Evolution
Introduction
Plesiosaurs belong to the larger group Sauropterygia, a monophyletic clade (Order) that consists of a variety of Mesozoic marine reptiles. This page will look at the origin and evolutionary trends of sauropterygians as a whole, and also focus in particular on the evolution of the plesiosaurs.
Sauropterygians were distributed worldwide (Rieppel, 1997). The last surviving members occur in the late Cretaceous, and the K-T boundary marks the group's final extinction (Benton, 1990). Compared with many other vertebrate groups, sauropterygians are generally well represented in the fossil record. This is mainly due to the primarily aquatic environment in which they lived and died. Aquatic environments are areas of deposition and so have high preservation potential and thus the fossil record of sauropterygians is relatively rich.
Carroll (1998) noteed that "Sauropterygians provide the most complete evidence of the sequence of events that leads to a specialised aquatic way of life". Yet, despite this, our understanding of the detailed phylogeny of the group is still in flux and research is ongoing to fully understand their evolution. A comprehensive phylogeny of basal non-plesiosaurian sauropterygians has been established during the last thirty years through the work of Rieppel (1998, 1999a, 1999b, 2000), Storrs, (1991, 1993) and Sues (1987). However, only in the last ten years have the inter-relationships of plesiosaurs begining to be understood in detail.
Ancestors
The origin of the Sauropterygia is an old problem (Taylor, 1989). Primitive diapsid eosuchian families, such as the younginoid family Tangasauridae, are the most likely ancestors (Carroll, 1988). Members of this group show modifications of the tail (lateral flattening) for an aquatic mode of life, which also puts them in the right ecological setting.
Claudiosaurus from the Upper Permian of Madagascar was considered sister taxon to the Sauropterygia by (Carroll, 1988; Storrs, 1993 and Piveteau, 1955 in Storrs, 1991). Similarities include an elongate neck, paddle-like distal limbs, and more diagnostically, lack of the lower temporal bar of the skull. The palate also resembles that of sauropterygians, especially the pachypleurosaur and "nothosaur" condition, in the loss of the transverse flange of the pterygoid, reduced suborbital fenestrae and interpterygoid vacuities and the upper temporal opening is smaller than the orbit as in pachypleurosaurs (Carroll 1988). The sternum shows little ossification (Carroll 1988), a feature common in aquatic reptiles termed "aquatic neotony" (Ricqles, 1975, Rieppel 1987b in Storrs 1993). However,
Distinguishing between analogous and homologous features
Until very recently and despite a widespread early interest in the group, the taxonomy within sauropterygia was in disarray. (Rieppel, 1997). Interrelationships were drawn based largely upon specimens without skulls (Weller, 1962 and White 1940 in Carpenter, 1997) and frequently on charachteristics that are no longer considered diagnostic, such as neck length and head size (Carpenter, 1997). Many taxa have been shoehorned into wastebasket genera such as Plesiosaurus of which there are hundereds of species, mostly undiagnostic or unique genera, known worldwide. This long history of Sauropterygian study is a hinderance to our current understanding. Many characters are also yet to be confirmed for all members of clades because of poor quality material (Sues, 1986). As a result there are numerous synonyms, for example, Tarlo (1960) winnowed 19 pliosaurs down to 7 . In the Plesiosauria the constant proportions of the body between taxa may have served as a reason to classify based on neck length (Carroll, 1988). The absence of articulated remains in the Triassic (Carroll, 1988; Storrs, 1993) limits our knowledge of the "nothosaur"- plesiosaur transition.Classification of plesiosaurs is currently under revision (Carpenter, 1997; Rieppel, 1997; Bakker, 1993; O'keefe, 2001). Traditional plesiosaur phylogenies comprise four main families: Pliosaurids, Plesiosaurids, Cryptoclidids and Elasmosaurids (Brown, 1981), regarded the Jurassic and Cretaceous a time of evolutionary continuity. This is an artefact of the traditional classification based on neck length (Bakker 1993). There have been numerous convergencies and extinctions as Bakker (1993) has identified and as such it is especially important to distinguish between, as Bakker (1993) terms it, 'heritage and habitus'. The post-crania is shaped by ecological need and thus its evolution is conservative (Carpenter, 1997). Except for elongation of the snout (Carpenter, 1997) and perhaps supporting structures (Forrest pers. comm. 2002), the skull is less effected by parameters of ecology. Based on evidence of the skull, short necked plesiosaurs such as Dolichorhynchops osborni have been recently reclassified as polycotylids, related closely to elasmosaurids rather than being pliosaurs (Carpenter 1997). Indeed, cladistic analyses (O'keefe, 2001) seem to confirm this. Plesiosaur evolution is more complicated than previously acknowledged.
SAUROPTERYGIAN EVOLUTION: ANATOMICAL DATA
Anatomical data is the most important and widely used source of data for determining relationships in prehistoric groups (as opposed to genetic data). Soft parts are very rareky preserved and so interpretations are based entirely on skeletal elements. And skeletons are very rarely complete preserfved in a perfect state.
All members of the sauropterygia share a generally similar form and several synpomorphies. Changes in body size, proportions, jaw mechanics and hearing are the result of speciation as the group adapted into a variety of niches. The size of animals becomes greater in increasingly derived clades so the tiny pachypleurosaurs range from less than 20 cm to 100 cm in length, "nothosaurs" range from approximately 1 meter to 4 metres, and the more derived plesiosaurs reached lengths of 15 metres or more.
CRANIAL DATA
There is widespread agreement that the absence of the lower temporal opening is in fact an unusual variation of the diapsid condition rather than being a unique "euryapsid" condition. (Taylor, 1989). However, the position of sauropterygians within the Diapsida is less well constrained. Rieppel (1989 in Taylor 1989) considered them to be either primitive diapsids yet to evolve the second opening, or to have lost the lower temporal opening via secondary derivation.
The upper temporal opening of the pachypleurosaur head is smaller than the orbit (Carroll, 1988) a plesiomorphic state, not a defining factor of the group (Storrs 1993) and the upper temporal bar is strong. The temporal opening is larger in the nothosaurs and in plesiosaurs increasingly so (Carroll 1988). The palate of pachypleurosauria and "nothosaurs" has posterior and medial extentions of the pterygoids, the interpterygoid vacuities are completely closed and ventrally the base of the braincase is covered. This is considered an extension of the condition seen in Claudiosaurus (see below) (Carroll, 1988). The pachypleurosaur skull is characterised by loss of the ectopterygoid (Rieppel, 1989 in Storrs, 1993) in addition to the reduction of the supratemporal and an impedence matching middle ear.
A distinguishing feature of the plesiosaur skull is the loss of the nasal bones, a synapomorphy above Pistosaurus (Carpenter, 1997). This is however slightly unreliable as misidentification of the prefrontals is common (Carpenter, 1997) and O'keefe (2001) proposed that this is only synapomorphic for the Plesiosauroidea (i.e. Pliosauroids posses nasals). The palate of Pistosaurus is basal (Sues, 1987) indicating that the plesiosaur lineage perhaps broke off before the pachypleurosaurs (Carroll, 1988).
Plesiosauroids tend to have upwards facing eyes while pliosauroids are laterally placed (Massare, 1988), this may have implications for feeding strategy.
In pliosauroids the mandibular symphysis may be short with 5-6 caniniform teeth or long with 10-12 caniniform teeth (Tarlo, 1960). Primitive elasmosaurs are identified by the presence of contacting frontals (Wagner 1914 in Carpenter, 1997). In the Cretaceous elasmosaurids and polycolylids, the pinneal foramen is dorsally closed, a synapomorphy, and in latest Cretaceous genera the frontals are separated by the premaxilla and form the dorsal rim of the orbit.
In plesiosaurs, the lachrymal is only present in pliosauroids and is absent in elasmosaurids and polycotylids (allegedly absent in plesiosaurids) (Carpenter, 1997), however, this may actually be the nasal bones (see above). In contrast to other Sauropterygia, the elasmosaurid jugal forms most of the edge of the orbit. In the elasmosaur Libonectes, a small fenestra indicates a remnant of the suborbital fenestra (Carpenter, 1997). The exoccipital-opithotic bone is not part of the occipital condyle in all plesiosaurs except Cryptoclidus and pliosaurs indicating an early divergence of this group. Synapomorphies between the Elasmosaurids and polycotylids include the presence of a vomeronasal fenestra, expansion of the pterygoids into plates below the braincase and loss of the pineal foramen and stapes.
In pliosauroids of Upper Jurassic sediments of Britain, the younger Oxfordian genera have circular cross-section teeth (patterns may be used for distinguishing species) while in the Kimmeridgian, all pliosaur teeth are trihedral in cross section (Tarlo, 1960).
POST-CRANIAL DATA
Pectoral Girdle
The pachypleurosaur and "nothosaur" pectoral girdle is poorly ossified. The reduced ventral portion of the pachypleurosaur girdle reflects their basal position (Carroll, 1988): neither the clavicles or the scapulae are expanded ventrally as they are in the "nothosaurs" and to an even greater extent in the Plesiosauria. However, derived features of the pachypleurosaur pectoral girdle include the massive scapula and the lack of posteromedian processes on the interclavicle. The coracoids are larger in "nothosaurs", a reflection of their increased reliance upon limbs for movement (Storrs, 1993).
There is a similarity between the pectoral girdles of adult plesiomorphic taxa such as Pachypleurosaurs and "nothosaurs" and the juvenile form seen in plesiosaurs (Storrs, 1993, Carroll, 1988). This is evidenced by the similarity between the pectoral girdles of juvenile Cryptoclidids, for example Cryptoclidus, with the adult forms of related but more primitive sauropterygians e.g Neusticosaurus and Ceresiosaurus. The limb girdles of plesiosaurs are ventrally expanded but the vertical components are reduced (Storrs, 1993). This is evidence of possible heterochrony within the sauropterygian lineage. Such peramorphosis (addition to form) also occurs in plesiosaur limbs. Typicaly elasmosaurids have a midline scapular bar. Further more, the pectoral girdles of placodonts such as Placodus show even less development, further evidence for their perceived primitive position.
Pelvic Girdle
This is reduced in the basal sauropterygians as they relied on asymmetrical undulatory motion, but expanded again in the plesiosaurs whose mode of locomotion involves the symmetrical use of limbs (Carroll, 1988).
Vertebrae
Pachypleurosaurs and "nothosaurs" have lost the intracentra except at the base of the tail and to the anterior of the atlas-axis. The number of sacral vertebrae in Sauropterygia ancestors is two, in pachypleurosaurs three, in "nothosaurs" six (Carroll, 1989). The neural spine of pachypleurosaurs is low and the neural arches are broad and flat and the tail is long, often longer than the rest of the body (Sues, 1987). This contrasts to the tall broad neural spines and sub-rectangular narrow neural arches, flat vertebrae faces and short laterally compressed caudals observed in "nothosaurs" (Sues, 1987). The thoraces of "nothosaurs" and plesiosaurs are rigid (Carroll, 1989) but those of pachypleurosaurs are less so. Densely packed gastralia occur in the "nothosaurs" (Storrs, 1993). This is also a feature of the Plesiosauria, a derived character as indicated by the lack of accessory vertebral articulations (Storrs, 1993). In pliosauroids the cervical vertebrae are distinguishable from other sauroptertgians as well as between inclusive species but unusually not between genera (Tarlo, 1960). It is the presence or absence of a ventral keel on the anterior cervical vertebrae which is an important distinguishing feature of pliosauroids (Tarlo, 1960). Pachyostosis is common in the ribs and vertebrae of many pachypleurosaurs e.g Neusticosaurus as a mechanism for achieving neutral buoyancy. This is rare in plesiosaurs but such thickening of bone has been observed in the pliosauroid genus Pachycostasaurus dawni from the Middle Jurassic (Cruikshank et al. 1996).
Limbs
Above. Limbs in Sauropterygians, epipodials are shaded (Modified from Storrs, 1993)
The limbs of sauropterygians are increasingly modified for aquatic life through proportional changes. In Pachypleurosaurs the humerus is large and the distal elements reduced. The condition in "nothosaurs" is similar but some forms e.g. Ceresiosaurus show some degree of hyperphalangy (Carroll, 1988; Storrs 1993). The number of digits remains 5 throughout all of the clades but the number of phalanges increases. Functional surfaces of the "nothosaur limbs" are increased by distal flattening and broadening of humerus and, shortened and flattened epipodials and reduced limb flexability. In the plesiosaurs, the limbs have been modified into hydrofoils, distally tapering paddles and extensive hyperphalangy occurs in the distal rays (Storrs, 1993; Sues, 1987). Further more, flexibility is limited: the only functional joint is the shoulder/hip joint. The bones of the limbs are analogous throughout these lineages.
Plesiosauroidss can be distinguished from pliosauroids on the relative size of their fore and aft limbs: the hind limbs in plesiosaurs are shorter whereas they are larger in pliosaurs (Bakker, 1993, Storrs 1993). In fact, the hind limbs are longer in all Sauropterygia except for the Plesiosauroidea (Sues 1987b). The propodials in the Pliosauroidea are of different length at different geological ages (Tarlo 1960) and the plesiosaur humeri especially, shows peramorphosis through ontogeny (Storrs 1993).
FUNCTIONAL EVOLUTION
Sauropterygian locomotion has been studied numerous times (see Robinson, 1975; Storrs, 1993). It is likely that the phylogeny of the Sauropterygia parallels an increasing adaptation to a fully aquatic existence via increasingly economic swimming regimes (Sues, 1987). The underwater flight employed by plesiosaurs is more economic than the undulatory locomotion employed by pachypleurosaurs and as such, the change in body proportions can be attributed to the gradual surpression of lateral truncal movement. This must have involved a radical shift in the central nervous system (Storrs, 1993). The reduction of appendicular girdle and limbs in the pachypleurosaurs may be a response to decrease drag (Storrs, 1993). A strong ancestral constraint is associated with this transformation in locomotory repertoire, and this is poorly understood.
