Description and Classification

The animal formerly known as Derasmosaurus is now understood to be Clidastes. Clidastes was a medium-sized mosasaur, typically growing to lengths of about 2 to 4 meters (around 6.5 to 13 feet), although some specimens could reach up to 6 meters (about 20 feet). It had a slender, streamlined body, which likely made it a fast and agile swimmer. Its limbs were modified into flippers, and it had a long, powerful tail that probably ended in a fluke, similar to a shark’s, to propel it through the water. Its jaws were lined with sharp, pointed teeth, well-suited for grasping slippery prey like fish and squid.

Mosasaurs, including Clidastes (and therefore the animals once called Derasmosaurus), were not dinosaurs. They were large marine lizards belonging to the order Squamata, which also includes modern lizards and snakes. In fact, mosasaurs are considered to be closely related to monitor lizards and snakes. Within the Mosasauridae family, Clidastes is placed in the subfamily Mosasaurinae, a group that includes other well-known genera like Mosasaurus itself. The reclassification of Derasmosaurus as Clidastes occurred as paleontologists studied more complete fossils and gained a better understanding of the variations within mosasaur species.

Distinguishing Features

Key features that characterized the mosasaur Clidastes (which includes fossils originally attributed to Derasmosaurus) include:

• A relatively slender and streamlined body compared to some larger contemporary mosasaurs like Tylosaurus, suggesting agility.
• A comparatively smaller adult size among mosasaurs, typically ranging from 2 to 4 meters in length.
• A skull that was moderately long and somewhat triangular, with a flexible (kinetic) structure that allowed it to swallow large prey whole.
• Specialized vertebrae in its back and tail. These vertebrae had extra joints that interlocked (the feature that gave Clidastes its name, meaning “locked vertebrae”), which stiffened parts of the spine. This adaptation likely contributed to more powerful and efficient swimming strokes driven by its tail.
• Four paddle-like flippers used for steering and balance, while the primary propulsive force came from its long, muscular tail.

Paleoenvironment and Diet

Fossils of Clidastes (the animal that Derasmosaurus refers to) are primarily found in marine sediments from the Western Interior Seaway. This was a vast, relatively shallow inland sea that divided North America into two landmasses during much of the Late Cretaceous period. The climate was warm, and this seaway was teeming with marine life. Clidastes shared these waters with a diverse community of animals, including other types of mosasaurs, long-necked plesiosaurs like Elasmosaurus, large sea turtles such as Archelon, various species of sharks including Cretoxyrhina, and abundant fish and cephalopods like ammonites.

As a carnivore, Clidastes was an active predator. Its sharp teeth and agile body indicate it likely hunted swift-moving prey. Its diet probably consisted mainly of fish, squid, and possibly smaller marine reptiles. Being a medium-sized predator, Clidastes would have also needed to be cautious of larger hunters in its ecosystem, such as bigger mosasaurs or large sharks.

Significance and Ongoing Research

While the name Derasmosaurus itself is no longer actively used by scientists for a distinct genus, its history is significant as it illustrates how scientific understanding of prehistoric animals evolves over time. Early paleontologists like Edward Drinker Cope often named new genera based on limited or fragmentary fossil material. As more complete skeletons are discovered and existing specimens are re-analyzed with new techniques, some of these early names are found to refer to animals already known by another name, leading to synonymization. This is a routine and important process in paleontology that helps to accurately classify ancient life.

Research on Clidastes, the genus that now includes the fossils once named Derasmosaurus, continues to provide valuable insights into the world of mosasaurs. Scientists study Clidastes fossils to learn more about mosasaur anatomy, their methods of swimming, their dietary preferences, and their role within the complex marine food webs of the Late Cretaceous. Discoveries related to Clidastes help paint a clearer picture of life in the Western Interior Seaway and the remarkable evolutionary journey of reptiles that adapted to marine environments and became apex predators before the mass extinction event at the end of the Cretaceous period.