o Lepidosauromorpha `--o Sauropterygia `--+?-Claudiosaurus `--+?-Thalattosauriformes |?-Placodontia `--o Eusauropterygia |--Pachypleurosauridae `--+--Nothosauridae `--+--Corosaurus `--+--Cymatosaurus `--+--Pistosauridae `--o Plesiosauria |==Pliosauroidea `--o Plesiosauroidea |--Plesiosaurus `--+--Elasmosauridae `--Cryptocleidoidea
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Plesiosaurus (Greek: plesios, near to + sauros, lizard) was a large (about 3 meters long), marine Sauropterygian reptile that lived during the early part of the Jurassic period, and is known by nearly complete skeletons from the Lias of England and Germany. It was distinguished 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 a larger group of reptiles as a whole Plesiosauria, of which it is an early but fairly typical member.
- Phylum: Chordata
- P. dolichodeirus
- P. guilelmiimperatoris
- P. brachypterygius (jr. synon.?)
- P. tournemirensis (jr. synon.?)
Plesiosaurus was one of the first of the "antediluvian reptiles" to be discovered (by Mary Anning), and excited great interest in Victorian England. It was so-named ("more lizard") by William Conybeare, 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 previously.
Plesiosaurus, along with Ichthyosaurus and the Pterodactyl Dimorphodon, was among the first Mesozoic reptiles to be recognized. Many beautiful specimens were unearthed along the cliffs of Lyme Regis by Mary Anning. Because this was one of the first prehistoric reptiles to be named, it ended up as something of a taxonomic wastebasket, and a great many other Plesiosaur remains were also identified as Plesiosaurus. Hence most "Plesiosaurus" species do not belong to this genus at all, and in fact belong to different families altogether (or are simply too incomplete to be properly identified). Valid species lived during the Early Jurassic only.
Characters: up to 3.5 m; head small; antorbital and temporal regions short; rostrum broad and pointed; pineal foramen prominent; parietals broad; sagittal crest present, sharp & narrow; jugal somewhat elongate; upper temporal fenestra circular; temporal bar narrow; cheek emargination weak; no real pterygoid flange; choanae placed almost directly below external nares; basicranium not covered by posterior expansion of pterygoids; mandibular symphysis not spatulate; 5 pairs of premaxillary teeth; ~24 pairs on dentary; teeth slender, pointed, homodont and without keels, round in cross-section; teeth ornamented with longitudinal ridges (?); neck long (150% body length) with ~40 cervical vertebrae; heads of cervical ribs separated by elongate longitudinal foramen (?); edges of centra articulations often rugose; prominent 'U'-shaped median notch at anterior of clavicular arch (for unossified interclavicle?); fenestrae (= coracoid foramina) of pectoral girdle are ovate and well-separated; scapula with a slender, posteriorly angled dorsal blade; coracoids moderately wide; "acute" (i.e. <45° from midline) angled posterolateral expansion of coracoids in adult; limbs narrow and very elongate; forelimbs slightly longer than hindlimbs (reversed in immature individuals); humerus with strong medial curvature in dorsal or ventral view; humerus with strong posterodistal expansion (entepicondyle?), but weak anterodistal corner (ectepicondyle?); humerus with interepipodial groove on distal end; robust, columnar anterior epipodials (radius & tibia) offset to extend beyond posterior epipodials (ulna & fibula); posterior epipodials crescentic; spatia interossea large and rhomboidal; 4 elements usually present in the proximal, 3 in the distal row of the carpus; 5 constricted metacarpals similar in form to phalanges; epipodials longer than broad; up to 9 phalanges in middle digit; digit IV longest; pubis large, slightly longer than ischium; pubis with generally convex anterior margin; puboischiadic fenestrae small, ovate to circular puboischiadic fenestra; pubis & ischium unfused across median bar; ilium with little twist to shaft; the fifth metapodial retained in the metapodial row.
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Plesiosaurus was exclusively marine, feeding on belemnites, fishes and other prey. It propelled itself by the paddles, the tail being too short to be of much use.
At one time, Plesiosaurus was a wastebasket taxon used to describe any Mesozoic plesiosaur of generally similar appearance. More recently there has been a number of revisions in sauropterygian taxonomy, and many species previously included here have been moved to other genera and families. Only two species are unambiguiously recognised, P. dolichodeirus and P. guilelmiimperatoris.
- Richard Owen, Fossil Reptili of the Liassic Formations, pt iii. (Monogr. Palaeont. Soc., 1865)
- Storrs, GW (1997), Morphological and taxonomic clarification of the genus Plesiosaurus, in JM Callaway & EL Nicholls (eds.), Ancient Marine Reptiles Academic Press, pp. 145-190.