Plesiosaurus

Credit: Mark Witton.

*Plesiosaurus* is an extinct genus of plesiosaur that lived in Europe during the Early Jurassic.

## History

The first complete *Plesiosaurus* was discovered by Mary Anning in Sinemurian rocks from the Lower Lias Group, December 1823. Other remains were found in Lias Group, Dorset rocks many years after until quarrying was ended in the 20th century. However, Henry De la Beche and William Conybeare named the species in 1821 based off less-complete material. Despite Conybeare's being found first, Anning's was considered the holotype. *Plesiosaurus* was one of the first "antediluvian reptiles", which excited and sparked great interest when it was discovered in Victorian England. Conybeare and De la Beche named it the "near lizard" to signify it was more similar to other reptiles than to the ichthyosaurs found in the same strata earlier. *Plesiosaurus* is the archetypical genus of pelsiosauria and the first described, and the order was named after it. Conybeare and De la Beche named it after scattered material from Bristol, Dorset and Lyme Regis in 1821. They erected the type, *P. dolichodeirus*, in 1824 after Anning's material. Most of the material and species previously assigned to the genus have been identified as undiagnostic plesiosaurian scraps dumped into *Plesiosaurus*.

## Description

*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 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.

*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.

*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.

## Classification

*Plesiosaurus* has been used as a wastebasket taxon, due to few anatomical or taxonomic studies. This has resulted in hundreds of species from most of the world and most of the Mesozoic being referred to *Plesiosaurus*. For example, no Younger Jurassic or Cretaceous specimens are *Plesiosaurus*, and only *P. dolichodeirus* remains. Benson et al. (2012) place *Plesiosaurus* in this cladogram:

## Species

### Reassigned Species

* *"P." balticus* (=Rhomaleosauridae)
* *"P." bernardi* (=Rhomaleosauridae, =*"Cimoliasaurus" cantabrigiensis*?)
* *"P." conybeari* (=Plesiosauridae)
* *"P." costatus* (=Rhomaleosauridae)
* *"P." hawkinsi* (=Rhomaleosauridae)
* *"P." macrocephalus*
* *"P." holmesi* (=*"P." hoodi*?, =Elasmosauridae gen. et sp. indet.)
* *"P." hoodi* (=*"P." holmesi*?, =Elasmosauridae gen. et sp. indet.)
* *"P." mauretanicus* (=Elasmosauridae?, gen. et sp. indet.)
* *"P." rostratus* (=*Rhomaleosaurus* and *Telmatosaurus*)
* *"P." smithi* (=Plesiosaurian indet.)
* *"P." latispinus* (=Plesiosauridae indet.)
* *"P." valdensis* (=Plesiosauridae)
* *"P." phillipsi* (=Plesiosauridae)
* *"P." ellipsospondylus* (=Plesiosauridae)
* *"P." giganteus* (=Pliosauridae)
* *"P." recentior* (=Pliosauridae)
* *"P." simplex* (=Pliosauridae)
* *"P." affinis* (=Plesiosauridae)
* *P.? subtrigonus* (=Plesiosauridae)
* *"P." macromus* (=Plesiosauridae)
* *P.? eleutheraxon* (=Plesiosauridae)
* *P.? bitractensis* (=Plesiosauridae)
* *"P." platydeirus* (=Plesiosauridae gen. et sp. indet.)
* *"P." rostratus* (=Rhomaleosauridae)
* *P.? coelospondylus* (=Plesiosaurian)
* *P.? erraticus* (=Plesiosaurian)
* *"P." houzeaui* (=Elasmosauridae)
* *"P." degenhardti* (=Plesiosauridae)
* *"P." limnophilus* (=Plesiosauridae)
* *"P." kanzleri* (=Plesiosauridae)
* *"P." pachyomus* (=Plesiosauria)
* *P.? dewalqi* (=Plesiosauridae)
* *P.? pentagonus* (=Plesiosauridae)
* *P.? trigonus* (=Plesiosauridae)
* *P.? bitractensis* (=Plesiosauridae)
* *"P." gurgitis* (=Plesiosauridae)
* *"P." guilelmi imperatoris* (=Plesiosauridae)
* *P.? nothosauroides*
* *P.? robustus* (=Plesiosauridae)
* *P.? keuperinus* (=Plesiosauridae)
* *P.? baruthicus* (=Plesiosauridae)
* *P.? suevicus* (=Plesiosauria)
* *P.? posidoniae* (=Plesiosauria)
* *"P. neocomiensis*" (=Plesiosauria)
* *"P." intermedius* (=Polycotylidae)
* *"P." gulo* (=Polycotylidae)
* *"P." gouldi* (=Polycotylidae)
* *"P." shirleyensis* (=Plesiosauridae)
* *"P." brevifemur* (=Elasmosauridae)
* *"P." mexicanus* (=Elasmosauridae)
* *"P." hesternus* (=Plesiosauria)
* *"P." neogaeus* (=Plesiosauria)
* *"P." mackayi* (=Plesiosauria)
* *"P." traversi* (=Plesiosauria)
* *"P." sutherlandi* (=*P.? ceolospondylus*)
* *"P." mudgei* (=*"P." affinis*)
* *"P." nordmanni* (=*"P." affinis*)
* *"P." vaccinulensis* (=*"P." affinis*)
* *"P." doedicomus* (="non-plesiosaurian")
* *"P." brachycephalus* (=*P.? baruthicus*)
* *"P." brachypterygius* (=*P.? baruthicus*)
* *"P." carinatus* (=*P.? baruthicus*)
* *"P." cliduchus* (=*P.? baruthicus*)
* *"P." etherirdgi* (=*P.? baruthicus*)
* *"P." hexagonalis* (=*P.? baruthicus*)
* *"P." hexatarsostinus* (=*P.? baruthicus*)
* *"P." ichthyospondylus* (=*P.? baruthicus*)
* *"P." infraplanus* (=*P.? baruthicus*)
* *"P." macrospondylus* (=*P.? baruthicus*)
* *"P." macrourus* (=*P.? baruthicus*)
* *"P." pentatarsostinus* (=*P.? baruthicus*)
* *"P." plicatus* (=*P.? baruthicus*)
* *"P." sterrodeirus* (=*P.? baruthicus*)
* *"P." subdepressus* (=*P.? baruthicus*)
* "P." winspitensis (=*P.? baruthicus*)
* *"P." (Eretmosaurus?) bavaricus* (=*"P." affinis*)

## Paleoecology

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* ate clams and snails, where it may have eaten belemnites, fish and other prey. It's jaw is U-shaped, and sharp  teeth would have acted like a fish trap. It propelled through water mainly with its flippers, since its tail was too small. It may have used its neck as a rudder to navigate while pursuing prey. Plesiosaurus birthed live young in water like sea snakes, where the young may have moved to estuaries to mature before moving out to sea. It has been suggests its long neck may have hindered the animal from gaining speed, with any bends in the neck causing turbulence.

*Plesiosaurus* would have had to straighten its neck to achieve acceleration, which would make hunting particularly difficult. Some hypothesize Plesiosaurus would wait for prey to come close rather than pursuit[*citation needed*].

## References

* https://en.wikipedia.org/wiki/Plesiosaurus
* https://paleoarchive.com/literature/Persson1963-RevisionClassificationPlesiosauria.pdf