| Plesiosaurus | |
|---|---|
 | |
| Mary Anning's plesiosaur: thePlesiosaurustype specimen on exhibit atNatural History Museum (specimen NHMUK OR 22656) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | †Sauropterygia |
| Order: | †Plesiosauria |
| Superfamily: | †Plesiosauroidea |
| Family: | †Plesiosauridae |
| Genus: | †Plesiosaurus De la Beche &Conybeare, 1821 |
| Species: | †P. dolichodeirus |
| Binomial name | |
| †Plesiosaurus dolichodeirus Conybeare, 1824 | |
Plesiosaurus (Greek:πλησίος (plesios), near to +σαῦρος (sauros), lizard) is a genus of extinct, large marinesauropterygianreptile that lived during theEarly Jurassic. It is known by nearly complete skeletons from theLias of England. Measuring around 3.5 m (11 ft) long, it is distinguishable by its small head, long and slender neck, broad turtle-like body, a short tail, and two pairs of large, elongated paddles. It lends its name to the orderPlesiosauria, of which it is an early, but fairly typical member. It contains only one species, thetype,Plesiosaurus dolichodeirus. Other species once assigned to this genus, includingP. brachypterygius,P. guilielmiimperatoris, andP. tournemirensis have been reassigned to new genera, such asHydrorion,Seeleyosaurus andMicrocleidus.
The first complete skeleton ofPlesiosaurus was discovered by early paleontologist and fossil hunterMary Anning inSinemurian (Early Jurassic)-age rocks of the lowerLias Group in December 1823 nearLyme Regis inDorset,England.[1][2] Additional fossils ofPlesiosaurus were found in rocks of the Lias Group ofDorset for many years,[3][4][5] "until the cessation of quarrying activities in the Lias Group, early in this [20th] century."[2] although less complete remains were used byHenry De la Beche andWilliam Conybeare to name the species two years earlier in 1821,[6] and despite being discovered first, Conybeare's remains were not theholotype; Anning's were.
Plesiosaurus was one of the first of the "antediluvian reptiles" to be discovered and excited great interest in 19th-century England. It was so-named ("near lizard") byWilliam Conybeare andHenry De la Beche, to indicate that it was more like a normal reptile thanIchthyosaurus, which had been found in the same rock strata just a few years earlier.Plesiosaurus is the archetypical genus of Plesiosauria and the first to be described, hence lending its name to the order. Conybeare and De la Beche coined the name for scattered finds from the Bristol region,Dorset, andLyme Regis in 1821.[6] Thetype species ofPlesiosaurus,P. dolichodeirus, was named and described by Conybeare in 1824 on the basis of Anning's original finds.
Compared to other plesiosaur genera,Plesiosaurus has a small head. The skull is much narrower than long,[7] reaching its greatest width just behind the eyes (thepostorbital bar).[8] Theanterior portion is "bluntly triangular".[8] In lateral view, the skull reaches its highest point at the rear of theskull table.[9] "The external nostrils overlie the internal nares".[8] They are not positioned at the tip of the snout, but farther back, nearer the eyes than the tip of the skull.[7] Unlike the nostrils ofRhomaleosaurus,[10] they do not appear to be adapted for underwater olfaction.[8] Theorbits (eye sockets) are roughly circular and are positioned about halfway along the length of the skull.[8] They face up and to the sides.[7][9] Just posterior to the orbits are thesupratemporal fenestrae, which are about the same size as the orbits and also roughly circular.[8] Between the four openings is thepineal foramen, and between the temporal fenestrae is a narrow sagittal ridge.[8] As in other plesiosaurs, thepterygoids of the palate are fused to thebasioccipital of thebraincase,[8] although the union is not as robust as in the pliosaursRhomaleosaurus andPliosaurus.[8][11] "The palatal bones are thin, but there is no suborbital fenestra."[8]
The tworami of the lower jaw make a "V" shape with an angle of about 45°.[7] The specialized region where they meet, thesymphysis, is robust. The two rami are fused at the symphysis, making a pointed, shallow scoop-like shape.[12]
The teeth ofPlesiosaurus are "simple, needle-like cones" that are "slightly curved and circular in transverse section". They are sharply pointed with fine striations running from tip to base, and point forward (procumbent). This procumbency becomes more pronounced near the leading end of the skull, where they may be only 10–15° above horizontal.[7] There are 20 to 25 teeth per upper jaw tooth row,[8] and 24 per low jaw tooth row.[7] Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.[12]
Plesiosaurus was a moderately sized plesiosaur that grew to 2.87–3.5 m (9.4–11.5 ft) in length.[13][14] There are approximately 40cervical vertebrae (neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae.[15] Of the rest of the vertebral column, there are a handful (four or five in theholotype specimen) of "pectoral" vertebrae from the neck-torso transition,[15] approximately 21 dorsal or back vertebrae, three or moresacral vertebrae, and at least 28caudal vertebrae.[16] Generally, thecentra of the cervical vertebrae are relatively elongated, being slightly longer than tall. The width, however, is usually greater than or equal to the length. The articular surfaces of the cervical centra are "slightly concave and kidney-shaped, with rounded, slightly rugose edges." Small holes called foramina subcentralia are found on the ventral surface of the centra. Some of the dorsals have rugose articular edges, like the cervicals; this feature is typically absent from the caudals.[15]
Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads.[15] Dorsal ribs are thick and have only one head. Sacral ribs are "short, robust, and blunt or knob-like on both ends." Caudal ribs have differentmorphologies depending on their location along the tail, with anterior examples being pointed and more distal examples being "broad and blunt."[15]Plesiosaurus also hasgastralia, also known as "belly ribs." Nine or more sets of gastralia are present between theshoulder andpelvis. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.[16]
The shoulder girdle is only partly known but appears to be typical for plesiosaurs. It includes fusedclavicles at the anterior end,scapulae (shoulder blades), and largecoracoids. The scapulae and coracoids both contribute to theglenoids (arm sockets). A pair of oval holes called pectoral fenestrae are found midway along the scapular/coracoid contacts.[16] The forelimbs are elongate and relatively narrow compared to those of most plesiosaurs. Thehumerus (upper arm bone) has distinctive curvature, which appears to be a retainedprimitive feature amongsauropterygians. MaturePlesiosaurus also have a distinctive groove along the ventral surface of the humerus. The forearm includes a flat, broad, crescent-shapedulna and a "robust and pillar-like"radius. The wrist includes six bones.[17] The hand paddle has five digits; thephalangeal formula is uncertain, but the count for one large individual, from "thumb" to fifth "finger", is 4-8-9-8-6.[18]
The pelvis includes equantpubic bones,ischia,[18] and blade-shapedilia connecting the pelvis to the vertebral column.[19] Theacetabulum is formed by surfaces on the pubic bones and ischia. Similar to the pectoral girdle, there is a pair of holes between the ischia and pubic bones.[18] The hindlimbs are long and narrow,[19] and in adults, they are much smaller than the forelimbs.[18] Thethigh bones are straight. The lower hindlimb includes two roughly equal-sized bones, the robusttibia and the semilunate-shapedfibula. There are six bones in the ankle. The foot paddle includes five digits. Like the hand, the phalangeal formula is uncertain, but is at least 3-7-9-8-7 from innermost to outer "toe".[19]
Plesiosaurus has historically been awastebasket taxon. This is due in part to few anatomical or taxonomic studies of the relevant fossils. Uncritical taxonomic work resulted in hundreds of species representing most of the world and most of the Mesozoic being assigned toPlesiosaurus. None of the youngerJurassic orCretaceous species belong toPlesiosaurus. Review of the Early Jurassic species indicates that the only English species properly assigned toPlesiosaurus isP. dolichodeirus.[2] Several other European Early Jurassic species have been assigned to new genera.P. brachypterygius,P. guilielmiimperatoris andP. tournemirensis, for example, were assigned to the new generaHydrorion,Seeleyosaurus andOccitanosaurus.
The followingcladogram follows an analysis by Bensonet al., 2012, and shows the placement ofPlesiosaurus within Plesiosauria.[20]
| Pistosauria |
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Plesiosaurus fed mainly on clams and snails, and is thought to have eatenbelemnites, fish and other prey as well.[21] Its U-shaped jaw and sharp teeth would have been like a fish trap. It propelled itself by the paddles, the tail being too short to be of much use. Its neck could have been used as a rudder when navigating during a chase.Plesiosaurus gave live birth to live young in the water like mostsea snakes. The young might have lived inestuaries before moving out into the open ocean.It has been postulated that the long neck ofPlesiosaurus would have been a hindrance when trying to speed up, any bend in the neck creating turbulences.[22] If that is the case thenPlesiosaurus would have had to keep its neck straight to achieve good acceleration, something that would make hunting difficult. For this reason it may be possible that these animals would actually lie in wait for prey to come close instead of trying to pursue them.
Unequivocal specimens ofPlesiosaurus dolichodeirus are limited to the Lyme Regis area of Dorset.[23] It appears to be the most common species of plesiosaur in the Lias Group of England.[24]Plesiosaurus is best represented from the "upper part of theBlue Lias, the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of theCharmouth Mudstone; using the Lias Groupammonite fossil zones, these rocks date to the earlySinemurian stage. Some otherPlesiosaurus fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from lateRhaetian or earlyHettangian rocks.[25]
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