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Bethany Latham, Jacksonville State University
Mosasaurs, as all members of the family Mosasauridae are known, were moderate- to giant-sized carnivorous marine lizards that lived during the Late Cretaceous period (roughly 100 to 66 million years ago). During this time, mosasaurs were probably the top predators in all the shallow seas that covered the continental shelf regions of the Earth. Their fossils have been unearthed on every continent, including Antarctica, but the majority of the finds occur in North America. The fossilized remains of numerous species of mosasaur have been found in Upper Cretaceous marine sediments in Alabama, especially in chalk deposits of Alabama's Black Belt in Dallas, Greene, Lowndes, and Perry Counties. One species, Selmasaurus russelli, is named for the Selma Group chalk deposits in which it was found.
Tylosaurus Teeth
The name "mosasaur" is Latin for "Meuse lizard," because the first fossils of this type of animal were discovered in a quarry on the Meuse River in Maastricht, The Netherlands. Scientists place the mosasaurs in the Squamata order of reptiles, which includes lizards and snakes, but are unsure about their relationship to lower taxa of lizards. Fossil evidence and their morphology suggest that they are most closely related to lizards of the family Varanidae, which includes the well-known Komodo dragon. Scientists generally agree that there are six subfamilies of Mosasauridae that comprise at least 29 separate genera. There are, however, significant challenges in accurately distinguishing among the known specimens to be sure how to classify all the fossils. This is especially true because some species were originally described from very scant remains.
Globidens Jaw
Mosasaurs ranged in size from the small Dallasaurus, at around three feet long (~1 meter) to the giant Mosasaurus at more than 50 feet (~16 meters). Like modern-day whales, mosasaurs evolved from terrestrial animals that exploited aquatic habitats and slowly became fully marine, and they breathed air like their terrestrial relatives. Most of them had hinged jaws that enabled them to swallow differently shaped prey animals whole. Mosasaurs propelled themselves through the water with their long and powerful tails and likely used their paddle-like, webbed limbs for steering and speed control. Fossils of several species of mosasaur have been found with downward-bent tail vertebrae, with evidence of a fleshy upper lobe forming a slightly fishlike propulsion surface. At least one species of Prognathodon is known to have had an even more pronounced fleshy, double-lobed tail, extending the swimming surface on a vertical plane like that of a shark. The additional thrust provided by this tail shape likely made them fast swimmers. Most of the mosasaur species found in Alabama probably fed primarily on a diet of fish, squid, and other marine animals. Two species, Globidens and Prognathodon, likely specialized in eating animals with shells, based on their rounded, crushing teeth. As far as scientists know from fossil evidence, mosasaurs gave birth to live young.
Tylosaurus Skin
The first mosasaur fossils discovered in Alabama—a few vertebrae and teeth—came from the Mooreville Chalk Formation (part of the Selma Group) in Lowndes County and were described by naturalist Robert W. Gibbes in 1850-1851. In 1869, a more complete specimen was described by paleontologist Edward D. Cope from a skull and nearly entire post-cranial skeleton of the species Clidastes propython. This specimen was collected from the Selma Group near Uniontown, Perry County, by Edward R. Showalter of Howard College (now Samford University) and given to the Academy of Natural Sciences of Philadelphia. C. propython is by far the most common mosasaur found in Alabama, comprising approximately 75 percent of the mosasaur remains collected. At least ten distinct species of mosasaur fossils have been unearthed in Alabama, however, and there are likely more yet to be identified. Specimens from Alabama primarily reside at the Academy of Natural Sciences at Philadelphia's Drexel University, the Auburn University Museum of Natural History, Chicago's Field Museum of Natural History, Birmingham's McWane Science Center, and the Alabama Museum of Natural History, which boasts the largest collection of mosasaur fossils in the state of Alabama.
Mosasaur Genera
Sweetie the Mosasaur
Clidastes ("locked vertebrae") was the smallest common genus, averaging about 10 feet (~3 meters) in length, although the largest discovered was close to 20 feet (~6 meters). They lived from around 89 to around 72 million years ago. Scientists believe that these animals were pursuit predators of fish and other fast and smaller prey animals. As with most mosasaurs, they had sharp teeth around the jaw margins that curved toward the back of their mouths and an additional set of teeth in the pterygoid bones of the palate farther back in their throats, presumably to grip prey as they moved their jaws. Three species have been identified in Alabama: C. propython, C. "moorevillensis," and C. liodontus.
Eonatator ("dawn swimmer") was also among the smaller types of mosasaur, averaging 8.5 feet (~2.6 meters). The genus was fairly recently distinguished from the similar genus Halisaurus. Its fossils have been found in the Eutaw Formation and the Mooreville Chalk Formation, and it probably lived from around 90 to around 70 million years ago. One species, Eonatator sternbergi, has been found in Alabama.
Globidens ("globe teeth") averaged 20 feet (~6 meters) in length and lived from about 72 to about 66 million years ago. Unlike most other mosasaurs, which had the pointed teeth typical of most predators, Globidens had short and rounded globe-shaped teeth (hence its genus name) used to crack open tough mollusk shells. Of the known species, one, G. alabamaensis, has been found in Alabama. It was discovered in 1912 in the Mooreville Chalk Formation of the Black Belt.
Halisaurus ("ocean lizard") was a smallish type of mosasaur, averaging 10 to 13 feet (~3 to 4 meters). Halisaurus lived from around 85 to 70 million years ago, and fossils tentatively identified as this species have been found in Perry and Greene Counties.
Mosasaurus ("Meuse lizard," like the descriptive name of the family) were among the larger mosasaurs, reaching up to 59 feet (~18 meters) in length. Based on the body shape and physical traits, scientists believe these animals were surface feeders. Three species have been identified in Alabama: Mosasaurus conodon, Mosasaurus maximus, and tentatively Mosasaurus missouriensis. Mosasaurus is notable for being featured in the 2015 film Jurassic World.
Platecarpus ("flat wrist") averaged 14 feet (~4 feet) long and lived from around 89 to 66 million years ago. Platecarpus fossils have been discovered in great states of preservation, allowing scientists to learn much about their physiology. For example, one specimen showed some evidence of having two lungs, reinforcing the evidence that mosasaurs are lizards. Three species have been identified in Alabama: Platecarpus ictericus (tentative), Platecarpus somenensis, and Platecarpus tympaniticus.
Plioplatecarpus ("more flat wrist") averaged 18 feet (~5.5 meters) and lived around 83 million years ago. Animals identified as this type of mosasaur had very large eyes and very recurved teeth and may have been deep-water predators. Fossils of these animals have been found in the Demopolis Chalk in Alabama.
Prognathodon ("forejaw tooth") species were large, ranging from around 20 feet (~6 meters) to 46 feet (~14 meters). These animals lived from about 83 to 66 million years ago. They had crushing and slicing teeth, which may be an adaptation for feeding on shelled animals such as ammonites and turtles. They also had bony rings in their eye sockets, which would have protected their eyes from pressure in deeper waters.
Selmasaurus ("Selma lizard," for its occurrence in the Mooreville Formation of the Selma Group) lived between 85 and 70 million years ago. Because skull parts are the most commonly found fossil material for this animal, its average size is unknown. The type specimen for this mosasaur is located in the Alabama Museum of Natural History in Tuscaloosa and was first identified as a separate genus in 1975 by University of Alabama graduate student Samuel Wayne Shannon.
Tylosaurus Hunting
It was not, however, formally named Selmasaurus until 1988. Selmasaurus russelli is the only species so far identified in Alabama, with the species name in honor of Dale A. Russell, a prominent mosasaur expert. It was first described in an article coauthored by Alabama paleontologist and award-winning fiction author Caitlin R. Kiernan.
Tylosaurus ("protuberance lizard") were also among the larger mosasaurs, reaching up to 50 feet (~15 meters) or more and with a long time range from around 89 to 66 million years ago. The Latin name comes from the elongated snout filled with pointed, recurved teeth. Like the equally large Mosasaurus species, the Tylosaurus would have been a top predator in Late Cretaceous seas.

Additional Resources

Ebersole, Jun, and Takehito Ikejiri. Contributions to Alabama Cretaceous Paleontology. Bulletin 31, Vol. 1. Tuscaloosa: Alabama Museum of Natural History, 2013.
Kiernan, Caitlin. “Stratigraphic Distribution and Habitat Segregation of Mosasaurs in the Upper Cretaceous of Western and Central Alabama, with an Historical Review of Alabama Mosasaur Discoveries.” Journal of Vertebrate Paleontology 22 (March 2001): 91-103.
Russell, Dale A., and Shelton P. Applegate. The Vertebrate Fauna of the Selma Formation in Alabama. Chicago, Ill.: Field Museum of Natural History, 1970.
Thurmond, John T., and Douglas E. Jones. Fossil Vertebrates of Alabama. Tuscaloosa: University of Alabama Press, 1981.
Wright, Kenneth R. [Caitlin R. Kiernan], and Samuel Wayne Shannon. " Selmasaurus russelli, a New Plioplatecarpine Mosasaur (Squamata, Mosasauridae) from Alabama." Journal of Vertebrate Paleontology 8 (March 1988): 102-7.
Published:  October 1, 2015   |   Last updated:  October 13, 2016