Where the elasmosaurs roam: Separating fact from fiction Original article published as: Everhart, M. J. 2002. Where the Elasmosaurs roam...... Prehistoric Times 53:24-27 Copyright © 2002-2009 by Mike Everhart; Last revised 09/18/2009 LEFT: Elasmosaurus adapted from a painting by Doug Henderson. Copyright © Doug Henderson; used with permission of Doug Henderson

Read more about KANSAS PLESIOSAURS

The warm, silt-laden water felt strange to the young, long-necked plesiosaur as he slowly worked his way upstream against the strong current of the river. His streamlined body seemed less buoyant in the fresh water and that made raising his head above water to breathe a bit more difficult. Unlike the ocean that was his home, he could not see very far in the murky water, so he moved cautiously along the sandy bottom, trying to stay in the deeper reaches of the main channel. Dimly he remembered being here several times before with a much larger member of his species. At the time, it was all he could do to stay close to her body and not be swept away by the current. Now sensory cells in his nasal passages ‘tasted’ strange new smells as the water passed in through his nose and exited his slightly opened mouth. Although he was hungry after his long journey, his instincts drove him up this river for something else.

Within a mile from where the river entered the ocean, the sand bottom of the river had turned to coarser gravel. The plesiosaur lowered his head and began to probe the gravel with his lower jaw. Clouds of silt and other debris rose wherever his chin touched the bottom. The sensations received through his lower jaw told him that this was not what he was looking for and he began swimming again. Holding his head and neck stiffly in front of his body, he relied on alternating beats of his wing-shaped front and rear flippers to provide the thrust that he needed to ‘fly’ forward through the water.

The river narrowed as he swam upstream and the current became faster. Swimming against a current was not something that he was used to doing. Such currents were hardly ever found in the calm, shallow ocean where he had spent almost all of his life. Within a short distance, he began to encounter large rocks that formed eddies in the current. It was behind a rock larger than himself that he finally found what he was looking for. Small round stones had collected on the downstream side of the boulder. Twisting his head slightly to the side, he was able to see that there were many more here than he would need. Slowly and delicately, he picked up one smooth rock after the other with his long, slender teeth. With each, he raised his head slightly and swallowed, feeling the hard stone move into his throat and start the slow trip down his 15 foot long esophagus to his stomach. He continued selecting and swallowing the round stones until he had satisfied his need. Raising his head above the surface briefly to breathe, his sharp underwater vision saw only a dark green blur that was the dense forest that crowded over the riverbank. Again he submerged, then turned and began his return journey back to the sea.

His movement flushed a small fish from its hiding place near the large rock. Before the fish could flee, the plesiosaur reacted and trapped it between jaws filled with long, interlocking teeth. Raising his head just above the surface, he swallowed his prey headfirst, then exhaled again and refilled his lungs. Vaguely satisfied, he submerged his head and began to swim downstream with the current with slow, deep strokes of his paddles.

Although isolated fragments of plesiosaurs had been described from North America earlier in the 19th Century by Joseph Leidy and others, the remains discovered by Dr. Theophilus H. Turner (1841-1869) near Fort Wallace in western Kansas in 1867 represented the first nearly complete specimen and the first known elasmosaur. Dr. Turner was an Army surgeon assigned to Fort Wallace. He found the remains of the huge animal while exploring exposures of the Pierre Shale along the right-of-way of the approaching Union Pacific Railroad near McAllaster Butte in present-day Logan County, Kansas. Dr. Turner gave two of the vertebrae of his "extinct monster" to John LeConte to take back for further examination in "eastern scientific institution." At the time, Dr. LeConte was a member of the group surveying an extension of the Union Pacific Railway. Upon completion of the survey, he returned to Philadelphia and took the vertebrae to E. D. Cope at the Academy of Natural Sciences in Philadelphia.

After receiving the shipment in March, 1868, Cope hurriedly assembled the bones into a strange and hereto unknown extinct animal which he named Elasmosaurus platyurus or roughly translated, "Flat-tailed plate-reptile." As was an accepted method of the times, Cope rushed the news of the discovery and a description into publication in a popular scientific journal without review by his peers. Unfortunately, in his haste, and following some of the early work on American plesiosaurs by his mentor, Joseph Leidy, Cope had placed the head of the plesiosaur on the wrong end. Contrary to popular belief, however, it was not Cope’s rival, O. C. Marsh, who discovered the mistake. It was Joseph Leidy himself, who announced two years later at another meeting of the Academy that Cope’s reconstruction was wrong and that his own, earlier work on plesiosaurs was also in error. Cope tried to minimize the damage by offering to buy back all copies of the journal but it was too late. In spite of Cope’s efforts and the immediate publication of a partially corrected description, O. C. Marsh would later make good use of the opportunity to embarrass his rival. This incident was probably the opening battle in the 19^th Century American paleontology conflict that later became known as "The Bone Wars".

The first nearly complete plesiosaur was discovered in the Jurassic rocks of Lyme Regis, England by Mary Anning in the winter of 1820-21. The name was given by the Reverend William Conybeare and means "near-reptile", a reference to the view that plesiosaurs were closer to reptiles than were the ichthyosaurs. Several complete specimens were already in European museums by the time that the American West was being settled. Some of these plesiosaurs like Plesiosaurus and Cryptoclidus had relatively long necks and small heads, and looked very much like their Cretaceous cousins. However, there is some evidence that many of these early plesiosaur lineages became extinct at the end of the Jurassic. Whatever the case, the numbers and diversity of plesiosaurs appears to have declined steadily through the Cretaceous.

Elasmosaurus platyurus was the first of several species of the very long-necked plesiosaurs (elasmosaurs) to be discovered and described from the late Cretaceous deposits in western Kansas, the Midwest and elsewhere in North America. Eventually, elasmosaurs would be found in Cretaceous marine deposits of almost every continent, including Australia and New Zealand, and even Antarctica. From where most of their fossils have been found, it appears that they preferred cooler waters found at higher latitudes to those of warmer equatorial climates. In North America, they are found more often in the Cretaceous deposits of Canada than in the central United States, and are extremely rare in the Gulf area.

Plesiosaurs are fascinating creatures, both for what we know and don’t know about them. By the Late Cretaceous, they had evolved into two distinct, but closely related groups that are generally referred to as short-necked (polycotylids) and long-necked (elasmosaurs). The short-necked variety were fairly small, reaching lengths of no more than 12-15 feet. They had long narrow jaws filled with slender, seizing teeth. It is likely that they were capable of swimming rapidly in pursuit of the small fish and cephalopods that were their prey. Elasmosaurs, on the other hand, had somewhat broader but much shorter heads on extremely long necks. They reached lengths of 45 feet or so, and weighed several tons as adults. They were probably slow moving, and not much of a threat to anything larger than a small fish. Their size, weight, and highly modified limbs probably meant that they could not have crawled ashore to lay eggs.

For various reasons, elasmosaurs seem to have captured the imagination of almost anyone interested in ancient creatures. In spite of their extinction more than 65 million years ago, their mystique lives on in Nessie, Champ and other "sea- and lake-serpents" around the world, basking shark carcasses netted by Japanese fishing boats and other unexplained sightings. All that really remains of them, however, are their bones. In the following text, I will try to put some flesh on these old bones.

Plesiosaurs have been described as "a snake drawn through the shell of a turtle"(*). While this is an exaggeration of sorts, the tear-drop shaped body of an elasmosaur was almost totally enclosed by long ribs, large pectoral and pelvic girdles and a continuous series of gastralia across the belly. A long neck was attached to the front and four wing-like paddles projected at nearly right angles. In one species (Elasmosaurus platyurus), the neck had seventy or more vertebrae and was more than half the length of the body. Since their discovery, this unusually long neck has produced a number of incorrect descriptions of its "snake-like" flexibility.

Many paleo-artists, including Charles Knight (Above), have drawn popular but anatomically impossible representations of elasmosaurs based on these early, fanciful descriptions. These ideas still persist in spite of publications by Williston (1914) and others that have shown that the cervical vertebrae of elasmosaurs had limited movement up and down and only slightly more in a horizontal plane There are good reasons for this limited flexibility which are discussed below.

Still another problem that becomes evident with the long neck of elasmosaurs is what happens when the head is moved to one side or the other while the animal is moving. Sitting twenty feet or more in front of the flippers, the head (and neck) would act as a highly effective rudder whenever it was moved away from the longitudinal axis of the animal. In other words, moving the head to the right while swimming would cause the whole body of the plesiosaur to turn in that direction. While it is possible that plesiosaurs used their head as a rudder to change directions while swimming, it would also be nearly impossible for them to swim in a straight line with their heads darting from side to side, or up and down, in search of prey as shown in many reconstructions. In any case, it seems likely that the tiny brain of an elasmosaur would have been very busy trying to maintain the position of the head during stationary feeding activities through fine movements of the large paddles located up to twenty-five and thirty-five feet to the rear. No disrespect for the mental capacity of plesiosaurs is intended here since plesiosaurs, and elasmosaurs in particular, were very successful predators for millions of years, even if we brainy humans don’t yet understand how they did it.

Thalassomedon haningtoni from the late Cretaceous (Turonian) Graneros Shale of Colorado and Nebraska, and Elasmosaurus platyurus from the Campanian Pierre Shale of Kansas were the longest of the elasmosaurs presently known: probably up to 14 meters or 45 feet in length. The length of the neck of elasmosaurs appears to have increased through time, conveying some adaptive advantage for this group that we do not fully understand. While other large marine predators of the time, including their short-necked, polycotylid cousins, snake-like mosasaurs, giant fish such as Xiphactinus, and huge Ginsu sharks (Cretoxyrhina mantelli), used speed to chase down or ambush their prey, it is more likely that elasmosaurs used stealth to stalk the schools of small fish that were their primary food source. A long neck would allow the large body of the plesiosaur to remain concealed far below a school of fish while the small head moved slowly at the end of its long neck to approach the unlucky victim from below. Having their eyes on top of their head, directed generally forward and upward, as is the case in most elasmosaurs, seems to support this method of feeding. The eyes were not unusually large but may have been capable of stereoscopic vision, something that would be useful for accurately locating small prey. It is likely that the head was moved fairly close to the prey before striking. Precise and rapid coordination between the eyes and the elasmosaurs paddles would have been essential for successful hunting of highly mobile fish and other marine life. Hunting from below may have also been used as a strategy to silhouette the prey of the elasmosaur against the sunlit surface while using the darker, deeper waters for concealment.

We know very little about the other senses of elasmosaurs. Some authors have suggested that the nostrils of plesiosaurs were well positioned to passively direct a continuous flow of water through the nasal cavity where the senses of taste and smell may have been useful in detecting prey. Williston notes that the inner ear (semi-circular canals) of plesiosaurs was large and well developed. This area serves to maintain equilibrium and coordinate muscular control and infers that plesiosaurs were probably quite graceful in their movements. While there is no evidence of external ears in plesiosaurs, they may have evolved a sensory line system that was used to locate prey from the vibrations generated by their movement. It appears likely, however, that they relied primarily on their eyesight to find food.

Evidence from stomach contents preserved in plesiosaur remains indicates that the prey that they ate was limited to small sized fish and invertebrates. Earlier plesiosaurs from the Jurassic and early Cretaceous appear to have preferred squid and other cephalopods, while the later and longer necked elasmosaurs fed on small fish such as Enchodus. While elasmosaurs were huge by any measure, their heads remained relatively small in relation to the size of their bodies A forty foot elasmosaur had a skull length of about 20 inches or about 4% of the total length of the animal. Unlike those of most mosasaurs, plesiosaur skulls were rigidly constructed. This meant that the cross-sectional size of the prey they could swallow was limited by the fixed distance between the hinge points (quadrates) of their jaws at the back of the skull. Even in the largest of the elasmosaurs this distance was no more than about 6-8 inches, and the prey that they fed on was probably no longer than about 18 inches in length. Their slender, interlocking teeth also appear to argue for trapping small prey. In all cases, plesiosaur teeth appear to have been used to seize small fish or invertebrates, and not to tear flesh or otherwise dismember larger prey. Once seized, the prey would have to be swallowed whole.

Because of their unusual body-plan, elasmosaurs were most likely slow moving animals, using their paddles as wings or foils to generate the lift necessary to ‘fly’ through the water. This type of swimming is more efficient than the rowing or paddling (i.e., a duck, Grebe or other diving bird) methods that are sometimes depicted. While modern underwater "fliers" such as penguins use only their front limbs, most researchers now believe that plesiosaurs used a coordinated movement of both pairs of limbs. Others think that the hind limbs did not have either the range of movement or the musculature necessary to effectively generate the lifting forces necessary for this method of swimming. Since the hind limbs are located well behind the center of gravity, they may have been more effectively used for steering. In that case, the rear limbs may have been rudders, paddles and/or stabilizers, especially to control the position of the body and head while remaining relatively motionless in the water during feeding.

The finger and toe bones (phalanges and tarsals) were hour-glass shaped. They fitted closely together with the bones of the adjoining fingers and were probably held tightly within a sheath of muscles, tendons and ligaments. Although plesiosaurs have five ‘fingers’, they exhibit a condition called hyperphalangy which means that they have many more individual finger bones than usual (humans have 4 bones in each finger; plesiosaurs had as many as 24!). The net effect of the compacted lower arm / leg and wrist / ankle structures, and the extra finger / toe bones was to modify what had been a walking limb into a long, tapering paddle that had limited flexibility. In cross-section, the distal portion of the limb was thicker on the leading edge than the trailing edge. It had literally become a wing or hydrofoil for flying through the water. Movement of the paddles in a manner similar to that of a bird’s wing in flight created lift and pulled the animal through the water. Unlike birds, however, the wrist was ‘locked’ and the paddles could not be folded or otherwise flexed.

Although plesiosaurs evolved from terrestrial reptiles with four legs, the limbs were highly modified into paddles, and lost most of the mobility of the elbow / knee, wrist / ankle and fingers / toes. The bones of the wrist became specialized to the point that they have had to be given different names (no longer carpals or tarsals). LEFT: A lateral (dorsal) view of the left front and rear limbs of Styxosaurus snowii (SDSMT 451) as adapted from Welles and Bump (1949, pl. 5)  Abbreviations:  Ra = Radius; Ul = Ulna; Ti = Tibia; Fi = Fibula; Re = Radiale; In = Intermedium; Ue = Ulnare; Te = Tibiale; Fe = Fibulare.

One other peculiarity that has been noted among long necked plesiosaurs since their discovery are the masses of gastroliths or stomach stones that are commonly found associated with their remains. These rounded and sometimes highly polished rocks are usually located inside the abdomen of complete specimens. There have been as many as several hundred stones of various sizes, from egg-sized down to pea-sized or smaller found associated with plesiosaurs. In the case of a recently discovered specimen from Antarctica, there were thousands of small quartzite pebbles (half-inch or less) in the stomach area. Typically, most gastroliths are hard rocks such as quartz, quartzite, or chert. Occasionally they are granite, basalt or other volcanics. Until recently, the largest gastroliths have been about as big as a tennis ball, but in two Kansas specimens found in the early 1990s, they were much larger. The largest stones in a University of Kansas specimen weighed about three pounds each and were nearly 4 inches in diameter (softball size!). These are the largest individual sizes and the greatest total weight of gastroliths ever documented from a plesiosaur or from any animal. The total weight of the stomach stones from this plesiosaur was about 30 pounds but considering the weight of the animal (about 3 tons), their relative weight is rather insignificant and they probably represent much less than weight of the daily food intake. More commonly, the total weight of gastroliths documented from individual plesiosaurs is less than ten pounds. It appears highly unlikely that these stones would have been effective in changing the buoyancy of large animals that had to consume a lot of food daily in order to sustain themselves while living in mid-ocean. Even the buoyancy changes caused by inhalation and exhalation through a 25-foot long trachea would have been more than could have been offset by the small amount of weight represented by the gastroliths.

This leads us to the question of what gastroliths were actually used for? While early fossil collectors such as Benjamin Mudge concluded from plesiosaurs found in Kansas in the 1870s that the stomach stones were used as "an aid to digestion similar to many living reptiles and birds," Samuel Williston and others initially believed that they were swallowed for other reasons, including a food craving or being mistaken for prey. Currently, most researchers believe that they were used in some manner for buoyancy control or for adjusting longitudinal balance. However, a nearly complete specimen of Styxosaurus snowii recently discovered in the Pierre Shale of western Kansas provides a rather convincing counter-argument. These remains included about a hundred gastroliths, fish bones, scales and teeth co-mingled in the stomach area. Most of the fish parts appeared to have been ground up into very small (0.1 in. or less) fragments. This suggests that whatever use they may have had for buoyancy control, they were definitely useful as an aid to digestion.

So where did these gastroliths come from? Mudge noted that the Kansas gastroliths "were the more curious, as we never found such pebbles in the chalk or shales of the Niobrara." Williston noted that some of the quartzite stones were similar to boulders found in northwestern Iowa or in the Black Hills of South Dakota. Both of these potential sources are 400 to 500 miles from where the plesiosaur remains that contained them were found. Whatever their origin, they certainly did not come from the soft mud bottom of the Western Interior Sea that covered Kansas. This in itself implies that the plesiosaurs had to travel long distances to the sources of these stones. The fact the stones are always rounded and smooth also means that the stones were eventually worn down to sizes small enough to pass though from the crop or stomach into the gut and had to be replaced periodically throughout the life of the plesiosaur.

A final subject is the issue of plesiosaurs crawling ashore to lay their eggs. It has been long assumed that because they are reptiles, they must have laid eggs. Consequently, plesiosaurs sunning themselves on the beach have been the subject of imaginative works by paleo-artists for more than 150 years. While almost all reptiles do lay eggs, it is highly unlikely that large marine reptiles could have reproduced in that manner. Turtles appear to be the only group of marine reptiles that still come ashore to lay their eggs. Unlike turtles, the limbs of plesiosaurs were so modified for use in underwater flying that they would have been too rigid to be used effectively for movement on land, much less for scooping out a nest for eggs in beach sand. In addition, the length and weight of an elasmosaur’s long neck would have effectively counter balanced the back half of the animal, lifting it off the ground and probably making the rear flippers useless for movement on land, or anything else, under the best of circumstances.

Ichthyosaurs have long been known to have given live birth to their young as evidenced by well preserved remains in the Holzmaden Shale of Germany. A mosasaur (Plioplatecarpus) specimen found recently in the Pierre Shale of South Dakota included the remains of several young of the same species in the pelvic region. There is also at least one specimen of a short-necked plesiosaur (Dolichorhynchops osborni) known with fetal material in the abdomen. It appears likely that live birth is one of the necessary adaptations that egg-laying reptiles had to make in order to successfully return to life in the ocean. If so, this raises additional questions as to how marine reptiles nurtured a fetus inside the mother’s body for a relatively long period of gestation. Did they simply retain the developing embryos and yolk sacs inside the mother’s body until they were ready to hatch as do some modern snakes? Or did had they developed some other method of nurturing baby plesiosaurs? We may never know. It is apparent that similarly-sized dinosaurs laid relatively small eggs which were limited in size by the ability of the porous shell to exchange oxygen and carbon dioxide for the developing fetus while supporting the weight of the egg. Plesiosaurs were highly successful animals that clearly had to have evolved some efficient method for reproduction in the marine environment.

Giving live birth to their young probably also meant that they provided some form of parental care. Turtles and other reptiles which lay many eggs at one time depend on safety in numbers for survival of a few of their young. Animals which invest their energy in birthing a few larger babies generally provide some sort of protection for those young as a means of improving their chances to survive to adulthood. Like some dinosaurs, plesiosaurs may have traveled together in small groups for that purpose. The survival rate for small, young animals in the middle of an ocean populated with giant predatory fish, Great White-sized Ginsu (Cretoxyrhina) sharks, and 30 to 50 foot long, hungry mosasaurs would have otherwise been close to zero. As it was, plesiosaurs may have been driven to the edge of extinction by the "mosasaur explosion" during the late Cretaceous. We do know from the bite marks on plesiosaur bones that their carcasses were scavenged by sharks, and that readily detachable parts, such a limbs, tails, heads and necks, were often carried away. The remains of a very large Cretoxyrhina shark at the University of Kansas includes more than a hundred polished gastroliths that most likely were the result of the shark scavenging on the carcass of a plesiosaur.

For all of their size and amazing adaptations to life in the ocean, it appears that plesiosaurs were an evolutionary dead end and were on their way out when the Cretaceous Period ended. Their greatest diversity and numbers probably occurred during the Jurassic or early Cretaceous. By early Maastrichtian time (78 mya) there were only a few species each of elasmosaurs and short-necked plesiosaurs left in Earth’s oceans. Like the ichthyosaur fish-lizards that became extinct in the early Cretaceous, plesiosaurs were probably losers of an evolutionary arms race for the same prey that first saw the rise of larger, faster bony fish and sharks as competitors during the early Cretaceous and then the explosive entry of the highly adaptable mosasaurs in the Late Cretaceous. Not only were they probably out-completed for food by larger and more aggressive mosasaurs, in at least one known case, they were also mosasaur food.

References and further reading:

Almy, K. J. 1987. Thof’s dragon and the letters of Capt. Theophilus Turner, M.D., U.S. Army, Kansas History Magazine, 10(3):170-200.  (Background on the discovery of Elasmosaurus platyurus, Cope’s "head on the wrong end" plesiosaur - historical)

Brown, B. 1904. Stomach stones and the food of plesiosaurs. Science, 20(501):184-185. (Gastroliths mixed with stomach contents in plesiosaurs)

Carpenter, K. 1994. Comparative cranial anatomy of two North American plesiosaurs (Reptilia: Sauropterygia) and a review of some North American elasmosaurs, unpublished draft.

Carpenter, K. 1994, A review of some short-necked plesiosaurs from the Cretaceous of North America, unpublished draft.

Carpenter, K. 1996. A review of  short-necked plesiosaurs from the Cretaceous of the Western Interior, North America, N. Jb. Geol. Palaont. Abh., (Stuttgart), 201(2):259-287.

Carpenter, K. 1997. Comparative cranial anatomy of two North American Cretaceous plesiosaurs, In Calloway J. M. and Nicholls, E. L., eds, Ancient Marine Reptiles, Academic Press, pp. 191-216

Carpenter, K. 1999. Revision of North American Elasmosaurs from the Cretaceous of the Western Interior, Paludicola, 2(2):148-173.

Cicimurri, D. J. and M. J. Everhart. 2001. An elasmosaur with stomach contents and gastroliths from the Pierre Shale (late Cretaceous) of
Kansas. Kansas Academy of Science, Transactions 104(3-4):129-143.

Cope, E. D. 1868. (A resolution thanking Dr. Theophilus Turner for his donation of the skeleton of Elasmosaurus platyurus). Proceedings of the Academy of Natural Sciences of Philadelphia 20:314.

Cope, E. D. 1868. On the genus Laelaps. American Journal of Science ser. 2, 46(138):415-417.

Cope, E. D. 1870. On Elasmosaurus platyurus Cope. American Journal of Science ser. 2, 50(148):140-141.

Cope, E. D. 1870. Additional note on Elasmosaurus. American Journal of Science ser. 2, 50(149):268-269.

Cope, E. D. 1870. Extinct Batrachia, Reptilia and Aves of North America. Transactions of the American Philosophical Society. New Series 14:1-253.

Darby, D. G. and R. W. Ojakangas. 1980. Gastroliths from an Upper Cretaceous plesiosaur, Journal of Paleontology, 54(3):548-556.

Everhart, M. J. 2000. Gastroliths associated with plesiosaur remains in the Sharon Springs Member of the Pierre Shale (late Cretaceous), Western Kansas. Kansas Academy of Science, Transactions 103(1-2):58-69.

Everhart, M. J. 2003.  First records of plesiosaur remains in the lower Smoky Hill Chalk Member (Upper Coniacian) of the Niobrara Formation in western Kansas. Kansas Academy of Science, Transactions 106(3-4):139-148.

Everhart, M. J.  2004. New data regarding the skull of Dolichorhynchops osborni (Plesiosauroidea: Polycotylidae) from rediscovered photos of the Harvard Museum of Comparative Zoology specimen. Paludicola 4(3):74-80.

Everhart, M. J. 2004. Plesiosaurs as the food of mosasaurs; new data on the stomach contents of a Tylosaurus proriger (Squamata; Mosasauridae) from the  Niobrara Formation of western Kansas. The Mosasaur 7:41-46.

Everhart, M. J. 2005. Bite marks on an elasmosaur (Sauropterygia; Plesiosauria) paddle from the Niobrara Chalk (Upper Cretaceous) as probable evidence of feeding by the lamniform shark, Cretoxyrhina mantelli. PalArch, Vertebrate paleontology 2(2): 14-24.

Everhart, M. J. 2006. The occurrence of elasmosaurids (Reptilia: Plesiosauria) in the Niobrara Chalk of Western Kansas. Paludicola 5(4):170-183.

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Leidy, J. 1870. (Remarks on Elasmosaurus platyurus). Proceedings of the Academy of Natural Sciences of Philadelphia 22:9-10. (for meeting of March 8, 1870)

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Martin, J. E. and L. E. Kennedy. 1988. A plesiosaur from the late Cretaceous (Campanian) Pierre Shale of South Dakota: A Preliminary Report. Proc. S.D. Acad. Sci., 67:76-79

Riggs, E. S. 1939. A specimen of Elasmosaurus serpentinus. Field Museum of Natural History, Geology 6:385-391.

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Schmeisser, R.L. and Gillette, D.D. 2009. Unusual occurrence of gastroliths in a polycotylid plesiosaur from the Upper Cretaceous Tropic Shale, southern Utah. PALAIOS 2009 24: 453-459.

Schumacher, B. A. and M. J. Everhart. 2004. A new assessment of plesiosaurs from the old Fort Benton Group, Central Kansas. Abstracts of oral presentations and posters, Joint Annual Meeting of the Kansas and Missouri Academies of Science, p. 50.

Storrs, G. W. 1981. A review of occurrences of the Plesiosauria (Reptilia: Sauropterygia) in Texas with Description of New Material. Masters Thesis, The University of Texas at Austin, 226 pages.

Storrs, G. W. 1984. Elasmosaurus platyurus and a page from the Cope-Marsh war. Discovery 17(2):25-27.

Storrs, G. W. 1997.  Morphological and taxonomic clarification of the genus Plesiosaurus. pp. 145-190, In Calloway, J. M. and Nicholls, E. L., eds, Ancient Marine Reptiles, Academic Press.

Storrs, G. W. 1999. An examination of Plesiosauria (Diapsida: Sauropterygia) from the Niobrara Chalk (upper Cretaceous) of central North America. The University of Kansas Paleontological Contributions, (N. S.), No. 11, 15 pp.

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Williston, S. W. 1904. The stomach stones of the plesiosaurs. Science (new series) 22:565.

Williston, S. W. 1906. North American Plesiosaurs: Elasmosaurus, Cimoliasaurus, and Polycotylus. American Journal of Science 4(21):221-236.

Williston, S. W. 1907. The skull of Brachauchenius, with special observations on the relationships of the plesiosaurs. United States National Museum Proceedings 32:477-489. pls. 34-37.

Williston, S. W. 1908. North American Plesiosaurs: Trinacromerum. Journal of Geology 16:715-735. figs. 1-15.

Williston, S. W. 1914. Water Reptiles of the Past and Present. Chicago Univ. Press. 251 pp. (Free, downloadable .pdf version here)