serrated bite marks on the remains. Collected by R.L. Bliss in April 1972 from the lower Smoky Hill Chalk of Gove County, Kansas. Here is a right lateral view of the premaxilla and the anterior portion of the right maxilla. Note the relatively short extension of the predentary portion of the rostrum. Click here for a photo of a healed rib facture including shark bite marks.

LEFT: The right quadrate of VP-3366 in lateral, medial, posterior and anterior views. The quadrates of Tylosaurus kansasensis lack (or have a very small) infrastapedial process.

LEFT (Upper): Left lateral view of the partial skull of VP-9350, including the left quadrate. (Lower): A close-up of the left lateral edge of the frontal, showing the anterior extension of the post orbitalfrontal over the orbit of the eye. In Tylosaurus, this flange extends over the posterior extension of the prefrontal and excludes the frontal from the orbit of the eye.

FHSM VP-13742, the weathered and fragmentary skull of an adult Tylosaurus including the premaxilla, portions of both maxillae, a complete frontal and both lower jaws (lacking both quadrates), and a few bones from the front limbs. The remains were discovered by the author in February 1990 on the surface of the low chalk (between mu4 and mu5) of Gove County. The tips of three shark teeth (Cretoxyrhina mantelli) were embedded in the top of the premaxilla and one each in the lateral side of both dentarys. In addition, the anterior end of the premaxilla had apparently been sheared off by the bite of a shark and the damaged bone shows no evidence of healing. The lower jaws measure 98 cm, suggesting a mosasaur of about 7 m in total length (Everhart, 2002). VP-13742 is the largest known example of Tylosaurus kansasensis.

LEFT: The partial frontal of VP-13742 in dorsal view.

LEFT: Left quadrate of VP-14848 in lateral and medial view. Note the scale (cm) .. this quadrate is less than 1.5 inches in height.

FHSM VP-15631, the complete skull (including premaxilla, frontal/parietal and partial right quadrate), pectoral girdle and most of both front limbs, several ribs and a series of forty-two consecutive cervical, dorsal and caudal vertebrae of an adult Tylosaurus. The specimen was collected by the author in November, 1979 from the lower chalk (MU1-MU2) of Ellis County. The overall condition is poor due to plant roots and subsurface weathering. The pre-dental process of the premaxilla is relatively short and the ventral surface appears to preserve some sort of bone modification due to an infection or disease (pathology).  This close-up of the parietal (pineal) foramen shows its characteristic location next to the frontal- parietal suture.  Evidence of scavenging by sharks is preserved in this deep cut across one of the front limb bones (radius or ulna).

LEFT: Frontal and partial parietal of VP-15631 in dorsal view.

FHSM VP-15632, a fragmentary skull of a juvenile Tylosaurus (including the premaxilla, frontal/parietal, both quadrates and both lower jaws) and five cervical vertebrae. The left lower jaw measures 42 cm. The specimen was discovered by P.A. Everhart in June 1990 in the lower chalk (between MU 4 and MU 5) of Gove County. A portion of the remains were exposed on the surface and some bones were damaged by weathering and plant roots. Quadrates of VP-15632 in lateral view (NOTE: This photo was published on page 147 of my recent book (Sea Monsters) with the caption identifying these bones as vertebrae(?)... I know the difference, but someone at National Geographic didn't).... The premaxilla and anterior portion of the left maxilla in left lateral view. Again, note the relatively short pre-dental process. Medial (lingual) view of a portion of the left maxilla, showing two old teeth (broken), a replacement tooth and a replacement for the replacement tooth. The rapid replacement of teeth may be an indication of how quickly these animals were growing... Go here for the earlier web page about this specimen

LEFT: A composite view of some of the elements of the skull of VP-15632, as labeled.

RIGHT: The four articulated cervical 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 dentarys in left lateral view. Note the relatively small size of the rostrum 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.  .

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 (V-43) 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

Bell, G. L., Jr. 1993. A phylogenetic revision of Mosasauroidea, unpublished doctoral dissertation, The University of Texas at Austin, 293 pages.

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.

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

Bell, G. L., Jr. and J. P. VonLoh. 1998. New records of Turonian mosasauroids from the western United States. Fossil vertebrates of the Niobrara Formation in South Dakota, Dakoterra 5:15-28.

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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Cope, E. D. 1869. [Remarks on Macrosaurus proriger.] Proceedings of the Academy of Natural Sciences of Philadelphia 11(81):123.

Cope, E. D. 1871. On the fossil reptiles and fishes of the Cretaceous rocks of Kansas. Art. 6, Pages 385-424 of Pt. 4, Special Reports, 4th Annual Report, U.S. Geological Survey of the Territories, (Hayden).

Cope, E. D. 1872. Note of some Cretaceous Vertebrata in the State Agricultural College of Kansas. Proceedings of the American Philosophical Society 12(87):168-170.

Cope, E. D. 1874. Review of the vertebrata of the Cretaceous period found west of the Mississippi River. U. S. Geological Survey of the Territories, Bulletin 1(2):3-48.

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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.

Everhart, M.J. 2008. A bitten skull of Tylosaurus kansasensis (Squamata: Mosasauridae) and a review of mosasaur-on-mosasaur pathology in the fossil record. Kansas Academy of Science, Transactions 111(3/4):251-262.

Everhart, M.J. and P.A. Everhart. 1997. Earliest occurrence of the mosasaur, Tylosaurus proriger, (Mosasauridae: Squamata) in the Smoky Hill Chalk (Niobrara Formation, Upper Cretaceous) of western Kansas. Journal of Vertebrate Paleontology 17(supplement to 3):44A.

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Marsh, O.C. 1872. On the structure of the skull and limbs in mosasaurid reptiles, with descriptions of new genera and species. American Journal of Science 3(18):448-464, pl. 10- 13.

Marsh, O.C. 1872. Note on Rhinosaurus. American Journal of Science 4(20):147.

Martin, L. D. and J.D. Stewart. 1977. The oldest (Turonian) mosasaurs from Kansas. Journal of Paleontology 51(5):973-975.

Russell, D.A. 1967. Systematics and morphology of American mosasaurs (Reptilia, Sauria). Yale Peabody Museum of Natural History Bulletin 23, 241 p.

Schumacher, B.A. 1993. Biostratigraphy of Mosasauridae (Squamata, Varanoidea) from the Smoky Hill Chalk Member, Niobrara Chalk (Upper Cretaceous) of western Kansas. Unpublished Masters thesis, Fort Hays State University, Hays, Kansas, 68 p.

Sheldon, M.A. 1996. Stratigraphic distribution of mosasaurs in the Niobrara Formation of Kansas, Paludicola 1:21-31.

Stewart, J.D. 1990. Niobrara Formation vertebrate stratigraphy. Pages 19-30, In Bennett, S. C. (ed.), Niobrara Chalk Excursion Guidebook. Univ. Kansas Museum of Natural History and Kansas Geological Survey.