Plesiosaurs (=Plesiosauria) are a group of secondarily aquatic carnivorous reptiles belonging to the clade Sauropterygia within the diapsid clade Euryapsida (Taylor 1989, Caldwell 1997a). Plesiosauria has retained phylogenetic validity for about 190 years, since the formal identification of the group by de Blainville (1835).
Sauropterygian clades
Cladogram for the main clades within Sauropterygia to show the relative position of Plesiosauria.
The order Plesiosauria consists of two superfamilies: Pliosauroidea and Plesiosauroidea. Members of the Pliosauroidea, often called pliosaurs, are typically short-necked, whereas members of the Plesiosauroidea are typically long-necked. However, generalisations based on neck length are unreliable, and recent research into plesiosaur evolution and phylogeny continues to reveal a more complicated story, with long and short-necked forms evolving independently many times. For example, the Late Cretaceous family Polycotylidae is a group of short-necked plesiosaurs that were traditionally regarded as pliosaurs, but research on their cranial anatomy(Carpenter 1997), and broad phylogenetic analyses of plesiosaurs, have shown that polycotylids are really plesiosauroids. Similarly, the genera Attenborosaurus and Thalassiodracon are pliosaurs with relatively long necks.
Plesiosaurian clades
This cladogram below shows the main clades (groups) within Plesiosauria. The Rhomaleosauridae is traditionally regarded as a pliosauroid (=pliosaur) family but some cladistic analyses have placed the Rhomaleosauridae outside of Pliosauroidea, in a sister relationship with a clade called Neoplesiosauria (Plesiosauroidea + Pliosauridae) (e.g. Benson et al. 2012), or in an unresolved polytomy with Plesiosauroidea and Pliosauroidea (e.g. Benson and Druckenmiller 2014). However, since the position of rhomalaeosaurids varies, I have used a traditional approach here and nested Rhomaleosauridae within pliosauroids.
Plesiosauroids are sometimes referred to colloquially as plesiosaurs, to the exclusion of pliosaurs, but not by academics and not on this website. That's a can of worms I explore here.
Position of Polycotylidae
The simplified cladogram below shows the main families within Plesiosauria and the alternative phylogenetic positions that have been proposed for the Polycotylidae relative to the other main families. The general consensus today is that polycotylids (=Polycotylidae) are plesiosauroids (=Plesiosauroidea).
Plesiosaur characteristics
The Plesiosauria is characterised by a suite of derived characters and 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 and plate-like) pectoral and pelvic girdles
- Nasals (nasal bones) absent
- No contact between ilium and pubis
- Relatively short trunk (body) and tail
- Well-developed gastralia ('belly ribs') forming a tight 'basket'
- Hyperphalangy (increase in the number of bones in the digits)
- Short and broad 'long bones', in the limbs, particularly the propodials and epipodials, which have transformed the limbs into flippers, of which both fore and aft are similar in shape and size.
- Paired nutritive foramina subcentralia (small openings for blood vessels or nerves) on the vertebral centra (one exception is Brachauchenius, and in some other pliosaurs the foramina are reduced)
The plesiosaur bauplan
Within the lineage, pliosauromorph forms with large heads and short necks were more rapid and manoeuvrable swimmers than plesiosauromorphs with long necks and small heads Robinson 1975, Massare 1988, O'Keefe 2001b). Despite these general morphotypes, the gross morphology of the postcrania remained broadly conservative throughout the evolution of the group (Carroll 1988, Storrs 1999). Famously described by many, including Owen (1860) "as a snake threaded through the trunk of a turtle" (p. 230). The typical plesiosauroid bauplan (body plan) was spectacular enough to defy the belief of scientists when Mary Anning discovered the first complete Plesiosaurus in 1823 (Taylor 1997, Cadbury 2000). The name Plesiosaurus was coined two years prior for remains "presenting many peculiarities of general structure" (De la Beche and Conybeare 1821)(p.560).
Are all plesiosaurs marine reptiles?
Contrary to some popular accounts (e.g. Lambert et al. 2001; Smith 2003a), plesiosaurs were not an exclusively marine group and plesiosaur remains are now widely known from freshwater and lagoonal deposits (e.g Wiffen and Moisley 1986, Cruickshank 1997, Sato 2002). Ambiguous plesiosaur material occurs in Middle Triassic deposits (Benton 1993a) but the first diagnostic plesiosaurs are from the uppermost Triassic (Taylor and Cruickshank 1993b; Storrs 1994, Storrs 1997). Plesiosaurs reached a cosmopolitan distribution by the Early Jurassic, achieved a maximum diversity in the Late Jurassic (Sullivan 1987), and persisted successfully to the Latest Cretaceous. Plesiosaur vertebrae of putative Palaeocene age were wrongly dated (Lucas and Reynolds 1993).