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
- pectoral and thyroid fenestration
- 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
Redundant Sauropterygian clade names
The order Plesiosauria (plesiosaurs) consists of two superfamilies. The Pliosauroidea (often termed pliosaurs) are typically short-necked, whereas the Plesiosauroidea (often termed plesiosaurs, but it gets complicated) are typically long-necked. However, generalisations based on neck length are today considered unreliable as research into plesiosaur evolution and phylogeny continues. The family Polycotylidae, for example, is a Late Cretaceous group of short-necked plesiosaurs that are traditionally regarded as pliosaurs. However, cranial evidence (Carpenter, 1997) points to a plesiosauroid affinity for them. Similarly, the pliosauroid genera Eurycleidus and Attenborosaurus are pliosaurs with relatively long-necks.
The Plesiosauria is characterised by the presence of a suite of derived characters and extreme adaptations to life in the water. All plesiosaurs have four large flippers and rigid barrel shaped bodies and share the following synapomorphies (Rieppel, 1997; Carroll, 1988; Storrs, 1993):
- Ventrally expanded (lengthened) pectoral and pelvic girdles
- Loss of nasal bones
- Absence of contact between illium and pubis
- Relatively short trunk and tail
- Very well developed gastralia
- Distinct hyperphalangy and shortening and broadening of the other limb bones transforming the limbs into paddles, of which both fore and aft are similar.
- Foramina subcentralia in their vertebrae (one exception: Brachauchenius, reduced in some other pliosaurs)
- A typical plesiosaur palate consists of vomers, largae palatines and pterygoids (Brown et al. 1994)