Plesiosaurians were large marine reptiles that attained global distribution throughout the world’s oceans during the Jurassic and Cretaceous periods. They became increasingly specialised to an open ocean lifestyle as they evolved from their more basal sauropterygian ancestors, the earliest of which were amphibious. The incomplete nature of the fossil record has made aspects of plesiosaurian palaeobiology hard to decipher. Recent phylogenetic, biomechanical, and anatomical studies have elucidated greater insights into how plesiosaurians evolved and lived in oceanic habitats. Here we discuss the findings of such studies that have enhanced our understanding of the life history of plesiosaurian lineages. Plesiosaurians evolved in the latest Triassic before diversifying into two major morphotypes: the plesiosauromorphs and pliosauromorphs. They adpoted three major locomotory methods: rowing, flying, and rowing flight, allowing efficient underwater navigation. Plesiosaurians thrived in freshwater and saltwater habitats, feeding on fish, invertebrates, and other marine tetrapods. Their skulls often possessed large orbits and specialised olfactory and auditory organs that aided underwater perception. They were endothermic, with a high metabolism, allowing for survival in high latitudes and deep water. These lines of evidence have vastly improved our understanding of the evolution and adaptations of plesiosaurians.
This article uses ‘plesiosaurians’ to denote all members of the clade Plesiosauria, which encompasses the plesiosauroids (and plesiosaurids within), pliosaurids, and rhomaleosaurids. ‘Plesiosaurs’ refers to members of Plesiosauroidea, while ‘pliosaurs’ refers to members of Pliosauridae. (Proposed phylogenetic relationships are illustrated below in Fig. 1.) Morphotype terms are hereinafter specified, with ‘plesiosauromorphs’ referring to the long-necked and small-headed body type and ‘pliosauromorphs’ referring to the short-necked and large-headed body type.
The first plesiosaurian was named in 1821 by Henry De la Beche and William Conybeare, who assigned an amassed selection of fragmentary remains to the new genus Plesiosaurus, meaning “near to reptile”. In 1823, Mary Anning became the first to find a near complete skeleton of a Plesiosaurus plesiosaur in Lyme Regis, thus confirming the validity of this new genus and showcasing a long-necked, small-headed body type. It was not until 1841 that Richard Owen coined the name Pliosaurus, meaning “more lizard”, to describe a new genus of large-headed and short-necked plesiosaurians. These two genera subsequently became points of reference for early plesiosaur and pliosaur classification.
Despite being such a well-known group of marine reptiles, the evolutionary history and palaeobiology of plesiosaurians is still not well understood due to the incompleteness of the fossil record. It is thought that Plesiosauria, a member of the larger clade Sauropterygia, evolved in the latest Triassic from more basal sauropterygians. These early eosauropterygians likely used their limbs for propulsion (as opposed to the ‘flying’ locomotion of plesiosaurians), perhaps along with tail movement. Such propulsion may have been used by the Triassic eosauropterygian nothosaurs, which were among the first sauropterygians to exhibit the long neck characteristic. Nothosaurs inhabited nearshore environments, possibly facilitating an amphibious lifestyle, although they were perhaps better suited to moving through water than on land given their largely inflexible forelimbs.
In the Triassic, some eosauropterygians developed into pistosauroids, which are recognised as a primitive, paraphyletic group of sauropterygians. Triassic pistosauroids, despite a lack of well-preserved specimens, have been considered the closest relatives of Plesiosauria, with Pistosauroidea eventually giving rise to Plesiosauria following the evolution of several key plesiosaurian characteristics, including a stiff neck, short stiff trunk, and reduced tail. This proposed phylogeny indicates that plesiosaurians evolved in the latest Triassic rather than appearing suddenly in the earliest Jurassic following the end-Triassic extinctions, as was believed for almost two centuries.
Plesiosaurians became the most derived clade of eosauropterygians, so their fossils exhibit progressive adaptations that made them more efficient at foraging in a pelagic environment. The limbs of plesiosaurians and earlier eosauropterygians exhibit an important difference: bones in the wrist and ankle regions of plesiosaurians are well ossified, whereas in other eosauropterygians these areas remained highly cartiliganous. This changed the limbs of plesiosaurians into rigid flippers, likely allowing plesiosaurians to swim more rigorously than other eosauropterygians.
Adapting into fully open marine tetrapods may have been what allowed Plesiosauria to survive into the Jurassic, as the Late Triassic marine regression wiped out the non-plesiosaurian sauropterygians that were not as specialised for the marine realm. Most of these non-plesiosaurian sauropterygians lived in coastal waters and around shallow carbonate platforms, which saw a calcification crisis during the end-Triassic extinctions, whereas pelagic prey on which plesiosaurians fed in the open sea were not as severely affected.
At least six plesiosaurian lineages survived across the Triassic-Jurassic boundary, indicating a Late Triassic radiation and divergence of plesiosaurs from pliosaurs. Plesiosaurians continued to thrive, reaching high diversity in marine ecosystems throughout the Jurassic and Cretaceous periods. In terms of diversity, the clade is comparable to modern cetaceans, with 114 valid species being used to assess the completeness of the plesiosaurian fossil record in 2017.
Throughout their evolutionary history, plesiosaurians underwent both iterative and convergent evolution of the plesiosauromorph and pliosauromorph body plans (Fig. 2), which has made it more difficult to provide a robust phylogenetic placement for specimens that exhibit characteristics from both lineages. For example, the apparently plesiosauromorph Attenborosaurus conybeari was classed as a basal member of Pliosauridae but has also been suggested to be a sister taxon to Plesiosauroidea (Fig. 1).
Plesiosaurians became the longest-surviving clade of secondarily aquatic tetrapods, but they were ultimately driven to extinction by the end-Cretaceous mass extinction, marking the end of the sauropterygians.
Locomotion and Hydrodynamics
As plesiosaurians evolved, they became more specialised for life in the open ocean. Plesiosaurian hydrodynamics have puzzled scientists since their earliest fossils were found, but various locomotory methods have since been proposed and can be summarised in three forms: rowing or paddling movement, rowing-flight, and underwater flight (Fig. 3).
Rowing or paddling movement involves the limbs spreading and pushing against the water to propel the body forward, where the back stroke has little water resistance. This form of locomotion allows greater manoeuvring, acceleration, and deceleration, which are necessary in complex environments.
Rowing-flight, similar to that used by sea lions, is a locomotory method that combines underwater flight and rowing movements. By combining lift and drag propulsion, the foreflippers are drawn ventrally, resulting in lift and propulsion. Once the foreflippers reach maximal ventral stroke, they then move backwards in a lateral rowing motion.
Lastly, the underwater flight locomotion generates an underpressure below and an overpressure above the arched flipper profile. This allows the flipper to be pushed and sucked forward simultaneously by the pressure gradient. This form of locomotion allows excellent long-distance swimming at moderate speed.
There is also a four-wing problem, as it is difficult to know exactly how plesiosaurians used those four flippers coordinately, especially how they avoided the vortices generated by the front flippers. It is likely that plesiosaurians used a suite of locomotory repertoires from the three proposed, where they changed their gaits (asynchronous, synchronous, semi-synchronous) according to various scenarios.
The plesiosaurian tail was reduced, and unlike basal members like nothosaurs that likely relied on their tails for forward locomotion, derived plesiosaurian tails had tail flukes for largely maintained streamlining. As for necks, thicker-necked pliosauromorphs were likely greater pursuit hunters than their longer necked contemporaries, which likely employed different foraging strategies.
Ecology and Diet
Plesiosaurians thrived in saltwater and freshwater habitats, from open marine to brackish and fluvial environments. Relative flipper size was likely linked to feeding strategies and local ecology. Plesiosauromorphs generally have larger foreflippers than hindflippers and are regarded as having been ambush predators and efficient, moderately fast long-distance swimmers. Pliosauromorphs usually have larger hindflippers than foreflippers at large body sizes and are generally considered to have been fast, powerful, and agile predators that pursued their prey.
All plesiosaurians were faunivorous but showed various dietary preferences, as indicated by their dentition and fossil gastric contents. Dentition is associated with functional morphology, where the shape of a tooth influences its purpose. Plesiosauromorphs overall had piercing and general-use teeth that are thin to moderately conical (Fig. 4A). Some aristonectid elasmosaurids and cryptocleidids are suggested to have been filter-feeders, supported by trace fossils interpreted as plesiosaur sieve marks through soft sediments to draw out elusive prey.
Regurgitalites (fossilised stomach contents) in plesiosaurians have been found with ammonite jaw parts (Fig. 4C), indicating cephalopod consumption (among other invertebrates). A small ichthyosaur neonate found in the gut region of a plesiosaur suggests they were also opportunists. Larger plesiosaurians, particularly large-headed pliosaurs, were able to feed on larger prey such as fish and other large marine reptiles. Plesiosaurian skeletons are also often associated with gastroliths, which would have influenced the animal’s buoyancy, aided in food processing, or both.
Pliosauromorphs are thought to have been macrophages and generalists, as evidence of gastric content has been found from pliosaurs with fish scales and pterosaur and dinosaur bones. Bite marks on plesiosaur bones have been attributed to pliosaurs, suggesting pliosauromorphs predated plesiosauromorphs (Fig. 4B).
As the highest ecological consumers, plesiosaurians were the apex predators of their habitats, although predation marks on their flippers may suggest Cretaceous plesiosaurians were preyed upon by mosasaurs. At the end of their life cycle, deceased plesiosaurs, like modern whale falls, would have fed benthic scavengers.
As plesiosaurians became better adapted for marine ecosystems, their sensory organs became highly specialised for thriving in marine environments. The skulls of some plesiosaurians such as Rhomaleosaurus megacephalus possess grooves running along the palate that supposedly guided water into the internal naris. It is thought that water would flow through the internal naris, where it would pass over the olfactory epithelium, allowing them to pick up scents in the surrounding water.
Plesiosaur skulls such as that of Thalassiodracon hawksini (Fig. 5) have notably enlarged orbits and large flattened sclerotic rings, suggesting its eyes were well adapted for seeing underwater and in low-light conditions. Additionally, the elongation of the stapes and their fusion to the exoccipital bones appear to be adaptations for hearing underwater. These adaptations are especially pronounced in cryptoclidids such as Abyssosaurus nataliae, which had an extremely shortened skull with wide orbits and huge exoccipital bones, which would have aided in navigating through the dark and withstanding the pressure of deep marine environments.
Foramina identified on the rostrum of Pliosaurus kevani suggest that some taxa possessed complex neurovascular channels which housed nerves that may have aided in prey detection, as seen in modern crocodilians.
Little is known about the cognitive function of plesiosaurians; however, endocasts of the elasmosaurid Libonectes morgani reveal a pronounced cerebellum and optic lobes, and a very long olfactory tract, providing additional evidence that plesiosaurians had well-developed senses of sight and smell.
30 different tetrapod lineages have independently colonised the marine realm, all of which are endothermic. Fossil evidence indicates that plesiosaurs were no exception and had very high metabolic rates among reptiles. Histological studies of the stylopodial bones (humerus and femur) of several plesiosaur taxa, including Plesiosaurus, Rheaticosaurus and Cryptoclidus, have revealed that plesiosaurs had very densely packed vascular canals within their bones, indicated by the density of primary osteons made of lamellar bone. These densely packed osteons also indicate that plesiosaurian bones grew quickly and, therefore, body growth rate was also high, comparable to modern birds and mammals.
Additionally, the estimated area and volume of plesiosaurian red blood cells is very high and significantly greater than those of basal sauropterygians. Together, these lines of evidence suggest that plesiosaurian bones were highly vascularised, which not only helped consistently supply their muscles with oxygen but also allowed for the storage of oxygen for diving (they needed to return to the surface to breathe air).
Plesiosaurian biogeography also provides evidence of endothermy. The Wallumbilla Formation, a unit of Lower Cretaceous sandstone and mudstone of Southwest Australia, has harboured the fossil remains of several plesiosaurian taxa such as Kronosaurus (a large pliosaur). Wallumbilla rocks were deposited at a very low palaeolatitude (~70°S) and sedimentological evidence, such as cryoturbated sediments and glacial tillites, implies that the Wallumbilla ecosystem was subjected to freezing temperatures. The presence of plesiosaurians here is evidence that they were capable of surviving in much cooler temperatures than what modern reptiles can tolerate and, therefore, were most likely endothermic.
Plesiosaurians were a highly successful clade of marine reptiles that prospered for around 135 million years, from the latest Triassic to the end of the Cretaceous. Their pelagic lifestyle helped them outlast their non-plesiosaurian sauropterygian relatives and survive multiple extinction events. They flew through the water on four powerful flippers which aided in seizing a diverse range of prey and allowed them to occupy a wide variety of ecological niches, from brackish to polar and from shallow to deep. Their highly acute senses allowed them to detect prey and predators alike and their high metabolism facilitated an active underwater lifestyle. They were the longest lasting known group of secondarily aquatic tetrapods and they may have persisted had it not been for the asteroid-induced cataclysm that ended their aquatic reign 66 million years ago.
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