Tanystropheus


Tanystropheus, was a reptile that dated 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 juvenile individuals are most abundant in the Besano Formation of Italy; other fossils have been found in the Middle East and China, dating to 232 million years ago during the middle Triassic period. A revision of Tanystropheus found that only two species are valid.

History

Tribelesodon, originally considered to be a pterosaur by Francesco Bassani in 1886, is now recognized as a junior synonym to Tanystropheus. The best-known species is Tanystropheus longobardicus. Other currently recognized species include T. conspicuus and T. meridensis. Another junior synonym of Tanystropheus is Procerosaurus, whose type species, P. cruralis, was erected by von Huene in 1902.
In 2005, Dr. Silvio Renesto described a specimen discovered in Switzerland which preserved the impressions of skin and other soft tissue.
Five new specimens of Tanystropheus longobardicus were described by Stefania Nosotti in 2007, allowing for a more comprehensive view of the anatomy of the species.
T. longobardicus specimens have also been found in the Guizhou province of China, as described by Li and Rieppel.
In 2014, fossils of Tanystropheus were discovered in the Wolfville Formation in the Bay of Fundy in Nova Scotia, Canada.

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 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 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 three-pronged, with a long and pointed central cusp and smaller cusps in front of and behind the central cusp. Wild considered these three-cusped teeth to be an adaptation for gripping insects. Cox noted that marine iguanas also had three-cusped teeth, and that Tanystropheus likely fed on marine algae like that species of lizard. Taylor 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, 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.

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 '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 which 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 forwards and then simultaneously retract them, creating a powerful 'jump' forwards. 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 called Gwyneddichnium. Some Gwyneddichnium tracks seem to represent a succession of paired footprints which 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 which 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 forwards 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.