Clidastes is an extinct genus of marine lizard belonging to the mosasaur family. It is classified as part of the Mosasaurinae subfamily, alongside genera like Mosasaurus and Prognathodon. Clidastes is known from deposits ranging in age from the Coniacian to the early Campanian in the United States. Clidastes means "locked vertebrae", which originates from the Greek noun κλειδί, or kleid meaning key (akin to Latin claudere meaning to shut). This refers to how the vertebral processes allow the proximal heads of the vertebrae to interlock for stability and strength during swimming. It was one of the earliest hydropedal mosasaurs, representing one of the first properly marine predatory forms alongside other early hydropedal genera like Tylosaurus and Platecarpus. It was likely an agile swimmer that preyed upon cephalopods, fish and other small vertebrates in shallow water. Isotopic analysis on teeth specimens has suggested that this genus and Platecarpus may have entered freshwater occasionally, just like modern sea snakes. Description Clidastes was the one of the smallest of the mosasaurs (the smallest known being Dallasaurus), averaging in length, with the largest specimens reaching long. The generic name refers to how the vertebral processes allow the proximal heads of the vertebrae to interlock for stability and strength during swimming. Even though the vertebrae lock together, the living animal would have still had a range of motion in the horizontal plane that is sufficient to allow for the high quality of swimming in shallow waters. Additionally the strengthening of the tail, and entire backbone, allowed for muscle attachments to help it swimming. It possessed a delicate and slim form with an expansion of the neural spines and chevrons near the tip of the tail and this enabled it to chase down the fastest of prey. Due to being a well-represented and well-studied genus, Russell (1967) could list a large range of unambiguous character states for the genus, including the following: "Premaxilla with or without small rostrum anterior to premaxillary teeth. Fourteen to eighteen teeth in maxilla. Prefrontal forms small portion of posterolateral border of external nares, broad triangular ala projects laterally from supraorbital wing. Prefrontal and postorbitofrontal widely separated above orbits. Lateral margins of frontal nearly straight and converge anteriorly, median dorsal ridge weak. Ventral process of postorbitofrontal to jugal confluent with broadly exposed dorsal surface of postorbitofrontal. No ventroposterior process on jugal. Parietal foramen small, located entirely within parietal. Margins of dorsal parietal