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Thalattosauria (Attic Greek for "sea lizards") is an extinct order of that lived during the Triassic. Thalattosaurs were diverse in size and shape, and are divided into two superfamilies: Askeptosauroidea and Thalattosauroidea.
Askeptosauroids were endemic to the Tethys Ocean, their fossils have been found in Europe and China, and they were likely semiaquatic 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 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 , their exact relationships are unresolved. They are widely accepted as , but experts have variously placed them on the reptile family tree among Lepidosauromorpha (Squamata, and their relatives), Archosauromorpha ( and their relatives), Ichthyosauria, , and/or other marine reptiles.
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 bones, and the nares (nostril holes) are shifted back close to the eyes. The premaxillae stretch back very far and are incised into the . This leads to an unusual trait that is characteristic of thalattosaurs, where the left and right are separated from each other and restricted to a small portion of the snout near the nares. The lacrimal bone is typically lost or fused to the large crescent-shaped prefrontal bone in front of the orbit, mirroring the postfrontal bone which is usually fused to the three-pronged postorbital bone behind the orbit.
Askeptosauroidea have narrow, straight-edged snouts which are often elongated and filled with conical teeth. One askeptosauroid, Endennasaurus, is entirely toothless 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 at the tip are downturned. Xinpusaurus and Concavispina also have downturned , but the end of the 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. Thalattosauroids also have Heterodont, 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. Palate dentition is extensive in thalattosauroids but absent in askeptosauroids.
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. 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 , and thin-shelled Ammonoidea. The low, robust teeth would have been useful for a "crushing" diet specialized in large Mollusca or other thick-shelled prey. Gunakadeit
The oldest known thalattosauroid is Wapitisaurus, followed by Thalattosaurus, Paralonectes, and Agkistrognathus. All four were found in British Columbia's Sulphur Mountain Formation and would have 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 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 Island arc and 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 Pistosauroidea and Ammonoidea. 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.
Further discussion by Merriam (1905) considered a relationship with Ichthyosauria 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) 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 when 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 Squamata as part of the group now known as Lepidosauromorpha.
An analysis by Müller (2004) has even considered thalattosaurs to belong just outside of Sauria. Unusually, thalattosaurs have an affinity to shift near Ichthyosauria (in the group Ichthyosauromorpha) 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. For example, Simões et al (2022) classify thalattosaurs as the sister group of the sauropterygians, with their clade being sister to the ichthyosauromorphs, and all three being basal Archosauromorpha. However, 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.
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
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 following cladogram represents the results of a thalattosaur ingroup Phylogenetics by Druckenmiller et al. (2020).
Blezingeria, a fragmentary marine reptile from the Muschelkalk of Germany, has also been considered a thalattosaur by some authors but this assignment is uncertain at best.Thalattosaurian fragments are known from Spanish Muschelkalk as well. Neosinasaurus, a poorly-known reptile from the Xiaowa Formation of China, has been identified as a possible thalattosaur. A previously unnamed specimen from Alaska was described as Gunakadeit in 2020.
| Agkistrognathus | 1993 | Sulphur Mountain Formation | Middle Triassic | () | A poorly-known thalattosauroid with strong jaws | |
| Anshunsaurus | 1999 | Zhuganpo Formation, Xiaowa Formation | Middle Triassic-Late Triassic (lateLadinian - early Carnian) | A large askeptosauroid known from three species. One of the few thalattosaurs for which a growth series is known | ||
| Askeptosaurus | 1925 | Grenzbitumenzone | Middle Triassic (late Anisian) | , | The namesake of Askeptosauroidea and one of the most well-known European thalattosaurs | |
| Clarazia | 1936 | Grenzbitumenzone | Middle Triassic (late Anisian) | A thalattosauroid related to Hescheleria | ||
| Concavispina | 2013 | Xiaowa Formation | Late Triassic (early Carnian) | The largest known thalattosauroid, a close relative of Xinpusaurus | ||
| Endennasaurus | 1984 | Zorzino Limestone | Late Triassic (middle Norian) | An unusual askeptosauroid with a pointed, toothless snout | ||
| Gunakadeit | 2020 | Hound Island Volcanics | Late Triassic (middle Norian) | () | A basal thalattosauroid, the most well-preserved specimen from North America | |
| Hescheleria | 1936 | Grenzbitumenzone | Middle Triassic (late Anisian) | A hook-snouted thalattosauroid | ||
| Miodentosaurus | 2007 | Xiaowa Formation | Late Triassic (early Carnian) | A very large askeptosauroid with a short snout | ||
| Nectosaurus | 1905 | Hosselkus Limestone | Late Triassic (Carnian) | () | One of the first thalattosaurs to be described, along with Thalattosaurus | |
| Paralonectes | 1993 | Sulphur Mountain Formation (Middle Triassic?) | Middle Triassic (late Ladinian) | () | A poorly-known thalattosauroid with a downcurved snout | |
| Pachystropheus | 1935 | Westbury Formation | Late Triassic (Rhaetian) | () | A small askeptosauroid, youngest known thalattosaur. | |
| Wapitisaurus | 1988 | Sulphur Mountain Formation | Early Triassic? (Olenekian?) | () | A tiny thalattosauroid initially described as a weigeltisaurid, and reinterpreted as a thalattosaur in 2023. | |
| Wayaosaurus | 2000 | "Wayao Member" (Xiaowa Formation) | Late Triassic (early Carnian) | A large askeptosauroid similar to Miodentosaurus. Initially described as a pachypleurosaur, and reinterpreted as a thalattosaur in 2023. | ||
| Thalattosaurus | 1904 | Hosselkus Limestone, Sulphur Mountain Formation | Middle Triassic-Late Triassic (Ladinian-Carnian) | (), () | 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 (lateLadinian - early Carnian) | A thalattosauroid with an unusual notched skull. Known from four species, though not all may be valid | ||
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