After dinosaurs, plesiosaurs are perhaps the most iconic of Mesozoic sauropsids. As obvious, while they became extinct in HE, they kept surviving on Spec, but unlike the extremely prevalent dinosaurs, plesiosaurs are reduced to two families, relics of their former reign.
Plesiosaurs evolved in the late Triassic, rendering them the longest lived marine tetrapods. They are the last of a clade known as Sauropterygia, one of the several clades of aquatic reptiles that evolved in the Triassic. Their exact relations in the fossil reccord are not known, although Spec plesiosaur DNA samples seem to suggest a closer relationship to archosaurs. The direct ancestors of the plesiosaurs were the paraphyletic "nothosaurs", sauropterygians that looked rather similar to the choristoderan hyphalosaurs, with long necks and webbed feet. Despiste their long tails, these animals appearently swam using the limbs like turtles, which would provide the unqie plesiosaurian swimming style. Eventually, the already flippered Pistosaurus evolved soon after nothosaurs diversified, and the presence of the late Triassic elasmosaur Alexeyisaurus indicates that plesiosaur diversity not only occured quickly, but that all major clades already evolved before the Jurassic began.
Surviving the Triassic/Jurassic extinction event, plesiosaurs diversified across most of the Mesozoic. Several species, like their ancestors, retained long necks and short skulls, being bottom feeders, but some taxa developed shorter necks and long jaws, being pelagic predators. Traditionally, long necked forms have been reffered as "true" plesiosaurs and the short necked varieties as pliosaurs, but while both groups did exist, long necked pliosaurs and short necked plesiosaurs did exist. Both clades competed with other marine sauropsids like ichthyosaurs and metriorhynchids, but none of these clades seem to have replaced the other ecologically.
The first obstacle to the plesiosaurian reign came with the Turonian thermal maximum, which decimated most marine tetrapods. Pliosaurs were completely exterminated, and plesiosaurs were reduced to two clades: the long necked elasmosaurids and the pliosaur like polycotylids. Both clades saw the arrival of new competitors, like the squamate mosasaurs and the dinosaurian hesperornithes, but they diversified nonetheless, occupying the niches left by the extinct plesiosaur, pliosaur and ichthyosaur clades, with only the larger macropredatory niches barred to them.
In Spec, the absence of a KT event allowed plesiosaurs to survive into the Cenozoic, and the Paleocene, being roughly an extension of the Maastrichtian, retained the status quo established. However, once it ended, things changed radically. The Paleocene/Eocene Thermal Maximum decimated over 90% of the marine tetrapod taxa, and plesiosaurs were not exempt. Nearly all elasmosaurids were gone, with the exception of the antarctican genus Australojormungandria, which survived 3 million years into the Eocene. Polycotylids were more lucky; the northern polar waters offered refuge to these animals, alongside with several other clades decimated elsewhere. The Azolla Event was not as powerful as in HE, rendering the Arctic Ocean a more stable environment, which produced an unique aquatic ecosystem.
By the Oligocene, the global cooling of the oceans allowed polycotylids to return to a cosmopolitan distribution. Some species diverged from the established dolphin like ecological niche, and expanded into forms with teeth not unlike those of ctenochasmatoid and boreopterid pterosaurs, trapping prey in a similar way, while pliosaur like macropredators patrolled the seas. This adaptative radiation continued well into the Miocene; while filter feeding forms gradually began to feel the effects of competition with balaeanateuthids, the macropredatory forms thrived in the presence of new prey like penguins, seaguins, sea mammals and saurocetes, culminating in the sperm whale sized Leopleurodon, the largest sauropterygian to have ever lived, dwarfing the largest known pliosaurs.
Evenually, however, the climatic changes of the Miocene began shaping the seas into their current faunal state, with the last of the unfit Mesozoic relics being wiped out. By the Pliocene, saurocetes, seaguins, penguins and neochoristoderes had taken the niches previously held by most polycotylids, with the larger filter feeding forms having surrendered to baleen squid supremacy, while the last of the giant macropredators gave way to new, more bizarre monsters like giant seaguins. Polycotylids survived, but with a few exceptions are now mostly restricted to freshwater relics. It is possible that they might reclaim the seas once again, since freshwater environments are more stable, but it is equally as likely that they'll vanish soon enough.
The longest lived known lineage of plesiosaurs, elasmosaurs are the stereotypical long necked "snake inside turtle body" beasts, thoughas with all plesiosaurs they're incapable of the pop cultural "swan naked pose", their necks being far too inflexible for that. They instead use them to forage in the sea bottom, their heads trailing across the sea bottom for small morsels while the body remains well above. In some species, the neck is twice the size of the body, proportionally the longest among non-pterosaurian vertebrates.
While elasmosaurs managed to reach some level of diversity during the Cenozoic, they have been largely replaced by cancridonts as early as the mid-Miocene, and nowadays only a few species remain, mainly in the polar regions.
Mullyewonk (Infeliciorisaurus novaeangliae)
The Mullyewonk is an average member of it's now very sparse clade, a 10-13 meter long beast with two thirds of it's length being neck, the largest living plesiosaur. Uusally of a dark blue colour with white patches, this animal occurs in the coastal waters all over the planet, though most common in the Pacific Ocean.
A frequently solitary animal, the Mullyewonk is cathemeral, foraging both during daylight and nighttime, though some populations have more diurnal or nocturnal schedules. It feeds on a variety of creatures it finds in the sea bottom by trailing with it's neck, most of it's diet being composed of various types of crustaceans, molluscs and bottom dwelling fish like rays. It will also occasionally forage pelagically on jellyfish, crustaceans, cephalopods and the occasional teleost. It's usually a slow moving animal, relying on it's size to defend itself, though it is more than capable of some bursts of speed.
As with most large cosmopolitain marine tetrapods, the Mullyewonk is nomadic, few parts of it's range having independent populations. Large breeding concentrations occur all over the world at all points of the year, coinciding with times and places of high productivity, sometimes even pelagic feeding events like baleen squid spawning. Males emit strong guttural calls, establishing small harems which they defend by attacking with their necks. After mating, females move to shallower waters to give birth, the young forming small creches protected by the male, while their mothers leave for good. The young growing up in estuaries is not uncommon, and adults may often enter freshwater to forage.
Vishnu (Hindustocleidus medius)
Another gangetic beast is the Vishnu (Hindustocleidus medius), a small elasmosaur, partial to crabs, worms, and shellfish. This beast is foul, to say the very least, with a stenching, leathery hide pungend and dark enough to wake the dead. It's bile is mildly poisonous, and is kept in check by numerous internal parasites.
Alaskan Whipneck (Anguillosaurus arcticus)
A species of elasmosaur from the Arctic Ocean. It is a top predator that preys on large fish, small balaenoteuths, and sharks. To help hunt prey much larger then what its ancestors ate, it has a jaw like a snake which can disconnect to help it swallow the larger prey. It is a very fast swimmer and, unlike other plesiosaurs, uses its tail to assist the flippers in propelling itself. It has silver skin with mottled golden markings.
Most modern plesiosaurs belong to this clade, also making up the majority of post-Paleocene plesiosaurs. While quire derived, these animals exemplify the unique anatomy of plesiosaurs.
The most unique feature of the whole of Plesiosauroidea (including the pliosaurs and their common "nothosaur" ancestors) is the swimming style; like penguins and sea turtles, plesiosaurs rely on the their paddle like flippers to swim, but unlike those animals both the forelimbs and the hindlimbs are elongated. As such, rather than just "fly" underwater with the forelimbs, plesiosaurs flap both flipper pairs in an alternating way, sometimes using all four limbs at once in order to gain bursts of speed. This propulsion method was unique in the Mesozoic and in HE, but the evolution of baleen squids and some derived cancridonts on Spec decreased the "specialness" of this.
To cope with this swimming style, the spine is inflexible, while the shoulder and pelvic girdles as well as the gastralia are extremely well developed. The paddle like limbs are mostly composed of the humerus/femur and the manus/pes, with the radius, ulna, fibula, tibia and wrist/ankle bones reduced to vestigial fragments that are pratically indistinct from each other. Polyphalangy is present, with each individual finger possessing tens of phalanges; this doesn't render the flipper any more flexible, and seems to be exclusively to elongate it. The tail is used to steer, and has a small fin; the skin is smooth, with small, non overlapping scales as in neochoristoderes.
Much like the iconic pliosaurs, polycotylids have long, gharial like jaws, although unlike several pliosaurs and the other lineage of pliosaur like plesiosaurs, leptocleidids, polycotylids, with a few extinct exceptions, retain a fairly long neck. Much like crocodyllians and pterosaurs, these animals lack lips, their teeth interlocking in plain sight. Much like mammals, plesiosaurs only have one temporal frenestrae, but this one is the supretemporal, not the infratemporal, and historically it had been used as evidence to classify sauropterygians and ichthyosaurs in a clade known as Euryapsida, before it was discovered that both groups were just derived disapsids. The zygomatic arch is far more well developed than in mammals, as to be expected from the loss of the infratemporal frenestrae, but like in synapsids a deep cranial crest is present, where the powerful jaw muscles are attached. The nostrils are located at the base of the snout; they are notably divided into internal and external nostrils, allowing the animal to distinct pathways for breathing and for using olfaction underwater. Indeed, the plesiosaurian sense of smeel is quite potent, allowing the animal to detect prey with extreme precision. The nostril system allows water to be in continuous motion, both in order to enhance the sense of smell and also to avoid harmfull organisms like Naegleriato be trapped in. The large eyes also allow a suberb vision, allowing the animal to hunt in dark hunters.
Plesiosaurs, being fully aquatic, are viviparous, and indeed the most conclusive evidence of viviparous plesiosaurs comes from Cretaceous polycotylids. The penis is strikingly similar to that of HE cetaceans, even being flexible; to avoid sperm loss, the cloaca is very tight, and the male discharges at the speed of a bullet. Unlike mosasaurs and elasmosaurid plesiosaurs, which give birth to many small young, polycotylids are K strategists, producing a single large baby that is protected by the pod. Polycotylids are very intelligent by sauropsid standards;t hey and pterosaurs are only outsmarted by maniraptors, neornithes in particular. Complex social behaviour has been observed in these animals, in some ways reminding specscientists of their HE cetaceans. They are cathemeral, and like birds they sleep very quickly; they spend a good part of the day resting in a series of naps.
Roughly 9 species survive currently.
Incredible Whulk (Insulaverpa dixoni)
The only fully marine modern polycotylid, this animal is predictably also the largest modern polycotylid and one of the largest plesiosaurs, with only the Mullyewonk providing competition for the largest plesiosaur spot. Reaching over 13 meters and weighting over 20 tonnes, this beast is a relic of the plesiosaur reign so recently lost. Colored in dark olive with a silver underside, this animal has a cosmopolitian distribution, generally spending the summers at higher latitudes and the winters near the tropics. It's teeth are long and needle like, similar to those of the boreopterid pterosaurs. They are poorly suited to grasp prey, but very efficient in trapping small fish and crustaceans in large numbers. During summer months it feeds in the ocean bottom of cold waters, much like grey whales in HE, filtering the sands in search of benthic crustaceans, worms and fish. In winter months, it opts for more pelagic environments, gathering in rich deep water zones to feed on crustaceans like krill, small fish and jellyfish. It will opportunistically switch between pelagic and benthic feeding if it needs, making it does quite adaptable and capable of co-existing with the more specialized baleen squid and seajar seaguins, specially since these tend to be exclusively pelagic animals.
Thanks to their enormous size, adults are generally protected from marine predators; only the largest mosasaurs and seaguins pose any sort of believable threat to these plesiosaurs. Indeed, through most of the year the Incredible Whulks can afford to forage alone, without disturbance. However, in winter months, these animals gather in large numbers. Males and females without young gather in rich feeding grounds, mostly pelagic spots above deep sea canyons and similar nutrient hotspots, often in the company of baleen squids, which also gather for the same reasons. Here, males fill the water with pheromones, indicative of their health status, in an attempt to convince females to mate. Direct confrontations can occur, but overall these beasts are far more diplomatic than their cephalopod contemporaries, that engage in violent battles. Indeed, much like in HE dolphins, bisexuality among males decreases agression significantly, and allows team work to occur in terms of mating with females, often in rather horrific rapes. The mating season is rather brief, but if baleen squids are present the animals stay until their mating season is over, taking advantage of the weak, dying male cephalopods like many other predators.
Gravid females may return during the conclusion of the baleen squid mating season as well, but mostly they don't migrate, spending the winters on summer feeding grounds until pregnancy is over; then they migrate to warm tropical waters, where they give birth and raise the young. The young forage for themselves, supervised closely by the adults, which eat very little, allowing the young to fatten up in these less productive waters. Once the winter ends, all animals return to colder waters, where the adults regain their lost weight and the young grow far more quickly. Not migrating further to tropical waters, the young stay with the pod for one more year, leaving then to form pods of juveniles. Action from marine predators might reduce these pods significantly, but 3 year old animals are generally safe from most predators. Sexual maturity is reached at 10 years of age. The animal might live up to 70 years.
Because of their cosmopolitian distribution, many specbiologists have wondered if the the Incredible Whulk is actually a number of different species. The Tasmanian Sea Incredible Whulk (I. d. reiputa) diverged from the other populations in the mid-Pleistocene, so it might represent an independent species from other populations. Despiste the name, it also occurs in South Africa and in between subantarctic islands. Another subspecies, the Patagonian Incredible Whulk (I. d. charlesdarwini), occurs in the temperate and cold coasts of South America, ranging across the Atlantic shore south of the La Plata River and in the pacific coast up to the Galapagos and as westwards as Rapa Nui; unlike the Tasmanian Sea Incredible Whulk, this one is clearly part of the main species, being closely related to the Californian Incredible Whulk (I. d. occidentalis).
Caspian Whulk (Insulaverpa auripteryx)
Despite being part of the once mighty Paratethys, the Caspian sea has been subjected to dry periods since it's formation. Hence, many of it's current fauna are recent migrants from freshwater ecosystems that feed the inner sea. However, a few of it's inhabittants are clearly marine animals. Selkies with little difference from northern species patrol these waters, as does the Caspian Whulk. This plesiosaur strongly resembles it's larger marine relatives, having diverged from it in the late Pliocene; it likely evolved from a population of their common ancestor that ended up in the Caspian Sea in interglacial periods, when vast lake systems formed between the Caspian sea, the Baikal Lake and the Arctic Ocean, allowing a brief passage for marine animals, which also explains the presence of selkies and seaguins in the Baikal. Fossils of polycotylids have been uncovered in the Black Sea, which was a vast lake during the glacial periods; it is possible that they might represent an extinct subspecies of this animal.
As to be expected, the Caspian Whulk is much smaller than it's marine relative; it reaches a maximum size of 7 meters, and even then most animals don't go beyond 6 meters. As the scientific name suggests, it has a golden colouration. Due to competition with sturgeons and due to the absence of baleen squids, it is a more specialised filter feeder, with thinner, more numerous teeth, although it still feeds on the bottom if there's less pelagic crustaceans and fish. These animals are more solitary than their marine cousins, only gathering together to raise their young, which are vulnerable to the Caspian's top predator, the Turk Lotan. The adults themselves are as big as the simoedosaurid, and a hungry Lotan won't hesitate to attack a Caspian Whulk if pressed. The polycotylid is, however, faster, and is relatively safe in deep waters, where the choristodere can't ambush as effectively.
Varuna (Siluradectes gangetica)
The Varuna is a 2 meter long polycotylid native to the river systems of the Ganges and Indus, the smallest living plesiosaur. This animal, coloured light grey, has not lost it's sight like HE's river dolphins and freshwater cancridonts, having instead developed bigger eyes to cope with the murky waters. It can see movements with extreme precision, and reacts with lightning fast reflexes. It also uses olfaction, which is also more well developed than that of it's relatives. Solitary, these small polycotylids don't usually form pods, although aren't territorial either. Breeding occurs year round; these are the most precocial of all polycotylids, the young leaving a week or so after birth. The main predators of these animals are the native crocodiles and simoedosaurids, which they easily evade thanks to their speed. During monsoon months, the Varuna swim up the river systems, hunting on the flooded territories. In particularly harsh dry seasons, they gather at the estuaries, where they are vulnerable to invading mosarks.
Kawarra (Parapterodelphis septentrionalis)
The Kawarra lives in the river systems of northern Australia and southern New Guinea; like most living polycotylids, it is mainly a freshwater animal, but it will venture on coastoal waters in search of new territories, and is quite common in the mangrove dominated bays. Reaching up to 5 meters, this animal is white, a strong contrast against the often murky waters it lives on. Breeding occurs during the dry seasons, when the animals gather in pods along estuaries and bays, while they venture inland to feed during the monsoons. With the exception of the largest crocodiles, these are the apex predators of the northern australian and new guinean river systems, and actually display facultative pack hunting behaviour, ganging up on large fish, turtles and even choristoderes. The Kawarra and the Varuna are part of a polycotylid diversity found in Asia/Australian rivers, of which several species are found in indonesian islands, which likely evolved from either a single marine ancestor that became isolated in multiple river systems, or from a single freshwater/estuarine species that gradually spread. The Kawarra seems to have diverged earlier from it's relatives, more or less 3 million years ago.
Turso (Tetraketos caucasica)
This plesiosaur likely evolved from a population that ended up in the Caspian Sea in interglacial periods, when vast lake systems formed between the Caspian sea, the Baikal Lake and the Arctic Ocean, allowing a brief passage for marine animals, which also explains the presence of selkies and seaguins in the Baikal. A possible ancestor of the Turso, Tetraketos moorei, was present around the european coastline as recently as the mid-Pleistocene, and probably occured in the high Arctic during interglacials. Polycotylid subfossil remains have been found on the Black Sea, which was a vast inland lake through most of the Pleistocene, and may belong to a closely related species if not an extinct subspecies. A population also lives in the Aral Sea, which seemingly diverged rather recently, probably having been transported there by the vast extensions of water formed by the melting of the last ice age.
The Turso has an average adult size of 7 meters, being the largest denizen of the Caspian, and has a distinct golden skin color. Like in it's extinct *Tetraketos* relatives, the Turso's teeth have converged with those of ctenochasmatoid pterosaurs, being very thin, long and extremely numerous, being baleen-like. It feeds by straining prey from the bottom, trapping crustaceans, insects and worms in it's teeth while biting through the mud and sand, much like HE's greywhales. Annually, swarms of crustaceans and insects inundate the waters of the Caspian and Aral seas, and without any other local filter feeder the Turso has the prime spot, foraging pelagically without competition from baleen-squid or pachycormids. The Turso only has one natural predator, the ravenous Caspian Sea Wolf, which only poses a threat to the pups and weakened adults. Breeding adults congregate in pods, surrounding the few young from attackers with their bodies, forming a living barrier against the selkies.