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Plesiosaurus (Greek: πλησίος ( plesios), near to + σαῦρος ( sauros), lizard) is a genus of extinct, large marine reptile that lived during the Early Jurassic. It is known by nearly complete skeletons from the Lias Group of England. 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 order Plesiosauria, of which it is an early, but fairly typical member. It contains only one species, the type species, Plesiosaurus dolichodeirus. Other species once assigned to this genus, including P. brachypterygius, P. guilielmiimperatoris, and P. tournemirensis have been reassigned to new genera, such as Hydrorion, Seeleyosaurus and Occitanosaurus.
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") by William Conybeare and Henry De la Beche, to indicate that it was more like a normal reptile than Ichthyosaurus, 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, and Lyme Regis in 1821. The type species of Plesiosaurus, P. dolichodeirus, was named and described by Conybeare in 1824 on the basis of Anning's original finds.
The two rami of the lower jaw make a "V" shape with an angle of about 45°. The specialized region where they meet, the Symphysis menti, is robust. The two rami are fused at the symphysis, making a pointed, shallow scoop-like shape.Storrs 1997 pp. 169.
The teeth of Plesiosaurus 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. There are 20 to 25 teeth per upper jaw tooth row, and 24 per low jaw tooth row. Up to four teeth of a lower jaw's tooth row are found in the symphyseal region.
Ribs are found from the neck to the tail. Cervical ribs are hatchet-shaped and have two articular heads. 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 different morphologies depending on their location along the tail, with anterior examples being pointed and more distal examples being "broad and blunt."Storrs 1997 pp. 170 Plesiosaurus also has gastralium, also known as "belly ribs." Nine or more sets of gastralia are present between the pectoral girdle and pelvis. Each set is composed of seven elements: a bone on the midline flanked by three lateral elements.
The pelvis includes equant pubic bones, ischium, and blade-shaped ilia connecting the pelvis to the vertebral column.Storrs 1997 pp. 178 The acetabulum 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. The hindlimbs are long and narrow, and in adults, they are much smaller than the forelimbs. The femur are straight. The lower hindlimb includes two roughly equal-sized bones, the robust tibia and the semilunate-shaped fibula. 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".
The following cladogram follows an analysis by Benson et al., 2012, and shows the placement of Plesiosaurus within Plesiosauria.
Vertebral column
Plesiosaurus was a moderately sized plesiosaur that grew to in length.Storrs 1997 pp. 149 There are approximately 40 cervical vertebrae (neck vertebrae), with different specimens preserving 38 to 42 cervical vertebrae.Storrs 1997 pp. 170 Of the rest of the vertebral column, there are a handful (four or five in the holotype specimen) of "pectoral" vertebrae from the neck-torso transition, approximately 21 dorsal or back vertebrae, three or more sacrum, and at least 28 .Storrs 1997 pp. 171 Generally, the centra 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.
Limbs
The shoulder girdle is only partly known but appears to be typical for plesiosaurs. It includes fused at the anterior end, (shoulder blades), and large . The scapulae and coracoids both contribute to the glenoid cavity (arm sockets). A pair of oval holes called pectoral fenestrae are found midway along the scapular/coracoid contacts. The forelimbs are elongate and relatively narrow compared to those of most plesiosaurs. The humerus (upper arm bone) has distinctive curvature, which appears to be a retained symplesiomorphy among . Mature Plesiosaurus also have a distinctive groove along the ventral surface of the humerus. The forearm includes a flat, broad, crescent-shaped ulna and a "robust and pillar-like" radius. The wrist includes six bones.Storrs 1997 pp. 173 The hand paddle has five digits; the phalanx bones is uncertain, but the count for one large individual, from "thumb" to fifth "finger", is 4-8-9-8-6.Storrs 1997 pp. 176
Classifications
Plesiosaurus has historically been a wastebasket 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 to Plesiosaurus. None of the younger Jurassic or Cretaceous species belong to Plesiosaurus. Review of the Early Jurassic species indicates that the only English species properly assigned to Plesiosaurus is P. dolichodeirus. Several other European Early Jurassic species have been assigned to new genera. P. brachypterygius, P. guilielmiimperatoris and P. tournemirensis, for example, were assigned to the new genera Hydrorion, Seeleyosaurus and Occitanosaurus.
Palaeobiology
Plesiosaurus fed mainly on clams and snails, and is thought to have eaten , fish and other prey as well. 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 most . The young might have lived in estuary before moving out into the open ocean.
It has been postulated that the long neck of Plesiosaurus would have been a hindrance when trying to speed up, any bend in the neck creating turbulences. If that is the case then Plesiosaurus 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.
Palaeoenvironment
Unequivocal specimens of Plesiosaurus dolichodeirus are limited to the Lyme Regis area of Dorset.Storrs 1997 pp. 148 It appears to be the most common species of plesiosaur in the Lias Group of England.Storrs 1997 pp. 179 Plesiosaurus is best represented from the "upper part of the Blue Lias, the 'Shales with Beef,' and the lower Black Ven Marls" the latter of which form part of the Charmouth Mudstone; using the Lias Group ammonite fossil zones, these rocks date to the early Sinemurian stage. Some other Plesiosaurus fossils are from later Sinemurian rocks. The oldest specimen may be a skull thought to come from late Rhaetian or early Hettangian rocks.Storrs 1997 pp.180
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