All rights reserved. Last updated May 2007. This site first opened in June 2001 and is continuously underconstruction.
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All plesiosaurs have four flippers, a short body, and a short tail. The skeleton below belongs to Rhomaleosaurus victor, one of the most complete specimens ever found. Click on a region of the skeleton to find out more about that part of plesiosaur anatomy. |
Above - Skeleton of Rhomaleosaurus victor in ventral view (underside) (redrawn from Fraas, 1910)
Postcranium
... tour of a typical plesiosaur postcranium (Muraenosaurus)...
The postcranial skeleton of plesiosaurs may be divided into the axial skeleton (the vertebral column and ribs) and the paraxial skeleton (girdles and limbs). A character common to nearly all plesiosaurs is foramina subcentralia (two small holes) on the ventral surface of the centrum. The axial skeleton can be further divided into regions as follows...
The cervical or neck vertebrae. Plesiosaur cervical ribs are borne on the centrum, and 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 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 finally migrate from the centrum (as in cervicals) to the neural arch, and become transverse process in the dorsal vertebrae. These support long ribs. the centra are often distinctly spool shaped. Dorsal vertebra from front:
Caudal or tail vertebrae. From the dorsal series we pass through the pelvic to the caudal series where the ribs return to the centrum. The caudal vertebrae also bear chevrons on their venral surfaces, and the vertebrae are usually more angular when viewed from end-on. Caudal vertebra:
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 accomodate the forelimb the scapulae may or may not meet on the midline depending on the species.
The pelvic girdle. As in the pectoral girdle the elements have expanded into plates, the pubis anteriorly and the ischium posteriorly. The illium is a small element which bridges the gap between the girdle and the sacral ribs - it only contacts the ischium in plesiosaurs.
Gastralia. Between the pectoral and pelvic girldles is a 'basket' of tightly packed belly ribs or gastralia.
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 s number of accessory bones in the epi-mesopodial area, depending on the species. Hindlimb (A) and forelimb (B):
There is a huge variation in skull anatomy in plesiosaurs: to examplify this, here is a small selection (mostly plesiosauroids), but there are also forms with long slender snouts (polycotylids) and forms with large robust jaws and teeth (some pliosaurids).(The scale bars =10 cm in the six top skulls and =5 cm in the lower two. The jugal is stippled grey - this is important for classification of plesiosaurs).
There is a similarity in the gross arrangement of bones throughout all sauropterygian clades although there are many differences too (see classification). Here is a highly simplified plesiosaurian Sauropterygia skull bone arrangement. The unlabelled red bone is the quadrate.(related page: Analysis of the skull of Liopleurodon ferox)