Tanystropheus

Tanystropheus (Greek τανυ~ “long” + στροφευς “hinged”) is an extinct reptile that dates from the Middle to Late Triassic epochs. It is recognizable by its extremely elongated neck, which measured long—longer than its body and tail combined. The neck was composed of 12–13 extremely elongated vertebrae. With its very long but relatively stiff neck, "Tanystropheus" has been often proposed and reconstructed as an aquatic or semi-aquatic reptile, a theory supported by the fact that the creature is most commonly found in semi-aquatic fossil sites wherein known terrestrial reptile remains are scarce. Fossils have been found in Europe. Complete skeletons of small individuals are common in the Besano Formation at Monte San Giorgio in Italy and Switzerland; other fossils have been found in the Middle East and China, dating from the Middle Triassic to the early part of the Late Triassic (Anisian, Ladinian, and Carnian stages).
History.
"Tribelesodon", originally considered to be a pterosaur by Francesco Bassani in 1886, is now recognized as a junior synonym to "Tanystropheus". Another junior synonym of "Tanystropheus" is "Procerosaurus", whose type species, "P. cruralis", was erected by von Huene in 1902. Several supposed "Tanystropheus" species described from the southwest United States in the late 19th century were later determined to belong to a new genus of theropod dinosaur, "Coelophysis".
The type species of "Tanystropheus", "T. conspicuus", is based on postcranial specimens from the Upper Muschelkalk of Germany and the Lower Keuper of Poland. These remains were previously known as "Macroscelosaurus" (a nomen oblitum) before being named as "Tanystropheus" in 1855. They were later regarded as undiagnostic, rendering "T. conspicuus" a nomen dubium.
"T. antiquus", which was named in the late 1900s from the Gogolin Formation of Poland, was based on vertebrae shorter than those of other "Tanystropheus" species. Its specimens were subsequently lost until they were rediscovered in the 21st century. "T. antiquus" is currently considered one of the few valid species of "Tanystropheus".
The best-known valid species of "Tanystropheus" is "T. longobardicus", which comes from the Besano Formation (also known as the Grenzbitumenzone), an Anisian-Ladinian formation exposed at Monte San Giorgio on the Italy-Switzerland border. The first "T. longobardicus" specimens were described in 1930s, and Monte San Giorgio continues to produce new specimens up to the present day. In 2005, Dr. Silvio Renesto described a "T. longobardicus" specimen from Switzerland which preserved the impressions of skin and other soft tissue. Five new specimens of "T. longobardicus" were described by Stefania Nosotti in 2007, allowing for a more comprehensive view of the anatomy of the species. 
"Tanystropheus" specimens from the Makhtesh Ramon in Israel were described as a new species, "T. haasi", in 2001. However, this species may be dubious due to the difficulty of distinguishing its vertebrae from "T. conspicuus" or "T. longobardicus". Another new species, "T. biharicus", was described from Romania in 1975. It has also been considered possibly synonymous with "T. longobardicus". A small but well-preserved skull and neck, specimen PIMUZ T 3901, was found in the Meride Limestone at Monte San Giorgio. It was given a new species, "T. meridensis", in 1980. The skull was later referred to "T. longobardicus", rendering "T. meridensis" a junior synonym of that species. Another species, "T. fossai" from the Norian-age Argillite di Riva di Solto in Italy, was given its own genus "Sclerostropheus" in 2019.
Specimens similar to "T. longobardicus" have been found in the Ladinian or early Carnian Zhuganpo Formation in the Guizhou province of China, as described by Li (2007) and Rieppel (2010). In 2014, fossils of an indeterminate species of "Tanystropheus" were discovered in the Wolfville Formation in the Bay of Fundy in Nova Scotia, Canada. Indeterminate "Tanystropheus" remains are also known from the Jilh Formation of Saudi Arabia and various Anisian-Ladinian sites in Spain, Italy, and Switzerland.
A 2019 revision of "Tanystropheus" found that only "T. longobardicus" and "T. antiquus" were valid species. In 2020, large "Tanystropheus" specimens from Monte San Giorgio originally assigned to "T. longobardicus" were given a new species, "T. hydroides".
Anatomy.
By far the most recognizable feature of "Tanystropheus" is its hyperelongate neck, equivalent to the combined length of the body and tail. The neck was composed of 12–13 hyperelongate cervical vertebrae. Cervical elongation reached its peak with cervical vertebra 9, which was ten times longer than it was tall. Weak development of cervical spines suggest that epaxial musculature was underdeveloped in "Tanystropheus" and that intrinsic back muscles (e.g., "m. longus cervicis") were the driving force behind neck movement. Subvertical placement of the pre- and postzygapophyses suggested limited lateral movement of the neck, whereas cervical ribs extending off these vertebrae would have formed a ventral brace that would transmit the forces from the weight of head and neck down to the pectoral girdle, providing passive support by limiting dorsoventral flexion. The body contained 13 hourglass-shaped dorsal vertebrae, two sacral vertebrae and approximately twelve caudal vertebrae with weakly developed dorsal and haemal spines along with well-developed transverse processes.
The pectoral and pelvic girdles are notably distinct. The forelimbs are smaller and more gracile than the hindlimbs, suggesting that the center of mass for "Tanystropheus" was closer towards the pelvic girdle. Attachment sites for the "m. caudofemoralis" muscle complex, coupled with soft-tissue preservation of relative muscle size, further support the proposition that "Tanystropheus" was a fairly bottom-heavy animal.
Paleoecology.
Diet.
The diet of "Tanystropheus" has been controversial in the past, although most recent studies consider it a piscivorous (fish-eating) reptile. The teeth at the front of the narrow snout were long, conical, and interlocking, similar to those of nothosaurs and plesiosaurs. This was likely an adaptation for catching aquatic prey. Additionally, hooklets from cephalopod tentacles and what may be fish scales have been found near the belly regions of some specimens, further support for a piscivorous lifestyle.
However, small specimens of the genus possess an additional, more unusual form of teeth. This form of teeth, which occurred in the rear part of the jaws behind the interlocking front teeth, were tricuspid (three-pronged), with a long and pointed central cusp and smaller cusps in front of and behind the central cusp. Wild (1974) considered these three-cusped teeth to be an adaptation for gripping insects. Cox (1985) noted that marine iguanas also had three-cusped teeth, and that "Tanystropheus" likely fed on marine algae like that species of lizard. Taylor (1989) rejected both of these hypotheses, as he considered the neck of "Tanystropheus" to be too inflexible for the animal to be successful at either diet.
The most likely function of these teeth, as explained by Nosotti (2007), was that they assisted the piscivorous diet of the reptile by helping to grip slippery prey such as fish or squid. Several modern species of seals, such as the hooded seal and crabeater seal, also have multi-cusped teeth which assist their diet to a similar effect. Similar teeth patterns have also been found in the pterosaur "Eudimorphodon" and the fellow tanystropheid "Langobardisaurus", both of whom are considered piscivores. Large individuals of "Tanystropheus", over in length, lack these three-cusped teeth, instead possessing typical conical teeth at the back of the mouth. They also lack teeth on the pterygoid and palatine bones on the roof of the mouth, which possess teeth in smaller specimens. The two morphotypes were originally considered to represent juvenile and adult specimens of "T. longobardicus". However, histology of the small specimens and restudy of the large specimens has shown that they each represent adult forms of two different species. The larger one-cusped morphotype was given a new species, "T. hydroides", while the smaller tricuspid morphotype retained the name "T. longobardicus".
Soft tissue.
The specimen described by Renesto in 2005 displayed an unusual "black material" around the rear part of the body, with smaller patches in the middle of the back and tail. Although most of the material could not have its structure determined, the portion just in front of the hip seemingly preserved scale impressions, indicating that the black material was the remnants of soft tissue. The scales seem to be semi-rectangular and do not overlap with each other, similar to the integument reported in a juvenile "Macrocnemus" described in 2002. The portion of the material at the base of the tail is particularly thick and rich in phosphate. Many small spherical structures are also present in this portion, which upon further preparation were revealed to be composed of calcium carbonate. These chemicals suggest that the black material was formed as a product of the specimen's proteins decaying in a warm, stagnant, and acidic environment. As in "Macrocnemus", the concentration of this material at the base of the tail suggests that the specimen had a quite noticeable amount of muscle behind its hips.
Lifestyle.
The lifestyle of "Tanystropheus" is controversial, with different studies favoring a terrestrial or aquatic lifestyle for the animal. In the 1980s, various studies determined that "Tanystropheus" lacked the musculature to raise its neck above the ground, and that it was likely completely aquatic, swimming by undulating its body and tail side-to-side like a snake or crocodile.
However, in 2005 Renesto found that "Tanystropheus" lacked many aquatic adaptations. Although the tail of "Tanystropheus" was flattened, it was compressed from top-to-bottom, so that it would have been useless for side-to-side movement. The long neck and short front limbs compared to the long hind limbs would have made four-limbed swimming inefficient and unstable if that was the preferred form of locomotion. Thrusting with only the hind limbs, as in swimming frogs, was also considered an inefficient form of locomotion for a large animal such as "Tanystropheus," although a later study found support for this hypothesis. Renesto's study also found that the neck was lighter than previously suggested, and that the entire front half of the body was more lightly-built than the rear half, which would have possessed a large amount of muscle mass. In addition to containing powerful hind limb muscles, this unusually large muscle mass would have shifted the animal's weight to its rear, stabilizing the animal as it swung and maneuvered its massive neck.
Renesto's analysis was the basis for later analyses of the genus. In 2015, paleoartist Mark Witton concluded that, although the neck made up half of the entire animal's length, it made up only 20% of the entire animal's mass due to having light and hollow vertebrae. In comparison, the heads and necks of pterosaurs of the family Azhdarchidae made up almost 50% of the animal's mass, yet they were clearly land based carnivores. The animal is also poorly equipped for aquatic life, with the only adaptation being a lengthened fifth toe, which suggests that it visited the water some of the time, though was not wholly dependent on it. This same research has also shown that "Tanystropheus" would have hunted prey like a heron. This interpretation is supported by taphonomic evidence, which indicates that the preservation of "Tanystropheus" specimens is more similar to the terrestrial "Macrocnemus" than the aquatic "Serpianosaurus" where all three co-occur.Renesto and Franco Saller's 2018 follow-up to Renesto (2005)'s study offered more information on the reconstructed musculature of "Tanystropheus". This study determined that the first few tail vertebrae of "Tanystropheus" would have housed powerful tendons and ligaments that would have made the body more stiff, keeping the belly off the ground and preventing the neck from pulling the body over. In addition, the hind limbs would have been quite flexible and powerful according to muscle correlations on the legs, pelvis, and tail vertebrae. This analysis shows that "Tanystropheus" was capable of a specific mode of movement while swimming. Namely, a "Tanystropheus" could extend its hind limbs forward and then simultaneously retract them, creating a powerful 'jump' forward. Further support for this hypothesis lies in the fact that a close relative of "Tanystropheus", likely the small "Tanytrachelos", is believed to have left an ichnogenus (trackway fossil) called "Gwyneddichnium". Some "Gwyneddichnium" tracks seem to represent a succession of paired footprints that can be assigned to the hind limbs, but with no hand prints. These tracks were almost certainly created by the same form of movement which Renesto and Saller theorized was the preferred form of swimming in "Tanystropheus".
With this information, the most likely lifestyle for "Tanystropheus" was that the animal was a shallow-water predator that used its long neck to stealthily approach schools of fish or squid without disturbing its prey due to its large body size. Upon selecting a suitable prey item, it would have dashed forward by propelling itself along the seabed or through the water, with both hind limbs pushing off at the same time. However, this style of swimming is most common in amphibious creatures such as frogs, and likewise "Tanystropheus" would also have been capable of walking around on land. The idea that "Tanystropheus" evolved this form of swimming over much more efficient, yet specialized, styles is evidence that it did not live an exclusively aquatic life as in most other marine reptiles such as ichthyosaurs or plesiosaurs.
A 2020 digital reconstruction of "Tanystropheus" skulls suggested that the animals, especially the "Tanystropheus hydroides", lived primarily in the water because of the position of the nostrils.