Plesiosaurus (Greek: πλησίος/plesios, near to + σαῦρος/sauros, lizard) was a large 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, Russia 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 the order Plesiosauria, of which it is an early, but fairly typical member. P. brachypterygius, P. guilielmiiperatoris and P. tournemirensis were assigned to new genera, Hydrorion, Seeleyosaurus, and Occitanosaurus. It lived from 200-65 mya (Jurassic-Cretaceous).
Discovery[]

Letter concerning the discovery of the genus from Mary Anning
The first complete skeleton of Plesiosaurus was discovered by early paleontologist and fossil hunter Mary Anning in Sinemurian (Early Jurassic)-age rocks of the lower Lias Group in December 1823. Additional fossils of Plesiosaurus were found in rocks of the Lias Group of Dorset for many years, "until the cessation of quarrying activities in the Lias Group, early in this [20th] century."
Plesiosaurus was one of the first of the "antediluvian reptiles" to be discovered and excited great interest in Victorian 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 genus Plesiosaurus is the sole member of the family Plesiosauridae. It is a rather poorly understood family, mostly because Plesiosaurus is a waste-basket taxon. This means that many different specimens have all been allocated to the genus Plesiosaurus, even when they were very different and probably deserve a name all of their own. This confusion was initiated during the 1800's when plesiosaurs were first being studied and described. Many of the species are now renamed and most of them do not even belong in the family Plesiosauridae. For example, 'Plesiosaurus' rostratus and 'Plesiosaurus' conybeari have been renamed Archaeonectrus and Attenborosaurus respectively; they are actually both pliosaurids. There are now no more than three valid species of Plesiosaurus (Storrs, 1997) although some remain e.g. 'Plesiosaurus' macrocephalus pending revision. The valid species (dolichodeirus, guilielmiiperatoris and brachypterygius) comprise the family Plesiosauridae.
Description[]

Plesiosaurus with a human to scale.
Plesiosaurus was a fairly typical member of its order, and measured around 3 to 5 metres (10 to 16 feet) in total length. The snout was short, but the mouth was able to open very wide, and the jaws were provided with a series of conical teeth in sockets, much like those of the living gavial. The neck was long and slender, but this seems to have been rather stiff, because the vertebrae are nearly flat-ended, which indicates that it could not have been bent in the swan-fashion represented in many old restorations. The other vertebrae are similarly almost flat-ended and firmly united, and there is no sacrum. The ribs are single-headed, and in the middle of the trunk, between the supports of the paired limbs, they meet a dense plastron of abdominal ribs. The short tail was straight and tapered rapidly at the end. The pectoral and pelvic girdles which supported the paired limbs are greatly expanded, the pectoral arch being similar to the corresponding bones of turtles. The limbs were elongated paddles, with five complete digits, although each consists of a very large number of phalanges. Some traces of skin discovered suggest that it was smooth, not scaly.
Plesiosaurus has a small head when compared to other plesiosaurs. The cranium is narrower more than long, with the width reaching its peak at the postorbital bar. The anterior is bluntly triangular. In the lateral, the skull peaks in height at the rear of the skulls table. The nostrils lie over the internal nares, but are not positioned near the tip of the snout. Instead, they are closer to the eyes. Unlike in Rhomaleosaurus, the nostrils do not appear adapted for underwater olfaction. The orbits are a rough circle, positioned near-halfway along the skull, facing up to the side. Posterior of the orbits are the supratemporal fenestrae, near the same size as the orbits and roughly circular. Between all four fenestrae is the pineal foramen, and between the temporal fenestrae is a thin sagittal ridge. The pterygoids in the palate to basioccipital of the braincase, like other plesiosaurs. Additionally, the union is not nearly as robust as Rhomaleosaurus and Pliosaurus. Though the palate bones are thin, no suborbital fenestra are present.

The type P. dolichodeirus, now at the Natural History Museum.
The two rami in the mandible form a V and an angle of ~45°, and the symphasis is robust with the rami fusing to it. This creates a pointed, shallow, scoop-like form. Plesiosaurus had simple conical needle-like teeth, slightly curving and circular in the transverse section. They sharply point and bear fine striations that run from tip-to-tip. They are procumbent, becoming more pronounced near the tip of the skull, where they reach ~10-15° above horizonal. 20-25 teeth line the upper jaw and 24 for the lower jaw. Up to 4 teeth in the mandible are in the symphyseal region.

Conybeare's illustration of Anning's Plesiosaurus in the naming paper.
Plesiosaurus was moderately-sized, at 3.5 meters (11 feet) long. ~40 cervical vertebrae exist, with some specimens preserving 38-42. There are a handful (4 or 5) oof pectoral vertebrae in the rest of the column in the neck-torso transition. ~21 dorsals, 3+ sacrals and at least 28 caudals are reported. Typically, the centra in the cervicals are relatively elongate, slightly longer than tall. However, the width is generally greater or equal to the length. The articular surfaces in the cervical centra are slightly concave, kidney-shaped and have rounded, slightly-rugose edges. The foramina subcentralia are on the ventral surface of the centra, and some dorsals have rugose articular edges, like the cervicals, but is usually absent in the caudals. Ribs are reported from the neck of the tail. The cervical ribs shape like hatchets, having two articular heads. The dorsal ribs are thick, and have one head. The sacral ribs are short, robust, and are blunt or knob-like at both ends. However, depending on their placement, the caudal ribs have varying morphologies. Their anterior morphology is pointed and distals are broad and blunt. Plesiosaurus bears gastralia, having 9 or more sets between the shoulder and pelvis. Each set has seven components, a bone at the midline having three lateral pieces at the flank.

Life restoration
Plesiosaurus has a typical plesiosaurian shoulder girdle; fused clavicles at the anterior, scapulae and large coracoids. The latter two contribute to the glenoids. Pectoral fenestrae sit midway down the scapular/coracoid contacts. The forelimbs are elongated and (relatively) narrow compared to most plesiosaurs. The humerus curves distinctly, which may be a retained primitive trait of sauropterygians. Mature individuals bear a distinct grove on the ventral surface of the humerus. The ulna are flat, broad and crescent-shaped and the radius is robust and pillar-like. In the wrist is 6 bones. The hand paddle has 5 digits, but the phalangeal formula is debated, One large individual, from the thumb to the 5h digit, has a formula of 4-8-9-8-6. The pelvis consists of equant pubic bones, ischia, and ilis that are blade-shaped and connect it to the vertebrae. The acetabelum forms on the pubic bones and ischia. Like the pectoral girdle, 2 holes sit between the ischia and pubic bones. The hindlimbs are long and narrow, and much smaller than the other pair in adult. The thigh bones are straight, and the lower hindlimb consists of 2 bones that are roughly equal in size. These are the robust tibia and semilunate-shaped fibula. Six bones make the ankle, and the foot paddle has 5 digit. Like in the hand, the phalangeal formula is uncertain, but is 3-7-9-8-7 at the least from the innermost to outermost digits.
Species[]

Skeleton of P. guilielmiiperatoris
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.
- Plesiosaurus dolichodeirus is the type species, known from the Lower Lias (Sinemurian) of Lyme Regis, which was about three metres long. Other plesiosaurs from the same formation measure between five to six meters in length.
- Plesiosaurus guilelmiimperatoris is known from a large almost complete skeleton from the Upper Lias (Toarcian) of Württemberg. There seems to be the impression of a rhomboidal flap of skin in a vertical plane; if so, many plesiosaurs may have been equipped in this way.
- 'P'. macrocephalus is quite distinct from other Plesiosaurus and deserves generic status of its own. It is currently under revision. An outdated description is given by Andrews (1896).
- 'P'. brachypterygius
Classification[]

Referred specimen, the first Plesiosaurus skeleton found - used to name the species in 1821

Historically important sculpture (left) in Crystal Palace Park

Modern restoration
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. guilielmiiperatoris and P. tournemirensis, for example, were assigned to the new genera Hydrorion, Seeleyosaurus and Occitanosaurus. For example, no Younger Jurassic or Cretaceous specimens are Plesiosaurus, and only P. dolichodeirus remains. Benson et al. (2012) place Plesiosaurus in this cladogram:
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Stratigraphy[]

Tentatively referred specimen in Calgary
Unequivocal specimens of P. dolichodeirus are limited to the Lyme Regis area of Dorset. It appears to be the most common species of plesiosaur in the Lias Group of England. Plesiosaurus is best represented from the "upper part of the Blue Lias, the 'Shales with Beef,' and the lower Black Ven Marls"; 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.
Paleoecology[]

Restoration
Unequivocal P. dolichodeirus are reported only to the Lyme Regis area in Dorset, and appears to be the most common plesiosaur of the Lias Group. This taxon is best known from the Upper Blue Lias, "Shales of Beef" and the Lower Black Ven Marls. Using ammonites from the Lias Group, the rocks are Early Sinemurian-aged, with some from later dates. The oldest specimen is Late Rhaetian or Early Gettangian.
Plesiosaurus fed mainly on clams and snails, and are thought to have eaten belemnites, 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 sea snakes. The young might have lived in estuaries 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.
Appearance in Jurassic Park and other media[]
Jurassic Park[]
- It can be created from DNA in Jurassic Park III: Park Builder.
- It can be created from DNA in Jurassic World: The Game.
- The Plesiosaur Pond was an attraction originally meant to appear in Jurassic World but was cut from the final film. It was to be a large pool like the Jurassic World Lagoon inhabited by Plesiosaurus and several unidentified species of ichthyosaurs. Visitors to Jurassic World would be able to visit the attraction by boarding a submarine attached to a monorail. Despite the attraction being cut from the film, Plesiosaurus is one of the animals that could be viewed on the Holoscape of the Innovation Center, possibly meaning it could have very well lived in Jurassic World.
Read more Plesiosaurus on Jurassic Park Wiki |