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Updated cranial measurements and body length of the mosasaur, Tylosaurus nepaeolicus (Squamata; Mosasauridae), from the Smoky Hill Chalk (Late Coniacian) of western Kansas

Copyright 2002-2010 by Mike Everhart  

Last updated 06/20/2010

 

 

 

LEFT: The left maxilla and attached premaxilla, and right maxilla of a large Tylosaurus nepaeolicus (FHSM VP-7262) in the Sternberg Museum of Natural History.

AUTHOR'S  NOTE: Please do not cite or quote from this webpage. This is a early draft of the published manuscript, with additional or updated figures. Please refer to: 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 Acad. Sci. Trans. 105(1-2):33-43.


Abstract

      Tylosaurus nepaeolicus (Cope 1874) is one of the least well known of the five species of mosasaurs that are recognized from the lower Smoky Hill Chalk Member (upper Coniacian) of the Niobrara Formation in western Kansas. In describing the type material, Cope stated that this species was one-third or less the size of T. proriger Cope 1869, a species that appeared during the Santonian and is well represented in the middle and upper chalk. Additional data provided by Russell from a review of specimens in the American Museum of Natural History and the Yale Peabody Museum showed that most T. nepaeolicus material is somewhat larger than the type specimen, but is significantly smaller than adult T. proriger specimens. Measurements of two additional T. nepaeolicus skulls, an articulated series of Tylosaurus sp. vertebrae and other material in the Sternberg Museum of Natural History suggest that this species approached 8-9 m in size by the end of the Coniacian and was approximately the same size as a large T. proriger reported from the lower Santonian. Although time and other morphological features separate T. nepaeolicus from T. proriger, the two species are closer in size than previously reported.

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Introduction

     Mosasaurs (Squamata; Mosasauridae) are extinct marine lizards that lived during the Late Cretaceous, from the Turonian (Martin and Stewart, 1977) through the Maastrichtian. Their remains are preserved in marine sediments around the world, but are especially abundant in the Smoky Hill Chalk Member of the Niobrara Formation in western Kansas.

     Between 1868 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 (Everhart, 2001) were described by the 19th Century Philadelphia paleontologist, Edward Drinker Cope (1840-1897). These include: the genus Clidastes Cope 1868 and its type species, C. propython Cope 1869 (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, T. proriger (Cope 1869), the first mosasaur described from Kansas, and T. nepaeolicus (Cope 1874). The genus Tylosaurus Marsh 1872 has a complicated history, including several name changes, which is too convoluted to be recounted here, save to say that Leidy (1873, p. 271) formally placed Macrosaurus proriger Cope 1869 into Tylosaurus.

     More than 125 years after its first description, Tylosaurus nepaeolicus remains relatively unknown. T. nepaeolicus is recorded only from the lower Smoky Hill Chalk Member (upper Coniacian) of the Niobrara Chalk in Kansas. According to records in the USNM (Smithsonian) collection, the first known specimen of Tylosaurus nepaeolicus was probably collected by Dr. George Miller Sternberg (1838-1915), an U. S. Army surgeon who was stationed at Fort Harker and elsewhere in western Kansas between 1866 and 1870. Dr. Sternberg, the older brother of Charles H. Sternberg, is credited with at least 40 other Kansas specimens in the USNM collection, but is best known for being the Surgeon General of the United States from 1893-1902. A mosasaur premaxilla (USNM V-3894) attributed to Dr. Sternberg was figured by Leidy (1873, p. 275, pl. 35) and initially identified as a T. proriger. Russell (1967, p. 176) examined the material and considered it to be T. nepaeolicus.

     Cope (1874, p. 37-38) described the type specimen of ‘Liodon’ nepaeolicus (AMNH 1565) from material discovered by Professor Benjamin F. Mudge (1817-1879) in the "gray shale of the Niobrara Cretaceous, a half mile south of the Solomon River," in north-central Kansas. The species name most likely comes from "Nepaholla," an earlier Indian name for the Solomon River (Solomon’s Fork of the Smoky Hill River) meaning "water on a hill" (Rydjord, 1972, p. 109). Cope (1871, p. 416) had earlier described the origin of material in the Mudge collection as "from the yellow chalk of the upper cretaceous (sic) of Kansas on the Solomon or Nepaholla River." The type locality is uncertain but probably is near the town of Stockton, in Rooks County.

     Mudge sent specimens to Cope as early as 1870 (Williston, 1898a, p. 29-30). During his 1871 trip to the West, Cope visited Mudge at the Kansas Agricultural College in Manhattan, Kansas (now Kansas State University) and examined Mudge’s collection of mosasaur and fish specimens (Cope, 1872). In 1874, Mudge and two assistants, including a young Samuel W. Williston (1851-1918), began collecting "vertebrates for Yale College" (Mudge, 1876, p. 216; Williston, 1898a, p. 31). Between 1874 and 1876, Mudge located eight additional specimens of Tylosaurus nepaeolicus for O. C. Marsh and the YPM collection, two of which also came from Rooks County. According to the YPM records, the others that he collected were from: Ellis County (1 specimen), Trego County (3 specimens), and Gove County (2 specimens). The exposures of the Smoky Hill Chalk Member in these counties generally are low in the chalk (upper Coniacian) with the exception of parts of northern Trego and western Gove County that are lower to middle Santonian. These localities are consistent with the biostratigraphic occurrence of T. nepaeolicus (Everhart, 2001, fig. 5; Schumacher, 1996, p. 56).

     According to Cope (1874, p. 37), the type specimen of Tylosaurus nepaeolicus (AMNH 1565) was represented by a right lower jaw, parts of the maxilla and premaxilla, a right quadrate and a dorsal vertebra. The 1874 description was repeated verbatim and illustrated in Cope (1875, p.177-178, pl. 35, figs. 11-14). Mudge's field measurement of the intact lower jaw (26 inches) was published by Cope (1874, p. 37). The lower jaw was subsequently damaged and incomplete by the time it was figured by Cope (1875, pl. 35, fig. 13). Cope (1874, p. 38) included measurements of the teeth, the quadrate and the vertebra, and stated that the species was "not more than one third the size or less" of T. proriger. He also indicated that the type specimen was not a "young individual" of T. proriger (Cope, 1874, p. 37). The new material described here places the type specimen low in the size range of T. nepaeolicus.

     Following the transfer of Macrosaurus proriger Cope 1869 and Liodon dyspelor Cope 1871 to Tylosaurus Marsh 1872 by Leidy (1873, p. 274 and 271 respectively), Merriam (1894, p. 24) placed Liodon nepaeolicus Cope 1874 in Tylosaurus. He also removed the Greek dipthong (), spelling the species name "nepeolicus", and included the name in his discussion of the mosasaur skull without further comment. According to E. Manning (pers. comm., 2001), if the diphthong was to be removed, the species name should probably have been changed instead to "nepaolicus", following the place name, Nepaholla, more closely.

     Williston (1898b, p. 176) apparently ignored the spelling change by Merriam (1894) and repeated much of the description provided by Cope (1874). He further suggested that "(t)he characters given - such as may be valid - are altogether too slight to distinguish the species, and I do not believe that T. nepaeolicus is entitled to recognition." Stewart (1990, p. 29) listed T. nepaeolicus as one of three species of mosasaurs typical of the lower chalk in the biostratigraphic zone of Protosphyraena perniciosa (upper Coniacian). Russell (1967, p. 187; 1993, p. 673) included T. nepaeolicus as one of three mosasaur species from the "low chalk." After examining the type specimen (AMNH 1565), C. Kiernan (pers. comm., 2001) concluded that it is not sufficient to serve as the basis for a taxon and is, in fact, indeterminate.

     Our knowledge of Tylosaurus nepaeolicus continues to be limited by the lack of specimens. Russell (1967, table A) provided measurements of several skulls of T. nepaeolicus in the AMNH and YPM collections. The most complete set of measurements is actually from a composite of two specimens (a complete skull, AMNH 124; and lower jaws, AMNH 134) that Russell (pers. comm., 2001) believed probably were from the same individual (Fig. 1). The length of the skull measured along the midline is 717 mm (from the tip of the premaxilla to the back of the occipital condyle) and the length of the lower jaw is 828 mm. A larger but less complete skull (YPM 3980, collected by Williston in 1875) has an estimated skull length of about 860 mm. Measurements of the length of the lower jaw of two complete specimens of T. proriger (AMNH 221 and FHSM VP-3) indicate that it is about 14% of the body length. Using this ratio as a starting point, the two largest T. nepaeolicus specimens in the AMNH and YPM collections would represent living mosasaurs that were about 5.9 m (19 ft) and 6.8 m (21 ft) in length, respectively. The largest T. proriger (AMNH 221) cited by Russell (1967, p. 210) measured just under 9 m (29 ft) long, or about one third larger than the largest T. nepaeolicus that he documented. Given that there was an even larger T. proriger specimen known at the time (the 12-13 m "Bunker" Tylosaur (KUVP 5033) at the University of Kansas Museum of Natural History, collected in 1911 from the upper chalk of Logan County by C. D. Bunker, University of Kansas), it is certainly understandable that T. nepaeolicus would have been considered to be a smaller species of Tylosaurus.

     Measurements of two more recently discovered Tylosaurus nepaeolicus skulls in the Fort Hays Sternberg Museum (FHSM) collection are provided here to update the size data available for this species, and to report what appears to be the intergradation in size of T. nepaeolicus and T. proriger from the upper Coniacian into the lower Santonian.

Institutional Abbreviations

     AMNH, American Museum of Natural History, New York, NY; FFHM: Fick Fossil and History Museum, Oakley, KS; FHSM, Fort Hays State University Sternberg Museum of Natural History, Hays, KS; KUVP, The University of Kansas, Museum of Natural History, Lawrence, KS; USNM, United States National Museum, Washington, D.C.; YPM, Peabody Museum of Natural History, Yale University, New Haven, CT.

Systematic Paleontology

                    Order Squamata Oppel, 1811

                        Family Mosasauridae Gervais, 1853

                            Genus Tylosaurus Marsh, 1872

                                Type species Tylosaurus nepaeolicus Cope, 1874

Discussion

     The calculation of a maximum adult size for any species from data in the fossil record is questionable at best because it is not always possible to obtain representative samples. Many collections include various age groups of individuals representing a growth series. In mosasaurs, growth was also probably indeterminate, with individuals growing throughout their life, albeit at a much slower pace as they got older. Additionally, there was a natural bias favoring the preservation of larger specimens in the Smoky Hill Chalk Member and there has been a human bias for collecting and reporting of the largest and best preserved individuals. Measurements obtained from a small sample of individuals may be statistically invalid, as well as being non-representative of the population being studied. In the case of Tylosaurus nepaeolicus, the total number of curated specimens is certainly fewer than 30, 13 of which are in the YPM collection. Nine additional specimens are in the United States National Museum (Smithsonian) collection.

     Several of these specimens consist of one or a small number of bones whose identification may be questionable. Limited data are available on the skulls of eleven of the specimens (Cope, 1874; Russell, 1967; this report). With the possible exception of YPM 124/134, there are no complete skulls and no reasonably complete skeletons of this species. Tylosaurus proriger, on the other hand, is well represented by dozens of skulls and many complete specimens in museums around the world.

     Although fragmentary remains of mosasaurs from Kansas first occur in Turonian rocks (Martin and Stewart, 1977), the earliest known remains identifiable to species occur in the Smoky Hill Chalk Member of the Niobrara Formation (upper Coniacian - lower Campanian). Tylosaurus nepaeolicus is one of five mosasaur species identified from the upper Coniacian of the Western Interior Sea (Everhart, 2001). T. nepaeolicus shared the shallow ocean that covered Kansas with Platecarpus tympaniticus (including the junior synonyms P. ictericus and P. coryphaeus), P. planifrons, and Clidastes liodontus Merriam 1894 (Everhart, Everhart and Bourdon, 1997) and another, undescribed primitive species of Tylosaurus (Stewart, 1990; Schumacher, 1993; Sheldon, 1996; Bell, 1997). Platecarpus tympaniticus and C. liodontus both continue into the upper chalk but the two early Tylosaurus species appear to be replaced by T. proriger in the lower Santonian. Platecarpus planifrons is not recorded past the lower Santonian (Everhart, 2001, p. 73-75). The earliest known T. proriger (FFHM 1997-10) was discovered in the Smoky Hill Chalk Member (middle Santonian) of Gove County in 1996 (Everhart, 2001, p. 71-73). The well-preserved, completely articulated skull is 1.2 m in length, with a total body length estimated at 8.7 m (28.5 ft.). Clidastes propython occurs for the first time in the middle Santonian of the Smoky Hill Chalk Member (Schumacher, 1993; Sheldon, 1996). Specimens of other species, including Halisaurus sternbergi (Wiman, 1920; see also Sternberg, 1922 regarding the discovery) and Ectenosaurus clidastoides Russell 1967, are too rare to determine their ranges, save to say that they apparently are restricted to the upper chalk.

     Mudge collected extensively from Rooks, Ellis, Trego and eastern Gove counties between 1874 and 1876 (Williston, 1898a, p. 31) where exposures are mostly in the lower half of the chalk. On the basis of his experience in the Niobrara, Williston (1897, p. 245-246) later observed that vertebrate remains were ten times more abundant in the "shallow water," upper chalk than in the "deeper water," lower chalk and spent little time collecting in the lower chalk. Most of the better known vertebrate specimens currently in museum collections come from the upper half of the chalk in western Gove, Logan, and Wallace counties (Bennett, 2000). Considering that Mudge collected a disproportionate number of the known specimens of Tylosaurus nepaeolicus during the time he spent in these low chalk exposures, it seems that a subsequent collecting bias against low chalk localities may partially account for the lack of specimens of this species.

FHSM VP-2209-1a.jpg (17261 bytes)    Tylosaurus nepaeolicus differs from T. proriger in the size and location of the parietal foramen (larger, closer to the frontal-parietal suture in the latter), the size of the infrastapedial process on the quadrate (larger in T. proriger), the lateral overlap of the anterior postorbital frontal over the prefrontal (absent in T. nepaeolicus), and other features (Bell, 1997, p. 297-310). Until recently, (Everhart, 2001), the two species also were separated chronologically by a gap of about 2 million years in the fossil record (upper Coniacian- upper Santonian).

LEFT: The reconstructed parietal of a large specimen of Tylosaurus nepaeolicus (FHSM VP-2209) in dorsal view.  The nearly complete specimen was collected M.V. Walker from near the town of Glade in Rooks County, KS, in 1966.

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Figure 1. Reconstructed skull and jaw of Tylosaurus nepaeolicus in right lateral view (adapted from Russell (1967), after AMNH 124 and 134): Abbreviations: a, angular; ar, articular; c, coronoid; d, dentary; e, epipterygoid; f, frontal; j, jugal; m, maxilla; p, parietal; pm, premaxilla; pof, post-orbitalfrontal; pr, prootic; prf, prefrontal; pt, pterygoid; q, quadrate; sa, surangular; sp, splenial; sq, squamosal; st; supratemporal. Adapted from text-figure 95 in Russell (1967).   Used with permission of Dale Russell.

     Two specimens of Tylosaurus nepaeolicus in the Sternberg Museum (FHSM VP-2209 and VP-7262) collected after Russell’s (1967) publication are significantly larger than the type specimen and most of the material cited by Russell (1967, p. 209). FHSM VP-2209 was collected in 1966 by M. Walker and O. Bonner from near Glade, in Phillips County, Kansas (Schumacher, 1993, p.40). The specimen includes both maxillae, the frontal, the anterior portion of the parietal, both squamosals, the quadrates, both mandibles, cervical and dorsal vertebrae, and some limb elements. According to J. D. Stewart (Schumacher, 1993, p.40), the site was in the biostratigraphic zone of Protosphyraena perniciosa (upper Coniacian). VP-7262 was collected in south-eastern Gove County by R. J. Zakrzewski in 1984 from near Hattin’s (1982) Marker Unit 5 (upper Coniacian). The remains consist of the premaxilla, complete right and left maxillae, the complete frontal, a large fragment of the posterior parietal, the left quadrate, both pterygoids, portions of both lower jaws including the complete left dentary, and numerous vertebrae. Many small fragments of shattered bone were collected at the site and it is possible that the skull was essentially complete prior to weathering.

     The same measurements reported by Russell (1967, p. 210) are provided for both of the new material (Table 1). Russell (1967) determined that the length of a Tylosaurus skull averaged 13.8% of the total body length. In practice, the length of the lower jaw is about 14% of the body length (AMNH 221 and FHSM VP-3). Because this measurement is more readily available for the less complete specimens of T. nepaeolicus, it was used to estimate total body length (Table 1, column I). In situations where only the length of the dentary was available (FHSM VP-7262), it was considered to represent 56% of the length of the lower jaw. Based on similar measurements of the width of the frontal between the orbits, the length between the anterior base of the first and the posterior base of the sixth maxillary tooth, and the height of the quadrate, the estimated body lengths for YPM 3980 and FHSM VP-7262 are identical (6.8 m, 22 ft.), with VP-7262 representing a much more complete specimen. With a lower jaw length of 1 m (3.3 ft.),VP-2209 appears to be the largest T. nepaeolicus known, fully half again as large as the type specimen. VP-2209 represents at 7.1 m (23 ft.) individual from near the Coniacian / Santonian boundary.

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A = Length of skull along midline; B = Length of premaxilla rostrum; C = Width of frontal between orbits; D = Length between first and sixth maxillary tooth; E = Height of quadrate; F = Length of lower jaw; G = Length of dentary; H = Length between first and sixth dentary tooth; I = Calculated length of body (14% of lower jaw length); * Estimated, this paper; ** = approximate. Measurements by (1) Cope (1874); (2) Russell (1967); (3) Osborn (1899)

     Three other specimens in the Sternberg Museum collection are relevant to this discussion. FHSM VP-2295 is a completely articulated skull and lower jaws of the undescribed species of Tylosaurus (lower jaw length = 720 mm). The specimen was recovered from the low chalk (upper Coniacian) of Ellis County, eight miles northeast of Ellis, by M. Walker (FHSM) and represents a 5 m (16 ft) adult. The frontal and right dentary have several long, deep gouges and punctures that seem to be unhealed bite marks from an fatal encounter with a much larger mosasaur. These bites are not typical of large sharks such as Cretoxyrhina mantelli and there are no other predators documented from the Western Interior Sea besides mosasaurs that could have inflicted these kinds of wounds on a 5 m Tylosaurus.

     FHSM VP-13742 is a large but badly weathered skull of an as yet unidentified Tylosaurus (Everhart, 1999) discovered within 0.5 km of the locality in Gove County that produced FHSM VP-7262, and at approximately the same stratigraphic level (upper Coniacian). The lower jaws measure 98 cm (39 in), suggesting a mosasaur of about 7 m (23 ft) in total length.

     FHSM VP-13908 is a series of 14 articulated Tylosaurus vertebrae from below Hattin's (1982) Marker Unit 4 (upper Coniacian) in Gove County. Although the species cannot be reliably identified from characteristics of the vertebrae, there is no reason to expect that it would be anything other than T. nepaeolicus or, less likely, the undescribed species of Tylosaurus (Stewart, 1990; Schumacher, 1993; Bell, 1997). The vertebrae were discovered in 1989 by my wife, Pamela Everhart, eroding from the edge of a dry stream bed. They were located within sight (about 200 m) of the FHSM VP-7262 locality, but are from a slightly lower stratigraphic level. The vertebrae are identified as posterior dorsals and pygals. Although laterally compressed somewhat during preservation, they are similar in length (110 mm) and diameter to those of FHSM VP-3 (an 8.8 m T. proriger). By comparing the length of the articulated segment (1.4 m) and the calculated percentage of this vertebral segment to the length of a complete 8.8 m Tylosaurus skeleton (FHSM VP-3) from the upper chalk (lower Campanian), a length of 8-9 m was estimated. This length is quite comparable to the calculated body size of another T. proriger (FFHM 1997-10) from the middle Santonian of Gove County.

Conclusion

     Since its description in 1874, Tylosaurus nepaeolicus has been considered to be significantly smaller than T. proriger. The lack of complete specimens of T. nepaeolicus appears to have discouraged further studies of this taxon. Although it remains one of the lesser known mosasaurs from the lower Smoky Hill Chalk Member in Kansas, a sufficient number of specimens is now available to provide a more accurate assessment of its size range in comparison to T. proriger. New material shows that adults of this species were significantly larger than the "one third" of the size of T. proriger originally estimated by Cope (1874) or even the specimens measured by Russell (1967). In addition, large vertebrae from the lower one-third of the Smoky Hill Chalk Member (upper Coniacian) indicate the presence of a Tylosaurus that was about 8-9 m in body length. Although minor morphological features separate the two species, the fossil record indicates clearly that by the end of the Coniacian (86 mya) T. nepaeolicus was approaching the same adult size observed in T. proriger remains from the lower to middle Santonian and later.

Acknowledgements

     I thank Dwayne and Marion Cheney, of Bellevue, NE, and Glenn and Arlene Bird of Quinter, KS, for allowing me access to their property, and for the donation of some of the specimens discussed in this paper. Many of the specimens and much of the biostratigraphic information reported in this study would not have been available without their cooperation and encouragement. I thank Donald Hattin (Dept. of Geology, Indiana University, Bloomington, IN) and J. D. Stewart (Los Angeles County Museum of Natural History, Los Angeles, CA) for their help over the years in understanding the stratigraphy and paleontology of the Smoky Hill Chalk Member. I thank Dale Russell (North Carolina State University, Raleigh) for his insight into this paper and our ongoing discussions about mosasaurs. The assistance of Robert Purdy and Michael Brett-Surman (United States National Museum, Washington, D. C.) in obtaining historical records on early mosasaur collections is appreciated. The advice and continued support of Jerry Choate, Rick Zakrzewski and Greg Liggett at the Sternberg Museum of Natural History, Hays, KS, is also appreciated. I thank Earl Manning (Geology Dept., Tulane Univ., New Orleans) for his review of the draft manuscript, his assistance in obtaining reference material and his views on the historical context that guided the discoveries made by 19th Century paleontologists in Kansas. I also thank C. Kiernan for her review and comments. John Jagt and Anne Schulp (Natuurhistorisch Museum Maastricht, Maastricht, The Netherlands) provided helpful comments on the final draft of the manuscript. Ben Creisler provided the reference material on the origin of the species name "nepaeolicus".


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