| Plesiopleurodon | |
|---|---|
 | |
| Holotype skull ofPlesiopleurodon wellesi | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Superfamily: | †Plesiosauroidea |
| Family: | †Polycotylidae |
| Genus: | †Plesiopleurodon Carpenter, 1996[1] |
| Species | |
Plesiopleurodon (/ˌpliːsiəˈplʊərədɒn/;Greek:plesios, meaning "near to",pleuro, meaning "side", anddon, meaning "tooth") is anextinctgenus ofMesozoicmarine reptiles, belonging to theSauropterygia, known from theLate Cretaceous of North America. It was named byKenneth Carpenter based on a completeskull with amandible,cervical vertebra, and acoracoid.[2] In naming the specimen, Carpenter noted "Of all knownpliosauroids,Plesiopleurodon wellesi most closely resemblesLiopleurodon ferox from theOxfordian of Europe, hence the generic reference."[2] It was initially described as a pliosauroid due to it short neck, a commontrait of the superfamily (although it is in theorderPlesiosauria).[2][3][4] However, later exploration into the relationships of both groups indicate that not all pliosauroids have short necks and not all plesiosauroids have long necks.[2] Later research indicates it is a member of thePolycotylidae, within thecladeOccultonectia.
It is distinguishable by its short neck, elongated head, and comparatively few teeth in thesymphysis when compared to other plesiosaurs.[2][5]
Theholotype,CM 2815, origin is uncertain, the original paper says it is fromBelle Fourche Shale in the Rattlesnake Hills of Wyoming (lowerCenomanian), but the Carnegie Museum collections data says its from the Hailey Shales Formation.[6][2] Regardless of its origin, it seems to be the only known fossil ofPlesiopleurodon.[2][7]
The skull is 71.2 cm long.[2] The roof was slightly crushed and aparietal is damaged, but is otherwise in good condition. Thepremaxilla forms the dorsal edge of the external naris, likePliosaurus andRhomaleosaurus.[2][8] InLiopleurodon ferox, the premaxilla does not share this connection.[2] LikeL. ferox, the premaxillaries separate thefrontals.[2] Thetemporal fenestra is longer than it is wide.[2] It retains the primitive plesiosaur feature of posterior sloping of the skull.[2] Thequadrate is firmly attached, unlike the attachments seen in mostpolycotylids.[2] The orbit has a reniform, or kidney-like, shape caused by posterior extension of themaxilla toward theprefrontals.[8] The pre-orbital skull length ratio is high, indicating a longer snout than other polycotylids.[7] The postorbital bar is reduced.[8]
Thedentary has many ridges and valleys, becoming especially prominent near the enlargedcaniniform teeth.[2][9] The tooth margin alternates between concave and convex on the dentary.[2] 19 teeth are in the dentaries, but there may be more that are not visible due to presence of thematrix it was mostly uncovered from.[2] The eight caniniform teeth are procumbent, or tilt forward, and smooth but are striated near where they attach to the jaw.[2][8] The skull was not prepared in such a way that allows for examination of thepalate.[5]
The cervical vertebrae are slightly wider than long and have equal lengths and heights.[2] Like other Cretaceous pliosaurs, thecervical ribs have one head.[2]
The coracoid has a long interpectoral bar that is longer than any seen inJurassic pliosaurs.[2]
HowPlesiopleurdon wellesi has been classified within Plesiosauria has varied historically, with this taxon initially being placed within Pliosauridae.[4][8] However, later authors have since classifiedPlesiopleurodon as a polycotylid.[8][7] Fischer and colleagues, in 2018, named a new clade, Occultonectia, to includePlesiopleurodon, the 'Richmond pliosaur' (museum number QM F18041), andSulcusuchus erraini.[8]
The earliest plesiosaurs are solely found in theNorthern hemisphere but diversified in the Jurassic.[10] By theEarly Jurassic, they are regarded ascosmopolitan and are primarily known from theTethys Realm ofEurope andChina.[10] This widespread distribution coincides with a globalsea level rise and the breakup ofPangaea, allowing for marine corridors to be used for dispersal.[10]
Plesiopleurodon, like all plesiosaurs, are obligatory aquatic animals that rely on paraxial propulsion to swim.[10] This meanslocomotion depends on the propulsion from paddle-like flippers, much like howsea turtles move.[10]
The Belle Fourche Shale of northeastern Wyoming has about 400 to 800 feet of noncalcareous shale between thesiliceousMowry Shale below and thecalcareousGreenhorn Formation above.[11] It contains largeironstoneconcretions in the lower 50 feet of theformation.[11]Bentonitebeds are present throughout theshale.[11] Of the shale that extends intoAlberta, the formation primarily consists of shale,siltstone, andsandstone.[12] The sandstones in southeastern Alberta and southwesternSaskatchewan are shallowgas reservoirs.[12]
While the Belle Fourche Shale is dated to the Cenomian, most of thefossils discovered there are from the uppermost portion, more than 500 feet above the base of the formation.[11] The primary fossils found include many species ofammonites andforaminifera, with few significantmacrofossils.[11]
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