First report of plesiosaurs from the lower Smoky Hill Chalk Member of western Kansas Copyright © 2003-2009 by Mike Everhart   Last updated 07/07/2009         LEFT: A drawing of Dolichorhynchops osborni  - Copyright © by Russell Hawley; Used with permission

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ABSTRACT: Although plesiosaurs are known to occur in the Upper Cretaceous Smoky Hill Chalk Member of the Niobrara Formation, their remains are uncommon, rarely complete and unequally distributed in the unit. There are only three reasonably complete plesiosaur skeletons (all Dolichorhynchops osborni) known from the Smoky Hill Chalk in Kansas. All three specimens are from the upper one-third (Early Campanian) of the chalk. Other remains are fragmentary and, in most cases, consist of body parts that could have been readily removed by predators or scavengers. Many of these specimens have teeth marks or appear to be partially digested. In spite of observations to the contrary by Williston and others, reliable stratigraphic and locality information for museum specimens exists only for plesiosaur remains in the upper portions of the Smoky Hill Chalk (Santonian – Lower Campanian). Recent discoveries reported here suggest that plesiosaurs were present throughout the deposition of the chalk, albeit in small numbers.

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Introduction

As a group, plesiosaurs (Sauropterygia) are well documented in the fossil record of the Western Interior Sea during the Late Cretaceous (Cope, 1868, 1869; Williston, 1906, 1908; Welles, 1943, 1952, 1962; Russell, 1993; Carpenter, 1996, 1997, 1999; Storrs, 1999). In Kansas, their remains include the holotype specimens of the elasmosaurs, Elasmosaurus platyurus Cope 1868, and Styxosaurus snowii (Williston 1890); the pliosaur, Brachauchenius lucasi Williston 1903; and the polycotylids, Polycotylus latipinnis Cope 1869, Trinacromerum bentonianum Cragin 1888, and Dolichorhynchops osborni (Williston 1902). All of the new specimens reported here are fragmentary and are not identifiable with certainty beyond Plesiosauridae. From their small size and other characteristics, the remains are here assumed to be from polycotylids and are most likely those of the genus, Dolichorhynchops.

Samuel W. Williston (1851-1918) was arguably the most knowledgeable paleontologist to work in the Smoky Hill Chalk Member of the Niobrara Chalk between 1875 and 1900. On the subject of plesiosaurs, he stated (1893, p.110) that they "are rare everywhere, but relatively more abundant in the lower strata." Four years later, (1897, p. 245), he elaborated somewhat when he wrote: "The Plesiosaurs occur in about equal abundance throughout the deposits. They are at best comparatively rare, but relatively far more abundant in the lower beds." Williston made these statements, however, without citing specific specimens or localities. Although it is possible that he was referring to the presence of plesiosaurs in the Cretaceous marine formations below the Niobrara, he made a point to define the "lower strata" of the Smoky Hill Chalk as the Rudistes Beds (1897, p. 239). Because he was one of the first to attempt to define the biostratigraphy of the chalk (Stewart, 1990, p. 19; Everhart, 2001, p. 61), it is unlikely that he was referring to remains from the underlying Fort Hays Limestone or older formations.

Both Williston (1903; p 73-74) and Moodie (1911, p. 95) discussed the presence of isolated embryonic and juvenile plesiosaur propodials from the Smoky Hill Chalk but did not provide further locality or stratigraphic information. Although Stewart (1990, p. 25) indicated that he was "aware of a few juvenile propodials... in the lowermost zones [Upper Coniacian] of the Smoky Hill Chalk Member," he saw no basis for Williston’s remarks concerning the relative abundance of plesiosaurs and did not document the occurrence of plesiosaur remains lower than the upper one-third (Upper Santonian – Lower Campanian) of the unit.

In his survey of fossil vertebrates of Kansas, Lane (1946, p. 303-310) reviewed plesiosaur specimens from Kansas in the University of Kansas Museum of Natural History (KUVP) and other museum collections. Among the polycotylids from the Smoky Hill Chalk, he lists (ibid., p. 306) three specimens of Polycotylus from Logan County (KUVP 5916, 1308, 1312) and four from Wallace County (KUVP 1320, 1321, 1323 and 1324). Bennett (2000) noted that exposures of the chalk in Logan and Wallace counties are in the upper one-half of the formation (Middle Santonian through Lower Campanian), while those in Rooks, Graham, Ellis, Trego and Gove counties are in the lower half of the formation (Upper Coniacian through Lower Santonian). Thus, all of the KUVP specimens listed by Lane are apparently from the upper chalk (Fig. 1).

More recently, Storrs (1999) examined specimens of plesiosaurs from the Niobrara Chalk of North America, and Carpenter (1996) reviewed the records of short-necked plesiosaurs from the Cretaceous of the Western Interior of North America. Neither Storrs nor Carpenter reported any plesiosaurs from the lower two-thirds of the Smoky Hill Chalk. Regarding one Gove County specimen, Storrs (1999; p. 5; see also Williston, 1906) concluded that Elasmosaurus sternbergi Williston 1906 (KUVP 1312), collected by Charles Sternberg in 1895, was from the upper chalk (Santonian). Storrs (ibid., p. 9) also indicated that the holotype of Dolichorhynchops osborni (KUVP 1300) came from the Smoky Hill Chalk (Campanian) of Logan County, Kansas. Carpenter (1996, p. 271) concurred, noting that KUVP 1300 was collected "east of Wallace" in Stewart’s (1990, p. 23) zone of Hesperornis. Bonner (1964, p. 41) reported that the Trinacromerum osborni (FHSM VP-404; now D. osborni, per Carpenter, 1997, p. 193) in the Sternberg Museum of Natural History at Fort Hays State University was collected from Logan County, a mile southwest of Russell Springs. A third, nearly complete D. osborni specimen (MCZ 1064) collected by G. F. Sternberg (Sternberg and Walker, 1957, p. 57) in 1926 was also discovered in Logan County (Carpenter, 1996, p. 271). Charles Sternberg (1922; p. 119; Everhart, in press) reported partially digested plesiosaur bones (USNM 9468) as gut contents within a large Tylosaurus proriger (USNM 8898) mosasaur from an exposure along [Twin] Butte Creek in Logan County. The plesiosaur has been tentatively identified as a juvenile polycotylid (O’Keefe, pers. com. 2001). The horizon of all these Logan County localities is in the upper one-third (Upper Santonian through Lower Campanian) of Smoky Hill Chalk.

According to the records of the Yale Peabody Museum (YPM), B. F. Mudge collected the remains of a polycotylid (YPM 1643) in 1875 from the Niobrara chalk in Trego County but no further locality information is available. Williston (1898, p. 31) recalled that he and Brous assisted Mudge in collecting for Yale College along the Saline and Smoky Hill Rivers in 1874 and 1875, most likely in Ellis and Trego Counties where Mudge had collected in 1873 (Peterson, 1987; p. 232). Exposures of the Niobrara in Trego County include the Fort Hays Limestone and lower one-third of the Smoky Hill Chalk (Upper Coniacian to Lower Santonian). Although YPM 1643 may be Upper Coniacian to Lower Santonian in age, there is not enough locality information available to be certain. A review of the literature, and the collections in the Sternberg Museum of Natural History at Fort Hays State University (FHSM), the University of Kansas Museum of Natural History (KUVP), the Denver Museum of Nature and Science, the University of Nebraska State Museum and the United States National Museum (USNM), suggests that the occurrence of plesiosaurs in the lower one third of the Smoky Hill Chalk (Upper Coniacian) has not been previously substantiated by specimens with well-documented locality or stratigraphic data.

In his discussion of the occurrence of short-necked plesiosaurs, Carpenter (1996, p. 284), determined that the genus Trinacromerum has a 3.3 million year range from Upper Cenomanian into the Turonian, and the genus Dolichorhynchops has a 4 million year range beginning in the earliest Campanian. This suggests that there is an almost 6 million year gap in the current fossil record of the Western Interior Sea that is unoccupied by polycotylids between the lower Middle Turonian and the beginning of the Campanian. A recent discovery of the remains of Trinacromerum bentonianum (FHSM VP-12059) in the Fairport Chalk Member of the Carlile Shale (Middle Turonian) may narrow this gap somewhat.

Within the Smoky Hill Chalk in Kansas, this gap extends from the Upper Coniacian through the end of the Santonian. This interval also corresponds to the greatest expansion of the Western Interior Sea (Hattin, 1982, p. 59), and the beginnings of the subsequent regression, a period in which the near-shore, shallow water environments, and possibly the preferred habitats of polycotylids, were the furthest away from the depositional area of the Smoky Hill Chalk in western Kansas. Here I report on six unassociated, fragmentary plesiosaur specimens from the Upper Coniacian of the Smoky Hill Chalk in Ellis, Gove and Lane counties, Kansas. Five of these specimens were collected by the author between 1988 and 1992, and the sixth was collected by J. D. Stewart of the Los Angeles County Museum of Natural History in my presence in 1990. Taken as a group, these specimens document the presence of plesiosaurs near the base of the chalk through the Upper Coniacian, and into the Lower Santonian. Although not providing a continuous record of plesiosaurs throughout the Smoky Hill Chalk (Fig. 2), this material indicates their presence in much of the unit. LEFT: Fig. 2 - Generalized stratigraphy of the Smoky Hill Chalk Member showing the correlation between Marker Units (MU: Hattin, 1982), biostratigraphic zones (Stewart, 1990) and the approximate horizon of Kansas plesiosaur specimens.

Abbreviations

The abbreviations for institutions referred to in the text are: ESU - Emporia State University Geology Museum, Emporia, KS; FHSM - Sternberg Museum of Natural History, Fort Hays State University, Hays, KS; and, USNM; KUVP – University of Kansas Vertebrate Paleontology Collection, Lawrence, KS; LACMNH - Los Angeles County Museum of Natural History, Los Angeles, CA.; MCZ – Museum of Comparative Zoology, Harvard, Cambridge, MA; and USNM - United States National Museum, Washington, D.C.

Materials

Discussion

All of the remains noted above represent body parts (head, limbs, tail) that were readily detachable from a living plesiosaur or a carcass by large predators / scavengers. Major portions of the outer surface of the bone on all of these remains, except LACMNH 148920, have been eroded away in a manner that is consistent with the surface pitting and erosion observed on partially digested bones. Examples include mosasaur bones apparently consumed by sharks (Shimada, 1997, p. 928; Everhart, 1999), plesiosaur bones reported as gut contents of a Tylosaurus (Everhart, in press), and hadrosaur bones described by Varricchio (2001, p. 402-404) as gut contents of a tyrannosaurid.

Prior to these discoveries, plesiosaurs were conspicuous by their absence in the lower Smoky Hill Chalk of western Kansas. Although Williston (1897, p.245-246), Bennett (2000, p. 2) and others have indicated that while most of the vertebrate remains from the chalk now in museum collections are from the upper one-half of the member, the lower chalk contains as many species (Stewart, 1990, p. 29-30). Based on their field experience, earlier collectors also believed that fossils from the upper chalk were more complete and in better condition than those in the lower chalk. This belief has created a bias regarding collecting in the lower chalk even though early workers, such as Professor Benjamin F. Mudge, discovered many specimens (Everhart, 2002a, p. 34-35) from the more eastern (older) exposures. E. D. Cope (1872, 1875) described a number of fish and reptile type specimens from Mudge’s lower chalk collection.

Propodials:

FHSM VP-13962 was recovered as float on a steep slope within 10 m of the base of the Smoky Hill chalk in northwest Ellis County. The specimen represents the proximal half of a propodial that would have been about 24 cm in length if complete. In comparison, the humerus of a 3 m Dolichorhynchops osborni (FHSM VP-404) is 33.6 cm in length (Bonner, 1964, p. 53). According to Moodie (1908, p. 451), the two nutrient foramina (7 mm and 4 mm diameter) located near the middle of the shaft are present only in immature specimens, and "disappear in the adult." The break in the shaft is fresh and probably indicates that the bone was somewhat more complete prior to eroding out of the chalk. LEFT: Fig. 3-1 FHSM VP-13962 - Proximal half of an immature plesiosaur propodial (Late Coniacian, Ellis County).

FHSM VP-13964 includes a fragment of the distal portion of a plesiosaur propodial and an associated piece of the pectoral or pelvic girdle that were discovered as float near Hattin’s (1982) marker unit 5 in southwest Gove County. Both fragments have a corroded exterior appearance but were not sun bleached, and did not appear to be otherwise weathered. The fragment compares favorably in size to the propodials of the two adult specimens of Dolichorhynchops osborni (KUVP 1300 and FHSM VP-404). LEFT: Fig. 3-3 FHSM VP-13964  Fragment of the distal end of a plesiosaur propodial (Late Coniacian, Gove County).

LACMNH 148920 consists of a left femur, two podials, four other small elements, either metapodials or phalanges (J.D. Stewart, pers. comm. 2001). It was collected in north-central Lane County, from just above Hattin’s (1982) marker unit 5. Prior to eroding out, the remains were articulated and the surface of the bones appeared unaltered by digestive processes. Field observations indicated serrated bite marks on the propodial that were attributable to the shark, Squalicorax falcatus. Schwimmer, et al., (1997, p. 79) noted evidence of Squalicorax scavenging on a large variety of marine vertebrates, including Dolichorhynchops sp. (FHSM VP-12059) and Trinacromerum willistoni (KUVP 5070, now T. bentonianum, per Carpenter, 1996, p. 284). LEFT: Fig. 3-6  LACMNH 148920  Field photo of an articulated plesiosaur hind limb, including femur, metapodials and epipodials (Early Santonian, Gove County)

Vertebrae:

FHSM VP-13963 is a single caudal (?) vertebra (3 cm in length by 3 cm in diameter) that was recovered as float near Hattin’s (1982) marker unit 1 in northwest Ellis County. The dorsal process is missing and the surface of the bone appears to be acid-etched. LEFT: FHSM VP-13963  Anterior and posterior views of a single, partially digested plesiosaur caudal vertebra (Late Coniacian, Gove County).

Three small caudal vertebrae (FHSM VP-13965) were discovered as float in western Gove County, from between Hattin’s (1982) marker unit 4 and 5. The measurements of the vertebrae are: 1) 2 cm in length by 3 cm in diameter; 2) 1.7 cm in length by 3 cm in diameter; and 3) 2 cm in length by 2.5 cm in diameter. All appear to have been partially digested. The dorsal processes are missing and the bone surfaces are corroded. The vertebrae were not articulated when collected but all appear to have come from near the end of the tail. LEFT: FHSM VP-13965   Anterior and posterior views of three (3) plesiosaur caudal vertebrae (Ellis County).

Skull:

Scattered elements from the back half of the skull of a single individual (FHSM VP-13966) were collected as float from just above Hattin’s (1982) marker unit 5 in southeastern Gove County. According to Ken Carpenter (pers. comm, 2001), the remains include the suborbital portion of the left maxilla, part of the supraoccipital, a braincase fragment possibly containing a portion of the auditory meatus, a cranial fragment including the parietal/frontal suture, an epipterygoid, the left articular, surangular and angular, the right articular, surangular, and angular, and three dentary fragments. LEFT: FHSM VP-13966  Labial and lingual vies of two partially digested fragments of the lower jaws of a small plesiosaur (Late Coniacian, Gove County).

Many fragmentary, partially digested remains of mosasaurs have been observed by the author from the same Upper Coniacian chalk localities as these plesiosaur remains. These are interpreted as the undigested portions of severed skulls, limbs and tails that were later regurgitated by sharks. In several cases, fragmentary mosasaur remains (FHSM VP-13283, VP-13742 and VP-13746) include embedded tooth fragments from the lamniform shark, Cretoxyrhina mantelli (Everhart, 1999). The partial remains of a shark (C. mantelli, KUVP 68797) that includes more than a hundred gastroliths and suggest predation or scavenging of an elasmosaur are documented from the Smoky Hill Chalk (Moodie 1912, p. 377-378; Shimada, 1997, p. 929; Everhart, 2000, p. 69). In addition, Shimada (1997, p. 927) described a specimen of Cretoxyrhina mantelli (KUVP 247) that contains the remains of a 4 m Xiphactinus audax as gastric contents. In another specimen (ESU 1047), the tip of a broken Cretoxyrhina tooth was embedded in a vertebra associated with the skull of a medium-sized Xiphactinus audax (Shimada and Everhart, 2002, in press). Cretoxyrhina was an apex predator in the Western Interior Sea and elsewhere during Turonian, Coniacian and Santonian time, and reached lengths of 6 m (Shimada, 1997, p. 930) or more. It is likely that they fed on plesiosaurs as well as mosasaurs and large fish.

The isolated, fragmentary plesiosaur remains reported here apparently resulted from feeding activities of large predators, most likely Cretoxyrhina mantelli. Five of the six specimens show a characteristic loss of the surface layer of bone due to corrosion by stomach acids and exhibit damage that is similar to that visible on hadrosaur bones identified as gut contents of a tyrannosaurid by Varricchio (2001). Sub-surface bone structures such as the "vascular canals and intertrabecular spaces have been enlarged, giving the bone a spongy appearance clearly distinct from fresh bone" (Ibid., p. 401-402). These bones are readily distinguishable from the well preserved marine reptile remains usually recovered from the Smoky Hill chalk.

Conclusion

Plesiosaurs were probably present in small numbers in the Western Interior Sea throughout the Late Cretaceous, contrary to the evidence in the fossil record reported by Carpenter (1996). The scarcity of their remains in the  lower (Upper Coniacian and Santonian) Smoky Hill Chalk of the Niobrara Formation may indicate that the open, deeper water of the middle portion of the seaway was not a preferred habitat for plesiosaurs. All of the fragmentary remains reported here are from plesiosaurs that appear to have been dismembered prior to burial, and possibly represent animals that made rare crossings of the seaway or "bloat and float" carcasses that were carried in from other locations by ocean currents. The partially digested condition of most of the remains implicates the lamnid shark, Cretoxyrhina mantelli, as the likely predator / scavenger, although no embedded teeth or bite marks have been associated with these corroded specimens. The serrated bite marks of Squalicorax have also been noted on plesiosaur remains. Further collection of plesiosaur material from this stratigraphic interval will enhance our understanding the paleoecology in the Western Interior Sea during the Late Coniacian.

Acknowledgements

I thank Robert Purdy and Michael Brett-Surman, United States National Museum, Washington, D.C. for access to and assistance in the Smithsonian collections; Larry Martin and Desui Maio, University of Kansas Museum of Natural History, for access to the collections and valuable discussions of Kansas plesiosaurs; Richard Zakrzewski, Fort Hays State University - Sternberg Museum of Natural History, for his support and access to the collections; Ken Carpenter, Denver Museum of Science and Nature, Denver, CO for his identification of the FHSM VP-13966 skull fragments and ongoing discussions of marine fossils; F. Robin O’Keefe, New York College of Osteopathic Medicine, Old Westbury, NY for his insight into the severely damaged remains of USNM 9468; Earl Manning, Tulane University, for access to older references and ongoing discussions of the history of discoveries in Kansas paleontology; and J. D. Stewart, Los Angeles County Museum of Natural History, Los Angeles, CA for the data on LACMNH 148920 and for sharing his knowledge of the Smoky Hill Chalk. The suggestions made by two anonymous reviewers improved the quality of this paper and are greatly appreciated.

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