Giant Sea Lizards Ruled the Waves While T. Rex Roamed on Land

The mosasaurs are a fearsome group of seagoing carnivores. The mosasaurs evolved into an extensive array of aquatic predators, which ate everything from dinosaurs washed up to the sea to other mosasaurs. While it was thought that these reptiles disappeared from Earth's oceans many years ago, North Dakota fossils are helping to rewrite their history.
The Hell Creek Formation in the west United States is best known for the fossils of Tyrannosaurus. Triceratops and Pachycephalosaurus are just a few of the many dinosaurs that inhabited the area 66 million years ago. Parts of the Hell Creek Formation contain pools of ocean that once belonged to North America, remnants of the vast Western Interior Seaway. A private landowner discovered a strange fossil in one of the pockets of marine strata and took it to North Dakota Geological Survey for identification. Further examination revealed that the bone was a large mosasaur and that it had been taken from the same site. The impact didn't cause the giant lizards to disappear. These bones supported the emerging view that ocean's apex predators were still roaming the Cretaceous waters until the impact. This was the conclusion of a paper published by PaleoBios in August.

Nathan Van Vranken (WVU Potomac State College paleontologist) notes that most of the mosasaur fossils from North America came from sites on the ancient Atlantic Coast or Gulf of Mexico. As the sea level fell and the Western Interior Seaway was drained from the continent, it became apparent that the once-thriving ecosystems were transformed into a "Strangelove ocean" where plankton and water quality declined. This harsh environment is thought to have been devoid of large predators and prey that would allow mosasaurs to survive. The new North Dakota discovery suggests that large mosaurs are still present in the remaining areas of the ancient seaway at the center of the continent. Van Vranken states that they "continued to exist until the end."

One of the difficulties in studying the history of mosasaurs lies in the fact that not all these animals lived in areas that were easily preserved. Amelia Zietlow from Richard Gilder Graduate School paleontology, said that the problem with understanding extinct animal diversity is that the fossil record is not perfect. She was not part of the new study. She notes that some mosasaur species were able to travel in deep, open water far away from the coasts. These species are more difficult to find than species that live near coastal areas where sediment can quickly bury their bodies. Multiple finds made in recent years show that mosaurs continued to thrive right up to the Cretaceous' end. A new study confirms that mosaurs are still alive in the Western Interior Seaway.

North Dakota's mosasaur was large and identified in the new paper as either Prognathodon, or Mosasaurus. This was a large animal that was an apex predator in its environment. Prognathodon can reach lengths of more than 33 feet. The largest Mosasaurus species is 59 feet. This is nearly 20 feet longer then the largest T. rex. Van Vranken states that while smaller mosasaurs probably ate fish and invertebrates but larger ones preferred larger prey like sharks and other marine reptiles. Zietlow states that they were "absolutely, positively, unquestionably apex Marine Predators, and the only thing an fully-grown Mosasaur had to fear was other larger mosasaurs." Van Vranken says that the North Dakota animal was probably caught by whatever it could because its last ocean pool was shrinking. After the massive asteroid impact caused environmental shocks that quickly and significantly shook up Earth's environment, mosaurs went extinct.

Over 30 million years of evolutionary history culminated in the fate of the last Mosasaurs. Van Vranken states that early mosaurs descended from an ancestor of monitor lizards who adapted to a marine life. This was possibly at a time when ocean levels rose and new food sources became available for reptiles who didn't mind getting wet. Dallasaurus and Russellosaurus, both fossils found in Texas' Cretaceous rocks, offer clues that the first mosaurs were a mix of a Komodo Dragon and Mosasaurus.

Over the millions of years of Cretaceous evolution that followed, the feet and scales of mosasaurs evolved into paddles. Their scales were streamlined and keeled, and some species developed tails with a shark-like tail that supported a downward-kinked tail. While some mosasaurs had bulbous, rounded teeth that crushed shells (such as the Globidens), most were sharp-toothed predators and ate almost anything they could get. One of the most striking features of mosasaurs is a second row with pointed teeth on their palate. This trap was extra and spiky, which prevented struggling prey from fleeing. Paleontologists discovered indentations on ammonites' shells that show the impressive teeth of the reptiles. This shows how they grabbed and crushed their shelly food.

The image of mosasaurs that paleontologists have assembled is different from the Hollywood version. This is just like the case with dinosaurs. The Hollywood version is kaiju-sized and has eaten sharks, pterosaurs dinosaurs, humans, and even humans in its rampages. Perhaps this is to keep true to its voracious mosaur reputation. Zietlow states that large mosasaurs, such as Tylosaurus and Mosasaurus, were smaller than the movie's version. They had snake-like scales, and would have had their lips covered instead of being a maw full of snaggletoothed chompers. The living animals were no less impressive than their movie counterparts. Zietlow states that the Mosasaurs are "real-life sea monsters." They have two rows of teeth, whale-like flippers and a shark-like tail. They are also the largest lizards ever to live. What's not love about them?