Plesiosaurs are a group of secondarily aquatic carnivorous reptiles belonging to the clade Sauropterygia within the diapsid clade Euryapsida (Tayor, 1989; Caldwell, 1997). The Plesiosauria has retained phylogenetic validity for well over 150 years, since the formal identification of the clade by de Blainville (1835). Unfortunately, this long history has hindered modern systematics of the group.
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 a more complicated story. The family Polycotylidae, for example, is a Late Cretaceous group of short-necked plesiosaurs that were traditionally regarded as pliosaurs. However, cranial ananomy (Carpenter, 1997) points to a plesiosauroid affinity for them. Similarly, the pliosauroid genus Attenborosaurus is a pliosaur with a relatively long-neck.
Cladogram of Plesiosauria
The simplified cladogram above shows the main families within Plesiosauria and the alternative phylogenetic positions proposed for the Polycotylidae. The general consensus today is that polycotylids (=Polycotylidae) are plesiosauroideans (=Plesiosauroidea) and this is the classification I have adopted for this website.
Contrary to popular accounts (e.g. Lambert et al. 2001; Smith, 2003a), plesiosaurs were not an exclusively marine group and remains are 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, 1993) but the first diagnostic plesiosaurs are uppermost Triassic in age (Taylor and Cruickshank, 1993b; Storrs, 1994, 1997). The lineage reached a cosmopolitan distribution by the Early Jurassic, a maximum diversity in the Late Jurassic (Sullivan, 1987) and persisted successfully to the Uppermost Cretaceous. Plesiosaur vertebrae of putative Palaeocene age were wrongly dated (Lucas and Reynolds, 1993).
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)
Within the lineage, pliosauromorph forms with large heads and short necks were more rapid and maneuverable 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 conservative throughout the evolution of the group (Carroll, 1988; Storrs, 1999). Famously described by many, including Owen (1860 p. 230), “as a snake threaded through the trunk of a turtle”. The typical plesiosauroid bauplan (body plan) was spectacular enough to defy the belief of scientists at the time (Taylor, 1997; Cadbury, 2000) when Mary Anning discovered the first complete Plesiosaurus in 1823. The name Plesiosaurus was coined two years prior for remains “presenting many peculiarities of general structure” (De la Beche and Conybeare, 1821, p. 560).
The chart below is a nested classification for Plesiosauria. Note that some classifications have placed Rhomaleosauridae outside of Pliosauroidea (=pliosaurs), but I have used a traditional approach and nested Rhomaleosauridae within Pliosauroidea.
The chart below is a nested classification for Sauropterygia to show the relative position of Plesiosauria.