Classification
Plesiosaurs are derived sauropterygians, a group that also includes the close relatives of the plesiosaurs: nothosaurs and pachypleurosaurs. Owen (1860) originally proposed the term Sauropterygia to encompass plesiosaurs and 'nothosaurs' (nothosaurs were a much broader concept at the time) (Storrs, 1991). Although the interrelationships of the constituent taxa has changed considerably, Owen's observations were accurate enough that this basic concept of Sauropterygia has prospered and the name represents a valid clade (natural group) today. Clades are defined by a number of common derived characters (synapomorphies) that indicate common ancestry. Synapomorphies of the clade Sauropterygia include (Carpenter, 1997; Sues, 1986; Storrs, 1993):
- Single (upper) supratemporal fenestra (the lower temporal fenestra was lost - an excavated cheek margin is a remnant in many taxa)
- Closed palate of which the pterygoids cover the basis cranii (brain case) ventrally.
- lack of the following bones of the skull supratemporal, postparietal, and tabular (and lachrimal?).
- retracted nares - situated close to the orbit rather than on the snout tip.
- large retroarticular process on the mandible
- three to six sacral vertebrae
- lack of [ossified] sternum (maybe this was a cartilaginous element).
- divided scapulocoracoid
- scapula lies superficially to the clavicle (i.e. the posterior part of the clavicle overlies the anterior part of the scapula)
- small ilium
- humerus lacks ectepicondylar foramen
Cladogram of Sauropterygia
The cladogram shows the relationships between the major groups within the Sauropterygia and indicates the derived position of plesiosaurs (click taxa to go to respective page). You can also select specific higher taxa and families from the drop down menus in the left side bar of the site
Redundant clade names
'Nothosauriformes', 'Cimoliasauridae'.
