Thalattosauria (Greek for "sea lizards") is an extinctorder ofmarine reptiles that lived during theTriassic Period.^[1] Thalattosaurs were diverse in size and shape, and are divided into two superfamilies:Askeptosauroidea andThalattosauroidea.

Thalattosauria Temporal range:Early?-Late Triassic(Olenekian?-Rhaetian) PreꞒ Ꞓ O S D C P T J K Pg N
A collage of thalattosaur fossils. Clockwise from upper left:Askeptosaurus italicus (an askeptosauroid),Endennasaurus acutirostris (an askeptosauroid),Gunakadeit joseeae (a thalattosauroid),Thalattosaurus alexandrae (a thalattosauroid)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Class:	Reptilia
Clade:	Neodiapsida
Order:	†Thalattosauria Merriam, 1904
Superfamilies
†Blezingeria? †Askeptosauroidea †Thalattosauroidea
Synonyms
ThalattosauriformesNicholls, 1999

Askeptosauroids were endemic to theTethys Ocean, their fossils have been found in Europe and China, and they were likelysemiaquatic fish eaters with straight snouts and decent terrestrial abilities.^[2] Thalattosauroids were more specialized for aquatic life and most had unusual downturned snouts and crushing dentition. Thalattosauroids lived along the coasts of bothPanthalassa and the Tethys Ocean, and were most diverse in China and western North America.^[3]

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 tolizards, their exact relationships are unresolved. They are widely accepted asdiapsids, but experts have variously placed them on the reptile family tree amongLepidosauromorpha (squamates,rhynchocephalians and their relatives),^[4][5]Archosauromorpha (archosaurs and their relatives),^[6]ichthyosaurs,^[7]sauropterygians,^[8] and/or other marine reptiles.^[9][10]

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 vialateral undulation. Thalattosauroids additionally possess short, wide limb bones poorly adapted for movement on land. In this superfamily, thehumerus is widest near the shoulder, thefemur is widest near the knee, theradius is reniform ("kidney-shaped"), andphalanges are long and plate-like. Askeptosauroids retain hourglass-shaped limb bones like land reptiles, but even they share specializations with thalattosauroids such as a shorttibia andfibula, with the latter expanding near the ankle.^[2][11][3]

Thalattosaurs arediapsid reptiles, meaning that they have temporal fenestrae, two holes in the head behind theorbit (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.^[12] Thalattosaurs lack aquadratojugal bone, leaving the lower temporal fenestra open from below. They also lackpostparietal andtabular bones, while thesquamosal bone is small, thesupratemporal bone is extensive, and thequadrate bone is large. When seen from above, the rear edge of the skull bears a large, triangular embayment that reaches further forwards than the quadrates.^[6]

Thalattosaurs have a rostrum (snout) significantly longer than the portion of the skull behind the eyes. A majority of this length is formed from thepremaxillary bones, and the nares (nostril holes) are shifted back close to the eyes. The premaxillae stretch back very far and are incised into thefrontal bones. This leads to an unusual trait that is characteristic of thalattosaurs, where the left and rightnasal bones are separated from each other and restricted to a small portion of the snout near the nares. Thelacrimal bone is typically lost or fused to the large crescent-shapedprefrontal bone in front of the orbit, mirroring thepostfrontal bone which is usually fused to the three-prongedpostorbital bone behind the orbit.^[12][13][6]

Askeptosauroidea have narrow, straight-edged snouts which are often elongated and filled with conical teeth. One askeptosauroid,Endennasaurus, is entirely toothless^[14] while another,Miodentosaurus, has a short, blunt snout.^[15] Most members of the second thalattosaur group,Thalattosauroidea, have more distinctive downturned snouts.Clarazia andThalattosaurus both have snouts that taper into a narrow tip. Most of the snout is straight, butpremaxillae at the tip are downturned.Xinpusaurus andConcavispina also have downturnedpremaxillae, but the end of themaxillae are sharply upturned, forming a notch in their skull. InHescheleria (and potentiallyNectosaurus andParalonectes), 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 adiastema. Thalattosauroids also haveheterodont dentition, with pointed piercing teeth at the front of the snout and low crushing teeth further back.^[13] The exception to this rule isGunakadeit, which has a straight snout and many slender teeth.^[3] 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-likecoronoid bone and anangular bone that extends far forwards along the lower edge of the jaw.Palatal dentition is extensive in thalattosauroids but absent in askeptosauroids.^[16][3]

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 vialateral undulation. Even so, they retained strong claws and functionaldigits which had not transformed into flippers, in contrast toichthyosaurs andsauropterygians. 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.^{[4][2][14][3][17]}

Thalattosaurs had diverse diets, though they probably all involved marine animals in one way or another.Endennasaurus probably predated small animals like fish fry or small crustaceans due to its lack of teeth.^[14] Various thalattosauroids (likeThalattosaurus,Xinpusaurus, andConcavispina) had large fang-like teeth at the front of the mouth and thick button-like teeth at the back of the mouth. Based onMassare (1987)'s^[18] technique of correlating diet with tooth shape, the taller teeth were suited for a "crunching" diet, involving armored fish, largecrustaceans, and thin-shelledammonites. The low, robust teeth would have been useful for a "crushing" diet specialized in largemolluscs or other thick-shelled prey.^[16][19]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 largehyoid apparatus which may have played a role insuction feeding.^[3] Thalattosaurs also fell prey to other marine reptiles: the torso of a ~4 meter (13 feet) longXinpusaurus xingyiensis has been found within the body cavity of a 5-meter (16 feet) long skeleton of the predatory ichthyosaurGuizhouichthyosaurus. This is the oldest known predatory interaction between marine reptiles, andXinpusaurus may also be the largest prey item preserved within another marine reptile.^[20]

It is not certain where thalattosaurs originated from. During the Triassic period, the earth had one giant supercontinent,Pangaea, which was surrounded by the superoceanPanthalassa. The eastern portion of Pangaea was incised by a massive tropical inland sea, theTethys Ocean, 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. AssumingEndennasaurus andAskeptosaurus were the mostbasal askeptosauroids, Askeptosauroidea would have originated in the Western Tethys Ocean, now the Alpine region of Europe.^[2] However, ifMiodentosaurus is more basal, a Western Tethys (European) origin would be significantly less likely.^[21] Although thesister group to Thalattosauria is still debated, one possibility, theicthyosauromorphs, seemingly evolved in the Eastern Tethys (China) during the early Triassic or earlier.^[10]

The oldest known thalattosauroid isWapitisaurus, followed byThalattosaurus,Paralonectes, andAgkistrognathus. All four were found inBritish Columbia'sSulphur Mountain Formation and would have lived in eastern Panthalassa, along what is now the western coast of North America.^[1][22] Müller (2005, 2007) argued that at least one branch of thalattosauroids had managed to spread worldwide early in their evolution.^[2][23] However, this is based on the hypothesis thatNectosaurus (from California),Xinpusaurus (from China), and an unnamed species fromAustria formed aclade basal to other thalattosaurs, a classification scheme which contrasts with many other studies.^[11]

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.^[23] Coastal "refuges" such asvolcanic island arcs andguyots may have facilitated the ability of thalattosaurs to spread between ocean basins.^[12]Hescheleria-like forms were previously only reported from North America and Europe,^[24] but in 2021 aHescheleria-like snout fragment was reported from China, indicating that they also had a widespread distribution.^[25] Trans-Panthalassa connections are also observed in other Triassic marine life such aspistosaurs andammonites.^[12] 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.^[3]

When first named by Merriam in 1904, Thalattosauria was only known by the speciesThalattosaurus alexandrae. Based primarily on the overall skull shape, it was hypothesized to have been close to the reptile orderRhynchocephalia, which includesSphenodon (the livingtuatara). Nevertheless,Thalattosaurus was recognized as distinct enough to be given its own order, and was tentatively grouped along with Rhynchocephalia in the groupDiaptosauria, a collection of various "primitive" reptiles now known to bepolyphyletic. Within Diaptosauria, thalattosaurs were also considered very closely related tochoristoderes and "Proganosauria" (parareptiles). Comparisons were also made with Parasuchia (phytosaurs), Lacertilia (lizards), andProterosuchus, but dismissed as incompatible with proposed evolutionary schemes.^[26]

Further discussion by Merriam (1905) considered a relationship withichthyosaurs 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 asPaliguana) was described in more detail, with thalattosaurs serving as a short-lived early attempt for near-lizards to return to the sea, an evolutionary process later repeated more successfully whenmosasaurs 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.^[4] Later 20th-century workers typically placed thalattosaurs close to rhynchocephalians orsquamates as part of the group now known asLepidosauromorpha.^[16]

The rising popularity ofcladistics in the late 1980s had some effects on thalattosaur classification. Continued research has helped cement some aspects of reptile classification, such as howSauria (a majorclade ofdiapsids including all living reptiles) split in the Permian into two branches: Lepidosauromorpha (which leads to lizards, snakes, and the tuatara) andArchosauromorpha (which leads to crocodilians and dinosaurs, including birds). While many paleontologists still consider thalattosaurs probable lepidosauromorphs, a few studies (such as aphylogenetic analysis byEvans, 1988) have instead suggested that they may be on the archosauromorph branch of Sauria.^[6]Rieppel (1998)'s re-evaluation of the thalattosaur-likepachypleurosaurHanosaurus argued that thalattosaurs have affinities with the aquatic reptile orderSauropterygia, which itself is aligned with turtles within an expansive interpretation of Lepidosauromorpha.^[5]

An analysis byMüller (2004) has even considered thalattosaurs to belong just outside of Sauria. Unusually, thalattosaurs have an affinity to shift nearichthyosaurs (in the groupIchthyosauromorpha) when certain basal saurians or near-saurians are excluded from the data set.^[7] 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. For example, Simõeset al (2022) classify thalattosaurs as thesister group of the sauropterygians, with their clade being sister to the ichthyosauromorphs, and all three being basalarchosauromorphs. However,cladograms generated by these analyses change in unpredictable ways through alterations to their methodology (such as including or excluding aquatic adaptations or switching betweenparsimony andbayesian inference), leading some to have concerns over the validity of the "marine superclade".^{[9][10][27][28][29]} While thalattosaurs are almost certainly diapsids, the large degree of uncertainty surrounding theiroutgroup relations has led most modern paleontologists to classify them as Diapsidaincertae sedis.

One of the first phylogenetic analyses specifically focusing on thalattosaurs was part ofNicholls (1999)'s reevaluation ofThalattosaurus andNectosaurus. She used a restricted definition of Thalattosauria which referred to aclade including all reptiles more closely related toNectosaurus andHescheleria than toEndennasaurus orAskeptosaurus. The more inclusive group includingAskeptosaurus,Endennasaurus, and traditional thalattosaurs was given the nameThalattosauriformes.^[16][2][23]

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 bothAskeptosaurus andThalattosaurus. In these studies, Thalattosauria is divided into two branches, one leading to relatives ofAskeptosaurus and the other leading to relatives ofThalattosaurus. The clade containing reptiles closer toThalattosaurus than to askeptosaurids is given the nameThalattosauroidea (and sometimes called Thalattosauridea^[11][21]). Meanwhile, the clade containing reptiles closer to askeptosaurids is termedAskeptosauroidea^[12][15][3] or Askeptosauridea.^[11][21]

Subsequent studies since Nicholls (1999) started to include more taxa, including newly described Chinese taxa such asAnshunsaurus andXinpusaurus.^[12][30] However, uncertainty overEndennasaurus's thalattosaurian ancestry led to it being excluded from these analyses. After Mülleret al. (2005) re-affirmed thatEndennasaurus was closely related toAskeptosaurus,^[14] all thalattosaurs known at the time were finally combined into phylogenetic analyses.^[11][23] Studies by Rieppel,Liu,Cheng,Wu, and others continued to identify new Chinese taxa such asMiodentosaurus and various species ofAnshunsaurus andXinpusaurus, thoughhomoplasy in these new taxa has led to little resolution in the structure of the two major branches of Thalattosauria.^[15][31] In an attempt to remedy this problem, new phylogenetic analyses were developed by Liuet al. (2013) during the description ofConcavispina,^[21] andDruckenmilleret al. (2020) during the description ofGunakadeit.^[3]

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 includeAskeptosaurus andAnshunsaurus,^[11] with a few studies also placingMiodentosaurus^[15] orEndennasaurus^[14] within it. Another family, Thalattosauridae, was originally used to groupThalattosaurus andNectosaurus,^[4] was later redefined to excludeNectosaurus,^[16] and later still encompassed practically all thalattosauroids.^[21] Many thalattosaur-focused paleontologists avoid using family names due to their inconsistent usage and questionable validity.

The followingcladogram represents the results of a thalattosaur ingroupphylogenetic analysis by Druckenmilleret al. (2020).^[3]

Other thalattosaurs include unnamed or indeterminate species from theKössen Formation ofAustria,^[23] theSulphur Mountain^[32] andPardonet Formations^[33] ofBritish Columbia, theNatchez Pass Formation ofNevada,^[33][24] and theVester Formation ofOregon.^[34][35]

Blezingeria, a fragmentary marine reptile from theMuschelkalk ofGermany, has also been considered a thalattosaur by some authors but this assignment is uncertain at best.^[2]Thalattosaurian fragments are known from Spanish Muschelkalk as well.^[36]Neosinasaurus, a poorly-known reptile from theXiaowa Formation of China, has been identified as a possible thalattosaur.^[15] A previously unnamed specimen fromAlaska^[37] was described asGunakadeit in 2020.^[3]