Just About Mosasaurs
tylo-col.gif (26006 bytes)


Just About Mosasaurs

Copyright 1996-2011 by Mike Everhart

Last revised 03/04/2011







Left: Tylosaurus proriger, a large marine reptile from the Western Interior Sea during the Late Cretaceous from about 90 to 65 million years ago).  Modified after an illustration by Gregory S. Paul.


First and most importantly, mosasaurs are not dinosaurs. They are extinct marine reptiles that are believed to be distantly related to monitor lizards such as the Komodo Dragon.  Based on recent evidence, however, it may be that they were even more closely related to snakes than monitor lizards.  The discovery and study of mosasaurs near Maastricht in Europe in the late 1700s predated the finding of dinosaurs by more than fifty years.  Many complete mosasaur specimens have been found in the Smoky Hill Chalk Member of the Niobrara Formation of Western Kansas, and some of the first mosasaur remains  were collected by Professor Benjamin Mudge and Dr. George M. Sternberg  more than 130 years ago.  A few years later, in a series of scientific expeditions sponsored by O. C. Marsh and Yale University, hundreds of specimens were collected. As a group, the fossilized remains of mosasaurs had been found all over the world, from Kansas to South Dakota to North Dakota to  Montana to  New Mexico to Colorado to Texas to Arkansas to Tennessee to Georgia to Alabama to New Jersey to  California, to Canada, from the Netherlands to Sweden, from Turkey to Israel to Africa, and from Brazil to Peru to Australia and New Zealand to the islands off the coast of Antarctica.  

Since 1998, Jim Martin,  a paleontologist from the South Dakota School of Mines and Technology has collected several partial sets of mosasaur remains from Vega Island off the eastern side of the Antarctic Peninsula.  According to Martin, the specimens included several juveniles which are very rare in the fossil record.  A new species of tylosaurine mosasaur was described from the same area by Fernando Novas and others in 2002. 

Shown below is a drawing from Williston (1898) that shows the skeletons of three common species of mosasaurs from Kansas; Clidastes propython, Platecarpus tympaniticus and Tylosaurus proriger. Although these three species are shown about the same size in the drawing, in life, Clidastes was the smallest (about 12-15 feet); Platecarpus was the next largest (about 24 feet) and Tylosaurus was the largest (30 plus feet):

1mosasa.jpg (48613 bytes)

New data on the skull and size of Tylosaurus nepaeolicus

A revised biostratigraphy of mosasaurs in the Smoky Hill Chalk of Western Kansas

Mosasaurs: Last of the Great Marine Reptiles


Click here for some excellent drawings of mosasaur skulls from Williston (1898)

Click here for the dig of an early Tylosaurus proriger (Kansas)

Click here for the dig of a Platecarpus tympaniticus (Kansas)

Click here for the dig of a Clidastes liodontus (Kansas)

Click here for the dig of a Platecarpus planifrons(?) (Kansas)

Click here for the dig of a rare Globidens (shell crushing) mosasaur (Kansas)

Click here for pictures of one of the largest mounted mosasaur specimens in the world (Kansas)

Click here for pictures of a very large Plioplatecarpus from North Dakota

Click here for the dig of a Mosasaurus conodon from South Dakota

The word "mosasaur" literally means "Meuse Reptile", and was so named because the first remains of a mosasaur to be discovered were found close to  the Meuse River, near the town of Maastricht in the Netherlands, inside an underground limestone mine sometime between 1770 and 1774. Eventually, this specimen was given the name Mosasaurus hoffmanni in honor of Dr. C. K. Hoffman who had studied the remains and believed them to be those of a giant crocodile.  For those who are interested in the early history of paleontology, the battle for possession of this specimen is a very unusual story, which includes being confiscated by the French army in 1795 in exchange for several cases of wine. The specimen is still in the French national museum in Paris, so the story continues even today.  

In 1845, a German naturalist by the name of Dr. George August Goldfuss published a paper describing a mosasaur from the wilds of North America (in what was to become South Dakota).  The specimen  (Mosasaurus Maximiliana) had been acquired by Prince Maximilian zu Wied during his visit to North America in the 1830s and is in the Goldfuss collection of the natural history museum at the University of Bonn,  Bonn, Germany.

For more  information about the origin and meaning of mosasaur names, go to Ben Creisler's Mosasauridae Translation and Pronunciation Guide, a recent addition to the The Dinosauria On-Line Dinosaur Omnipedia.

Many millions of years ago, probably during the early part of the Cretaceous Period, the ancestors of mosasaurs left the land and began to adapt to the marine environment in much the same manner that the ancestors of modern whales returned to the sea.  Like whales, mosasaurs also had to surface periodically to breathe.  Instead of an "up and down" movement their tail like whales and porpoises, however, mosasaurs used a sinuous, undulating movement of their tails to propel themselves rapidly through the water. This movement would have been much like that of a swimming alligator or snake.  Check out this excellent, technical paper by Richard Cowen if you would like more information about the Locomotion and Respiration in Marine Air Breathing Vertebrates.

This picture of a re-constructed Mosasaurus skull  was adapted from a photograph provided by Greg Livaudais.

By the time of the deposition of the Smoky Hill Chalk (beginning about 87 mya), mosasaurs were quickly becoming the dominant carnivores in many marine environments around the world, feeding on fish, squid, ammonites, turtles, small plesiosaurs, and even other mosasaurs. Although some other reptiles, including plesiosaurs (and Kronosaurus),  and marine crocodiles like Deinosuchus, were as large and certainly as dangerous, none were as successful in terms of numbers, or as wide spread as the mosasaurs. Another kind of marine reptile, the Ichthyosaurs , were almost extinct by the time the mosasaurs were evolving. This diverse and highly specialized group had ruled the oceans for millions of years during the Triassic and Jurassic periods, but became extinct well before the end of the Cretaceous.  The remains of some of the largest Ichthyosaurs known (Shonisaurus popularis) were found in the western United States (Nevada).  It is very possible that mosasaurs were so successful because they were able to move into the ecological niches left vacant by the extinction of the Ichthyosaurs.

Like the Ichthyosaurs,   mosasaurs gave live birth to their young and may have even provided them some form of parental protection.  Evidence exists that mosasaurs of all sizes and age groups were living in mid-ocean during the deposition of the Smoky Hill Chalk in the Western Interior Seaway.

The Cretaceous ocean must have been a very hostile environment for all creatures.   Mosasaurs shared the ocean with giant turtles like Protostega, huge fish like Xiphactinus, a creature that grew to a length of more than 18 feet and had three inch long teeth.  There were giant sharks like Cretoxyrhina mantelli, that cruised the shallow waters of the Inland Sea and may have grown to as large as 22 feet in length. These sharks looked much like (but were not closely related to) the Great White Shark in the movie, Jaws. Swarms of smaller sharks called Squalicorax scavenged carcasses of dead animals, and may have preyed on the sick and injured.

The air above the ocean was also filled with threats to small creatures on or near the surface.  Giant Pteranodons, flying reptiles with wingspreads of more than 20 feet and mouths that gapped open more than three feet, were always on the prowl for food for themselves and their young. Since the remains of these Pteranodons are found in the middle of the seaway, hundreds of miles from the nearest shore, these large flying reptiles must have been able to soar over the ocean for hours or days at a time, much like a modern albatross. There was even a six foot tall, flightless sea bird, called Hesperornis regalis, that swam in the Late Cretaceous seas and dived to catch fish much like a modern penguin. We know that it was carnivorous because its jaws were still equipped with many sharp teeth. It must have preyed on whatever smaller animals that it could catch. Smaller birds, like Ichthyornis, would have looked and acted much like the seagulls and terns of today. Even these small sea birds had sharp teeth in their jaws.

Mosasaurs  preyed on smaller individuals of other species, including ammonites, fish, birds and even smaller mosasaurs. One genus, called Tylosaurus,  reached lengths of 30 to 40 feet in the Western Interior Sea during the deposition of the chalk . By the Late Cretaceous, just before the end of the Age of Dinosaurs 65 million years ago, mosasaurs were becoming larger and more specialized.  Some kinds of mosasaurs such as Mosasaurus and Hainosaurus, were as large as 45-50 feet in length while others, like Globidens, had developed specialized, ball shaped teeth for crushing clams or other hard shelled invertebrates.  All things considered, it was not a very safe or friendly place to live.  Mosasaurs were at the top of the food chain, but life could be very short for the unwary.  Fortunately for humankind, the mosasaurs also became extinct along with the dinosaurs at the end of the Cretaceous Period. They are not something you would want to share your oceans with.

Click here to read about "Mosasaurs: Last of the great marine reptiles"

Click here to visit the Oceans of Kansas Paleontology Virtual Mosasaur Museum

Click here for some excellent drawings of mosasaur skulls from Williston (1898)

Click here for a list of references in my library about mosasaurs and plesiosaurs.

Click here for more information about the meaning and origin of mosasaur names in Ben Creisler's Translation and Pronunciation Guide