Plesiosaur Stomach Contents

AUTHORS NOTE: Do not cite or quote from this webpage. This is a early draft of the published manuscript, with additional or updated figures and references. Please refer to: Cicimurri, D. J. and M. J. Everhart, 2001. An elasmosaur with stomach contents and gastroliths from the Pierre Shale (late Cretaceous) of Kansas. Kansas Acad. Sci. Trans 104(3-4):129-143. "Feeding Styxosaurus" (above) is copyright © 2001 by Russell Hawley.

A nearly complete skeleton of an elasmosaurid plesiosaur (NJSM 15435) from the Sharon Springs Member (Middle Campanian) of the Pierre Shale, Logan County, Kansas, is intimately associated with fragmentary fish remains and numerous gastroliths. The fish bones and gastroliths were located just behind the pectoral girdle in the abdominal region. Identifiable prey includes Enchodus and other small clupeomorph fishes. An isolated tooth of the anacoracid shark Squalicorax cf. S. pristodontus was also recovered in this area. Ninety-five gastroliths (6.8 kg) were present, with the largest stone measuring 15.1 x 8.5 x 5.7 cm (5.0 x 3.3 x 2.2 in.) and weighing 1.06 kg (2.3 lb.). Many of the gastroliths are composed of pink or gray Sioux Quartzite, which suggests that the source of these stones (O) was over 600 km (475 mi.) to the northeast of where the elasmosaur remains (X) were discovered. The association of fragmentary fish remains and gastroliths within the abdomen of NJSM 15435 supports the contention that the stones aided in the breakdown of food in plesiosaurs.

Determination of the diet of extinct animals from the fossil record is usually indirect, especially within a marine ecosystem. One can infer dietary preferences by comparing tooth morphology to that of extant animals (Massare, 1987). Shed teeth and cut marks on bone can indicate that certain species of sharks fed upon fish and marine reptile carcasses (Schwimmer et al.; 1997; Shimada, 1997; Everhart, 1999). Some food items may be the result of opportunistic scavenging, and may not necessarily represent the typical prey for those particular sharks (Schwimmer, 1997). Coprolites offer an accurate glimpse into the diet of fossil vertebrates, but it is often difficult to identify the predator taxon (Martin and Bjork, 1987). For that reason, gastric residues or stomach contents associated with fossil skeletons are particularly valuable because both the predator and prey taxa can often be identified.

Occurrences of fossil vertebrates with preserved stomach contents are rare. While gastroliths are often associated with plesiosaur skeletons (Williston, 1904, 1914), only six sets of plesiosaur remains containing gastric residues have previously been reported from the late Cretaceous Pierre Shale (Table 1). The vertebrae, scales, and teeth of six species of fish, including Enchodus, were associated with the type specimen of Elasmosaurus platyurus (ANSP 10081) discovered by Captain Theophilus Turner, an Army physician at Fort Wallace, in the Pierre Shale of Logan County, Kansas in 1867 (Cope, 1868). Cope (1872) reported the remains of a juvenile mosasaur (Clidastes) beneath the vertebral column of a much larger elasmosaur (the type of Plesiosaurus gulo) from the Pierre Shale near McAllaster, Logan County, Kansas (see also Storrs, 1999, for the identification and stratigraphic occurrence of this specimen). Brown (1904) described plesiosaur remains (AMNH 5803) from the Ft. Pierre Formation, at the "head of Hat Creek Basin, 18 mi. southwest of Edgemont, South Dakota" that contained fish vertebrae, broken pterodactyl bones, and Scaphites as stomach contents. Martin and Kennedy (1988) documented the remains of a plesiosaur from the Pierre Shale of South Dakota (SDSM 14267) with stomach contents (teleost fish vertebrae and scales) and gastroliths. A specimen of Trinacromerum bonneri (now Dolichorhynchops osborni) containing teeth of the enchodontid Apateodus as gastric residue, and an elasmosaur associated with gastroliths and macerated fish remains were reported by Nicholls (1988) from the Pembina Member of the Pierre Shale in Manitoba, Canada.

In a general way, records of pre-Campanian plesiosaur food items (Table 1) tend to include mostly cephalopods (squid, belemnite and ammonite) and few or no fish. Cephalopod beaks associated with gastroliths were preserved as stomach contents in a pliosaur specimen from the late Cenomanian of Japan (Sato and Tanabe, 1998; Sato and Storrs, 2000). An elasmosaur skeleton from the early Cenomanian of Nebraska (Thalassomedon haningtoni Welles 1943, UNSM 50132) at the University of Nebraska State Museum includes fragments of ammonite shells mixed with gastroliths in the abdominal region. Tarlo (1959) documented cephalopod hooklets associated with the remains of Pliosaurus brachyspondylus from the late Jurassic Kimmeridge Clay of England. Taylor, et al. (1993) described the dermal scutes of an ornithischian dinosaur associated with another specimen of the pliosaur Pliosaurus brachyspondylus. Wahl (1998) reported the remains of a hybodont shark (teeth, dorsal spines and skin denticles) and coleoid cephalopod hooklets from a late Jurassic plesiosaur skeleton from Wyoming.

Fossilized remains of marine reptiles other than plesiosaurs have also included stomach contents. Martin and Bjork (1987) described the remains of a shark (Cretalamna(?) teeth), a teleost fish (Bananogmius), a marine bird (Hesperornis) and another mosasaur (Clidastes) in the preserved stomach contents of a large Tylosaurus proriger from the Pierre Shale of South Dakota. Camp (1942) reported teleost fish remains associated with a California mosasaur (Plotosaurus tuckeri Camp 1942, CIT 2750). Everhart (in preparation) observed partially digested fish vertebrae (Cimolichthys nepaholica) and scales in the abdominal region of a 10 m (33 ft.) Tylosaurus proriger skeleton (FFHM 1997-10) from the Smoky Hill Chalk Formation, Gove County, Kansas. Pollard (1968) recovered cephalopod hooklets mixed with small quartz grains from ichthyosaur remains discovered in the Lower Lias (early Jurassic) of Lyme Regis, England.

The rarity of such specimens may result from several factors, including: the decay and disarticulation of the carcass prior to burial (Schäfer, 1972; Nicholls, 1988; Schwimmer, 1997; Everhart, 2000); the scavenging of the carcasses by sharks which results in the removal of the visceral elements (Shimada, 1997); or the death of the animal long after the last ingestion of food.

The abbreviations for institutions referred to in the text are as follows: AMNH – American Museum of Natural History, New York; ANSP – Academy of Natural Sciences of Philadelphia; CIT - California Institute of Technology, Pasadena, CA; FFHM – Fick Fossil and History Museum, Oakley, KS; FMNH – Field Museum of Natural History, Chicago, IL; KUVP – University of Kansas Vertebrate Paleontology Collection, Lawrence, KS; NJSM – New Jersey State Museum, Trenton, NJ; NZGS – New Zealand Geological Survey, Lower Hutt, NZ; SDSM South Dakota School of Mines and Technology, Rapid City, SD; SMUSMP – Southern Methodist University, Shuler Museum of Paleontology, Dallas, TX; TMM – Texas Memorial Museum, University of Texas at Austin; UMUT – University Museum, University of Tokyo, Japan; UNSM – University of Nebraska State Museum, Lincoln, NE.

NJSM 15435 was discovered in 1991 by Pete Bussen, a local rancher, from elements of the right front limb eroding from the edge of a gully in an area locally referred to as "Coal Oil Canyon" in western Logan County, Kansas. The plesiosaur was lying along a northeast to southwest axis (Figure 1). Dr. David Parris of the New Jersey State Museum was offered the opportunity to dig the remains as the result of an earlier contact with Mr. Bussen. Staff and volunteers from the NJSM recovered the specimen on three successive digs in 1991-1992. Stratigraphically, the specimen was located below the mid-point of the Sharon Springs Member of the Pierre Shale (Baculites maclearni [mclearni] biozone). The type section of this member is located about 22 km northeast of the excavation, near McAllaster Buttes, and is named for the nearby town of Sharon Springs, Kansas (Elias, 1931). The Sharon Springs Member was deposited in a shallow inland sea that covered much of the central United States and Canada during middle Campanian time (Parrish and Gaultier, 1993). This lithostratigraphic unit is composed of soft, fissile, bituminous shale and contains numerous bentonites and septarian concretions (Gill et al., 1972). Extensive field work in the Sharon Springs Member in South Dakota by the SDSMT has documented a diverse vertebrate assemblage consisting of fish, marine turtles, plesiosaurs, mosasaurs, pterosaurs, and birds (Martin and Bjork, 1987). Although such rigorous surveys have not yet been conducted in the Sharon Springs Member in Kansas, the same groups of vertebrates are also well represented (Carpenter, 1990).

* Postscript: the proper spelling of the species name, contrary to Cobban (1993), is mclearni; from B. mclearni Landes 1940

NJSM 15435 (Figure 1) represents the nearly complete skeleton of a large elasmosaurid plesiosaur that was tentatively identified in the field as "Alzadasaurus snowii." on the basis of the characteristic shape of the pelvic girdle (D. Parris, pers. comm., 1991). Since that time, however, Carpenter (1999) has proposed that Alzadasaurus is a junior synonym of Styxosaurus. Field measurements of the complete and fully articulated left rear limb (1.7 m) and caudal vertebrae (2.0 m) indicate that the plesiosaur was about 10 m (33 ft.) in length and would have weighed over 2,800 kg (Everhart, 2000). The specimen includes nearly complete pectoral and pelvic girdles, most of the vertebral column, ribs and gastralia, and almost all of the limb elements. Unfortunately, surface erosion prior to the discovery of the remains had removed the area of shale that may have contained the anterior-most portion of the neck and the skull. Gastroliths, teleost fish bones, teeth and scales were located in an area of discolored shale between the pectoral and pelvic girdles on the right side of the skeleton. The position of the gastroliths and the ribs, and the location of the pectoral girdle in relation to the right front limb, indicate that NJSM 15435 came to rest on its chest. The right rear limb and pelvic girdle elements were displaced anomalously to the left side of the vertebral column before fossilization. While the distal ends of the right rear limb and left front limb are missing (possibly lost to scavengers), the right front limb appeared to have been essentially complete prior to eroding out of the shale. Although the ribs and gastralia were somewhat scattered within the abdominal region of the elasmosaur, the relative completeness of the skeleton, especially the left rear limb, suggests that the animal settled on the bottom shortly after death and that significant scavenging did not occur. Scavenging may have been inhibited if, as suggested by Parrish and Gaultier (1993), oxygen levels were reduced near the sea floor. Articulated limbs, girdle elements, and vertebrae suggest that currents were absent or insufficient to disturb the remains. The scattering of ribs, gastralia and gastroliths within a relatively small area indicates that the gut may have bloated and burst while the carcass was lying on the bottom. Most of the dorsal vertebrae were enclosed in a series of limestone concretions. The concretions may have formed where they did as the result of carbonate ions being drawn to a locally restricted reducing environment produced by the decay of the gut (E. Manning, pers. comm. 2001).

Conditions were especially favorable within the Sharon Springs Member of the Pierre Shale in Kansas for the secondary formation of selenite. As a result of anoxic bottom conditions during and after burial, pyrite crystals were likely present throughout both the bones and the surrounding clay. They provided the sulfur which, when combined with the calcium from dissolved aragonitic bivalve shells, produced the calcium sulfate crystals (E. Manning, pers. comm., 2001). These crystals can grow within bones and often result in poorly preserved, highly fragmented and fragile fossils. There was substantial degradation of the smaller bones of NJSM 15435 by the growth of selenite crystals. In this instance, even the large, dense propodials were affected.

A fish bone hash was concentrated in a one by two-meter area just behind the pectoral girdle on the right side of NJSM 15435. Gastroliths and gastralia were scattered randomly throughout the bone hash within this area. The gray shale in this region was stained red-orange, a diagenetic artifact possibly related to the decomposition of the carcass. Matrix containing the stomach contents was collected in two plaster jackets in 1992. Disarticulated fish vertebrae, fragments of fish bone and scales, as well as numerous small (1-2 mm) flecks of fish bone were dispersed throughout the discolored matrix (Figure 2). Small pieces of pulverized fish bone or scale adhered to some the gastroliths. Many of the ribs and gastralia were located in a layer under the gastroliths. There were no occurrences of gastroliths beneath this lower layer of bones. Nicholls (1988) reported macerated fish bone associated with several plesiosaur skeletons from the Pembina Member of the Pierre Shale, with fragments ranging in size from 1 mm to 3 cm. According to Nicholls (ibid.), the surface of these bone fragments showed no evidence of chemical corrosion by stomach acids.

Fragments of fish bone were the most common elements recovered from the stomach contents of NJSM 15435 during preparation. Materials that were considered to be diagnostic and were sent to J. D. Stewart for identification. Cranial fragments of small clupeomorph fishes were present and several teeth, vertebrae, and scales of the common Cretaceous teleost Enchodus sp. Agassiz, 1835 were also identified (J. Stewart, pers. comm., 1996). A nearly complete tooth of the shark Squalicorax cf. S. pristodontus was also recovered from this area, as was an isolated, but unidentified, elasmobranch vertebra. Storrs (1981) reported shark teeth associated with the thoracic ribs, gastralia and gastroliths of a Thalassomedon sp. (TMM 42245-1) plesiosaur and concluded that it was unlikely that the shark teeth were part of the stomach contents. As only one shark tooth was recovered with NJSM 15435, it is regarded here only as possible evidence of scavenging.

Enchodus remains are common throughout the Smoky Hill Chalk and Pierre Shale. Based on measurements of a complete Enchodus skeleton (Pete Bussen, private collection, pers. comm., 2000) preserved as stomach contents inside a larger fish (Cimolichthys nepaholica) from the Pierre Shale, a 75 cm (30 in) Enchodus has vertebrae with an average length of 15 mm and diameter of 10 mm. Most of the fish vertebrae associated with NJSM 15435 have a length of about 8 mm and a diameter of about 6 mm, suggesting a prey length of 35 to 45 cm (14 to 18 in).

Lithophagy (stone swallowing) is well documented in many groups of animals (see review in Whittle and Everhart, 2000) including plesiosaurs and dinosaurs. Seeley (1877) reported "about a peck (1/3 cu. ft.) of ovate and rounded pebbles" in the abdominal region of a Mauisaurus gardneri from England. Williston (1904) noted "at least 30 instances" of gastroliths associated with the remains of plesiosaurs from Europe and North America. "Small polished and rounded pebbles" were reported by Ji, et al. (1998) in the crop of a feathered dinosaur, Caudipteryx zoui. from China. Taylor (1999) concluded that Caudipteryx was insectivorous and used gastroliths to grind up the hard exoskeletons of its prey. Kobayashi, et al. (1999) documented masses of small siliceous grains in the crops of 12 associated ornithomimid dinosaur skeletons. While the function of gastroliths in plesiosaurs is still uncertain, it is probable that the grains served a similar function in the digestive process of these dinosaurs as the grit in the crops of some modern birds.

Many of the NJSM 15435 gastroliths are pink quartzite. Other materials include gray quartzite, quartz, chert, and sandstone. Williston (1893) reported red quartzite gastroliths from a large Kansas plesiosaur from Ellsworth County, and concluded that they were lithologically identical to Sioux Quartzite outcroppings in northwestern Iowa. Welles and Bump (1949) described gastroliths associated with the type specimen of Alzadasaurus pembertoni (SDSM 451), now Styxosaurus snowii, as being composed of pink and gray quartzite, siltstone, sandstone, and flint. The lithology of the gastroliths from NJSM 15435 appears to be similar to that of SDSM 451, and it is possible that these plesiosaurs acquired their stones from the same general geographic areas. Based on the presence of pink and gray quartzite, the source of some of the stones may have been from islands or other localities along the eastern shore of the Western Interior Sea where Precambrian quartzites were exposed during the time of deposition of the Sharon Springs Member (Bretz, 1987). The gastroliths reported by Darby and Ojakangas (1980) from the middle Campanian Bearpaw Shale were composed of gray quartzite, chert, granite, basalt and other volcanics, and silicified shale, which suggests different source areas than for NJSM 15435 and SDSM 451. Riggs (1939) reported that all of the gastroliths in the specimen he studied from the Fort Benton Shale of Montana were composed of granite. Most of the gastroliths in TMM 42245-1 (Storrs, 1981) from the early Turonian of Texas were composed of a dark chert, with a few others identified as siltstone and quartzite.

Long/intermediate/short axis measurements were taken with dial calipers. The largest gastrolith from NJSM 15435 measures 151 x 85.4 x 57.4 mm, and the smallest 8.6 x 6.6 x 5.9 mm. The average size of the gastroliths was about 30 x 25 x 15 mm. The measurements of the intermediate and short axes of the largest gastrolith (85.4 x 57.4 mm) of NJSM 15435 may provide an idea of the cross-sectional area of the largest prey that could have been ingested by this elasmosaur. Shuler (1950) noted that a similar sized specimen of Libonectes morgani would have only been able to swallow a fish with a cross section no larger than 13 cm (5 in) because on the narrow width and inflexible articulation (fused symphysis) of the lower jaws. According to Massare (1987), the maximum skull width of many plesiosaurids was < 10 cm (4 in.). The inside of the reconstructed lower jaw of Thalassomedon haningtoni (DMNH 1588) at the Denver Museum measures about 15 cm (6 inches) wide where it articulates with the quadrates. Despite the huge size of their bodies, the relatively small skull size and slender, seizing teeth suggests that food items for elasmosaurs were limited to small diameter fish and soft-bodied invertebrates.

Although the shape of the gastroliths of NJSM 15435 varies, they can be categorized into three general types: disks (near equal long and intermediate-axis length, shorter short-axis length), spheres (near equal long/intermediate/short-axis lengths), and cylinders (greater long-axis length, near equal intermediate and short axis length). The majority of the stones are well rounded, with smooth surfaces, and even those with flat sides have rounded corners. They also have a thin exterior coating of iron oxide-stained clay that tends to obscure the composition of the stone and the degree of polish. The gastroliths composed of quartzite have a dull pitted texture, whereas those of chert are highly polished. Those identified as quartz are dull and smooth. These observations appear to correlate well with those of other authors (Welles and Bump, 1949; Darby and Ojakangas, 1980; Storrs, 1981; Wiffen and Moisley, 1986). The cause of the polishing has usually been attributed to grinding against each other in the stomach or crop of the animal (Shuler, 1950; Gillette, 1990).

The function of gastroliths in plesiosaurs has been and continues to be a controversial topic. Benjamin F. Mudge (1878), the first State Geologist of Kansas, noted that "in the Plesiosauri, we found another interesting feature, showing an aid to digestion similar to many living reptiles and some birds. This consisted of well worn siliceous pebbles, from one-fourth to one-half inch in diameter. They were the more curious, as we never found such pebbles in the chalk or shales of the Niobrara." Because plesiosaurs probably swallowed their prey whole, these stones may have been utilized in the mechanical breakdown of food (Brown, 1904; Andrews, 1910; Williston, 1914; Shuler, 1950; Martin and Kennedy, 1988). In contrast, the fossilized remains of mosasaurs regurgitated by large sharks (Everhart, 1999), predatory fish preserved with intact prey such as the famous "fish in a fish" Xiphactinus specimen (FHSM VP-333) with a Gillicus (FHSM VP-334) inside at the Sternberg Museum of Natural History, and a mosasaur (Martin and Bjork, 1987) with preserved stomach contents from the Western Interior Sea all suggest that prey, or parts of prey, swallowed whole tends to remain more or less articulated during the early stages of digestion in animals without gastroliths.

Cott (1961) concluded that extant crocodiles use stones for buoyancy regulation, and Denton et al. (1997) proposed that this was the case in the extinct Cretaceous/ Paleocene marine crocodile Hyposaurus. Many authors have suggested that plesiosaurs used gastroliths to adjust buoyancy (Williston, 1893; Darby and Ojakangas, 1980; Taylor, 1981; Storrs, 1981). Currie (1981) demonstrated that the weight of the gastroliths in the lower abdomen of the aquatic eosuchian, Hovasaurus boulie, would have lowered the center of gravity, increased the specific gravity of the animal, and aided in maintaining an upright orientation for swimming. Carroll (1985) proposed that the use of gastroliths and heavy gastralia for buoyancy control in plesiosaurs was one of several adaptations made by diapsid reptiles returning to life in the ocean. Reiss and Frey (1991) reviewed the evolution of underwater flight in a variety of animals and concluded that ballasting would not have been required for underwater flight in plesiosaurs. Storrs (1993) suggested that plesiosaur gastroliths functioned in a dual role of helping to maintain a neutrally buoyant position in the water column while also serving as a grinding aid to digestion. Sander et al. (1997) suggested that a mass of sand grains discovered in the abdomen of a pistosaurid sauropterygian from Triassic marine sediments in Nevada was used for buoyancy control.

Taylor (1993, 1994) discussed the physics of buoyancy in marine animals, and proposed that a relatively small mass of these stones would be sufficient to influence the position of the animal in the water column. Taylor (1993) also noted that gastroliths can be quickly swallowed or vomited by some modern marine mammals to adjust buoyancy as needed. It is doubtful, however, that this would have been a useful strategy for an elasmosaur with a 5-6 m (16-20 ft.) long neck, especially one that was living and feeding hundreds of kilometers from the nearest sources of such stones. Lingham-Soliar (2000) suggested that gastroliths may have been used more to adjust the longitudinal balance or center of gravity of the plesiosaur rather than simply reducing overall buoyancy. In the case of NJSM 15435, the effect of 6.8 kg of stones as ballast on the buoyancy of a marine reptile weighing several thousand kilograms is considered to have been negligible.

Ninety-five gastroliths (6.8 kg) were recovered from within the abdominal region of a large elasmosaurid plesiosaur (NJSM 15435) in direct contact with matrix containing whole, fragmentary, and finely ground teleost fish bones, teeth and scales. The stomach contents of NJSM 15435 indicate a piscivorous diet, with Enchodus and other small (less than 50 cm / 1.6 feet) clupeomorph fishes being the main food sources. The location of the gastroliths within the abdominal region of NJSM 15435 and their intimate association with the fragmentary remains of small fish supports the hypothesis that these stones aided the plesiosaur in the breakdown of food. Whether or not this was their primary function or was secondarily related to buoyancy control remains a matter of debate.


Matrix Sample 2 (2 gastroliths)	Matrix Sample 2 with additional gastroliths	Matrix Sample 2- Close up of the same three gastroliths
		Three samples of matrix containing gastroliths and fish bone 'hash' associated with a plesiosaur from the Pierre Shale of Kansas (scale = mm)
Matrix Sample 1 (2 gastroliths)	Matrix Sample 3 (2 gastroliths lying on top of gastralia). A 3rd gastrolith is underneath

We thank Jerome (Pete) Bussen of Wallace, Kansas for donating this unique specimen to the New Jersey State Museum. This paper would have not been possible without the collective help of the many volunteers who participated in the digs. The authors also thank David Parris, Bill Gallagher, and Barbara Grandstaff of the New Jersey State Museum for the opportunity to prepare and describe the stomach contents, and for their critical review of the initial manuscript. J. Calcagni provided assistance in preparing the stomach contents. Charlotte Holton (American Museum of Natural History) provided unpublished locality data for the Barnum Brown (1904) specimen. L. F. Noè (University of Derby, Derby, England) provided information on stomach contents and gastroliths associated with English plesiosaurs. Comments provided by Christian Cicimurri, Philip Currie (Royal Tyrrell Museum of Paleontology, Earl Manning (Tulane Univ., New Orleans), Glenn Storrs (University of Cincinnati), and Richard Zakrzewski (Sternberg Museum, Fort Hays State University) greatly improved upon the final version of the manuscript. Special thanks are also extended to J. D. Stewart of the Los Angeles County Museum of Natural History for his identification of the fish remains.

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