Plesiosaurians (= plesiosaurs) are derived sauropterygians. This group also includes close relatives of the plesiosaurs: nothosaurs, pachypleurosaurs, and the pistosaurids and other non-plesiosaurian pistosaurians. Owen (1860) originally proposed the term Sauropterygia to encompass plesiosaurs and ‘nothosaurs’ (the concept of nothosaurs was much broader at the time than it is now)(Storrs, 1991). Although the interrelationships between different sauropterygian taxa has changed considerably with time, Owen’s early observations were so accurate that this 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).
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, 1986; 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)
This nested classification for Sauropterygia shows the position of Plesiosauria relative to the other major sauropterygian clades.