Plesiosaurs (=Plesiosauria) are derived sauropterygians. This group includes close relatives of the plesiosaurs: nothosaurs, pachypleurosaurs, and the pistosaurids (and other non-plesiosaurian pistosaurians). Owen (1860) originally coined Sauropterygia to encompass plesiosaurs and ‘nothosaurs’. The concept of nothosaurs then was much broader than it is now (Storrs 1991), and the interrelationships between different sauropterygian taxa has changed considerably. However, Owen’s early observations were accurate enough that the basic concept of Sauropterygia has endured to the present day, and the group is a valid clade (i.e. a natural group with a shared common ancestor).

Skulls of various basal (non-plesiosaurian) sauropterygians in dorsal view (Compiled and modified from Storrs 1993)

Clades are defined by a number of common derived characters (synapomorphies) that indicate their shared ancestry. Synapomorphies of the clade Sauropterygia include (Carpenter 1997, Sues 1987, Storrs 1993):

  • A single (upper or supra) temporal fenestra in the skull roof (the lower temporal fenestra was lost
    and so an excavated cheek margin is a remnant of this feature in many taxa).
  • ‘Closed’ palate (without openings) in which the pterygoids cover the basis cranii (braincase)
  • Absence of the following skull bones: supratemporal, postparietal, and
    tabular (and lachrimal?).
  • Retracted nares – the nostrils are situated close to the orbit rather than on the snout tip.
  • Large retroarticular process on the mandible (for opening the jaws)
  • Three to six sacral vertebrae
  • Absence of an ossified sternum (maybe a reduced cartilaginous sternum was present).
  • Divided scapulocoracoid (separate scapula and coracoid)
  • Pectoral fenestra in the pectoral girdle and and thyroid fenestra in the pelvic girdle.
  • Scapula lies superficial to the clavicle (the posterior part of the
    clavicle overlies the anterior part of the scapula)
  • small ilium
  • Absence of humerus ectepicondylar foramen (opening in humerus)

Sauropterygian classification

This nested classification for Sauropterygia shows the position of Plesiosauria relative to the other major sauropterygian clades. The term ‘basal sauropterygian’ is often applied to non-plesiosaurian sauropterygians.