Plesiosaurs are a clade of Sauropterygian reptiles that were first observed in the late Triassic surviving until the Cretaceous mass extinction event (Benton, 1990). As a result of the high deposition rates in the marine environments they inhabited, the fossil record of plesiosaurs is particularly complete in relation to other groups. Therefore, we are aware of several convergences and extinctions in their evolution. For example, we know that long and short-necked morphotypes developed independently more than once (O’Keefe, 2002). The evolution of plesiosaurs is complex, and there is still ambiguity over the classification of some species. This makes plesiosaur evolution a topic of much interest in current research.
The exact origin of Sauropterygians and therefore Plesiosaurs is uncertain. It was once thought that Claudiosaurus was the most closely related group to Sauropterygians, however, it was found that the postcranial skeleton was too basic to allow for the range of aquatic motion seen in plesiosaurs (Carroll, 1981). More recently it is believed that the closest ancestors are most likely an early ‘diapsid eosuchian family’, for example, the Tangasauridae (Carroll, 1988). The closest relative of Plesiosaurs are Triassic Pistosaurids (Cheng et al., 2006) which are plesiosaur-like, but lack adaptations to a marine lifestyle seen in later Plesiosaurs.
From skeletal analysis, we know that there was great variation in size and body proportions between Sauropterygians. Size increased with geological time with several lineages evolving a giant size during the Jurassic and Cretaceous.
As mentioned Plesiosaurs are thought to be primitive diapsids with a euryapsid condition. Within the cranium, a unique feature of the group is the loss of the nasal bones. Additionally, plesiosaurs usually have upward facing orbits, and the upper temporal fenestra becomes larger in Plesiosaurs compared to previous groups such as Pachypleurosaurs and Nothosaurs (Carroll, 1988).
As can be seen in figure 1, the limbs of Sauropterygians, D being a Plesiosaur, become increasingly adapted to a marine lifestyle over time. The number of digits in the limbs is five in all species, but hyperphalangy occurs in more derived plesiosaurs. Additionally, flattening and broadening of the humerus occurs with time as well as reduced flexibility in derived plesiosaur limbs as a result of a change in the position and shape of bones in the limb. Finally, the only functional joints in plesiosaurs were the shoulder and hip joints (Storrs, 1993) which distinguishes them from previous groups of Sauropterygians.
As previously mentioned, Plesiosaurs are derived Sauropterygians; a basic cladogram is given in figure 2 which shows where they fit in relation to their relatives. There are several characteristics, or synapomorphies, which indicate common ancestry. For example, the loss of nasal bones is a synapomorphy between Plesiosaurs and Pistosaurs. In addition to this, Plesiosaurs can be broken down further into two superfamilies known as the Pliosauroidea, which are typically short-necked and the Plesiosauroidea that are typically long-necked. However, there are some Plesiosauroids that are short-necked, most notably, the late-Cretaceous group Polycotylidae. They were originally thought to be Pliosaurs, but it …
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