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Mosasaurs: Last of the great marine reptiles


Copyright © 2000-2009 by Mike Everhart

Last updated: 07/18/2009

Original article published as: Everhart, M. J. 2000. Mosasaurs: Last of the great marine reptiles. Prehistoric Times. 44:29-31




Copyright © 1999 by Dan Varner; used with permission of Dan Varner

I was pleased to see mosasaurs being featured in the June/July 2000 (#42) edition of Prehistoric Times and wanted to take the opportunity to update the article with some additional information. As webmaster at Oceans of Kansas Paleontology, and an Adjunct Curator of Vertebrate Paleontology at the Sternberg Museum of Natural History, my interests are centered on late Cretaceous marine life in general and mosasaurs in particular. Mosasaurs were the dominant predator in the oceans of the late Cretaceous and were certainly the marine equivalent of Tyrannosaurus rex. The finding of the first mosasaur remains in the Netherlands about 1780 (Mosasaurus hoffmanni) predates the discovery of dinosaurs by more than fifty years, yet they have been frequently ignored in the media and text books on paleontology in favor of the more popular ichthyosaurs and plesiosaurs. Maybe it is because they look superficially like alligators and crocodiles, or it might be that they are simply too common, since they are represented in the fossil record from every continent by thousands of specimens from more than 40 known species. In spite of this lack of good press, however, there have been a number of recent, exciting discoveries that add to our knowledge of these magnificent animals.

UPDATE 2005: This article was extensively rewritten and cited, and now is available as Chapter 9 of my book Oceans of Kansas - A Natural History of the Western Interior Sea

See also: Everhart, M. J. 2005. Rapid evolution, diversification and distribution of mosasaurs (Reptilia; Squamata) prior to the K-T Boundary. Tate 2005 11th Annual Symposium in Paleontology and Geology, Casper, WY, p. 16-27 (not peer-reviewed).

     The ancestors of mosasaurs were probably related to small terrestrial lizards called aigialosaurs that lived close to the ocean from the late Jurassic through the Cretaceous. In the mid-Cretaceous, the ancestral mosasaurs began to rapidly adapt to life in the ocean. They were not the first reptiles to leave the land and return to the sea, but they were possibly the most successful. Ichthyosaurs and plesiosaurs had inhabited the oceans since the Triassic, evolving into many diverse forms and surviving several major extinction events. For unknown reasons, ichthyosaurs declined significantly in early Cretaceous and are thought to have been extinct about the time that the earliest mosasaurs re-entered the water. Plesiosaurs were also less numerous in the late Cretaceous than during the Jurassic, and had evolved into some very specialized forms like the long-necked elasmosaurs. Even the short-necked plesiosaurs (pliosaurs and polycotylids) were much smaller than their Jurassic cousin, Liopleurodon, and an early Cretaceous relative, Kronosaurus. It is possible that both the ichthyosaurs and the plesiosaurs were losing the evolutionary battle of "who eats who" to faster, larger and more advanced varieties of fish such as Xiphactinus and the giant Ginsu sharks (Cretoxyrhina mantelli). Several other groups of reptiles, including marine crocodiles, teleosaurs, placodonts and turtles had also enjoyed limited successes in the marine environment, but none approached the world-wide domination that mosasaurs would attain in the late Cretaceous.

     The early ancestors of mosasaurs probably fed in the ocean and returned to land much like the marine iguanas that are found today in the Galapagos Islands. Over a relatively short period of time, however, these ancestral mosasaurs became larger and more specialized, evolving rapidly into several genera of highly successful predators. By the beginning of Coniacian time (about 90 million years ago - mya), there were three major genera (Tylosaurus, Platecarpus and Clidastes) living in the Western Interior Seaway. Tylosaurs were the largest of these early mosasaurs, reaching about 10 meters (30 feet) in length by Santonian time (85 mya). Platecarpus was probably the most common of the first wave of mosasaurs and reached lengths of about 8 meters (24 feet). Clidastes, the smallest and probably the most primitive of the group, was generally less than 5 meters in length when full grown (specimens of larger individuals are known). By the Campanian, Tylosaurs were even larger (13-14 meters) and many more species were making their appearance. Within the space of a few more million years, by Maastrichtian time (70 mya), mosasaurs were truly huge, with several lineages (Mosasaurus and Hainosaurus – a close relative of Tylosaurus) reaching 15 meters (50 feet) or more. One giant specimen (Hainosaurus bernardi) found in Europe was 17 meters (almost 55 feet) in length. There was no doubt who were the biggest and baddest predators in the oceans 70 million years ago.

     As mosasaurs increased in size and diversity during the last 25 million years of the Cretaceous, they also spread around the world. While their fossilized remains have been found most often in Kansas and South Dakota, their bones are known from every continent, and were most recently found in Antarctica. It is interesting to note that the word "mosasaur" means Meuse River lizard, a tribute to the locality where the first mosasaur was found in the Netherlands. (Read a translation of the original story here) Soon afterwards, mosasaur remains were discovered in England and as far away as New Zealand. The first North American mosasaur remains (mostly teeth) were described from the greensands of New Jersey. In the early 1830s, a German nobleman, Prince Alexander Philip Maximilian von Wied and a Swiss artist, Karl Bodmer, traveled among the Indians in wilds of Nebraska and South Dakota. Along the way, they acquired the remains of a Mosasaurus that had been found along the Missouri River in what would become South Dakota. That specimen eventually found its way back to Germany where it was prepared and described in an 1845 paper by Dr. August Goldfuss. The skull of this North American Mosasaurus is still in the collection of the Goldfuss Museum in Bonn, Germany. In 1869, E. D. Cope began briefly described and named the first mosasaur from Kansas and the rush for fossils was on. His rival, O. C. Marsh, organized the Yale College Scientific Expeditions of the 1870s and collected about two thousand mosasaur specimens from Kansas for the Peabody Museum.

     Although they probably preferred the shallower coastal waters where their prey was most abundant, they were also excellent open water swimmers. Mosasaurs were certainly capable of traveling across large bodies of water as evidenced by the numerous fossils found in the chalks and shales deposited in the middle of the Western Interior Seaway, hundreds of miles from the nearest land. From the number of remains that are found in these deposits, it seems apparent that mosasaurs were living in mid-ocean, and not just migrating through it. Fossils found recently in Canada and in Africa suggest that near the end of the Cretaceous, some species were adapting to freshwater habitats that had been the primary domain of alligators and crocodiles.

     Mosasaurs used a side to side, undulating motion of their long, sinuous tails to swim through the water. They were probably not capable of swimming long distances at high rates of speed but rather hunted from ‘ambush’, surprising and out-accelerating or outrunning their prey over a short distance. The tail of Clidastes is somewhat shorter in proportion to its body length than that of other mosasaurs, but has elongated vertebral processes that would have supported a much deeper tail. This may indicate that this genera was faster than other mosasaur genera. Speed may have been a very useful adaptation when your larger cousins (tylosaurs) saw you as their next meal. However, it was not always enough as demonstrated by fossilized stomach contents of a tylosaur found in South Dakota which included a smaller Clidastes.

     Because mosasaurs were so well adapted to living in the ocean, it is improbable that they ever voluntarily came out on land. As they adapted to life in the ocean, their legs and feet became broad, flat paddles used primarily for steering. Their limbs were no longer solidly connected to their backbones and could not support the weight of their large bodies out of the water. While they may have been able to slither through shallow water much like a snake, they would have been nearly helpless on dry land. Once beached, they would have most likely died of suffocation from the collapse of their lungs, or from overheating.

     While their bones show that mosasaurs were certainly reptiles, the discovery of a Plioplatecarpus ‘mother mosasaur’ in South Dakota with the remains of several unborn young in her abdomen appears to show conclusively that they bore their young live. Like ichthyosaurs, mosasaurs probably only gave birth to a maximum of 4 or 5 babies at time. This is a very small number compared to other reptiles such as crocodiles, alligators and turtles that lay dozens of small eggs, and depend on sheer numbers for the survival of their young. Early paleontologists, such as Samuel Williston, concluded that young mosasaurs must have lived in estuaries or other near-shore, protected environments because their remains had not been found along with those of the adult mosasaurs. Whether those remains were simply overlooked or ignored by Cope, Marsh, Sternberg and others in the rush for bigger, more spectacular specimens, we know now that mosasaurs of all ages lived and died in the middle of the Western Interior Seaway.

     The presence of the remains of young individuals in the Smoky Hill chalk of Kansas indicates that mosasaurs were giving birth in mid-ocean, 200 miles or more from the nearest land. It also suggests that mosasaurs, out of necessity, may have lived together in groups for the protection of their young. There were too many other hungry predators around, including 20 foot long sharks, giant predatory fish (Xiphactinus) and other species of mosasaurs, for young mosasaurs to have survived long without some assistance. While it is possible that the Western Interior Seaway had masses of floating seaweed that young animals could hide in, there is no fossil evidence to support this theory and the presence of many fast swimming predators, such as Xiphactinus, seems to argue for large areas of open, unobstructed water. One variety of primitive swordfish (Protosphyraena) had long pectoral fins that extended two to three feet outward on either side of its body, hardly a good design for efficient hunting in a kelp forest. Most other marine reptiles, including turtles, polycotylids and plesiosaurs, also moved through the water with long, outstretched limbs.

     Mosasaurs probably fed primarily on fish, but are known to have eaten just about anything that they could swallow. The preserved stomach contents of a large tylosaur on exhibit in the Museum of Geology at the South Dakota School of Mines and Technology contains the bones of a smaller mosasaur (Clidastes), a bird (Hesperornis), a fish (Bananogmius) and possibly a shark. Fish remains were found in a specimen of Plotosaurus from California and a Tylosaurus from Kansas. The remains of a turtle are preserved in an European specimen, and the exhibit specimen of Tylosaurus proriger at the Smithsonian was discovered with a plesiosaur as its last meal. Ammonite shells have been found with what appear to be mosasaur bite marks, but there is some question as to whether or not these holes were caused by mosasaur bites or far more likely, by the attachment of limpets. A Globidens specimen found in South Dakota has the ground of remains of clams in its abdomen.

     It is important to note here that, with the exception of a few specialized and late evolving species, mosasaur jaws and teeth were used for seizing and holding of prey until it could be swallowed. Most mosasaur teeth are cone shaped and do not have the cutting edges typical of shark or theropod teeth. Short of dismembering its prey by the sheer force of its bite, a mosasaur had to swallow whatever it caught whole, much like a modern snake. Mosasaurs and snakes are also similar in the way that their skulls are constructed, including a lower jaw with a flexible joint that was used to ‘ratchet’ prey into the mosasaur’s throat with the aid of an additional, double row of teeth in the roof of their mouth. Living in mid-ocean, such adaptations were necessary for survival, and mosasaurs were winners in that regard. Only a few of the later genera of mosasaurs developed specialized teeth for cutting or tearing flesh (Leiodon), or for crushing clams and other hard shelled invertebrates (Globidens).

    "They were furnished, like snakes, with four rows of formidable teeth on the roof of the mouth. Though these were not designed for mastication, and, without paws for grasping, could have been little used for cutting, as weapons for seizing their prey they were very formidable. And here we have to consider a peculiarity of these creatures, in which they are unique among animals. Swallowing their prey entire like snakes, they were without that wonderful expandability of throat due in the latter to an arrangement of levers supporting the lower jaw. Instead of this each half of that jaw was articulated or jointed at a point nearly midway between the ear and the chin. This was of the ball-and-socket type, and enabled the jaw to make an angle outward, and so widen by much the space enclosed between it and its fellow. The arrangement maybe easily imitated by directing the arms forward, with the elbows turned outward and the hands placed near together. The ends of these bones were in the Pythonomorpha as independent as in the serpents, being only bound by flexible ligaments. By turning the elbows outward and bending them, the space between the arms becomes diamond-shaped and represents exactly the expansion seen in these reptiles, to permit the passage of a large fish or other body. The arms, too, will represent the size of jaws attained by some of the smaller species. The outward movement of the basal half of the jaw necessarily twists in .the same direction the column-like bone to which it is suspended. The peculiar shape of the joint by which the last bone is attached to the skull depends on the degree of twist to be permitted, and therefore to the degree of expansion of which the jaws were capable."

E. D. Cope, 1872

     Life in the oceans of the Cretaceous was dangerous at best and mosasaurs did not always have things their way. New evidence, including sharks teeth embedded in partially digested mosasaur bones, from the late Coniacian Smoky Hill Chalk of Kansas indicates that mosasaurs were frequently fed upon by the large ginsu sharks (Cretoxyrhina mantelli). While it is not possible at this point to determine whether this feeding activity was the result of an attack on a live mosasaur or the scavenging of a carcass, it is probable that a five or six meter Ginsu would behave much like a modern Great White shark, or an extinct Carcharodon megalodon. More than likely, Ginsu sharks attacked small, injured or sickly mosasaurs when they most vulnerable, but probably would not have passed up a free meal either. The rest of the story, however, indicates that Cretoxyrhina became extinct over the space of a few million years at the same time that mosasaurs were becoming bigger and much more wide spread. Did mosasaurs cause the extinction of these large sharks? No one knows for sure, but we do know that modern sharks suffer greatly from over-fishing because they do not reproduce rapidly. It seems reasonable that bite-sized, juvenile sharks were on the menu for growing numbers of large, hungry mosasaurs.

Mosasaurs ruled the oceans of the Late Cretaceous and were beginning to invade fresh water environments such as estuaries, swamps and rivers when the Age of Dinosaurs ended. Did they die suddenly due the catastrophic effects of an asteroid impact in the Yucatan, or was their extinction more gradual following the general collapse of the marine ecosystem? We may never know.

Go here for a webpage that describes the meteor impact that caused the Manson Crater in Iowa, 74 million years ago

Acknowledgments: Although not mentioned in the published article, I would like to recognize several of the individuals who have in some way have helped and encouraged my work with mosasaurs.  I thank Gordon Bell, Pete Bussen, Kenneth Carpenter, Donald Hattin, John Jagt, Greg Liggett, Johan Lindgren, Theagarten Lingham-Soliar, Earl Manning, Larry Martin, James Martin, Judy Massare, Dan Merriam, Eric Mulder, Elizabeth Nicholls, David Parris, Dale Russell, Bruce Schumacher, David Schwimmer, Kenshu Shimada, J. D. Stewart, Glenn Storrs, Dan Varner and Richard Zakrzewski.

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