Thalattosauria
Thalattosauria is an extinct order of marine reptiles that lived during the Triassic Period. 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 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 diapsids, but experts have variously placed them on the reptile family tree among Lepidosauromorpha, Archosauromorpha, ichthyosaurs, sauropterygians, 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. 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 is widest near the knee, the radius is reniform, 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 and fibula, with the latter expanding near the ankle.Skull
Thalattosaurs are diapsid reptiles, meaning that they have temporal fenestrae, two holes in the head behind the orbit. 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, leaving the lower temporal fenestra open from below. They also lack postparietal and tabular bones, while the squamosal bone is small, the supratemporal bone is extensive, and the quadrate 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.Thalattosaurs have a rostrum 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 are shifted back close to the eyes. The premaxillae stretch back very far and are incised into the frontal bones. 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 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 premaxillae at the tip are downturned. Xinpusaurus and Concavispina also have downturned premaxillae, but the end of the maxillae are sharply upturned, forming a notch in their skull. In Hescheleria, 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 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 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 lateral undulation. Even so, they retained strong claws and functional digits which had not transformed into flippers, in contrast to ichthyosaurs 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 probably predated small animals like fish fry or small crustaceans due to its lack of teeth. Various thalattosauroids 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 's technique of correlating diet with tooth shape, the taller teeth were suited for a "crunching" diet, involving armored fish, large crustaceans, and thin-shelled ammonites. The low, robust teeth would have been useful for a "crushing" diet specialized in large molluscs or other thick-shelled prey. Gunakadeit
Distribution
It is not certain where thalattosaurs originated from. During the Triassic period, the earth had one giant supercontinent, Pangaea, which was surrounded by the superocean Panthalassa. The eastern portion of Pangaea was incised by a massive tropical inland sea, the Tethys 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. Assuming Endennasaurus and 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 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 during the early Triassic or earlier.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 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, Xinpusaurus, 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 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 ammonites. 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.