Plesiosaurians (= plesiosaurs) are derived sauropterygians. This group also includes close relatives of the plesiosaurs: nothosaurs, pachypleurosaurs, and pistosaurids. 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 remained and 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):
- 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) ventrally.
- 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)
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 also termed plesiosaurs, but it gets complicated) are typically long-necked. However, generalisations based on neck length are now considered unreliable as research into plesiosaur evolution and phylogeny continues to reveal their a more complicated story. 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 secondary adaptations to life in the water. All plesiosaurs have four large flippers and rigid broad bodies. They all possess the following synapomorphies (shared derived characters) in their skeleton (Rieppel, 1997; Carroll, 1988; Storrs, 1993):
- Ventrally expanded (lengthened) pectoral and pelvic girdles
- Absence of nasal bones
- Absence of contact between illium and pubis
- Relatively short trunk (body) and tail
- Very well-developed gastralia ('belly ribs') that form a tight 'basket'
- Hyperphalangy (increase in the number of bones in the digits)
- Shortened and broadened other limb bones (in particular the propodials and epipodials) that have transformed the limbs into flippers, of which both fore and aft are similar in shape and size.
- Paired nutritive foramina subcentralia (small openings, presumably for blood vessels or nerves) on their vertebral centra (one exception: Brachauchenius, and the foramina reduced in some other pliosaurs)