Anatomy


All plesiosaurs have four flippers, a short body and a short tail. The skeleton below belongs to Meyerasaurus victor, one of the most complete plesiosaur specimens ever found. It shows the ventral (underside) surface of the animal (redrawn from Fraas, 1910). Click on a region of the skeleton to skip to information about that part of the anatomy.

Cranium and jaws

There is considerable variation in plesiosaur skull anatomy. N.B. This section of the page is currently under construction.

Postcranium

The postcranial bones illustrated on this page belong to Muraenosaurus (from Andrews, 1910).

The postcranial skeleton of plesiosaurs can be divided into the axial skeleton the vertebral column and ribs) and the paraxial skeleton (limb girdles and limbs). A character common to nearly all plesiosaurs is a pair of foramina subcentralia (two small openings for blood vessels and/or nerves) on the ventral surface of the centrum. The axial skeleton can be further divided as follows…

The cervical or neck vertebrae

Plesiosaur cervical ribs are borne on the centrum but the rib facets migrate dorsally as one moves from anterior (front) vertebrae to posterior (back) vertebrae. The number of rib heads is usually two in early plesiosaurs and one in later forms, the cervical ribs themselves are short. The articular surfaces range from amphicoelus (concave) to platycoelus (flat) depending on the species.

Anterior cervical vertebra

Posterior cervical vertebrae

Dorsal or back vertebrae

The cervical series merges into the dorsal series through a transitionary series of pectoral vertebrae. During this transition the rib facets move from the centrum (as seen in the cervicals) to the neural arch, becoming elongate transverse processes in the dorsal vertebrae. These support long ribs. The centra are often distinctly spool shaped.

Dorsal vertebra from front

Caudal or tail vertebrae

In the caudal series, or tail, the ribs return to the centrum. The caudal vertebrae also bear chevrons on their venral surfaces and thar articular faces of the centra are more angular.

caudal vertebra

The pecotral girlde

The pectoral (shoulder) girdles of plesiosaurs are expanded in thin plates ventrally, the main element in the pectoral girdle are the coracoids and the scapulae – where these meet on the lateral (outside) surface they form a glenoid cavity to accommodate the forelimb. The scapulae may or may not meet on the midline depending on the species. A large clavicle-interclavicle complex is present in early plesiosaurs but is lost in later forms.

The pectoral girdle from A. above, and B. the side

The pelvic girdle

As in the pectoral girdle the elements form expanded plates. The pubis is situated anteriorly and the ischium posteriorly. The ilium is a small element which bridges the gap between the pectoral girdle and the sacral ribs – it contacts the ischium in plesiosaurs only.

The pelvic girdle from A. above, and B. the side

Gastralia

Positioned between the pectoral and pelvic girldles is a basket of tightly packed ‘belly ribs’ or gastralia. Each row consists of a boomerang-shaped central element and about three simple gastralia on each side.

The limbs

Both the fore and hind limbs have developed into wing-like flippers, and closely resemble each other. The propodial (humerus/femur) is relatively long, the epipodials (radius, ulna/ tibia, fibula) are shortened, becoming wider than long is some taxa. The mesopodials are disk-like. The metapodials and phalanges (finger bones) are quite short and waisted. Plesiosaurs show hyperphalangy (increase in the number of finger bones). There may or may not be a number of accessory bones in the epi-mesopodial area, depending on the species.

Plesiosaur limbs from below: A. hindlimb, and B. forelimb