Choristoderes are a very interesting clade of aquatic sauropsids, largely overlooked even by many prehistoric life enthusisasts. Much like in pterosaurs, the exact relationships of these animals are not very clear, although most studies either place them somewhere in Archosauromorpha or as sister taxa to Lepidosauria; it has also been suggested that they are more basal diapsids, perhaps even at the very base of this clade. The exact date when these animals evolved is not clear either; the earliest unambiguous fossils come from the late Triassic, but the known taxa are already sufficiently derived to suggest a much earlier origin, possibly dating back to the Permian, which is supported given how we know lepidosaurs, archosauromorphs and obviously more basal diapsids were diverse well in the middle of the Permian. These animals are well known for their longevity; they not only survived the KT event (and likely passed through the Permian extinction), but positively thrived during the Paleocene and Eocene periods before a sudden disappearence of taxa, and even then they occured in Europe until the Miocene, and are suspected to have only become extinct by the Pliocene or even the early Pleistocene, when european temperatures were too cold even for them. These last taxa, such as Lazarussuchus, were the most primitive known choristoderes, implying one or more ghost lineages extending back to the Triassic or even Permian periods; even individual genera are known to have had long lifespans, like Cteniogenys, which lived from the middle Jurassic all the way to the Campanian. And there is a possibility that this clade endured until the Holocene (see below).
Longevity is not the only trait worth of noting in these reptiles, however. While many clades were very conservative in terms of ecological roles, by the Cretaceous several forms had expanded into new niches.
Giant salamander and plesiosaur wannabes
All known choristoderes are known to have been quite adapted to an aquatic lifestyle. The least derived taxa like Lazarussuchus might had been adapted to climbing, but likely as means to bask as in modern freshwater turtles; forms as derived as Cteniogenys (a quite basal form) already spent most of their lives on water. All known choristodere skin impressions show very small, rounded and non-overlapping scales (with the exception of Monjurosuchus, which exhibits two rows of dorsal scutes), giving the animal a very smooth skin texture; all species have conical teeth, a mark of aquatic predatory reptiles, apropriate to grasp slippery prey like fish and pierce through the shells of crustaceans and molluscs. They had quite dense bones, heavily ossified gastralia and short, yet robust, ribs, adaptations seen in diving reptiles.
Basal lineages, as well as the fairly derived monjurosuchids, are often compared to the modern Shinisaurus lizards, aquatic squamates that ambush fish, tadpoles and invertebrates from shallow water or from vegetation above during daylight. While they are a good analogue for more basal taxa, more derived species like monjurosuchids have small claws and almost paddle-like feet, unlike Shinisaurus lizards; we also are not certain if they were diurnal. A closer analogue would had been probably giant salamanders and freshwater turtles, also ambush predators that spend most of their time in water, and have cathemeral or nocturnal lifestyles. In particular, giant salamanders and turtles can ambush in deeper waters, in the bottom, unlike Shinisaurus lizards; given the adaptations for diving present in choristoderes, it is more logical that monjurosuchids and Cteniogenys hunted in a similar way. They likely used their tail to swim, but the limbs likely had a role in propulsion as well, like in turtles and nothosaurs. They likely were oviparous, but ovovivipary could had evolved (see below).
By the Cretaceous, however, choristoderes began exploring new niches. Two lineages in particular became very distinct from their salamander like ancestors: the long necked hyphalosaurids, and the long jawed neochoristoderes.
Hyphalosauridae is known by two genera, the titular Hyphalosaurus and the less known Shokawa. Alongside monjurosuchids (Hyphalosauridae and Monjurosuchidae may be sister clades), they are exclusively asian, with Hyphalosaurus occuring in the famous Liaoning formations, while Shokawa is known from a single specimen from the Okurodani Formation.
Hyphalosaurids are notable for their long necks, remaniscent of those of sauropterygians and tanystrophids. Indeed, it is thought that hyphalosaurids occupied a similar ecological niche to that of the marine plesiosaurs; these animals come exclusively from areas that were deep lagoons and lakes in the time that they were alive, absent from nearby areas thought to have been shallower. With flatter and smoother scales than their relatives, clear evidence of ovovivipary and an eel-like tail fin, hyphalosaurids were clearly well adapted to a freshwater pelagic lifestyle, and are thought to have chased after prey actively, unlike the monjurosuchids and more basal forms, which ambushed prey. The exact function of the long neck in Mesozoic aquatic reptiles is unknown, and likely induced drag, but it is thought that allowed bottom based stalking, an apropriate transition between ambush and active predation. In some plesiosaurs, we now know that the long neck might have offered an advantage in finding food in the bottom, skimming the sands in search of benthic prey; the same was likely true about hyphalosaurids. Like in plesiosaurs, hyphalosaurids had inflexible spines, but unlike them they used their tail to swim, although the limbs could also have offered propulsion, like in nothosaurs and turtles.
While leptocleidid plesiosaurs did colonise some freshwater ecosystems, any freshwater dwelling sauropterygians are notably absent from Asia, while other fauna associted with marine ecosystems were very prevalent in Liaoning, like sharks, lampreys, sturgeons and even the albatross like ornithocheirid pterosaurs. It is possible, thus, that these choristoderes managed to prevent the colonisation of plesiosaurs, having the competitive advantage, although it could equally be that plesiosaurs never colonised asian freshwater habitats for some other reason.
Hyphalosaurids and monjurosuchids are not known from after the very end of the Aptian, and at least the former likely became extinct in the Turonian extinction events that caused the demise of many aquatic and terrestrial reptiles. However, considering that the more basal, but likely similar Cteniogenys survived well into the late Cretaceous, it is possible that monjurosuchids survived much longer than we currently know.
The pliosaur mimics
Of all choristoderes, the most iconic are the members of Neochoristodera. These animals first evolved in the Aptian, and already showed their notable features that set them apart of the other choristoderes: long, slender jaws, not unlike those of gharials, as well as body sizes that no other choristodere reached.
Like monjurosuchids and hyphalosaurids, these animals evolved in Asia, evidencing that most choristodere diversification occured there. However, unlike their relatives, we know that champsosaurs were successful, and eventually colonised the remaining laurasian landmasses, remaining diverse until the Eocene in freshwater and brackish water ecosystems; evidence of fossils in then insular landmasses such as Europe and Greenland suggests that these choristoderes occured in marine environments at least occasionally, and tolerated colder waters than modern crocodyllians (see below). And appearently outcompeted their crocodyllian competitors; only alligatorids, animals less specialised in piscivory, co-existed with neochoristoderes, while dyrosaurids, gharials and crocodiles are absent until after the demise of neochoristoderes (the true crocodile Asiatosuchus did exist in Eurasia, and the crocodile-like Borealosauchus existed in North America, but appearently neither competed with neochoristoderes, having both been more specialised in terrestrial prey, a common trend in early Cenozoic crocodyllians).
Neochoristoderes are divided into two clades: Champsosauridae, which includes the very widespread and successfull Champsosaurus as well as the poorly described Eotomistoma, and Simoedosauridae, which includes a wide variety of species dating from Aptian and the early late Cretaceous of Asia, as well as the Cenozoic Simoedosaurus, widespread across Laurasia. It has been suggested that the long, gharial like jaws evolved independently in the two clades, with Simoedosaurus having shorter and broader crocodile like jaws that might had been the ancestral condition, but both clades share the fundamental skull morphology unique to this clade.
Despiste their frequent comparations to crocodyllians, these animals weren’t amphibious, but rather fully aquatic, much like hyphalosaurids and monjurosuchids, and this is well reflected in the skull morphology; while they have their nostrils near the distal end of the skull, they aren’t positioned dorsally like in crocodiles, but rather at the very tip of the maxilla, and the eyes are also less positioned dorsally and more positioned forward in the skull. As a result, these animals didn’t float on the surface like modern crocodyllians, and instead laid on the bottom to rest, with only the very tip of the snout reaching the surface like a snorkel; as a result, when alive these creatures would have been very cryptic. The position of the eyes indicates that sight was the most important sense; the presence of features like pressure sensors (present in crocodyllians and spinosaurs, and likely present in several marine reptiles) has not been studied, but was likely to have occured, given their hunting methods.
Ambush hunting has been inferred for neochoristoderes, but simoedosaurids exhibit cranial features that seem suggestive of hydrodynamic efficiency, suggesting that they presumably chased after prey; likely, variation occured among neochoristodere genera, with forms like Champsosaurus relying more on ambush while forms like Tchoiria showed more proficiency in using speed to hunt. Neochoristoderes, having strong paddle like limbs, likely relied on the limbs for propulsion at least as much as the tail, like other choristoderes as well as nothosaurs and turtles. Despiste having slender jaws, neochoristoderes had tremendous bite forces, estimated to have been close in strength to those of modern crocodiles, of over 1910 N (~430 lbs of force), courtesy of the extremely powerful jaw muscles housed within the enormous temporal fresnestrae. This is considerably superior to the known bite force of the modern slender jawed gharials, which amounts to 1182 N (~266 lbs of force). This seems to be an indicator that neochoristoderes delivered extremely fast bites into their prey, as means of both grasping them and severely injuring them before they had time to react. This may also mean they were capable of killing large aquatic vertebrates; large fish like sarcopterygians, freshwater sharks and paddlefish were common in the Cretaceous and the early Cenozoic, and the Eocene saw the evolution of many large freshwater tetrapods, such as trionyxid turtles, pantolestids and suliforme birds (in the Cretaceous, hesperornithes and possibly stagodontids [the latter being now suspected of being otter-like] occupied similar ecological niches to those of suliformes and pantolestids respectively).
Like hyphalosaurs, neochoristoderes were completly aquatic, living the vast majority of the hours in their lives underwater. They were the only choristoderes known to have occured in marine ecosystems; fossils of Champsosaurus have turned up in Greenland, in what was once the shores of the Arctic Ocean, and both Champsosaurus and Simoedosaurus managed to colonise Europe (an archipelago way into the Eocene) and Appalachia (western North America, seperated from eastern North America – Laramidia – by the Western Interior Seaway, and from Europe and Greenland by the Atlantic and Arctic oceans); curiously, at least one of these marine journeys can be inferred to have occured through the North Atlantic, suggesting a neochoristodere stronghold on Arctic waters. The polar oceans were much warmer in the Cretaceous and early Cenozoic than they are today, but even then they were prone to polar nights extending for several months, with currently unknown effects on the temperature and ecosystems of those seas. It appears neochoristoderes were exceptionally adapted to colder waters; their distribution on Paleocene North America is strongly suggestive of this, occuring in lattitudes where crocodyllians are very rare, even the generally cold tolerant alligatorids. How exactly did neochoristoderes tolerated colder water temperatures is unknown; hibernation is how alligatorids generally prevail in temperate climates, but fully aquatic reptiles generally solve this problem by endothermy.
Despiste being fully aquatic, neochoristoderes did not completly remove their ties to land. Champsosaurus is notable for displaying sexual dimorphism in the limb anatomy, with females displaying sacral fusion and more robust limbs; this suggests that females were more capable of terrestrial locomotion than males. Thus, despiste being otherwise fully aquatic reptiles, champsosaurs had to return to land to lay eggs, just like modern turtles. However, these differences in limb morphology are not present in simoedosaurids, suggesting that, much like hyphalosaurids, they became ovoviviparous, and therefore spent all of their lives submerged.
Alongside crocodiles, alligatorids, gharials, dyrosaurids and turtles, neochoristoderes were among the large sized aquatic reptiles that survived the KT event; their luck over plesiosaurs and mosasaurs has been attributed to the fact that freshwater ecosystems remained more stable than the marine ones, and might actually had also benefitted from the input of carcasses of animals that died such as dinosaurs. Turtles, crocodyllians and the smaller, more basal choristoderes like Lazarussuchus also had slower metabolisms than mosasaurs and plesiosaurs, but it is uncertain if the same applied to neochoristoderes, and leatherback sea turtles and lamniforme sharks survived despiste being endothermic.
During the Paleocene and Eocene periods, neochoristoderes thrived in Laurasia, even preventing gharials and dyrosaurids from having much success in the northern continents, and greatly reducing crocodile diversity there as well; they also increased in size. However, by the Oligocene, these animals vanished from the face of the Earth, felling victims to a series of extinction events far less intense than the KT event. Other victims were the marine dyrosaurid crocodyllians, which also survived through the KT event, and the also marine palaeophid sea snakes. These extinctions also claimed the vast majority of primitive whales, leaving only the mysceti and the odontoceti, as well as the aquatic pantolestid mammals and several aquatic bird taxa and both fresh and salt water ecosystems. These series of extinctions, known as the Eocene-Oligocene extinction event, if thought to have occured due to global cooling linked to Antartica’s progressive isolation, the Azolla Event and several meteorite impacts, resulting in a drier climate; the most affected biomes were freshwater and coastoal habitats, while the open seas became more fertile.
This ironic twist is not the first time where pelagic fauna survived while coastoal and freshwater taxa became extinct; the same occured in the Triassic/Jurassic boundary, resulting in the extinction of all aquatic reptiles with the exception of the specialised ichthyosaurs and the pliosaur/plesiosaur sauropterygians, as well as obviously the then small choristoderes and perhaps some coastoal phytosaurs (crocodiles, while surviving, did not become aquatic until several tens of millions of years into the Jurassic, well after phytosaurs were gone, even if they survived until the early Jurassic instead of dying out right in the end of the Triassic).
After the demise of neochoristoderes, Choristodera was represented by basal forms like Lazarussuchus, that dwelt in european wetlands, beforing meeting their demise at the Pliocene or possibly as late as the Pleistocene, when the climate cooled beyond the capacities of tolerance to cold displayed by choristoderes.
Choristodera in cryptozoology
If there is any chance that the thousands of aquatic reptile cryptids are neither hoaxes or bizarre aquatic mammals or fish, chances are they are highly derived choristoderes, not plesiosaurs, pliosaurs or mosasaurs (certainly as hell they’re not ichthyosaurs or metriorhynchids); it is highly unlikely that any of those clades survived the KT event, otherwise cetacean evolution would never have occured. If any of those creatures were sauropsids, instead of mammals like bizarre cetaceans, pinnipedes or monotremates, choristoderes are the only logical option, seeing as they are the only marine reptiles of the Cenozoic that are neither turtles, crocodyllians or sea snakes (marine iguanas, being a recent species, is defenitely out of question); furthermore, the fact that they could produce ovovivipary (hence having the capacity of loosing all connection to land), had smooth skin and produced the nothosaur like hyphalosaurids and polycotylid/metriorhynchid like neochoristoderes would make a choristodere easily mistaken for mesozoic marine sauropsids.
Of course, considering the lack of fossil reccord for lineages of marine choristoderes, even this would be hard to swallow. There’s also no evidence of bizarre marine mammals that fit the descriptions of many sea monsters, and Lazarussuchus proved that choristoderes can not show in the fossil record for extremely long periods of time, but even this world would seem contrived.
A good chunk of reptilian lake cryptids might also perhaps be choristoderes, although the iconic Nessie and other dwellers of glacial lakes certainly aren’t, and most likely don’t really exist either.
Through a friend of mine, I came to know of a particularly peculiar report from China:
“Farmers and fishers living in Eastern China have reported a strange aquatic reptile living in the rivers;
There is an account of a family who captured a strange creature near the rivers when fishing. They went fishing in a river near their house, and they spotted a large animal basking on the riverbank. The creature attempted to escape into the water, but they managed to entangle the animal in a net. The animal was about 1.5 meter long, and looked like a crocodile, except it had a ‘slender snout’, ‘strangely-shaped head’ and ‘smooth, lizard-like skin’. They brought it home alive. One family member suggested it was an aligator, but the animal didn’t look like any creature they had seen before. The grandmother thought the animal was a dragon, and they freed it in the place where they found it before the ‘dragon’ brought bad luck.
And another account of a farmer who saw a large (about 2 meter long?), reptilian and strange-looking animal slide into the water when he approached the riverbank.”
The description offered is obviously remaniscient of a neochoristodere; the identity of the creature as a monitor lizard has been suggested, but monitors don’t have slender snouts, and while I can’t claim to know what is considered “strange” in an reptile to people in China, it is worth to note that, as seen before, neochoristodered have very wide skull bases, due to the enormous temporal fresnestrae and associated musculature, which would defenitely make the head “strangely shaped”.
Unfortunately, I have no idea if the previous reports were forgery or actually happened, so people more in outch with the cryptozoological community are welcomed to lend a hand.
Ji Q., Ji, S., Lü, J., You, H. and Yuan, C. (2006). “Embryos of Early Cretaceous Choristodera (Reptilia) from the Jehol Biota in western Liaoning, China.” Journal of the Paleontological Society of Korea, 22(1): 111-118.
Gao, K.-Q. and Ksepka, D.T. (2008). “Osteology and taxonomic revision of Hyphalosaurus (Diapsida: Choristodera) from the Lower Cretaceous of Liaoning, China.” Journal of Anatomy, 212(6): 747–768.
Ji Q., Wu, X.-C. and Cheng, Y.-N. (2010). “Cretaceous choristoderan reptiles gave birth to live young.” Naturwissenschaften, 97(4): 423-428.
Gao, K.; Evans, S.; Ji, Q.; Norell, M.; and Ji, S. (2000). “Exceptional fossil material of a semi-aquatic reptile from China: the resolution of an enigma”. Journal of Vertebrate Paleontology20 (3): 417–421.