Choristodera is an unusual linage of diapsid sauropsids. Having evolved in the late Triassic, at the very end of the reptilian radiation that preceeded the reign of the dinosaurs, these animals diversified as aquatic animals. Their relationships with other reptiles aren't well known; some have related them to lepidosaurs, alongside sphenodontians and squamates. Others classified them as part of the archosauromorph radiation. Evidence doesn't seem to favor one group over the other in this, sharing features with both.
After their appearence in the Triassic, through most of the Mesozoic these aquatic reptiles were generally restricted to relative small, lizard like forms, occupying an ecological niche roughly similar to that of the modern Chinese Crocodile Lizard (Shinisaurus crocodylurus),whose evolution in our world occured after the extinction of small choristoderes in Asia; a linage of choristoderes, Hyphalosauridae, managed to go further, elongating their necks, becoming miniature plesiosaur analogues from the rivers of Cretaceous Asia. Eventually, however, even they disappeared, as did all of Asia's small choristoderes, presumably after the drying in the Late Cretaceous; by the Eocene there were none of these small lizard like forms.
By the early late Cretaceous, choristoderes diversified in a new, until then unusual way. Increasing in size, they became more crocodile like, developing longer rostrums. This change seems to coincide with the decline of crocodylians that took place at that time in Asia. Eventually, however, crocodilians returned, and since then both groups began wrestling over their ecological niche; generally, these large choristoderes favoured fish eating niches akin to those of gharials, competing thus with both true gharials and the now gone dyrosaurid crocodilians.
Being a mostly laurasian clade, pratically no choristodere fossils come from the southern continents; on the other hand, it seems that gavialids had their stronghold over there, as had true crocodiles. When the Cretaceous ended, choristoderes had a monopoly over aquatic predator niches in laurasian water bodies, gavialids restricted to Africa and South America and the only aquatic crocodilians in the northern hemisphere being the generalist alligatorids and terrestrial crocodiles, neither competition for the mostly piscivorous choristoderes. However, as the Oligocene dawned, climatic changes did not favour these aquatic reptiles; the gharial like forms declined severely as their aquatic environments dried away, being then replaced by invading gharials from the south. The smaller lizard like choristoderes had a much longer longevity on the european continent, but even then disappeared as the ice ages came.
On Terra Alternativa, on the other hand, things went quite differently.
At the beginning of the Cenozoic, champsosaurs took advantage of the extinction of mosasaurs and plesiosaurs to explore marine ecosystems. They crossed the sea into South America, and then to Africa, while colonising marine ecosystems; being more adapted to an aquatic lifestyle than their crocodilian competitors, they triumphed over dyrosaurids, and prevented any serious colonisation by gharials of South America's freshwater ecosystems. By the end of the Eocene, they had an essencially cosmopolitian presence, both dominant families (see below) having fossils on both hemispheres. When the Oligocene cooling occured, they were far more efficient at surviving, given their presence in freshwater bodies on all landmasses. Returning to the seas well before the Miocene, the last gharials were finally outcompeted, and much like their crocodilian counterparts did in our world choristoderes spread across the world's oceans. Their success would eventually meet another downfall, the Pliocene cooling, followed by the ice ages, but their diversity is still overwhelming, being present in lakes, rivers and coast waters all over the world.
Over the hundreds of millions of years they evolved on earth, choristoderes became quite specialised to life in the water. Their skeleton is quite robust, as in many aquatic tetrapods; they have heavily ossified gastralia, and the ribs are short and massive. Their legs became paddles, used in swimming and steering, whereas the tail became laterally compressed for the same reasons. The thorax is dorso ventrally flattened. Their skin is smooth with small, non-overlapping scales, making it more hydrodynamic, as opposed to crocodillians and their scutes. Being so adapted to live in the water, they are ovoviparous, retaining the eggs inside and giving birth to live young rather than coming ashore to lay eggs. Indeed, most choristoderes are fully aquatic, rather than amphibious like crocodillians, and such is reflected on their sensory organs. In crocodillians, such as gharials, the nostrils and the eyes are placed in a very dorsal position, allowing the animal to float without anything but the eyes and nostrils visible. In choristoderes, the eyes are directed in a frontal position instead, and the nostrils are at the very tip of the snout. Is is because they preffer to rest on the bottom rather than float, and thus the only part of the body that needs to surface is the tip of the snout. Being ectothermic and obtaining some of the oxygen they need from the skin, as turtles and sea snakes do (much like some turtles, they also obtain oxygen from cloacal tissue), choristoderes can spend long periods of time without surfacing, being thus quite cryptic animals. Laying in the bottom waiting for prey, they have proportionally huge jaw muscles, allowing extremely fast bites. Their skull is quite similar to the standard lepidosaurian skull, minus the complex quadrate.
All living choristoderes belong to Neochoristodera, the forms outside the clade being relic european forms, which died out by the Pliocene as in our world.
A mainly eurasian clade, simoedosaurids began as typical, gharial like piscivores during the Cretaceous; having their origins as far back in time as the lowest early Cretaceous, it is possible they evolved their long rostrums independently from the champsosaurs, although it is clear both groups are sister linages. They seem to have entered the Cenozoic with something of a heavy blow, as only Simoedosaurus is known from the beginning of this period, although with a wide distribution on Asia and North America. In Terra Alternativa, there's the additional taxa Nepots, courtesy of a more complete fossil record of early Cenozoic India. The first known gondwannan choristodere, the earliest fossils of this genus come from the Paleocene of India, just after the KT event; within the Paleocene and the Eocene, it seems to have spread into Asia, Europe (then an isolated archipelago), Africa and Madagascar. Whereas Simoedosaurus died on the onset of the Eocene/Oligocene extinction event, Nepots survived, and diversified. Its descendents would take once more the seas, reaching North America when their champsosaur competitors were at their lowest (see below), and were the first choristoderes to reach Australia, by the middle Miocene, although this conquest was very brief. Eventually, though, they were gradually replaced by champsosaurs, and the Pliocene cooling took a large blow on them. Still, while gone from most of their former range, the five living species enjoy some success, in spite of competition from both champsosaurs as well as crocodillians and aquatic mammals.
Caribbean Pelican Lotan (Mourasuchomimus caribensis)
The last member of a linage of cosmopolitian range, this animal occurs obviously in the Caribbean; capable of living confortably in either salt or freshwater, it is most often found in estuaries and coastoal lakes and bays. It is most distinct by the nature of its rostrum; its jaws are wider than those of its relatives, being quite broad at the base before narrowing in a triangular shape. As the name indicates, it has a pelican like pouch on the lower jaw, that uses to catch prey; it lost the teeth in that same jaw, while the upper one has many thin teeth, remaniscent of baleen, albeit much shorter of course. It feeds by opening the jaws as wide as possible while attacking a fish school, closing them and expelling the water while the teeth retain the prey inside, a hunting style used both by our myscetes and pelicans, as well as the Cretaceous stomatosuchid crocodilians as well as the Miocene Mourasuchus (which did not evolve on Terra Alternativa). Like most choristoderes it is still a benthic animal, waiting or stalking in the bottom, relying on surprise to seize prey, and resting in the bottom rather than floating, rendering it, like most choristoderes, a very cryptic animal. Adults are generally brown coloured, with bright orange pouches; during the breeding season the males develop purple and red patterns. Being viviparous like all choristoderes, the green and black juveniles are born with a typical gharial like snout filled with normal teeth, which gradually becomes the specialised pelican like jaws of the adults. It is the largest of the living choristoderes, reaching a full grown size of ten meters, atlhough the average is 7-8.5 meters.
Iberian Lotan (Neosimoedosaurus duberdicus)
In both Spec and HE, Europe was subjected to intense glaciations in the Pliocene and Pleistocene, which caused the extinction of most of it's reptiles, amphibians and fish. Combined with it's relative geographical isolation, this meant that few species recolonised the continent during the warmer interglacial periods; as such, in both worlds it's current herpetofaunas are greatly impoverished compared to those of North America and eastern Asia. This has affected most noticebly the continent's freshwater environments: as recently as the early Pliocene, both timelines displayed large aquatic predators such as crocodillians and cryptobranchid salamanders, wiped out when the climate cooled. In HE, the interglacials saw only the emerge of a few pike species as the apex predators in the numerous european wasterways, with a single large catfish evolving in eastern Europe. In Spec, however, western Europe's waterways are still patrolled by one large "reptillian" species, an animal truly remaniscent of HE's folkloric river and loch monsters.
The Iberian Lotan, Neosimoedosaurus duberdicus, as the name implies, patrols the waterways of the Iberian Peninsula, where it is the apex predator. It is a representative of the unusual clade of aquatic sauropsids known as Choristodera, the only lineage of post-Triassic non-archosaur, non-lepidosaur reptiles aside from turtles, plesiosaurs and ichthyosaurs, which in both timelines survived across the Cenozoic. In HE, the very last choristodere was the european Lazarrusuchus, a basal lizard-like form which held on in it's wetlands until the glaciations, after the rest of it's kin disappeared from the rest of the world. The Iberian Lotan, however, is a more derived form of choristodere, part of the iconic long-snouted, fully aquatic Neochoristodera, which in Spec managed to survive beyond the Eocene, and diversify across the world's freshwater and coastoal environments.
In particular, the Iberian Lotan is part of Simoedosauridae, a clade of neochoristoderes well represented in the fossil reccord of Asia across the Cretaceous, and in the Paleogene of Europe and North America by the genus Simoedosaurus. In Spec, simoedosaurids managed to survive in the Caribbean, as represented by the Oligocene genus Caribbodraco, and later spread across the world's coastoal and freshwater environments in the Miocene. However, simoedosaurids never reached the same success as their sister clade, Champsosauridae, and eventually witnessed a decline in the late Miocene and Pliocene, with only 5 species alive today. The genus Neosimoedosaurus seemingly evolved in the shores of the Paratethys Seaway in the late Miocene and Pliocene, and is currently also represented by the Turkish Lotan (N. anatolica), occuring in eastern Europe and Turkey, and the Nile Lotan (N. niloticus), as well as a now extinct Pleistocene species, †N. barbaricina. The Iberian Lotan seemingly diverged from it's closest relatives in the early Pleistocene, around 800,000 years ago.
The oldest Iberian Lotan remains come from the Ebro's basin from around 760,000 years ago; older specimens have been found in the Balearic Islands, but it's unclear whereas they belong to the Iberian Lotan, †N. barbaricina, or another species altogether. Tagus and Guadiana basin specimens are known from 699,000 years ago, presumably reaching the other river systems directly via changing tributaries and/or interglaciary mass floods, and since then the species has established itself across the Iberian Peninsula, often disappearing from most of it during glaciations but always remaining in these rivers. Currently, it occurs in the Douro, Tagus, Sado, Guadiana, Guadalquivir, Ebro, Júcar and Segura river systems as well as a few large lakes like the Sanabria, having spread across the Peninsula during the end of the last glaciation, as well as more recently via marine incursions.
The Iberian Lotan is the largest freshwater animal in the Euoprean continent, reaching a maximum length of 6 meters and weighting over 700 kg, but it usually smaller at an average length of 4.7-5.3 meters. Like most neochoristoderes, it can best be described as a reptile's answer to a garfish, it's limbs converted into powerful flippers, the tail into a large, rotund tail fin, and the jaws being very long (they fit the broader, more Atractosteus like profile typical of Simoedosaurus rather than the finner shape of champsosaurids and basal simoedosaurids). The torso is dorso-ventrally flattened, allowing it to rest at the bottom and increasing it's hydrodinamic profile, the nostrils are a single opening at the very tip of the snout, the orbits are oriented forwards (crocodiles, for instance, have the eyes positioned dorsally instead), the rear of the skull is proportionally massively bulbous as the temporal fenestrae form massive arches where the immense jaw muscles are located. The scales are very small and non-overlapping, and a thick layer of blubber covers the entire body, obscuring it's body contours significantly; it's skin is overall very smooth and provoking little drag, generally coloured black.
Like most neochoristoderes, the Iberian Lotan is a rather cryptic animal, spending most of if not it's entire life completely submerged; with the nostrils at the very tip of the snout, the animal basically uses it's jaws as a snorkel, meaning that only a few inches of it's body regularly surface. Like nearly all choristoderes, it is viviparous and does not come ashore to breed, being completly aquatic, and like most of it's neochoristodere kin it is endothermic, hence being able to survive in the colder waters of it's home. In these aspects, neochoristoderes resemble somewhat river dolphins, but unlike them they retained good eyesight, their vision in fact being their primary sense, alongside gustation. It favours shallow, relatively clear waters, but it can be found in most riverine and lacustrine habitats as long as the water is at least 1.3 meters deep and it can move around.
The Iberian Lotan is the largest carnivore and indeed animal in it's habitat, with pups only being occasionally targetted by selkies. It feeds on a variety of prey items, favouring fish of various sizes as well as crayfish, but also regularly feeding on turtles, amphibians, waterfowl and other aquatic birds, riverine selkies and multies, washed out carrion and even terrestrial dinosaurs and mammals that come to drink. In estuaries, small mosasaurs, Hesperornithes and marine mammals like cancridonts may be targetted. The Lotan usually hunts by ambush or slow stalking, hidding itself near the bottom where it may use it's flippers to "walk", then striking at lightning fast speeds. The powerful neochoristodere jaw muscles usually simply ensure extremely fast bites in most species, but the more robust jaws of derived simoedosaurids mean that they can target larger prey; when this happens, the fast and extremely powerful bites cause severe tissue damage and even bone fracture, meaning that the prey suffers terrible injury and slowly bleeds to death. The Iberian Lotan usually hunts solitarily, but in some regions, particularly in Winter months, the animals may hunt in loose cooperation, joining in "mobs" to trap fish shoals in the shallows or overwhelming large prey.
Like HE river dolphins, most neochoristoderes tend to have migratory habits, and the Iberian Lotan is no exception. During Spring months, it swims upriver, reaching well inland and even taking advantage of seasonal floods to travel around into lakes or other rivers. It is during this time of the year that females give birth, usually From late March to late April, usually producing one or two pups, that stay in the vicinity of their mother for a month or so. Around September or so, they migrate downriver, with populations in the smaller river systems like the Sado often congregating entirely in the estuary. During this time of year, the Iberian Lotan is at it's most social, males producing infrasounds as means to attract mates. Reproductive mating occurs primarily between late September and early October - though same-sex intercourse may be found year-round -, resulting in a pregnancy that lasts throught the Winter.
Turk Lotan (Neosimoedosaurus anatolica)
Neosimoedosaurus had its origins in Central Asia, when what is now mostly steppe and desert was once the vast inland branch of the Tethys sea, still remaining in the Caspian, Aral and Black seas. Spreading westwards once the Mediterranean was filled again, its range met difficulties with the Pliocene cooling, forcing retreat into North Africa, in the Nile. Nonetheless, they managed to expand even during the ice ages; first to diverge was the mentioned iberian species, colonising western Europe, and considerably later the Nile and Turk species diverged.
The Turk Lotan resembles its iberian relative, the main difference is that it is more bluish in colouration. It occurs in the major rivers and lakes of Asia Minor and Greece, occuring frequently also in the seperating Aegean and southern Black seas, being thus the most marine of the genus. There are also several populations in the islands of the Aegean Sea, some exclusively marine while others have the benefit of at least one river; the population on Cyprus might be a distinct species, although further studies are necessary. In times this animal occured all over the Black Sea, when it was a large freshwater body; nowadays still some fully marine populations occur in the south of this sea. Like its iberian relative it has no competitors in the freshwater ecosystems of its range, and in the sea it is also only bothered by some sharks, marine mammals and particularly large hesperornithes.
Nile Lotan (Neosimoedosaurus niloticus)
Brown and Black coloured, this animal is much akin to its relative, although it shares its habitat with crocodiles, which offer it predation and competition, although being more piscivorous than its relatives usually lessens the later. It occurs all over the Nile, from its delta to its source deep in Africa's heart; it occurs as far south as the big african lakes. Because desertification was never as extense as in our world due to the lack of anthropogenic influcence, North Africa is far wetter, allowing several other minor river systems where this animal occurs, although for obvious reasons its range is far more limited than that of its crocodillian competitors.
One of the two native malagasy choristoderes, this one is endemic to lake Alaotra and the rivers that feed it. About 3 meters at length, this brown-redish coloured animal is less impressive than its relatives, and it is within the prey range of the native Voay crocodiles. It diverged from its simoedosaurid relatives quite early, and it appears to be pretty much identical to the long gone Nepots.
The other main linage of modern choristoderes, champsosaurs are the most famous choristoderes, descended from Champsosaurus itself. During the Paleocene, unlike in our world, TA's champsosaurs reached South America, where they became dominant, preventing successful colonisation by both gharials, dyrosaurids and simoedosaurids. During the Eocene they were quite successfull, occuring in Europe and north Africa; via South America they were also present in Antartica and Australia. With the Oligocene cooling they disappeared from most of their range, but they managed to survive in South America, building their a stronghold. Eventually, they gradually began to spread elsewhere, but it wasn't until the middle Miocene that they truly expanded. By then, they exploded in diversity, spreading across the seas and colonising the other landmasses, at the expense of simoedosaurid diversity and ending the last gharials. By the time of the Pliocene cooling, the fatal blow wasn't extreme as they had already made their presence in virtually all landmasses but Antartica.
At the present there are about 11 recognised champsosaur genera, with a +20 number of species.
Jacobson's Champsosaur (Tursiopsaurus jacobsoni)
Occuring in Australia's northern waters, this champsosaur is fully marine. It has a typical colour scheme for a marine animal, with a black upperside and a silver underside, with silver dots along the head and the sides. It is the fastest living choristodere, being able to swim at speeds of over 40 km/ph, although obviously only for short periods of time. It is about 4 meters long
Sargasso Sea Champsosaur (Pelagotomistoma sargassum)
A pelagic species, this animal occurs mostly in the Sargassum Sea, as the name indicates. It is coloured olive green with a grey underside, with black and brown dots appearing all along the body, the exact amount, location, shape and size of such dots being different in each individual. It is one of the few choristoderes that leads a completly pelagic lifestyle, never touching the bottom. Resting in the floating sargassum natural rafts, they hide themselves in them as well, in order to both ambush prey and to protect themselves from predators. Their breeding season coincides with the arrival of migratory fish, such as eels, which come in huge numbers to lay their eggs. They are about 2 meters long.
Malagasy Champsosaur (Asiatochampsosaurus madagascarensis)
One of the two malagsy choristoderes, this one has a far wider distribution than the native simoedosaurid, occuring in both the major rivers as well as costoal waters; it preffers freshwater habitats, although some populations like the one in the Comoros are fully marine. There are also some populations in east Africa as well. Reaching an adult size of 5 meters, this creature is black in colour. Its closest relatives are actually asian species like the Yangtzee Champsosaur (Asiatochampsosaurus sinensis), having appearently crossed the Indian Ocean somewhere in the Pliocene.