Thalattosauria (

Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...

for "sea lizards") is an extinct order of prehistoric marine reptiles that lived in the middle to late

Triassic The Triassic ( ) is a geologic period and system (stratigraphy), system which spans 50.6 million years from the end of the Permian Period 251.902 million years ago (Year#Abbreviations yr and ya, Mya), to the beginning of the Jurassic Period 201.36 ...

period. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea and Thalattosauroidea. Askeptosauroids were endemic to the

Tethys Ocean The Tethys Ocean ( el, Τηθύς ''Tēthús''), also called the Tethys Sea or the Neo-Tethys, was a prehistoric ocean that covered most of the Earth during much of the Mesozoic Era and early Cenozoic Era, located between the ancient continents ...

, their fossils have been found in Europe and China, and they were likely

semiaquatic In biology, semiaquatic can refer to various types of animals that spend part of their time in water, or plants that naturally grow partially submerged in water. Examples are given below. Semiaquatic animals Semi aquatic animals include: * Ve ...

fish eaters with straight snouts and decent terrestrial abilities. Thalattosauroids were more specialized for aquatic life and most had unusual downturned snouts and crushing dentition. Thalattosauroids lived along the coasts of both

Panthalassa Panthalassa, also known as the Panthalassic Ocean or Panthalassan Ocean (from Greek "all" and "sea"), was the superocean that surrounded the supercontinent Pangaea, the latest in a series of supercontinents in the history of Earth. During th ...

and the Tethys Ocean, and were most diverse in China and western North America. The largest species of thalattosaurs grew to over 4 meters (13 feet) in length, including a long, flattened tail utilized in underwater propulsion. Although thalattosaurs bore a superficial resemblance to lizards, their exact relationships are unresolved. They are widely accepted as

diapsid Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years a ...

s, but experts have variously placed them on the reptile family tree among

Lepidosauromorpha Lepidosauromorpha (in PhyloCode known as ''Pan-Lepidosauria'') is a group of reptiles comprising all diapsids closer to lizards than to archosaurs (which include crocodiles and birds). The only living sub-group is the Lepidosauria, which cont ...

(

squamates Squamata (, Latin ''squamatus'', 'scaly, having scales') is the largest order of reptiles, comprising lizards, snakes, and amphisbaenians (worm lizards), which are collectively known as squamates or scaled reptiles. With over 10,900 species, it ...

, rhynchocephalians and their relatives), Archosauromorpha ( archosaurs and their relatives),

ichthyosaurs Ichthyosaurs (Ancient Greek for "fish lizard" – and ) are large extinct marine reptiles. Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia ('fish flippers' – a designation introduced by Sir Richard Owen in 1842, alt ...

, and/or other marine reptiles.

Description

Thalattosaurs have moderate adaptations to marine lifestyles, including long, paddle-like tails and slender bodies with more than 20 dorsal vertebrae. There are few unique traits of the postcranial skeleton shared by all thalattosaurs, but the skeleton is still useful for distinguishing between askeptosauroids and thalattosauroids. Askeptosauroids are characterized by elongated necks with short neural spines and at least 11 vertebrae, while thalattosauroids have shorter necks sometimes involving as few as four vertebrae. Thalattosauroids also have tall neural spines on their neck, back, and especially the tail vertebrae, increasing the surface area for swimming via

lateral undulation Undulatory locomotion is the type of motion characterized by wave-like movement patterns that act to propel an animal forward. Examples of this type of gait include crawling in snakes, or swimming in the lamprey. Although this is typically the ...

. Thalattosauroids additionally possess short, wide limb bones poorly adapted for movement on land. In this superfamily, the humerus is widest near the shoulder, the

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates wit ...

is widest near the knee, the

radius In classical geometry, a radius ( : radii) of a circle or sphere is any of the line segments from its center to its perimeter, and in more modern usage, it is also their length. The name comes from the latin ''radius'', meaning ray but also the ...

is reniform ("

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blo ...

-shaped"), and phalanges are long and plate-like. Askeptosauroids retain hourglass-shaped limb bones like land reptiles, but even they share specializations with thalattosauroids such as a short

tibia The tibia (; ), also known as the shinbone or shankbone, is the larger, stronger, and anterior (frontal) of the two bones in the leg below the knee in vertebrates (the other being the fibula, behind and to the outside of the tibia); it connects ...

and

fibula The fibula or calf bone is a leg bone on the lateral side of the tibia, to which it is connected above and below. It is the smaller of the two bones and, in proportion to its length, the most slender of all the long bones. Its upper extremity i ...

, with the latter expanding near the ankle.

Skull

Thalattosaurs are

reptiles, meaning that they have temporal fenestrae, two holes in the head behind the

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...

(eye socket). However, many thalattosaurs have a vestigial upper temporal fenestra which is slit-like, and some have it fully closed up by surrounding bones. Thalattosaurs lack a

quadratojugal bone The quadratojugal is a skull bone present in many vertebrates, including some living reptiles and amphibians. Anatomy and function In animals with a quadratojugal bone, it is typically found connected to the jugal (cheek) bone from the front and ...

, leaving the lower temporal fenestra open from below. They also lack

postparietal Postparietals are cranial bones present in fish and many tetrapods. Although initially a pair of bones, many lineages possess postparietals which were fused into a single bone. The postparietals were dermal bones situated along the midline of the ...

and tabular bones, while the

squamosal bone The squamosal is a skull bone found in most reptiles, amphibians, and birds. In fishes, it is also called the pterotic bone. In most tetrapods, the squamosal and quadratojugal bones form the cheek series of the skull. The bone forms an ancestral c ...

is small, the

supratemporal bone The supratemporal bone is a paired cranial bone present in many tetrapods and tetrapodomorph fish. It is part of the temporal region (the portion of the skull roof behind the eyes), usually lying medial (inwards) relative to the squamosal and latera ...

is extensive, and the

quadrate bone The quadrate bone is a skull bone in most tetrapods, including amphibians, sauropsids (reptiles, birds), and early synapsids. In most tetrapods, the quadrate bone connects to the quadratojugal and squamosal bones in the skull, and forms upper pa ...

is large. When seen from above, the rear edge of the skull bears a large, triangular embayment that reaches further forwards than the quadrates. Thalattosaurs have a rostrum (snout) significantly longer than the portion of the skull behind the eyes. A majority of this length is formed from the premaxillary bones, and the nares (nostril holes) are shifted back close to the eyes. The premaxillae stretch back very far and are incised into the

frontal bones The frontal bone is a bone in the human skull. The bone consists of two portions.'' Gray's Anatomy'' (1918) These are the vertically oriented squamous part, and the horizontally oriented orbital part, making up the bony part of the forehead, pa ...

. This leads to an unusual trait that is characteristic of thalattosaurs, where the left and right nasal bones are separated from each other and restricted to a small portion of the snout near the nares. The

lacrimal bone The lacrimal bone is a small and fragile bone of the facial skeleton; it is roughly the size of the little fingernail. It is situated at the front part of the medial wall of the orbit. It has two surfaces and four borders. Several bony landmarks of ...

is typically lost or fused to the large crescent-shaped

prefrontal bone The prefrontal bone is a bone separating the lacrimal and frontal bones in many tetrapod skulls. It first evolved in the sarcopterygian clade Rhipidistia, which includes lungfish and the Tetrapodomorpha. The prefrontal is found in most modern a ...

in front of the orbit, mirroring the

postfrontal bone The skull is a bone protective cavity for the brain. The skull is composed of four types of bone i.e., cranial bones, facial bones, ear ossicles and hyoid bone. However two parts are more prominent: the cranium and the mandible. In humans, th ...

which is usually fused to the three-pronged

postorbital bone The ''postorbital'' is one of the bones in vertebrate skulls which forms a portion of the dermal skull roof and, sometimes, a ring about the orbit. Generally, it is located behind the postfrontal and posteriorly to the orbital fenestra. In some v ...

behind the orbit. Askeptosauroidea have narrow, straight-edged snouts which are often elongated and filled with conical teeth. One askeptosauroid, ''

Endennasaurus ''Endennasaurus'' is an extinct genus of thalattosaurian from the Upper Triassic of Italy. It was found in and named after the Endenna cave, composed of Zorzino Limestone in Lombardia. while another, '' Miodentosaurus'', has a short, blunt snout. Most members of the second thalattosaur group, Thalattosauroidea, have more distinctive downturned snouts. '' Clarazia'' and '' Thalattosaurus'' both have snouts that taper into a narrow tip. Most of the snout is straight, but

premaxilla The premaxilla (or praemaxilla) is one of a pair of small cranial bones at the very tip of the upper jaw of many animals, usually, but not always, bearing teeth. In humans, they are fused with the maxilla. The "premaxilla" of therian mammal has ...

e at the tip are downturned. '' Xinpusaurus'' and '' Concavispina'' also have downturned

e, but the end of the

maxilla The maxilla (plural: ''maxillae'' ) in vertebrates is the upper fixed (not fixed in Neopterygii) bone of the jaw formed from the fusion of two maxillary bones. In humans, the upper jaw includes the hard palate in the front of the mouth. T ...

e are sharply upturned, forming a notch in their skull. In '' Hescheleria'' (and potentially '' Nectosaurus'' and '' Paralonectes''), the premaxillae are abruptly downturned at the end of the snout, nearly forming a right angle with the rest of the jaw. In these forms, the end of the snout is a toothy hook separated from the rest of the jaw by a space called a

diastema A diastema (plural diastemata, from Greek διάστημα, space) is a space or gap between two teeth. Many species of mammals have diastemata as a normal feature, most commonly between the incisors and molars. More colloquially, the condition ...

. Thalattosauroids also have heterodont dentition, with pointed piercing teeth at the front of the snout and low crushing teeth further back. The exception to this rule is '' Gunakadeit'', which has a straight snout and many slender teeth. Thalattosaurs often have a pronounced retroarticular process at the rear of the mandible. Thalattosauroids are more specialized than askeptosauroids in jaw anatomy, as they have evolved a large peak-like coronoid bone and an angular bone that extends far forwards along the lower edge of the jaw.

Palatal The palate () is the roof of the mouth in humans and other mammals. It separates the oral cavity from the nasal cavity. A similar structure is found in crocodilians, but in most other tetrapods, the oral and nasal cavities are not truly separ ...

dentition is extensive in thalattosauroids but absent in askeptosauroids.

Paleobiology

Thalattosaurs are only known from marine deposits, indicating that they were all primarily aquatic reptiles. The retracted nostrils and long, paddle-shaped tail are further evidence for aquatic habits. Thalattosauroids seemingly spent all of their time in the water, with short, wide limbs, poorly developed wrist and ankle bones, and tall vertebrae adapted for swimming via

. Even so, they retained strong claws and functional digits which had not transformed into flippers, in contrast to

and sauropterygians. Unlike these other marine reptiles, there is no evidence that thalattosaurs fully adapted to a pelagic life out in the open ocean, and instead they probably all lived in warm waters close to the coast. Askeptosauroids had stronger limbs more typical of terrestrial reptiles, indicating they would have been capable of moving around on land to some extent. They likely primarily used their tails when swimming, while thalattosauroids may have utilized their body and tail in conjunction. Thalattosaurs had diverse diets, though they probably all involved marine animals in one way or another. ''

Endennasaurus ''Endennasaurus'' is an extinct genus of thalattosaurian from the Upper Triassic of Italy. It was found in and named after the Endenna cave, composed of Zorzino Limestone in Lombardia. Various thalattosauroids (like '' Thalattosaurus'', '' Xinpusaurus'', and '' Concavispina'') had large fang-like teeth at the front of the mouth and thick button-like teeth at the back of the mouth. Based on Massare (1987)'s technique of correlating diet with tooth shape, the taller teeth were suited for a "crunching" diet, involving armored fish, large

crustaceans Crustaceans (Crustacea, ) form a large, diverse arthropod taxon which includes such animals as decapods, seed shrimp, branchiopods, fish lice, krill, remipedes, isopods, barnacles, copepods, amphipods and mantis shrimp. The crustacean gro ...

, and thin-shelled

ammonites Ammonoids are a group of extinct marine mollusc animals in the subclass Ammonoidea of the class Cephalopoda. These molluscs, commonly referred to as ammonites, are more closely related to living coleoids (i.e., octopuses, squid and cuttl ...

. The low, robust teeth would have been useful for a "crushing" diet specialized in large

molluscs Mollusca is the second-largest phylum of invertebrate animals after the Arthropoda, the members of which are known as molluscs or mollusks (). Around 85,000 extant species of molluscs are recognized. The number of fossil species is estim ...

or other thick-shelled prey. '' Gunakadeit'''s slender teeth correlated with the "Pierce II" guild of Massare (1987), indicating it likely fed on soft, fast-moving fish and squid. It also had a large

hyoid apparatus The hyoid apparatus is the collective term used in veterinary anatomy for the bones which suspend the tongue and larynx. It consists of pairs of stylohyoid, thyrohyoid, epihyoid and ceratohyoid bones, and a single basihyoid bone. The hyoid appara ...

which may have played a role in

suction feeding Aquatic feeding mechanisms face a special difficulty as compared to feeding on land, because the density of water is about the same as that of the prey, so the prey tends to be pushed away when the mouth is closed. This problem was first identifi ...

. Thalattosaurs also fell prey to other marine reptiles: the torso of a ~4 meter (13 feet) long ''Xinpusaurus xingyiensis'' has been found within the body cavity of a 5 meter (16 feet) long skeleton of the predatory ichthyosaur ''

Guizhouichthyosaurus ''Guizhouichthyosaurus'' is an extinct genus of ichthyosaur which is known primarily from the Xiaowa Formation of the lower Carnian stage of the Late Triassic in southwest China. The type species of this genus is ''Guizhouichthyosaurus tangae'', ...

.'' This is the oldest known predatory interaction between marine reptiles, and ''Xinpusaurus'' may also be the largest prey item preserved within another marine reptile.

Distribution

It is not certain where thalattosaurs originated from. During the Triassic period, the earth had one giant supercontinent,

Pangaea Pangaea or Pangea () was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It assembled from the earlier continental units of Gondwana, Euramerica and Siberia during the Carboniferous approximately 335 million y ...

, which was surrounded by the superocean

. The eastern portion of Pangaea was incised by a massive tropical inland sea, the

, which extended all the way from China to Western Europe. While thalattosauroids are known from worldwide Triassic marine deposits, askeptosauroids are only known in Tethyan deposits. Assuming ''

Endennasaurus ''Endennasaurus'' is an extinct genus of thalattosaurian from the Upper Triassic of Italy. It was found in and named after the Endenna cave, composed of Zorzino Limestone in Lombardia.Askeptosaurus'' were the most basal askeptosauroids, Askeptosauroidea would have originated in the Western Tethys Ocean, now the Alpine region of Europe. However, if '' Miodentosaurus'' is more basal, a Western Tethys (European) origin would be significantly less likely. Although the sister group to Thalattosauria is still debated, one possibility, the icthyosauromorphs, seemingly evolved in the Eastern Tethys (China) during the early Triassic or earlier. The oldest known thalattosauroids (''Thalattosaurus'', '' Paralonectes'', and '' Agkistrognathus'' of

British Columbia British Columbia (commonly abbreviated as BC) is the westernmost province of Canada, situated between the Pacific Ocean and the Rocky Mountains. It has a diverse geography, with rugged landscapes that include rocky coastlines, sandy beaches, ...

Sulphur Mountain Formation The Sulphur Mountain Formation is a geologic formation of Early to Middle Triassic age. It is present on the western edge of the Western Canada Sedimentary Basin in the foothills and Rocky Mountains of western Alberta and northeastern British Col ...

) lived in eastern Panthalassa, along what is now the western coast of North America. Müller (2005, 2007) argued that at least one branch of thalattosauroids had managed to spread worldwide early in their evolution. However, this is based on the hypothesis that '' Nectosaurus'' (from California), ''Xinpusaurus'' (from China), and an unnamed species from

Austria Austria, , bar, Östareich officially the Republic of Austria, is a country in the southern part of Central Europe, lying in the Eastern Alps. It is a federation of nine states, one of which is the capital, Vienna, the most populous ...

formed a clade basal to other thalattosaurs, a classification scheme which contrasts with many other studies. The worldwide distribution of Thalattosauroidea is intriguing considering that thalattosaurs are considered to be poorly adapted for traversing open oceans, which would have been a necessity for spreading between the eastern coast of Panthalassa and the Tethys Ocean. Coastal "refuges" such as volcanic island arcs and guyots may have facilitated the ability of thalattosaurs to spread between ocean basins. '' Hescheleria''-like forms were previously only reported from North America and Europe, but in 2021 a ''Hescheleria''-like snout fragment was reported from China, indicating that they also had a widespread distribution. Trans-Panthalassa connections are also observed in other Triassic marine life such as pistosaurs and

. Evidently thalattosaurs were capable of dispersing throughout major marine regions multiple times before the group's extinction, with thalattosauroids likely more prolific at spreading than askeptosauroids due to their greater aquatic adaptations.

Classification

Early hypotheses

When first named by Merriam in 1904, Thalattosauria was only known by the species ''Thalattosaurus alexandrae''. Based primarily on the overall skull shape, it was hypothesized to have been close to the reptile order Rhynchocephalia, which includes ''Sphenodon'' (the living

tuatara Tuatara (''Sphenodon punctatus'') are reptiles endemic to New Zealand. Despite their close resemblance to lizards, they are part of a distinct lineage, the order Rhynchocephalia. The name ''tuatara'' is derived from the Māori language and m ...

). Nevertheless, ''Thalattosaurus'' was recognized as distinct enough to be given its own order, and was tentatively grouped along with Rhynchocephalia in the group Diaptosauria, a collection of various "primitive" reptiles now known to be

polyphyletic A polyphyletic group is an assemblage of organisms or other evolving elements that is of mixed evolutionary origin. The term is often applied to groups that share similar features known as homoplasies, which are explained as a result of conver ...

. Within Diaptosauria, thalattosaurs were also considered very closely related to choristoderes and "Proganosauria" (

parareptiles Parareptilia ("at the side of reptiles") is a subclass or clade of basal sauropsids ( reptiles), typically considered the sister taxon to Eureptilia (the group that likely contains all living reptiles and birds). Parareptiles first arose near t ...

). Comparisons were also made with Parasuchia (

phytosaurs Phytosaurs (Φυτόσαυροι in greek) are an extinct group of large, mostly semiaquatic Late Triassic archosauriform reptiles. Phytosaurs belong to the order Phytosauria. Phytosauria and Phytosauridae are often considered to be equivalen ...

), Lacertilia ( lizards), and ''

Proterosuchus ''Proterosuchus'' is an extinct genus of archosauriform reptiles that lived during the Early Triassic. It contains three valid species: the type species ''P. fergusi'' and the referred species ''P. alexanderi'' and ''P. goweri''. All three spec ...

'', but dismissed as incompatible with proposed evolutionary schemes. Further discussion by Merriam (1905) considered a relationship with

due to their similar ecology, but questioned why their skull and vertebral anatomy would diverge so widely if they had a close common ancestor. He proposed that potential similarities were best explained as convergent evolution. The possibility that thalattosaurs diverged from reptiles close to lizards (such as ''

Paliguana ''Paliguana'' is an extinct genus of lizard-like lepidosauromorph reptile from the earliest Triassic Katberg Formation (Beaufort Group) in the upper ''Lystrosaurus'' Assemblage Zone of South Africa South Africa, officially the Republic ...

'') was described in more detail, with thalattosaurs serving as an short-lived early attempt for near-lizards to return to the sea, an evolutionary process later repeated more successfully when

mosasaurs Mosasaurs (from Latin ''Mosa'' meaning the 'Meuse', and Greek ' meaning 'lizard') comprise a group of extinct, large marine reptiles from the Late Cretaceous. Their first fossil remains were discovered in a limestone quarry at Maastricht on th ...

evolved from true lizards. Nevertheless, Merriam found no clear evidence that any previously known reptile group was directly ancestral to thalattosaurs or vice versa. They were probably descended from land-dwelling Permian reptiles, and not closely related to other marine reptile groups which first evolved in the Triassic. Later 20th-century workers typically placed thalattosaurs close to rhynchocephalians or

as part of the group now known as

Modern classification and outgroup relations

The rising popularity of

cladistics Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived cha ...

in the late 1980s had some effects on thalattosaur classification. Continued research has helped cement some aspects of reptile classification, such as how

Sauria Sauria is the clade containing the most recent common ancestor of archosaurs (such as crocodilians, dinosaurs, etc.) and lepidosaurs ( lizards and kin), and all its descendants. Since most molecular phylogenies recover turtles as more closely ...

(a major clade of

diapsids Diapsids ("two arches") are a clade of sauropsids, distinguished from more primitive eureptiles by the presence of two holes, known as temporal fenestrae, in each side of their skulls. The group first appeared about three hundred million years ag ...

including all living reptiles) split in the Permian into two branches: Lepidosauromorpha (which leads to lizards, snakes, and the tuatara) and Archosauromorpha (which leads to crocodilians and dinosaurs, including birds). While many paleontologists still consider thalattosaurs probable lepidosauromorphs, a few studies (such as a

phylogenetic analysis In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

Evans Evans may refer to: People *Evans (surname) *List of people with surname Evans Places United States *Evans Island, an island of Alaska *Evans, Colorado *Evans, Georgia *Evans County, Georgia *Evans, New York *Evans Mills, New York *Evans City, ...

, 1988) have instead suggested that they may be on the archosauromorph branch of Sauria. Rieppel (1998)'s re-evaluation of the thalattosaur-like

pachypleurosaur left, 220px, '' Pachypleurosaurus'' Pachypleurosauria is an extinct clade of primitive sauropterygian reptiles that vaguely resembled aquatic lizards, and were limited to the Triassic period. They were elongate animals, ranging in size from , w ...

'' Hanosaurus'' argued that thalattosaurs have affinities with the aquatic reptile order

Sauropterygia Sauropterygia ("lizard flippers") is an extinct taxon of diverse, aquatic reptiles that developed from terrestrial ancestors soon after the end-Permian extinction and flourished during the Triassic before all except for the Plesiosauria became ...

, which itself is aligned with turtles within an expansive interpretation of Lepidosauromorpha. An analysis by Müller (2004) has even considered thalattosaurs to belong just outside of Sauria. Unusually, thalattosaurs have an affinity to shift near

(in the group

Ichthyosauromorpha The Ichthyosauromorpha are an extinct clade of marine reptiles consisting of the Ichthyosauriformes and the Hupehsuchia, living during the Mesozoic. The node clade Ichthyosauromorpha was first defined by Ryosuke Motani ''et al.'' in 2014 as t ...

) when certain basal saurians or near-saurians are excluded from the data set. Some analyses derived from Müller (2004) group thalattosaurs in a "marine superclade" with ichthyosauromorphs and sauropterygians, and sometimes with turtles, archosauromorphs, or lepidosauromorphs as well. However,

cladograms A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

generated by these analyses change in unpredictable ways through alterations to their methodology (such as including or excluding aquatic adaptations or switching between parsimony and bayesian inference), leading some to have concerns over the validity of the "marine superclade". While thalattosaurs are almost certainly diapsids, the large degree of uncertainty surrounding their outgroup relations has led most modern paleontologists to classify them as Diapsida ''

incertae sedis ' () or ''problematica'' is a term used for a taxonomic group where its broader relationships are unknown or undefined. Alternatively, such groups are frequently referred to as "enigmatic taxa". In the system of open nomenclature, uncertain ...

''.

Ingroup relations

One of the first phylogenetic analyses specifically focusing on thalattosaurs was part of Nicholls (1999)'s reevaluation of '' Thalattosaurus'' and '' Nectosaurus''. She used a restricted definition of Thalattosauria which referred to a clade including all reptiles more closely related to ''Nectosaurus'' and '' Hescheleria'' than to ''

Endennasaurus ''Endennasaurus'' is an extinct genus of thalattosaurian from the Upper Triassic of Italy. It was found in and named after the Endenna cave, composed of Zorzino Limestone in Lombardia. However, most studies focusing on the group have preferred to retain a broader definition of Thalattosauria equivalent to Nicholls' Thalattosauriformes clade, including reptiles close to both ''Askeptosaurus'' and ''Thalattosaurus''. In these studies, Thalattosauria is divided into two branches, one leading to relatives of ''Askeptosaurus'' and the other leading to relatives of ''Thalattosaurus''. The clade containing reptiles closer to ''Thalattosaurus'' than to askeptosaurids is given the name Thalattosauroidea (and sometimes called Thalattosauridea). Meanwhile, the clade containing reptiles closer to askeptosaurids is termed Askeptosauroidea''''or Askeptosauridea. Subsequent studies since Nicholls (1999) started to include more taxa, including newly described Chinese taxa such as '' Anshunsaurus'' and '' Xinpusaurus''.'''' However, uncertainty over ''Endennasaurus'''s thalattosaurian ancestry led to it being excluded from these analyses. After Müller ''et al''. (2005) re-affirmed that ''Endennasaurus'' was closely related to ''Askeptosaurus'', all thalattosaurs known at the time were finally combined into phylogenetic analyses. Studies by Rieppel,

Liu / ( or ) is an East Asian surname. pinyin: in Mandarin Chinese, in Cantonese. It is the family name of the Han dynasty emperors. The character originally meant 'kill', but is now used only as a surname. It is listed 252nd in the classic text ...

, Cheng, Wu, and others continued to identify new Chinese taxa such as '' Miodentosaurus'' and various species of ''Anshunsaurus'' and ''Xinpusaurus'', though

homoplasy Homoplasy, in biology and phylogenetics, is the term used to describe a feature that has been gained or lost independently in separate lineages over the course of evolution. This is different from homology, which is the term used to characterize ...

in these new taxa has led to little resolution in the structure of the two major branches of Thalattosauria. In an attempt to remedy this problem, new phylogenetic analyses were developed by Liu ''et al''. (2013) during the description of '' Concavispina'', and Druckenmiller ''et al.'' (2020) during the description of '' Gunakadeit''. The internal relationships of thalattosaurs is still considered tentative and inconclusive, although the fundamental structure of the group (a monophyletic Thalattosauria clade split into askeptosauroids and thalattosauroids) is very stable. Some paleontologists have attempted to divide thalattosaurs into families. One family, Askeptosauridae, is typically considered to include ''Askeptosaurus'' and ''Anshunsaurus,'' with a few studies also placing ''Miodentosaurus'' or ''Endennasaurus'' within it. Another family, Thalattosauridae, was originally used to group ''Thalattosaurus'' and ''Nectosaurus'', was later redefined to exclude ''Nectosaurus'', and later still encompassed practically all thalattosauroids. Many thalattosaur-focused paleontologists avoid using family names due to their inconsistent usage and questionable validity. The

cladogram A cladogram (from Greek ''clados'' "branch" and ''gramma'' "character") is a diagram used in cladistics to show relations among organisms. A cladogram is not, however, an evolutionary tree because it does not show how ancestors are related to ...

presented here is based on the largest and most recent analysis of thalattosaur ingroup relations, Druckenmiller ''et al.'' (2020).

List of genera

Other thalattosaurs include unnamed or indeterminate species from the Kössen Formation of

, the Sulphur Mountain and Pardonet Formations of

, the Natchez Pass Formation of

Nevada Nevada ( ; ) is a state in the Western region of the United States. It is bordered by Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the 7th-most extensive, ...

, and the Vester Formation of

Oregon Oregon () is a state in the Pacific Northwest region of the Western United States. The Columbia River delineates much of Oregon's northern boundary with Washington, while the Snake River delineates much of its eastern boundary with Idaho. T ...

. '' Blezingeria'', a fragmentary marine reptile from the

Muschelkalk The Muschelkalk (German for "shell-bearing limestone"; french: calcaire coquillier) is a sequence of sedimentary rock, sedimentary rock strata (a lithostratigraphy, lithostratigraphic unit) in the geology of central and western Europe. It has a Mid ...

Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...

, has also been considered a thalattosaur by some authors but this assignment is uncertain at best. '' Neosinasaurus'' and '' Wayaosaurus'', a pair of poorly-known reptiles from the

Xiaowa Formation The Xiaowa Formation is a Carnian-age geological formation found in southern China. It is a sequence of limestone and marls from the Carnian stage of the Triassic. Its lower section was previously known as the Wayao Formation or Wayao Member of th ...

of China, have also been considered thalattosaurs. Thalattosaurian fragments are known from Spanish Muschelkalk. A previously unnamed specimen from

Alaska Alaska ( ; russian: Аляска, Alyaska; ale, Alax̂sxax̂; ; ems, Alas'kaaq; Yup'ik: ''Alaskaq''; tli, Anáaski) is a state located in the Western United States on the northwest extremity of North America. A semi-exclave of the U.S. ...

was described as '' Gunakadeit'' in 2020.

References

{{Taxonbar, from=Q1123322 Triassic reptiles Triassic first appearances Late Triassic extinctions