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Thalattosauria

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Not to be confused withThalattosuchia.
Thalattosauria
Temporal range:Middle-Late Triassic,Anisian–Rhaetian
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 classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Neodiapsida
Order: †Thalattosauria
Merriam,1904
Superfamilies
Synonyms

Thalattosauria(Greekfor "sea lizards" ) is an extinctorderofmarine reptilesthat lived in the Middle to LateTriassic.Thalattosaurs were diverse in size and shape, and are divided into two superfamilies:AskeptosauroideaandThalattosauroidea.Askeptosauroids were endemic to theTethys Ocean,their fossils have been found in Europe and China, and they were likelysemiaquaticfish eaters with straight snouts and decent terrestrial abilities.[1]Thalattosauroids were more specialized for aquatic life and most had unusual downturned snouts and crushing dentition. Thalattosauroids lived along the coasts of bothPanthalassaand the Tethys Ocean, and were most diverse in China and western North America.[2]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,rhynchocephaliansand their relatives),[3][4]Archosauromorpha(archosaursand their relatives),[5]ichthyosaurs,[6]and/or other marine reptiles.[7][8]

Description

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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, thehumerusis widest near the shoulder, thefemuris widest near the knee, theradiusis reniform ( "kidney-shaped "), andphalangesare long and plate-like. Askeptosauroids retain hourglass-shaped limb bones like land reptiles, but even they share specializations with thalattosauroids such as a shorttibiaandfibula,with the latter expanding near the ankle.[1][9][2]

Skull

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A diagram of the skull ofThalattosaurus alexandrae

Thalattosaurs arediapsidreptiles, 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.[10]Thalattosaurs lack aquadratojugal bone,leaving the lower temporal fenestra open from below. They also lackpostparietalandtabularbones, while thesquamosal boneis small, thesupratemporal boneis extensive, and thequadrate boneis large. When seen from above, the rear edge of the skull bears a large, triangular embayment that reaches further forwards than the quadrates.[5]

Thalattosaurs have a rostrum (snout) significantly longer than the portion of the skull behind the eyes. A majority of this length is formed from thepremaxillarybones, 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 bonesare separated from each other and restricted to a small portion of the snout near the nares. Thelacrimal boneis typically lost or fused to the large crescent-shapedprefrontal bonein front of the orbit, mirroring thepostfrontal bonewhich is usually fused to the three-prongedpostorbital bonebehind the orbit.[10][11][5]

Askeptosauroideahave narrow, straight-edged snouts which are often elongated and filled with conical teeth. One askeptosauroid,Endennasaurus,is entirely toothless[12]while another,Miodentosaurus,has a short, blunt snout.[13]Most members of the second thalattosaur group,Thalattosauroidea,have more distinctive downturned snouts.ClaraziaandThalattosaurusboth have snouts that taper into a narrow tip. Most of the snout is straight, butpremaxillaeat the tip are downturned.XinpusaurusandConcavispinaalso have downturnedpremaxillae,but the end of themaxillaeare sharply upturned, forming a notch in their skull. InHescheleria(and potentiallyNectosaurusandParalonectes), 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.[11]The exception to this rule isGunakadeit,which has a straight snout and many slender teeth.[2]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 boneand anangular bonethat extends far forwards along the lower edge of the jaw.Palataldentition is extensive in thalattosauroids but absent in askeptosauroids.[14][2]

Paleobiology

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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 functionaldigitswhich had not transformed into flippers, in contrast toichthyosaursandsauropterygians.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.[3][1][12][2][15]

Thalattosaurs had diverse diets, though they probably all involved marine animals in one way or another.Endennasaurusprobably predated small animals like fish fry or small crustaceans due to its lack of teeth.[12]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[16]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 largemolluscsor other thick-shelled prey.[14][17]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 apparatuswhich may have played a role insuction feeding.[2]Thalattosaurs also fell prey to other marine reptiles: the torso of a ~4 meter (13 feet) longXinpusaurus xing yiensishas 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, andXinpusaurusmay also be the largest prey item preserved within another marine reptile.[18]

Distribution

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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. AssumingEndennasaurusandAskeptosauruswere the mostbasalaskeptosauroids, Askeptosauroidea would have originated in the Western Tethys Ocean, now the Alpine region of Europe.[1]However, ifMiodentosaurusis more basal, a Western Tethys (European) origin would be significantly less likely.[19]Although thesister groupto Thalattosauria is still debated, one possibility, theicthyosauromorphs,seemingly evolved in the Eastern Tethys (China) during the early Triassic or earlier.[8]

The oldest known thalattosauroids (Thalattosaurus,Paralonectes,andAgkistrognathusofBritish Columbia'sSulphur Mountain Formation) 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.[1][20]However, this is based on the hypothesis thatNectosaurus(from California),Xinpusaurus(from China), and an unnamed species fromAustriaformed acladebasal to other thalattosaurs, a classification scheme which contrasts with many other studies.[9]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.[20]Coastal "refuges" such asvolcanic island arcsandguyotsmay have facilitated the ability of thalattosaurs to spread between ocean basins.[10]Hescheleria-like forms were previously only reported from North America and Europe,[21]but in 2021 aHescheleria-like snout fragment was reported from China, indicating that they also had a widespread distribution.[22]Trans-Panthalassa connections are also observed in other Triassic marine life such aspistosaursandammonites.[10]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.[2]

Classification

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Early hypotheses

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Many 20th-century paleontologists considered thalattosaurs to be an independent offshoot of formerly terrestrial reptiles closely related tosquamates(such aslizards) orrhynchocephalians(such as thetuatara,pictured)

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,Thalattosauruswas 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 tochoristoderesand "Proganosauria" (parareptiles). Comparisons were also made with Parasuchia (phytosaurs), Lacertilia (lizards), andProterosuchus,but dismissed as incompatible with proposed evolutionary schemes.[23]

Further discussion by Merriam (1905) considered a relationship withichthyosaursdue 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 an short-lived early attempt for near-lizards to return to the sea, an evolutionary process later repeated more successfully whenmosasaursevolved 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.[3]Later 20th-century workers typically placed thalattosaurs close to rhynchocephalians orsquamatesas part of the group now known asLepidosauromorpha.[14]

Modern classification and external relationships

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Thalattosaurs have been proposed to be related to various reptile groups includingarchosauromorphs,[5]sauropterygians,[4]andichthyosaurs[6][7](such asTemnodontosaurus,pictured)

The rising popularity ofcladisticsin the late 1980s had some effects on thalattosaur classification. Continued research has helped cement some aspects of reptile classification, such as howSauria(a majorcladeofdiapsidsincluding 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 analysisbyEvans,1988) have instead suggested that they may be on the archosauromorph branch of Sauria.[5]Rieppel(1998)'s re-evaluation of the thalattosaur-likepachypleurosaurHanosaurusargued that thalattosaurs have affinities with the aquatic reptile orderSauropterygia,which itself is aligned with turtles within an expansive interpretation of Lepidosauromorpha.[4]

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.[6]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 groupof the sauropterygians, with their clade being sister to the ichthyosauromorphs, and all three being basalarchosauromorphs.However,cladogramsgenerated by these analyses change in unpredictable ways through alterations to their methodology (such as including or excluding aquatic adaptations or switching betweenparsimonyandbayesian inference), leading some to have concerns over the validity of the "marine superclade".[7][8][24][25][26]While thalattosaurs are almost certainly diapsids, the large degree of uncertainty surrounding theiroutgrouprelations has led most modern paleontologists to classify them as Diapsidaincertae sedis.

Internal relationships

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One of the first phylogenetic analyses specifically focusing on thalattosaurs was part ofNicholls(1999)'s reevaluation ofThalattosaurusandNectosaurus.She used a restricted definition of Thalattosauria which referred to acladeincluding all reptiles more closely related toNectosaurusandHescheleriathan toEndennasaurusorAskeptosaurus.The more inclusive group includingAskeptosaurus,Endennasaurus,and traditional thalattosaurs was given the nameThalattosauriformes.[14][1][20]

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 bothAskeptosaurusandThalattosaurus.In these studies, Thalattosauria is divided into two branches, one leading to relatives ofAskeptosaurusand the other leading to relatives ofThalattosaurus.The clade containing reptiles closer toThalattosaurusthan to askeptosaurids is given the nameThalattosauroidea(and sometimes called Thalattosauridea[9][19]). Meanwhile, the clade containing reptiles closer to askeptosaurids is termedAskeptosauroidea[10][13][2]or Askeptosauridea.[9][19]

Subsequent studies since Nicholls (1999) started to include more taxa, including newly described Chinese taxa such asAnshunsaurusandXinpusaurus.[10][27]However, uncertainty overEndennasaurus's thalattosaurian ancestry led to it being excluded from these analyses. After Mülleret al.(2005) re-affirmed thatEndennasauruswas closely related toAskeptosaurus,[12]all thalattosaurs known at the time were finally combined into phylogenetic analyses.[9][20]Studies by Rieppel,Liu,Cheng,Wu,and others continued to identify new Chinese taxa such asMiodentosaurusand various species ofAnshunsaurusandXinpusaurus,thoughhomoplasyin these new taxa has led to little resolution in the structure of the two major branches of Thalattosauria.[13][28]In an attempt to remedy this problem, new phylogenetic analyses were developed by Liuet al.(2013) during the description ofConcavispina,[19]andDruckenmilleret al.(2020) during the description ofGunakadeit.[2]

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 includeAskeptosaurusandAnshunsaurus,[9]with a few studies also placingMiodentosaurus[13]orEndennasaurus[12]within it. Another family, Thalattosauridae, was originally used to groupThalattosaurusandNectosaurus,[3]was later redefined to excludeNectosaurus,[14]and later still encompassed practically all thalattosauroids.[19]Many thalattosaur-focused paleontologists avoid using family names due to their inconsistent usage and questionable validity.

Thecladogrampresented here is based on the largest and most recent analysis of thalattosaur ingroup relations, Druckenmilleret al.(2020). It shows all thalattosaur genera except for the fragmentaryAgkistrognathus.[2]

Thalattosauria

List of genera

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Other thalattosaurs include unnamed or indeterminate species from theKössen FormationofAustria,[20]theSulphur Mountain[29]andPardonet Formations[30]ofBritish Columbia,theNatchez Pass FormationofNevada,[30][21]and theVester FormationofOregon.[31][32]Blezingeria,a fragmentary marine reptile from theMuschelkalkofGermany,has also been considered a thalattosaur by some authors but this assignment is uncertain at best.[1]Neosinasaurus,a poorly-known reptiles from theXiaowa Formationof China, has also been considered a thalattosaur.[13]Thalattosaurian fragments are known from Spanish Muschelkalk.[33]A previously unnamed specimen fromAlaska[34]was described asGunakadeitin 2020.[2]

Name Year Formation Location Notes Images
Agkistrognathus 1993 Sulphur Mountain Formation(Middle Triassic?) Canada(British Columbia) A poorly-known thalattosauroid with strong jaws
Anshunsaurus 1999 Zhuganpo Formation,Xiaowa Formation(Middle Triassic-Late Triassic,Ladinian?-Carnian?) China A large askeptosauroid known from three species. One of the few thalattosaurs for which a growth series is known
Askeptosaurus 1925 Grenzbitumenzone(Middle Triassic,Anisian?) Switzerland,Italy The namesake of Askeptosauroidea and one of the most well-known European thalattosaurs
Clarazia 1936 Grenzbitumenzone(Middle Triassic,Anisian?) Switzerland A thalattosauroid related toHescheleria
Concavispina 2013 Xiaowa Formation(Late Triassic,Carnian?) China The largest known thalattosauroid, a close relative ofXinpusaurus
Endennasaurus 1984 Zorzino Limestone(Late Triassic,Norian) Italy An unusual askeptosauroid with a pointed, toothless snout
Gunakadeit 2020 Hound Island Volcanics(Late Triassic,Norian) United States(Alaska) A basal thalattosauroid, the most well-preserved specimen from North America
Hescheleria 1936 Grenzbitumenzone(Middle Triassic,Anisian?) Switzerland A hook-snouted thalattosauroid
Miodentosaurus 2007 Xiaowa Formation(Late Triassic,Carnian?) China A very large askeptosauroid with a short snout
Nectosaurus 1905 Hosselkus Limestone(Late Triassic) United States(California) One of the first thalattosaurs to be described, along withThalattosaurus
Paralonectes 1993 Sulphur Mountain Formation(Middle Triassic?) Canada(British Columbia) A poorly-known thalattosauroid with a downcurved snout
Pachystropheus 1935 Westbury Formation(Late Triassic,Rhaetian) United Kingdom(Somerset) A small askeptosauroid, youngest known thalattosaur.[35]
Wapitisaurus 1988 Sulphur Mountain Formation(Early Triassic) Canada(British Columbia) A thalattosauroid initially described as aweigeltisaurid.[36]
Wayaosaurus 2000 "Wayao Member"(Late Triassic,Carnian?) China A large askeptosauroid similar toMiodentosaurus.Initially described as apachypleurosaur.[37]
Thalattosaurus 1904 Hosselkus Limestone,Sulphur Mountain Formation(Middle Triassic-Late Triassic) United States(California),Canada(British Columbia) The namesake of Thalattosauria and the first genus to be described. Known from at least two species
Xinpusaurus 2000 Zhuganpo Formation,Xiaowa Formation(Middle Triassic-Late Triassic,Ladinian?-Carnian?) China A thalattosauroid with an unusual notched skull. Known from four species, though not all may be valid

References

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