Clidastes

Julius T. Csotonyi Artwork

***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.

## 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*.

### Teeth and tooth replacement

Mosasaur teeth are of rather uniform morphology (with a few exceptions, such as in *Globidens*) with a pointed and curved tooth crown that sits on a pedicel composed of bone. The enamel surface is smooth and the crown is subdivided into a lingual and labial surface while the outer surface of the crown is made of enamel and the inner layer is made of dentine. Fossil specimens show evidence of upright vertically positioned developing replacement teeth. Snakes have been thought of as the only squamates with replacement teeth that develop in a horizontal posteriorly inclined position. Snakes deviate from the usual varanoid pattern of tooth replacement, in that their replacement teeth develop in a horizontal inclined position and rotate, however snakes differ from Mosasaurs because they do not possess the resorption pits found in Mosasaurs.

Mosasaurs, including *Clidastes*, and snakes both share the traits of thecodont tooth implantation, and a recumbent position of replacement teeth. However mosasaurs develop replacement teeth by rotating within the resorption pits that are at the base of functional teeth. This is different from snakes because snakes have recumbent replacement teeth that lay horizontal and rotate into functional position when needed. In mosasaurs like *Clidastes*, once the functional tooth is lost, a new tooth pedicel develops for the replacement tooth. In the case of mosasaurs though, they differ from the thecodont dentition pattern of archosaur and mammals because mosasaurs show true ankylosis and not a fibrous tooth attachment via periodontal ligament that's usually found in mammals and archosaurs.

The marginal tooth rows in mosasauroids like *Clidastes* are found on the premaxilla, maxilla and the dentary. On the dorsal surface of the dentary there is an interdental ridge that separates successive teeth labially. These interdental ridges serve to separate succeeding teeth that grow upward between existing teeth.

## 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:

## Appearance in other media

### *Jurassic Park*

Clidastes appears in *Jurassic Park III: Park Builder*.