Clidastes is a mosasaur, a carnivorous sea reptile. It is unique because it was the one of the smallest of the mosasaurs (the smallest being Dallasaurus), averaging 2–4 meters (6.6–13.1 ft) in length, with the largest specimens reaching 6.2 meters (20 feet) long. Clidastes was an agile swimmer that probably swam by undulating its tail, and preyed on cephalopods, fish and other small vertebrates in shallow water.
Description[]
Clidastes was the one of the smallest of the mosasaurs (the smallest known being Dallasaurus), averaging 2–4 meters (6.6–13.1 ft) in length, with the largest specimens reaching 6.2 meters (20 feet) 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 surface parallel one another and cranial midline to posterior base of diverging suspensorial rami, forming narrow rectangular field medially on parietal. Squamosal sends abbreviated wing medially to contact ramus irom parietal. Otosphenoidal crest on prootic covers exit for cranial nerve VII laterally. Fourteen to sixteen teeth in pterygoid. Suprastapedial process of quadrate moderately large; tympanic ala very thick. Stapcdial pit elliptical in form. Sixteen-18 teeth in dentary. Small projection of dentary aritcrior to first dentary tooth. Medial wing Irom angular contacts or nearly contacts coronoid. Dorsal. edge of surangular very thin Iamina of bone rising anteriorly to position high on posterior surface of coronoid. Retroarticular process of articular triangular in outline with heavy dorsal crest. Mandibular teeth usually compressed, bicarinatc and with smooth enamel surfaces." Russell noted that his diagnosis was exclusively based on C. propython and C. liodontus and might not necessarily apply to C. sternbergii (later referred to its own genus, Eonatator) or C. iguanavus.
Occurrences[]
Clidastes is currently found in marine deposits in the US. In past, however, specimens were referred to this genus from Sweden, Germany, Russia, Mexico, and the Maastrichtian of Jordan. However, Lively (2019) questioned the referral of these remains to Clidastes due to their fragmentary nature and lack of apomorphies placing them in the genus to the exclusion of other mosasaurs.
Discovery[]
E. D. Cope discovered the first specimens of Clidastes propython in 1869 from the Mooreville Chalk in Lowndes County, Alabama. The remains unearthed were that of a juvenile but are one of the best preserved and most complete mosasaurs collected from the state and is regarded as the generic holotype of Clidastes. In 1918, Charles H. Sternberg and his son found additional remains of Clidastes in Kansas. They were surprised to see that it had humeri and femora with round heads, similar to that of mammals. Due to good preservation of the caudals, Sternberg noted that the chevrons along the vertebrae were ankylosed to the center, which is not observed in other mosasaurs. This synapamorphy was believed to aid in fitting the proximal heads snugly into the basins that hew out from the vertebrae almost locking them in place.
Classification and species[]
The dental and vertebral morphology of Clidastes is closer to that of Mosasaurus than to any other mosasaur, firmly placing it within the subfamily Mosasaurinae. Besides being different in size, the teeth of Campanian species of Mosasaurus (namely M. missouriensis and M. conodon) differ from those of Clidastes in having a large number of facets that are also more distinct than those in Clidastes. The cervical vertebrae of Clidastes are also different from those in Mosasaurus by being more elongated.
Clidastes is most frequently recovered as one of the most basal mosasaurines, and the most basal hydropedal mosasaurine genus, being more derived than the plesiopedal Dallasaurus but less derived than later genera like Prognathodon or Globidens. The cladogram below is modified from Aaron R. H. Leblanc, Michael W. Caldwell and Nathalie Bardet, 2012:
