Rapid evolution, diversification and distribution of mosasaurs (Reptilia; Squamata) prior to the K-T boundary Michael J. Everhart Sternberg Museum of Natural History, Fort Hays State University, Hays, Kansas, 67601.       Webpage created 12/03/2005; updated 06/11/2010 Copyright © 2005-2010  by Mike Everhart    LEFT: Here a Late Cretaceous Mosasaurus hobetsuensis Suzuki (1985) cruises along the rocky shores of what would become modern day Japan.  Copyright © 2002 by Dan Varner; used with permission of Dan Varner.  This picture was painted for me as a gift for my host at the Morphogenesis conference I attended in Nagoya, Japan in 2002.

Rapid evolution, diversification and distribution of mosasaurs (Reptilia; Squamata) prior to the K-T boundary

Michael J. Everhart

Sternberg Museum of Natural History, Fort Hays State University, Hays, Kansas, 67601. 

ABSTRACT

Highly-adapted marine reptiles called mosasaurs became the apex predators of the Earth’s oceans in the last 25 million years of the Late Cretaceous. Returning to the sea during Cenomanian - Turonian time, they evolved from 1 m, shore-dwelling lizards into a variety of large (up to 17 m), fully marine carnivores. Their remains have been found in marine deposits on all continents, including Antarctica. Evidence of their rapid radiation, worldwide distribution and dominance appears to contrast sharply with that of other marine reptiles such as ichthyosaurs and plesiosaurs, the dinosaurs and even other marine predators such as sharks. Mosasaurs were highly adaptable and apparently able to fill many of the ecological niches left vacant by the extinction of the ichthyosaurs, marine crocodiles and plesiosaurs. Their rapid evolution may have also contributed to the extinction of several species of sharks, and they may have been competing with crocodilians in estuarine and freshwater environments at the end of the Cretaceous. Mosasaurs are an example of adaptive radiation prior to the K-T boundary extinction.

INTRODUCTION

Following the extinction event(s) near the end of the Permian, the evolution and diversification of reptiles produced a great variety of extinct and modern forms including dinosaurs, birds, pterosaurs, lizards, turtles, snakes, and a diverse group that is generally referred to as marine reptiles. During the Triassic, some marine reptiles, including ichthyosaurs and plesiosaurs, evolved into a number of successful families that persisted through most of the Mesozoic. By the end of the Early Cretaceous, however, Ichthyosaurs had become extinct. During the Turonian, plesiosaurs were reduced to two families, the elasmosaurids and polycotylids, which apparently survived into the Maastrichtian. Mosasaurs were the last major group of reptiles to return to the sea, evolving from at least two (Russell, 1967), or possibly three or more (Bell and Polcyn, 2004) lineages of small, terrestrial lizards called aigialosaurs. They diversified rapidly, spread quickly and flourished in the Earth’s oceans during the last 25 million years of the Late Cretaceous. Their rapid rise and dominance occurred over a much shorter period of time than other marine reptiles and sharks, or terrestrial groups such as dinosaurs and mammals.

The remains of mosasaurs were initially collected in the Netherlands in the mid-1700s, some fifty years before the discovery of dinosaurs, and were among the first recognized fossils of large animals. Adrian Camper (1800) is credited as the first to recognize the relationship between the remains of “Le Grand Animal of Maastricht” (Mosasaurus hoffmanni) and varanid lizards (e. g., Komodo dragon). By 1850, mosasaur remains had been found in Cretaceous marine deposits around the world, including England, New Jersey, South Dakota and New Zealand. The 1868 discovery of the type specimen of Tylosaurus proriger in western Kansas brought two well known paleontologists of the day (O. C. Marsh and E. D. Cope) to the state, and resulted in the collection of literally thousands of specimens from the Smoky Hill Chalk over an interval of about ten years. Since that time, mosasaur remains have been discovered on every continent, including Antarctica, and new species are being described and named at a steady pace (Table 1).

By the end of the Cretaceous, two genera of mosasaurs (Mosasaurus and Hainosaurus) had grown to lengths of more than 15 m, and represented a group of marine carnivores that were as dominant in their environment as Tyrannosaurus rex was on land. Unlike T-rex, these giant mosasaurs were more widely distributed geographically and more numerous. During Maastrichtian time, mosasaurs were diversifying rapidly and entering many niches left vacant by the demise of other marine reptiles, and beginning to enter freshwater environments. From their fossil record, it is evident that mosasaurs were a highly successful group that became the apex predators of the Earth’s oceans rather suddenly during the Late Cretaceous. For all their success, however, they may have become too specialized and became extinct at or near the end of the Cretaceous, most likely due to a collapse of the marine ecosystem.

The first mosasauroids appear in the fossil record during Cenomanian time in Europe and the Western Interior Sea. Small terrestrial lizards considered to be ancestral to mosasaurs (aigialosaurs and coniasaurs) have been found in the Adriatic region of southeastern Europe and in North America. Carroll and Debraga (1992) reported three mosasaur-like aigialosaur specimens from Cenomanian-Turonian (93 mya) deposits in Yugoslavia, and Bell and Polcyn (1996) documented the distribution of coniasaurs in the Western Interior Sea. Russell (1967) proposed two stem groups of aigialosaurids that were “Clidastes-like” (including Mosasaurus) and “Platecarpus-like” (including Tylosaurus). Martin and Stewart (1977), Bell (1995), and Bell and VonLoh (1998) documented mosasauroid and early mosasaur remains from Kansas, South Dakota and Texas. While it is likely that the first mosasaurs evolved from several different aigialosaurid lineages, the geographic origin(s) of the earliest mosasaurs is still uncertain. Their closest modern relatives are probably monitor lizards (varanids) like the Komodo dragon and quite possibly snakes (Caldwell, 1999), although the exact relationships are currently a matter of debate among mosasaur workers. Wherever they may have first entered the sea, it appears that they were able to spread rapidly around the world by migrating via shallower coastal waters. Similar species found as far apart as North America and New Zealand suggest that the initial spread of mosasaurs occurred rapidly, while other, highly derived species found in California and Africa provide evidence of the relatively rapid diversification of isolated populations.

One of the issues related to the study of North American mosasaurs since 1868 has been the sheer number of specimens in collections, and the tendency of earlier workers to name new species from non-diagnostic material. Cope (1871) noted that a total of 6 species had been discovered in the “cretaceous beds of Kansas.” The following year, Cope (1872) reported 17 species from the Kansas chalk and a grand total of 42 species from Kansas, Alabama and New Jersey. Marsh (1880) noted that the “Museum of Yale College contains remains of not less than 1,400 distinct individuals.” Williston (1891) observed that there had been “twice too many generic names given; so, too, it is pretty evident that there is an even greater number of synonyms among the specific names.” Many of the early Kansas specimens had been collected by Professor B. F. Mudge, and by Williston (1898, p. 200) himself, who said “I have seen altogether not far from 2000 specimens of Mosasaurs, and have collected with my own hands not less than 400.” Williston (ibid., p. 169-170) also wrote that the “determination of the species described by early authors [mostly E. D. Cope and O. C. Marsh] is in large part clearly impossible in the absence of the type specimens” and “four-fifths of all described species must be abandoned.” Since Williston’s time, the number of mosasaur genera and species has grown at a slower, more measured pace.

In his study of the systematics and morphology of mosasaurs, Russell (1967, p. 121) indicated that a major problem “was the proper application of 86 described species names of American mosasaurs to the hundreds of fragmentary to nearly complete skeletons in the collections of the American Museum and the Peabody Museum at Yale.” Not only had Marsh and Cope named new species of mosasaurs from fragmentary material that would be considered non-diagnostic today, they had neglected to properly curate many of their “type specimens.” Russell (ibid.) reduced the number of genera and species of mosasaurs to more realistic levels. However, during the last two decades, and especially the last 10 years, discoveries of new material in various parts of the world have added greatly to our knowledge of mosasaurs, and to the number of valid species that are now recognized.

DISCUSSION

Although the pre-Coniacian remains of mosasaurs are rare, and usually fragmentary, two new, relatively complete specimens are currently being described from Texas by Bell and Polcyn (2005, in press) and Polcyn and Bell (2005, in press). It does appear that the divergence between the Clidastes-like and Platecarpus-like mosasaurs had occurred prior to the Turonian (Bell, 1995). Martin and Stewart (1977) described two sets of vertebrae and a jaw fragment from the middle Turonian Fairport Chalk Member of the Carlile Shale Formation in Kansas and noted their affinities with Clidastes. Another skull element (a Platecarpus-like frontal; KUVP 97200; Bell, pers. comm., 2004) from the same strata in Ellis County, Kansas is also in the University of Kansas collection. Lingham-Soliar (1994) reviewed mosasaur remains from the Upper Turonian of Angola in western Africa. Bell (1995) and Bell and VonLoh (1998) reported on fragmentary specimens of mosasauroids from the Boquillas Formation of western Texas and the Greenhorn Formation (Early Turonian) of South Dakota. A more detailed discussion of the stratigraphic occurrence of mosasaurs and their discovery worldwide is provided by Bell (1997). The first (and earliest) Tylosaurus remains from Kansas have recently been described from the Lower Campanian Fort Hays Limestone (Everhart, (2005b). Tylosaurus is certainly the earliest genus of large mosasaurs.

The rise of mosasaurs can be visualized as occurring in three distinct waves (Table 2), each of which radiated outward from their point of origin. By middle Coniacian time (87 mya), the "first wave" of mosasaurs (Tylosaurus, Platecarpus, and Clidastes) was well established in the Western Interior Sea that covered Kansas and most of the Midwest portion of North America (Williston, 1898; Russell, 1967; Everhart 2001). As mosasaurs continued to evolve, growing larger and diversifying rapidly, a "second wave" of genera and species, including Hainosaurus, Plioplatecarpus, Mosasaurus, Globidens, and Prognathodon appeared near the beginning of the Campanian (83.5 mya). Following a possible near-extinction event near the middle of the Campanian reported by Lindgren (2004), mosasaurs rebounded, and a "third wave" was just getting underway during the final years of the Cretaceous, shortly before their final extinction.

Camp (1942) reported on two highly derived species of mosasaurs (Plotosaurus and Plesiotylosaurus) from the Maastrichtian age Moreno Formation of California that are quite unlike any other known species and indicative of an isolated population. Modifications to the skull, paddles and tail of Plotosaurus give it a decidedly ichthyosaurian appearance, and hint at a life style or habitat preferences that differ from most other mosasaurs (See Lindgren, Caldwell and Jagt (2008) for a more recent analysis). The crocodile-like Goronyosaurus nigeriensis (Lingham-Soliar, 1999a) and ichthyosaur-like Pluridens walkeri (Lingham-Soliar, 1998) from the Maastrichtian of Africa also represent highly divergent lineages. The durophagous mosasaurs (Globidens, Compressidens, Carinodens and to some extent, Prognathodon) represent a group that moved away from the “normal” mosasaur diet of fish and cephalopods (squid) and evolved stronger, more rigid skulls and specialized teeth adapted for feeding upon hard-shelled prey.

Size, body shape and integument

Aigialosaurs, the ancestors of mosasaurs, were small terrestrial reptiles that reached lengths of about 1-2 m during Cenomanian time. By the early Coniacian, the largest genus of mosasaurs, Tylosaurus, grew to lengths of about 7 m, with Platecarpus and Clidastes being considerably smaller (Table 3). Following Cope’s Law, mosasaurs generally increased in size through the last 25 million years of the Cretaceous, with two species, Hainosaurus (a tylosaurine) and Mosasaurus reaching, reaching nearly 17 m (Lingham-Soliar, 1999b). As mosasaurs diversified, however, there were a number of smaller, durophagous species (e. g., Carinodens) that were occupying other ecological niches during Campanian and Maastrichtian time.

More than anything else, the long, slender body shape and skeleton of a mosasaur resembled that of a snake. However, mosasaurs retained their front and rear limbs, and the chest region was noticeably expanded, a possible indication that, unlike snakes, they retained two lungs. The head was conical in shape and was extremely long and narrow in some species (e.g., Ectenosaurus clidastoides). While their body shape might be considered relatively inefficient for high-speed swimming compared with the models provided by ichthyosaurs and plesiosaurs, it appears likely that mosasaurs were more of an ambush predator than a pursuit predator (Massare, 1988). In that regard, the body shape, larger, more flexible skull, and swimming style evolved by mosasaurs appears to have been a more successful strategy for competing in the marine environment.

Although Marsh (1872) claimed to have discovered “dermal scutes” on mosasaurs, they were re-examined by Williston (1891) and found to be fragments of the thin sclerotic ring that covered much of the outer surface of the eye of the mosasaur. Snow (1878), at the University of Kansas, was the first to report on the dermal covering of mosasaurs, citing as an example the remains of an estimated 8 m long Tylosaurus proriger specimen (KUVP 1075) he had discovered in Gove County in 1877. Snow published a photograph showing the impressions of about 3000 scales and noted (ibid., p. 56) that there were about 90 scales per inch, being somewhat smaller than those of a large rattlesnake (80 scales per inch). The scales are diamond shaped (3.3 mm x 2.5 mm), with a raised ridge (carina) on the long axis. Scales of a similar size and shape, but lacking the central ridge, are also known from a specimen of Ectenosaurus clidastoides (FHSM VP-401) in the collection of the Sternberg Museum of Natural History. Numerous patches of small, diamond-shaped scales are visible on the bones on the underside of a Platecarpus tympaniticus specimen from Kansas reported by Geist et al. (2002). All of these specimens are of late Santonian to early Campanian age, and lived some 20-18 million years before the end of the Cretaceous. Skin impressions, however, are unknown from younger specimens of mosasaurs. Given the apparent smooth integument of ichthyosaurs and plesiosaurs, it appears likely that mosasaurs may have also lost their scales as they evolved further. (NOTE:  Lindgren, et al., 2009 demonstrated that Plotosaurus bennisoni from the Middle Maastrichtian of California had scales)

Locomotion

Early mosasaurs were somewhat more conservative in the evolution of their body plan than the highly derived ichthyosaurs and plesiosaurs. Although they used the same side-to-side movement of their tail as the ichthyosaurs (compared to the wing-shaped paddles and underwater flight of plesiosaurs), they did not develop the lobed, semi-lunate tail. In overall appearance, they resembled a crocodile with paddles instead of legs and feet. It appears likely that most mosasaurs used their limbs for steering and attitude control, although the flexible, highly cartilaginous paddles of Tylosaurus may have had a slightly different functionality compared to the much more solid limb bones of Clidastes and Mosasaurus (Figure 1). In most early mosasaurs, the widely-spread digits were loosely joined by a webbing to form a flexible paddle, while in later genera (e. g., Plioplatecarpus and Plotosaurus) the digits were arranged tightly together to form a stiffer, wing-like structure. These highly derived paddles may have been more useful for propulsion in other than open-water environments (Lingham-Soliar, 1992: for a counter argument, see Nicholls and Godfrey, 1994). In either case, mosasaur limbs and limb girdles were rather quickly modified to the point that they would no longer support the weight of the animal out of the water. Based on the discovery of embryos in an aigialosaur (Caldwell and Lee, 2001) from the Cenomanian-Turonian of Slovenia, it is likely that live birth had evolved in the early mosasauroids at some before they returned completely to the sea. Freed from the necessity of laying their eggs on land, mosasaurs were probably completely marine animals by the early Turonian.

Figure 1. The stoutly constructed right forelimb of Mosasaurus hobetsuensis Suzuki, 1985 from the lower Maastrichtian, Hokkaido, Japan is nearly indistinguishable from those of M. conodon (Museum of Geology, South Dakota School of Mines and Technology) and M. hoffmanni (Natuurhistorisch Museum Maastricht). (HMG-12, Hobetsu Museum, Hokkaido, Japan)

The muscular tail of mosasaurs was flattened laterally and was used to propel the animal through the water with a sinuous, side-to-side movement. Although somewhat shorter proportionally than in most terrestrial lizards, the tail of mosasaurs was still quite long, up to 42% of body length in Clidastes liodontus and 52% in Tylosaurus proriger (Russell, 1967). In some genera (i.e. Clidastes and Plotosaurus), the surface area of the tail was increased by lengthening the neural spines of the vertebrae. This may have occurred as a means of increasing thrust in compensation for the relatively shorter length. Unlike modern sea snakes that undulate their entire body while swimming, the pre-caudal vertebrae of mosasaurs were relatively inflexible laterally and provided a stable base for the muscular tail. The head and neck were probably held in line with the dorsal vertebrae while swimming while the paddles were folded against the side of the animal except to change directions. Although capable of swimming long distances and remaining at sea indefinitely, mosasaurs probably were not as fast as ichthyosaurs and the polycotylid plesiosaurs (Massare, 1988). Where they did excel apparently, was as ambush predators, using surprise and rapid acceleration to overtake and capture their prey (Massare, 1987). The fact that thousands of mosasaur specimens have been collected from the Niobrara Chalk in western Kansas, a locality that would have been hundreds of km from the nearest land from Coniacian through early Campanian time, indicates that they were as well adapted to life in the sea as modern sea mammals. Jacobs, et al. (2004) reported that modern sea snakes and marine iguanas are limited to habitats where the surface water temperature ranges between 20º and 35º C. Even so, iguanas in the Galapagos Islands have to sun themselves for extended periods to increase their body temperature between feeding forays. This raises questions regarding changes in the ancestral reptilian metabolism and circulatory system of mosasaurs that would both support swimming over long periods of time and the continuous loss of body heat to the surrounding seawater. Additional studies are necessary to address these issues. (Added June 11, 2010 - See Bernard, et al. 2010 in Science for new information regarding the body temperature of marine reptiles.  See also Motani 2010 for comments)

Feeding adaptations

Marine creatures face different problems than terrestrial forms in acquiring and swallowing prey because of the weightless, three-dimensional environment in which they live. Captured prey, if released, can float away, sink or be taken by competitors. As land dwelling reptiles that returned to the sea, mosasaurs had to adapt feeding strategies that addressed these issues. In the case of most of the ichthyosaurs and plesiosaurs, the solution was simply feeding on small fishes and cephalopods that could be captured and swallowed intact. While the body plans of the fish-like ichthyosaurs and highly streamlined plesiosaurs appear to be advantageous to capturing prey, both of these groups also had relatively small, rigid skulls that limited the maximum size of the prey that could be swallowed. Massare (1988) considered them to be pursuit predators, depending on their speed to chase down and acquire prey. As a result, their feeding strategy required a major expenditure of energy to catch large numbers of small fish and cephalopods. In the case of ichthyosaurs, and possibly plesiosaurs, their eventual extinction may have been related to the evolution of faster fishes and the competition from larger teleosts for the same prey.

Mosasaurs, on the other hand, retained a relatively large skull that was 10-14% of their total body length (Russell, 1967; Everhart, 2001). In addition, the skull in the earlier genera (Tylosaurus, Platecarpus and Clidastes) was highly kinetic (Russell, 1967), an adaptation seen in modern snakes that made the skull of these animals flexible enough to swallow much larger prey. Other feeding adaptations in mosasaurs included: a mobile quadrate that provided additional fore and aft movement to the lower jaw; an intermandibular hinge that allowed the lower jaws to bow outward as the prey was pulled into the mouth; pterygoid teeth that kept the prey from escaping as the lower jaw disengaged and moved forward; and a symphysial hinge between the tips of the lower jaws that allowed some degree of independent movement (Figure 2). The similarities between the jaw mechanics of mosasaurs and snakes are discussed in detail by Lee, et al. (1999).

Figure 2. Tylosaurus kansasensis: The skull of a new (Everhart, 2005a) mosasaur species from the Niobrara Chalk of western Kansas. (Ang – angular; Art – articular; Cor – Coronoid; Den – Dentary; Fron – Frontal; Jug – Jugal; Max – Maxilla; Par – Parietal; PF – Prefrontal; POF – Postorbitalfrontal; Pt – Pterygoid; Quad – Quadrate; Spl – Splenial; Sq – Squamosal; Sur – Surangular)

Cope (1872) was one of the first to describe the feeding mechanism in mosasaurs: “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 teeth of the earliest mosasaurs were simple cones that were slightly recurved posteromedially. See Leidy (1858) for an early description of the teeth of Mosasaurus. Smaller species, such as Clidastes and Platecarpus, tended to retain slender, grabbing teeth for capturing small fish and cephalopods while the teeth of the larger Tylosaurus were much more robust and were probably used to seize and kill larger prey. Carina, if present, were small, and occasionally serrated. Mosasaur teeth are generally indicative of prey preferences, with Tylosaurus and Prognathodon being generalists that were capable of taking a variety of prey including large fish, birds, other mosasaurs (Martin and Bjork, 1987), and small plesiosaurs (Sternberg, 1922; Everhart, 2004).

Only a few, poorly known species (e. g., Leiodon) appear to have developed narrow teeth with efficient cutting edges to cut flesh or dismember prey. At the other extreme, genera like Prognathodon evolved heavily built conical teeth that were capable of crushing hard shelled prey such as ammonites and turtles (Dollo, 1887; See bitten turtle shell fragment here). First described by Gilmore (1912), Globidens had a heavily built skull and rounded, ball-shaped teeth that were well adapted to feeding on clams and other shellfish. A relatively complete, but as yet undescribed Globidens specimen [NOTE: Globidens schurmanni was described by Martin (2007)] from the Pierre Shale of South Dakota was found with fragments of several species of bivalves in the abdominal area (Martin and Fox, 2004). The feeding mechanics of another, much smaller mosasaur with an unusual assortment of grasping and crushing teeth (Carinodens belgicus; Figure 3) were described by Schulp, et al. (2004). Other species, such as Plotosaurus (Camp, 1942) and Pluridens (Lingham-Soliar, 1998) appear to have evolved longer jaws that held a large number of smaller teeth for more efficient feeding on smaller prey, similar to the feeding mechanism of many ichthyosaurs.

Figure 3. Restoration of Carinodens belgicus by Wouter Verhesen: A small (2 m) mosasaur from the Maastrichtian of the Netherlands with a highly derived dentition (Schulp, et al. 2004). Used with permission of the artist and the Natuurhistorisch Museum Maastricht.

A highly derived African species (Goronyosaurus nigeriensis) developed functional canine teeth and a heavily built skull that resembles that of a crocodilian more than a mosasaur (Lingham-Soliar, 1999a). Remains of this Maastrichtian species, as well as those of a Plioplatecarpus from Canada (Holmes, et al., 1999) have been found in freshwater deposits, and may be evidence of competition between mosasaurs and terrestrial crocodilians. LEFT: (Added figure): Reconstruction of the skull of Goronyosaurus nigeriensis, based on the holotype (BMNH R14153) and referred material. Adapted from Lingham-Soliar (1999a) (Scale = 10 cm). Another figure from Lingham-Soliar (2002) is HERE:

Live birth

It has been known for more than a century that ichthyosaurs gave live birth to their young. An as yet undescribed polycotylid plesiosaur specimen from the Pierre Shale of western Kansas shows indications of young within the body of an adult (Rothschild and Martin, 1993). Although Williston (1898) and Russell (1967) argued against live birth in mosasaurs, they did not cite evidence of nesting sites or protected areas where smaller mosasaurs had been found. More recently, Bell, et. al. (1996) and Bell and Sheldon (2004) reported the discovery of a mother mosasaur with skeletal elements of at least four babies in her abdomen. Caldwell and Lee (2001) described a mosasaurid specimen from southeastern Europe that contained embryonic material. Sheldon (1990) and (Everhart, 2002) noted the presence of many specimens of small mosasaurs from the Smoky Hill Chalk, an indication of mosasaurs were giving birth in mid-ocean. It is likely that the reproductive strategy of mosasaurs involved the investment of the mother’s resources in a relatively small number of larger, well-developed babies instead of laying a large number of eggs like marine turtles or crocodiles. From the remains that have been found, it appears that baby mosasaurs were between 1 and 2 m in length at birth (Everhart, 2002; Bell and Sheldon, 2004). In the case of the Niobrara Chalk (Coniacian through lower Campanian) of western Kansas, the remains of very young mosasaurs were found in an area that would have been more than 300 km from the nearest coast on the eastern edge of the Western Interior Sea. While it has been speculated that there may have something similar to a kelp forest or seaweed mat to provide shelter for small animals but no evidence for such a scenario has been found. Even with larger sizes at birth, the survival rate of young mosasaurs was probably low in an environment shared with large sharks, giant teleosts and other mosasaurs. However, the fossil record indicates that enough individuals reached reproductive age to maintain population growth over a long span of time.

Diversity and Distribution

As noted earlier, mosasaurs spread rapidly around the world during the later stages of the Cretaceous, quickly becoming the apex predator of the Earth’s oceans and occupying various other niches left vacant by the extinction of the ichthyosaurs, the reduction in numbers of plesiosaurs, and possibly even the extinction of large, pelagic shark species like Cretoxyrhina mantelli. While Williston (1898) and Russell (1967) reduced the inflated number of species reported by Cope and Marsh, the number of recognized mosasaur species continues to grow as new specimens are found and new localities are explored (Table 1). Lingham-Soliar (1999b) estimated that there were about 20 genera and 45 species of mosasaurs living at the end of the Cretaceous. Based on recent discoveries, it is likely that those numbers may be underestimated by a factor of 2 or more. Mosasaurs appear to have spread from the Western Interior Sea north and west along the rim of the western Pacific, with similar genera showing up in Japan and across the equator in Australia and New Zealand (Figure 4). Movement probably occurred in both directions across a much narrower northern Atlantic between the east coast of North America and Europe. Similar migrations appear to have occurred through the Tethys Ocean over submerged portions of Europe and Africa, and into the Middle East. A new species of tylosaurine mosasaur (Lakumasaurus antarcticus) discovered in upper Campanian – lower Maastrichtian deposits on an island off the coast of Antarctica (Novas, et al., 2002) provides additional data on the worldwide distribution of this genus. The presence of Mosasaurus hobetsuensis in Japan (Suzuki, 1985b) and a recent discovery of a mosasaur much like Mosasaurus hoffmanni in Turkey (Bardet and Tunoglu, 2002) indicates that, like tylosaurines, this genus was also living in many places around the world.

Figure 4. A generalized map of the Earth showing the approximate locations of known mosasaur genera during Campanian-Maastrichtian time. The dashed lines indicate the probable extent of epicontinental seas at the same time. (Adapted from Suzuki, 1985a). Click here for a more detailed map of the Western Interior Sea of North America.

It is likely that the success of mosasaurs as predators led to a population explosion and created pressure to expand into new territories. In the case of two of the largest and most ubiquitous genera, Tylosaurus and Mosasaurus, the distances traveled would have been roughly half way around the world (roughly 20,000 km). While this is a relatively large distance, an expanding population could easily move that far in 4000 years at an average rate of 5 km per year. For an animal that was well adapted to living completely at sea, it is more likely that mosasaurs spread around the world at a much faster rate. Although the origin, timing and direction of these “migrations” is still uncertain, relict populations of highly derived mosasaurs such as those found in the Maastrichtian of California and Africa may eventually shed some light on these issues. New discoveries continue to add both to the number of species that are known and the extent to which the various genera dispersed across the oceans of the Late Cretaceous.

CONCLUSION

During the last 25 million years of the Cretaceous, mosasaurs evolved relatively quickly from small shore-dwelling lizards into the dominant marine predators in the oceans of the Earth. Their adaptations to life in the ocean included a highly kinetic skull and jaws, major modifications to the axial skeleton and limbs, changes in body shape and covering, growth to very large size, and live birth. The evolution of mosasaurs can be considered as a pre-Tertiary model of rapid adaptive radiation.

ACKNOWLEDGEMENTS

I thank Gorden Bell, Mike Polcyn and Dale Russell for our continuing discussions of all things mosasaur, J.D. Stewart and Donald Hattin for their insights into the stratigraphy and the appearance of mosasaurs in the Niobrara Chalk of western Kansas. I am also grateful to Richard Zakrzewski (Sternberg Museum of Natural History), Larry Martin and Desui Miao (University of Kansas Museum of Natural History), James Martin and Carrie Herbel (Museum of Geology, South Dakota School of Mines and Technology), Earle Spamer and Ted Daeschler (Academy of Natural Sciences of Philadelphia), Robert Purdy and Michael Brett-Surman (United States National Museum), Anne Schulp (Natuurhistorisch Museum Maastricht) and Kazuhiko Sakurai (Hobetsu Museum, Hobetsu, Hokkaido, Japan) for access to specimens in their care. Yoshiyuki Usami (Kanagawa University, Yokohama, Japan) facilitated my trip to the conference in Nagoya, Japan in 2002.

LITERATURE CITED

Bardet N., X. P. Suberbiola, and N-E Jalil. 2003. A new mosasauroid (Squamata) from the Late Cretaceous (Turonian) of Morocco. Comptes Rendus Palevol 2(8): 607-616.

Bardet N., X. P. Suberbiola, M. Iarochene, F. Bouyahyaoui, B. Bouya, and M. Amaghzaz. 2005. A new species of Halisaurus from the Late Cretaceous phosphates of Morocco, and the phylogenetical relationships of the Halisaurinae (Squamata: Mosasauridae). Zoological Journal of the Linnean Society 143: 447-472.

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