''Clidastes'' is an extinct

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of marine lizard belonging to the

mosasaur Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Ancient Greek, Greek ' meaning 'lizard') are an extinct group of large aquatic reptiles within the family Mosasauridae that lived during the Late Cretaceous. Their first fossil remains wer ...

family. It is classified as part of the

Mosasaurinae The Mosasaurinae are a subfamily of mosasaurs, a diverse group of Late Cretaceous marine Squamata, squamates. Members of the subfamily are informally and collectively known as "mosasaurines" and their fossils have been recovered from every contin ...

subfamily, alongside genera like ''

Mosasaurus ''Mosasaurus'' (; "lizard of the Meuse (river), Meuse River") is the type genus (defining example) of the mosasaurs, an extinct group of aquatic Squamata, squamate reptiles. It lived from about 82 to 66 million years ago during the Campanian an ...

'' and ''

Prognathodon ''Prognathodon'' 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 ''Clidastes''. ''Prognathodon'' has been recovered from depos ...

''. ''Clidastes'' is known from deposits ranging in age from the

Coniacian The Coniacian is an age or stage in the geologic timescale. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series and spans the time between 89.8 ± 1 Ma and 86.3 ± 0.7 Ma (million years ago). The Coniacian is preceded by ...

to the early

Campanian The Campanian is the fifth of six ages of the Late Cretaceous epoch on the geologic timescale of the International Commission on Stratigraphy (ICS). In chronostratigraphy, it is the fifth of six stages in the Upper Cretaceous Series. Campa ...

in the

United States The United States of America (USA), also known as the United States (U.S.) or America, is a country primarily located in North America. It is a federal republic of 50 U.S. state, states and a federal capital district, Washington, D.C. The 48 ...

. ''Clidastes'' means "locked vertebrae", which originates from the

Greek Greek may refer to: Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group *Greek language, a branch of the Indo-European language family **Proto-Greek language, the assumed last common ancestor of all kno ...

noun κλειδί, or kleid meaning key (akin to

Latin Latin ( or ) is a classical language belonging to the Italic languages, Italic branch of the Indo-European languages. Latin was originally spoken by the Latins (Italic tribe), Latins in Latium (now known as Lazio), the lower Tiber area aroun ...

''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 hydropedalIn mosasaurs, the terms "hydropedal" and "plesiopedal" refers to varying limb conditions and varying degrees of adaptations for marine life. Plesiopedal mosasaurs, such as ''

Dallasaurus ''Dallasaurus'' ("Dallas County, Texas, Dallas lizard") is a basal mosasauroid from the Upper Cretaceous of North America. Along with ''Russellosaurus'', ''Dallasaurus'' is one of the two oldest mosasauroid taxa currently known from North America ...

'' or '' Tethysaurus'' were primitive and largely coastal, while later hydropedal mosasaurs were streamlined and well-adapted to marine life. mosasaurs, representing one of the first properly marine predatory forms alongside other early hydropedal genera like ''

Tylosaurus ''Tylosaurus'' (; "knob lizard") is a genus of Russellosaurina, russellosaurine mosasaur (an extinct group of predatory marine Squamata, lizards) that lived about 92 to 66 million years ago during the Turonian to Maastrichtian stages of the Late ...

'' and ''

Platecarpus ''Platecarpus'' ("oar wrist") is an extinct genus of aquatic lizards belonging to the mosasaur family, living around 84–81 million years ago during the middle Santonian to early Campanian, of the Late Cretaceous period. Fossils have been found ...

''. It was likely an agile swimmer that preyed upon

cephalopod A cephalopod is any member of the molluscan Taxonomic rank, class Cephalopoda (Greek language, Greek plural , ; "head-feet") such as a squid, octopus, cuttlefish, or nautilus. These exclusively marine animals are characterized by bilateral symm ...

fish A fish (: fish or fishes) is an aquatic animal, aquatic, Anamniotes, anamniotic, gill-bearing vertebrate animal with swimming fish fin, fins and craniate, a hard skull, but lacking limb (anatomy), limbs with digit (anatomy), digits. Fish can ...

and other small vertebrates in shallow water. Isotopic analysis on teeth specimens has suggested that this genus and ''

'' may have entered freshwater occasionally, just like modern sea snakes.

Description

''Clidastes'' was one of the smallest of the mosasaurs (the smallest known being ''

''), averaging in length, with the largest specimens reaching long.Cope, E.D. 1868. On new species of extinct reptiles. Proceedings of the Academy of Natural Sciences of Philadelphia 20: 181 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. The Osteology of the Reptiles p181 Fig-146

The Osteology of the Reptiles p181 Fig-146

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.Olivier, R., & Maureen, K. (December 01, 2005). Tooth Replacement in the Late Cretaceous Mosasaur Clidastes. Journal of Herpetology, 39, 4.) 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 Sweden, formally the Kingdom of Sweden, is a Nordic countries, Nordic country located on the Scandinavian Peninsula in Northern Europe. It borders Norway to the west and north, and Finland to the east. At , Sweden is the largest Nordic count ...

,Lindgren, J., & Siverson, M. (January 01, 2004). The first record of the mosasaur Clidastes from Europe and its palaeogeographical implications. Acta Palaeontologica Polonica, 49, 219-234.

Germany Germany, officially the Federal Republic of Germany, is a country in Central Europe. It lies between the Baltic Sea and the North Sea to the north and the Alps to the south. Its sixteen States of Germany, constituent states have a total popu ...

,Caldwell, M.W., & Diedrich, C.G. 2005. Remains of ''Clidastes'' Cope, 1868, an unexpected mosasaur in the upper Campanian of NW Germany. (Igitur.) Igitur.

Russia Russia, or the Russian Federation, is a country spanning Eastern Europe and North Asia. It is the list of countries and dependencies by area, largest country in the world, and extends across Time in Russia, eleven time zones, sharing Borders ...

Mexico Mexico, officially the United Mexican States, is a country in North America. It is the northernmost country in Latin America, and borders the United States to the north, and Guatemala and Belize to the southeast; while having maritime boundar ...

, and the

Maastrichtian The Maastrichtian ( ) is, in the International Commission on Stratigraphy (ICS) geologic timescale, the latest age (geology), age (uppermost stage (stratigraphy), stage) of the Late Cretaceous epoch (geology), Epoch or Upper Cretaceous series (s ...

Jordan Jordan, officially the Hashemite Kingdom of Jordan, is a country in the Southern Levant region of West Asia. Jordan is bordered by Syria to the north, Iraq to the east, Saudi Arabia to the south, and Israel and the occupied Palestinian ter ...

. 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 Edward Drinker Cope (July 28, 1840 – April 12, 1897) was an American zoologist, paleontologist, comparative anatomist, herpetologist, and ichthyologist. Born to a wealthy Quaker family, he distinguished himself as a child prodigy interested i ...

discovered the first specimens of ''Clidastes propython'' in 1869 from the Mooreville Chalk in

Lowndes County, Alabama Lowndes County is in the central part of the U.S. state of Alabama. As of the 2020 census, the county's population was 10,311. Its county seat is Hayneville. The county is named in honor of William Lowndes, a member of the United States Con ...

. 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 The humerus (; : humeri) is a long bone in the arm that runs from the shoulder to the elbow. It connects the scapula and the two bones of the lower arm, the radius and ulna, and consists of three sections. The humeral upper extremity consists of ...

and

femora The femur (; : femurs or femora ), or thigh bone is the only bone in the thigh — the region of the lower limb between the hip and the knee. In many four-legged animals the femur is the upper bone of the hindleg. The top of the femur fits in ...

with round heads, similar to that of mammals. Due to good preservation of the caudals, Sternberg noted that the

chevrons Chevron (often relating to V-shaped patterns) may refer to: Science and technology * Chevron (aerospace), sawtooth patterns on some jet engines * Chevron (anatomy), a bone * '' Eulithis testata'', a moth * Chevron (geology), a fold in rock la ...

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 ''

'' than to any other mosasaur, firmly placing it within the subfamily

. 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 ''

'' but less derived than later genera like ''

'' or '' Globidens''. The cladogram below is modified from Aaron R. H. Leblanc, Michael W. Caldwell and Nathalie Bardet, 2012: Clidastes BW

There is only one named species of ''Clidastes'' that is valid, ''C. propython''. ''Clidastes iguanavus'' Cope, 1868 was the original type species, but the ICZN was petitioned to make ''C. propython'' the new type species by virtue of that species being based on diagnostic remains, which it did vis-à-vis Opinion 1750 (1993).

Invalid species

There is also an undescribed form from the

Mooreville Chalk Formation The Mooreville Chalk is a geological formation in North America, within the U.S. states of Alabama and Mississippi, which were part of the subcontinent of Appalachia. The strata date back to the early Santonian to the early Campanian stage of th ...

Alabama Alabama ( ) is a U.S. state, state in the Southeastern United States, Southeastern and Deep South, Deep Southern regions of the United States. It borders Tennessee to the north, Georgia (U.S. state), Georgia to the east, Florida and the Gu ...

that likely represents a new taxon on its own, informally dubbed ''"Clidastes moorevillensis"'', which can be distinguished from both ''C. propython'' and ''C. liodontus'' based on its dental characteristics. ''Clidastes liodontus'' was described from the late Coniacian to early

Smoky Hill Chalk Member of the Niobrara Formation in

Kansas Kansas ( ) is a landlocked U.S. state, state in the Midwestern United States, Midwestern region of the United States. It borders Nebraska to the north; Missouri to the east; Oklahoma to the south; and Colorado to the west. Kansas is named a ...

. There are also earlier occurrences of the species, dated to the

, and it might thus be ancestral to the later ''C. propython''. ''C. liodontus'' grew to about 3–4 meters in length compared to the 4-5 meter (and on occasion larger) length of ''C. propython''. The type specimen of ''C. liodontus'', consisting of

maxilla In vertebrates, the maxilla (: maxillae ) is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. The two maxil ...

e, a

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammals h ...

and dentaries from the

Niobrara Formation The Niobrara Formation , also called the Niobrara Chalk, is a geologic formation in North America that was deposited between 87 and 82 million years ago during the Coniacian, Santonian, and Campanian stages of the Late Cretaceous. It is com ...

of Kansas, was housed at the Bayerische Staatssammlung für Paläontologie and may have been destroyed in the

Second World War World War II or the Second World War (1 September 1939 – 2 September 1945) was a World war, global conflict between two coalitions: the Allies of World War II, Allies and the Axis powers. World War II by country, Nearly all of the wo ...

. Russell (1967) diagnosed the species in general as follows: "Premaxilla V-shaped in horizontal cross-section, small rostrum present anterior to premaxillary teeth. Posteroventral portion of root of second premaxillary tooth not exposed on sutural surface with maxilla. Premaxillo-maxillary suture rises posteriorly to position varying from dorsal to fourth to dorsal to sixth maxilIary tooth and parallels longitudinal axis of cranium. Fourteen to fifteen teeth in maxilla. Median dorsal surlace of parietal narrow. Parietal foramen small, close to or distinctly separated from frontal suture. Parietal foramen opens ventrally into brain cavity without broadening into wide excavation. Anterior border of prootic descends beneath prootic incisure without forming shelf. Foramen for cranial nerve VII leaves brain cavity through medial wall of prootic. Infrastapedial process absent on quadrate. Sixteen teeth in dentary." Lively (2019) declared ''Clidastes liodontus'' a ''

nomen dubium In binomial nomenclature, a ''nomen dubium'' (Latin for "doubtful name", plural ''nomina dubia'') is a scientific name that is of unknown or doubtful application. Zoology In case of a ''nomen dubium,'' it may be impossible to determine whether a ...

'', while taking note of the

nomen nudum In Taxonomy (biology), taxonomy, a ''nomen nudum'' ('naked name'; plural ''nomina nuda'') is a designation which looks exactly like a scientific name of an organism, and may have originally been intended to be one, but it has not been published ...

status of "moorevillensis", recommending that ''Clidastes'' be restricted to ''C. propython''.Joshua R. Lively (2019). "Taxonomy and historical inertia: Clidastes (Squamata: Mosasauridae) as a case study of problematic paleobiological taxonomy". Alcheringa: An Australasian Journal of Palaeontology. in press. doi:10.1080/03115518.2018.1549685.

''Clidastes propython''

''C. propython'' is the best studied species of the genus, and was for this reason chosen by the ICZN to replace ''C. iguanavus'' as the type species. ''C. propython'' is known from the

of the

(

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

Texas Texas ( , ; or ) is the most populous U.S. state, state in the South Central United States, South Central region of the United States. It borders Louisiana to the east, Arkansas to the northeast, Oklahoma to the north, New Mexico to the we ...

and

South Dakota South Dakota (; Sioux language, Sioux: , ) is a U.S. state, state in the West North Central states, North Central region of the United States. It is also part of the Great Plains. South Dakota is named after the Dakota people, Dakota Sioux ...

) and of

. The earliest known occurrences of the species are middle

Santonian The Santonian is an age in the geologic timescale or a chronostratigraphic stage. It is a subdivision of the Late Cretaceous Epoch or Upper Cretaceous Series. It spans the time between 86.3 ± 0.7 mya ( million years ago) and 83.6 ± 0.7 m ...

in age and from the

of Kansas, whilst the latest are Middle to Late

in age, coinciding with a poorly understood middle Campanian intercontinental mosasaur extinction event, which seems to have heavily affected genera such as ''Clidastes''. Russell (1967) listed the following unambiguous character states for the species: "Premaxilla V-shaped in horizontal cross-section, small. rostrum present anterior to premaxillary teeth. Posteroventral portion of root of second premadlary tooth exposed on sutural surface with maxilla. Premaxillo-maxillary suture rises posteriorly in gentle curve to terminate at point above seventh maxillary tooth. Premaxillary suture of maxilla smoothly keeled and paraIIels longitudinal axis of maxilla. Sixteen-18 teeth in maxilla. Median dorsal surface of parietal moderately broad. Parietal foramen small, lies close to suture with frontal and opens ventrally into elliptical excavation in parietal, length of which exceeds that of dorsal opening by about five times. Anterior border of prootic forms shelf beneath prootic incisure, then descends abruptly to basisphenoid. Foramen for cranial nerve VII leaves brain cavity through medial wall of prootic. Infrastapedial process present on quadrate. Seventeen to eighteen teeth in dentary.". Tylosaurus sp kansas

Russell (1967) also referred a large number of fragmentary species of ''Clidastes'' to ''C. propython'' on the basis of that those with good cranial material were morphologically indistinguishable from the type specimen of ''C. propython''. Among these former species now seen as synonyms of ''C. propython'' are ''C.'' "''cineriarum''", ''C.'' "''dispar''", ''C.'' "''velox''", ''C.'' "''wymani''", ''C.'' "''pumilus''", ''C.'' "''tortor''", ''C.'' "''vymanii'', ''C.'' "''stenops''", ''C.'' "''rex''", ''C.'' "''medius''" and ''C.'' "''westi''".

''Clidastes iguanavus''

The

''C. iguanavus'' is the original type species of ''Clidastes'' and poorly known in comparison to ''C. propython'' and ''C. liodontus''. The type specimen consists of a single vertebra from the anterior thoracic region, YPM 1601, collected in a marl pit near Swedesboro,

New Jersey New Jersey is a U.S. state, state located in both the Mid-Atlantic States, Mid-Atlantic and Northeastern United States, Northeastern regions of the United States. Located at the geographic hub of the urban area, heavily urbanized Northeas ...

. The vertebra is similar to that of the other species in its general proportions and the strong zygosphene-zygantrum articulation. ''C. iguanavus'' can be differentiated in its central articulations, which are kidney-shaped in outline, with a stronger emargination dorsally for the spinal cord, and in the relatively stout proportions of the centrum.

References

* Callison, G. (1967). Intracranial mobility in Kansas mosasaurs. Lawrence * Charles H. Sternberg Transactions of the Kansas Academy of Science Vol. 30 (Apr. 18, 1919 - Feb. 19, 1921), pp. 119–120 * Cope, E.D. 1868. On new species of extinct reptiles. Proceedings of the Academy of Natural Sciences of Philadelphia 20: 181 * Dobie, J. L., Daniel, R. W., & Bell, G. L. (June 19, 1986). A Unique Sacroiliac Contact in Mosasaurs (Sauria, Varanoidea, Mosasauridae). Journal of Vertebrate Paleontology, 6, 2, 197–199. * Kiernan, C. R. (January 1, 2002). Stratigraphic distribution and habitat segregation of mosasaurs in the Upper Cretaceous of western and central Alabama, with an historical review of Alabama mosasaur discoveries. Journal of Vertebrate Paleontology, 22, 1, 91-103. * Lindgren, J & Schulp, A. (September 1, 2010). New material of Prognathodon (Squamata: Mosasauridae), and the mosasaur assemblage of the Maastrichtian of California, U.S.A. Journal of Vertebrate Paleontology, 30, 5.) * Wright, K. R. (September 23, 1988). The First Record of Clidastes liodontus (Squamata, Mosasauridae) from the Eastern United States. Journal of Vertebrate Paleontology, 8, 3, 343-345

Notes

{{Taxonbar, from=Q134597 Mosasaurines Mosasaurs of North America Mosasaurs of Europe Fossil taxa described in 1868 Taxa named by Edward Drinker Cope Mooreville Chalk