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 was discovered that they share an affinity with plesiosaurs (Carpenter, 1997). Figure 3 shows a cladogram giving both possible positions of the Polycotylidae.
The morphology of plesiosaurs and their relatives is very well known because of the amount of fossils available to study. I believe this allows for fairly accurate positioning within the phylogenies as well as giving the most complete and convincing evidence for an example of the development of a specialised way of life in the water. The differences in the cranial and post-cranial skeletons between the species within Plesiosauria are likely to lead to further debate as to the exact placement, but this can only be resolved with further study. To further this point, during my research, I found that there is a distinct lack of fossils from the Late Triassic, which restricts our understanding of the transition from Nothosaurs to Plesiosaurs. This would be an interesting area for further study if the fossil records would allow for it.
Figure 1 – Evolution of the limb in Sauropterygians, A is the oldest, D the youngest. (modified from Storrs, 1993 taken from Smith, 2014).
Figure 3 – Simplified cladogram of Plesiosauria showing the different possible positions for Polycotylidae. (from Smith, 2014)