Mosasaurs: The Deadliest Marine Reptiles

The Evolution, and Extinction, of Mosasaurs

proganthodon fossil

Roland Tanglao/Wikimedia Commons/CC BY 2.0

Although they weren't technically dinosaurs, the marine reptiles known as mosasaurs hold a unique place in paleontological history: it was the discovery of a specimen of Mosasaurus in 1764, in a Dutch quarry, that galvanized scientists into the realization that species could become extinct (and that the earth used to be populated by some very strange creatures well before Biblical times). Mosasaurus ("lizard from the Meuse River") was soon named by the renowned naturalist Georges Cuvier, and the general name "mosasaur" attached to other members of this ancient family.

In evolutionary terms, mosasaurs were distinct from three other famous groups of marine reptiles, ichthyosaurs ("fish lizards"), long-necked plesiosaurs, and short-necked pliosaurs. These sleek, reptilian predators may have been responsible for the extinction of the ichthyosaurs by the end of the Cretaceous period (not necessarily by eating them, but by out-competing them for food), and their quick, agile, hydrodynamic builds gave plesiosaurs and pliosaurs a run for their money. Essentially, mosasaurs ruled the seas for about 20 million years, until the K/T Extinction expunged most giant reptiles (and all the marine varieties) from the face of the earth 65 million years ago.

Mosasaur Evolution

While it would be tempting to speculate that mosasaurs evolved from ichthyosaurs and plesiosaurs, this doesn't appear to be the case. The recent discovery of the small, amphibious Dallasaurus, which was capable of swimming as well as walking on land, hints that mosasaurs evolved from early Cretaceous reptiles very similar in appearance to modern monitor lizards (another transitional candidate is the European Aigialosaurus). Less certain is the proposed evolutionary relationship between ancient mosasaurs and modern snakes; the two reptile families share sleek body plans, scaly skin and the ability to open their mouths extra-wide, but the rest is a matter of debate.

In geological terms, one of the odd things about mosasaurs is that their fossils tend to turn up far inland, especially in the western United States and the interior of western Europe, along with other continents. In the case of the U.S., this is because, back in Cretaceous times, much of North America was covered by the "Great Interior Sea" (or the Sundance Sea, as it's also called), a broad but shallow body of water that swamped large portions of modern-day Kansas, Nebraska, and Colorado. Kansas alone has yielded three major mosasaur genera, Tylosaurus, Platecarpus, and Clidastes.

Mosasaur Lifestyles

As you might expect with such a long-lasting family of marine reptiles, not all mosasaurs were in the same weight class or pursued the same diet. The largest individuals of Mosasaurus attained lengths of 50 feet and weights of 15 or so tons, but other genera were considerably sleeker: Tylosaurus, for example, packed only about seven tons into its 35-foot length, and Platecarpus (judging by its fossil remains, the most common mosasaur of North America) was only about 14 feet long and a few hundred pounds.

Why these variations? Reasoning by analogy with modern marine predators, like the Great White Shark, it's likely that bigger mosasaur genera like Mosasaurus and Hainosaurus feasted on their fellow mosasaurs and marine reptiles, while smaller species like Clidastes made do with relatively harmless prehistoric fish. And to judge by the round, pebbly shapes of their teeth, it seems that other mosasaurs like Globidens and Prognathodon specialized in gobbling down shelled prey, ranging from small mollusks and ammonites to larger (and tougher) sea turtles.

At the time they went extinct, mosasaurs were facing increased competition from prehistoric sharks, a good example being Cretoxyrhina (aka the "Ginsu Shark"). Not only were some of these sharks sleeker, faster and more vicious than the likes of Tylosaurus and Globidens, but they may have been smarter as well. The mass extinction of marine reptiles in the wake of the K/T Extinction allowed sharks, the new apex predators, to evolve to bigger and bigger sizes in the course of the Cenozoic Era. The culmination of this trend was the truly enormous (up to 50 feet long and 50 tons) Megalodon.

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Strauss, Bob. "Mosasaurs: The Deadliest Marine Reptiles." ThoughtCo, Aug. 25, 2020, Strauss, Bob. (2020, August 25). Mosasaurs: The Deadliest Marine Reptiles. Retrieved from Strauss, Bob. "Mosasaurs: The Deadliest Marine Reptiles." ThoughtCo. (accessed June 25, 2021).