Tanystropheus

''Tanystropheus'' (Greek ~ 'long' + 'hinged') is an extinct archosauromorph reptile from the Middle and Late Triassic epochs. It is recognisable 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).Dal Sasso, C. and Brillante, G. (2005). ''Dinosaurs of Italy''. Indiana University Press. , .

History

Monte San Giorgio specimens

19th century excavations at Monte San Giorgio, on the Italy-Switzerland border, revealed a fragmentary fossil of an animal with three-cusped teeth and elongated bones. Monte San Giorgio preserves the Besano Formation (also known as the Grenzbitumenzone), a Anisian-Ladinian formation recognised for its spectacular fossils.''Tanystropheus''
Vertebrate Palaeontology at Milano University. Retrieved 2007-02-19. In 1886, Francesco Bassani interpreted the unusual fossil as a pterosaur, which he named ''Tribelesodon longobardicus''. It would take more than 40 years for this misconception to be resolved. Though this holotype specimen of ''Tribelesodon longobardicus'' was destroyed in World War II, excavations by Bernhard Peyer in the late 1920s and 1930s revealed many other complete specimens from Monte San Giorgio.

Peyer's discoveries allowed ''Tribelesodon longobardicus'' to be recognised as a non-flying reptile, with its supposed elongated finger bones recognized as neck vertebrae. These vertebrae were compared favorable with those previously described as ''Tanystropheus'' from Germany and Poland. Thus, ''Tribelesodon longobardicus'' was renamed to ''Tanystropheus longobardicus'' and its anatomy was revised into a long-necked, non-pterosaur reptile. Specimen PIMUZ T 2791, which was discovered in 1929, has been designated as the neotype of the species.Well-preserved ''T. longobardicus'' fossils continue to be recovered from Monte San Giorgio up to the present day. Rupert Wild reviewed and redescribed all specimens known at the time via several large monographs in 1973/4 and 1980. 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.

A small but well-preserved skull and neck, specimen PIMUZ T 3901, was found in the slightly younger Meride Limestone at Monte San Giorgio. It was given a new species, ''T. meridensis'', in 1980. The specimen was later referred to ''T. longobardicus'', rendering ''T. meridensis'' a junior synonym of that species. A 2019 revision of ''Tanystropheus'' found that ''T. longobardicus'' and ''T. antiquus'' were the only valid species in the genus. In 2020, large ''Tanystropheus'' specimens from Monte San Giorgio originally assigned to ''T. longobardicus'' were given a new species, ''T. hydroides''.

Other specimens

The first ''Tanystropheus'' specimens to be described were several large vertebrae found in the mid-19th century. They were recovered from the Upper Muschelkalk of Germany and Lower Keuper of Poland. Though initially given the name ''Macroscelosaurus'' by Count Georg Zu Münster, the publication containing this name is lost and its genus is considered a ''nomen oblitum''. In 1855, Hermann von Meyer supplied the name ''Tanystropheus conspicuus'', the type species of ''Tanystropheus'', to the fossils. They were later regarded as ''Tanystropheus'' fossils undiagnostic relative to other species, rendering ''T. conspicuus'' a ''nomen dubium'' possibly synonymous with ''T. hydroides''. In the 1880s, E.D. Cope named three supposed new ''Tanystropheus'' species from the southwest United States. However, these fossils were later determined to belong to theropod dinosaurs, which were given the new genus '' Coelophysis''.

In the 1900s, Friedrich von Huene named several ''Tanystropheus'' species from Germany and Poland. ''T. posthumus'', from the Norian of Germany, was later considered an indeterminate theropod vertebra and a ''nomen dubium''. ''T. antiquus'', from the Gogolin Formation of Poland, was based on cervical vertebrae which were proportionally shorter than those of other ''Tanystropheus'' species. Long considered destroyed in World War II, several ''T. antiquus'' fossils were rediscovered in the late 2010s. ''T. antiquus'' is currently considered one of the few valid species of ''Tanystropheus.'' As the Gogolin Formation is upper Olenekian to lower Anisian in age, ''T. antiquus'' fossils are likely the oldest in the genus. Specimens likely referable to ''T. antiquus'' are also known from Germany and The Netherlands. Several more von Huene species, including "''Procerosaurus cruralis''", "''Thecodontosaurus latespinatus''", and "''Thecodontosaurus primus''", have been reconsidered as indeterminate material of ''Tanystropheus'' or other archosauromorphs.

''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''. The most complete ''Tanystropheus'' fossils outside of Monte San Giorgio come from the Guizhou province of China, as described by Li (2007) and Rieppel (2010). They are also the youngest and easternmost fossils in the genus, hailing from the upper Ladinian or lower Carnian Zhuganpo Formation. They include a large morphotype (''T. hydroides'') specimen, GMPKU-P-1527, and an indeterminate juvenile skeleton, IVPP V 14472. In 2015, a large ''Tanystropheus'' cervical vertebra was described from the Anisian to Carnian Economy Member of the Wolfville Formation, in the Bay of Fundy of Nova Scotia, Canada. Indeterminate ''Tanystropheus'' remains are also known from the Jilh Formation of Saudi Arabia and various Anisian-Ladinian sites in Spain, France?, Italy, and Switzerland. One of the youngest ''Tanystropheus'' fossils is a vertebra from the lower Carnian Fusea site in Friuli, Italy.

Several new tanystropheid genera have been named from former ''Tanystropheus'' fossils. In 2006, possible ''Tanystropheus'' material from the Anisian Röt Formation in Germany was named as '' Amotosaurus''. In 2011, fossils from the Lipovskaya Formation of Russia were given the genus '' Augustaburiana'' by A.G. Sennikov. He also named the new genus ''Protanystropheus'' for ''T. antiquus,'' but other authors continue to keep that species within ''Tanystropheus''. ''Tanystropheus fossai,'' from the Norian-age Argillite di Riva di Solto in Italy, was given its own genus '' Sclerostropheus'' in 2019.

Anatomy

Neck

By far the most recognisable feature of ''Tanystropheus'' is its hyperelongate neck, equivalent to the combined length of the body and tail. ''Tanystropheus'' had 13 massive cervical (neck) vertebrae, though the first two were smaller and less strongly developed. The atlas (first cervical), which connects to the skull, is a small, four-part bone complex. It consists of an atlantal intercentrum (small lower component) and pleurocentrum (large lower component), and a pair of atlantal neural arches (prong-like upper components). There does not appear to be a proatlas, which slots between the atlas and skull in some other reptiles. The intercentrum and pleurocentrum are not fused to each other, unlike the fused atlas of allokotosaurs. The tiny crescent-shaped intercentrum is overlain by a semicircular pleurocentrum, which acts as a base to the backswept neural arches. The axis (second cervical) is larger, with a small axial intercentrum followed by a much larger axial pleurocentrum. The axial pleurocentrum is longer than tall, has a low neural spine set forwards, and small prezygapophyses (front articular plates). The large postzygophyses (rear articular plates) are separated by a broad trough and support pointed epipophyses (additional projections).

The third to eleventh cervicals are hyperelongate in ''T. longobardicus'' and ''T. hydroides'', ranging from three to 15 times longer than tall. They are somewhat less elongated in ''T. antiquus'', less than 6 times longer than tall. The cervicals gradually increase in size and proportional length, with the ninth cervical typically being the largest vertebra in the skeleton. In general structure, the elongated cervicals resemble the axial pleurocentrum. However, the axis also has a keel on its underside and an incomplete neural canal, unlike its immediate successors. In the rest of the cervicals, all but the front of each neural spine is so low that it is barely noticeable as a thin ridge. The zygapophyses are closely set and tightly connected between vertebrae. The epipophyses develop into hooked spurs. The cervicals are also compressed from the side, so they are taller than wide. Many specimens have a longitudinal lamina (ridge) on the side of each cervical. Ventral keels return to vertebrae in the rear half of the neck. The 12th cervical and its corresponding ribs, though still longer than tall, are notably shorter than their predecessors. The 12th cervical has a prominent neural spine and robust zygapophyses, also unlike its predecessors. The 13th vertebra has long been assumed to be the first dorsal (torso) vertebra. This was justified by its general stout shape and supposedly dichocephalous (two-headed) rib facets, unlike the cervicals. However, specimen GMPKU-P-1527 has shown that the 13th vertebra’s rib simply possessed a single wide articulation and an unconnected forward branch, more similar to the cervical ribs than the dorsal ribs.

All cervicals, except potentially the atlas, connected to holocephalous (single-headed) cervical ribs via facets at their front lower corner. Each cervical rib had a short stalk connecting to two spurs running parallel to their vertebrae. The forward-projecting spurs were short and stubby, while the rear-projecting spurs were extremely narrow and elongated, up to three times longer than their respective vertebrae. This bundle of rod-like bones running along the neck afforded a large degree of rigidity.

Other vertebrae

There are 12 dorsal (torso) vertebrae, which are smaller and less specialised than the cervicals. Though their neural spines are taller than those of the cervicals, they are still usually rather short. The dorsal ribs are double-headed close to the shoulder and single-headed in the rest of the torso, sitting on stout transverse processes in the front half of each vertebra. More than 20 angled rows of gastralia extend along the belly, each gastral element represented by a pair of segmented rods which intermingle at the midline.

The two sacral (hip) vertebrae are low but robust, bridging over to the hip with expanded sacral ribs. The latter sacral rib is a single unit without a bifurcated structure. The tail is long, with at least 30 and possible up to 50 caudal vertebrae. The first few caudals are large, with closely interlinked zygapophyses and widely projecting pleurapophyses (transverse processes without ribs). The length of the pleurapophyses decreases until they disappear between the eighth and thirteenth caudal. The height of the neural spines also decreases gradually down the tail. Two pairs of large, curved bones, known as heterotopic ossifications, sit behind the hips in about half of known specimens preserving the area. These bones are possibly sexually dimorphic, and have also been reported in ''Tanytrachelos''. They may be linked to reproductive biology, supporting reproductive organs (if they belong to males) or an egg pouch (if they belong to females). A row of long chevrons is present under a short portion of the tail, behind the heterotopic bones or the space they would have occupied.

Pectoral girdle and forelimbs

The clavicles have a fairly standard form, as curved and slightly twisted rods. They lie on the interclavicle, a bone which has a rhombic (broad, diamond-shaped) front part followed by a long stalk. The interclavicle is rarely preserved and its connections to the rest of the pectoral (shoulder) girdle are mostly inferred from ''Macrocnemus''. The scapula has the form of a large semicircular plate on a short, broad stalk, similar to other tanystropheids. The coracoid is a large oval-shaped plate with a broad glenoid facet (shoulder socket).

The humerus is straight and slightly constricted at the middle. Near the elbow it is expanded and twisted, with ectepicondylar groove on its outer edge. The radius is slender and somewhat curved, while the ulna is similar in shape to the humerus and lacks a distinct olecranon. There are four carpals (wrist bones): the ulnare, radiale, and two distal carpals. The ulnare and radiale are large and cuboid, enclosing a small foramen between them. The larger outer distal carpal connects to metacarpals III and IV, while the much smaller inner one connects to metacarpals II and III. Metacarpals III and IV are the largest bones in the hand, followed closely by metacarpal II. Metacarpals I and V are both short. The hand’s phalangeal formula (joints per finger) is 2-3-4-4-3. The terminal phalanges (fingertips) would have formed thick, blunt claws.

Hip and hindlimbs

The components of the pelvis (hip) are proportionally small, though their shape is fairly standard for tanystropheids. The ilium is low and extends to a tapered point at the rear. The pubis is vertically oriented, with a small but distinct obturator foramen and a concave rear edge. The lower edge of the large, fan-shaped ischium converges towards (but does not contact) the pubis, nearly encompassing a large gap known as the thyroid foramen.

The hindlimbs are significantly larger than the forelimbs, though similar in overall structure and proportions. The femur is long, slender, and sigmoid (curved at both ends). It has a longitudinal muscle scar (the internal trochanter) on its underside and a broad joint at the acetabulum (hip socket). The tibia and fibula are straight, with the former much thicker and expanded at the knee. The large proximal tarsals (ankle bones) include a rounded calcaneum and a blocky astragalus, which meet along a straight or shallowly indented contact in most specimens. Unlike most early archosauromorphs, ''Tanystropheus'' has only two pebble-like distal tarsals: the larger fourth distal tarsal and minuscule third distal tarsal. There are five closely-appressed metatarsals, with the fourth and third being the longest. Though the first four metatarsals are slender and similar in length, the fifth (outermost) is very stout and subtly hooked, slotting into the ankle along a smooth joint. The estimated phalangeal formula is 2-3-4-5-4, and first phalange of the fifth toe was very long, filling a metatarsal-like role as seen in other tanystropheids.

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.

Terrestrial habits and locomotion

Major studies on ''Tanystropheus'' anatomy and ecology by Rupert Wild in the 1970s argued that it was an active terrestrial predator, keeping its head held high with an S-shaped flexion.Wild, R. 1973. Tanystropheus longbardicus (Bassani) (Neue Egerbnisse). in Kuhn-Schnyder, E., Peyer, B. (eds) — Triasfauna der Tessiner Kalkalpen XXIII. Schweiz. Paleont. Abh. Vol. 95 Basel, Germany. Though this interpretation is not wholly consistent with its proposed biomechanics, more recent studies have found some support for land-based movement in ''Tanystropheus''.

Renesto (2005) argued that ''Tanystropheus'' lacked clear adaptations for underwater swimming. The tail of ''Tanystropheus'' was compressed vertically (from top-to-bottom) at the base and thinned towards the tip, so that it would have been useless for lateral (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 strengthening the hind limbs, the large hip and tail muscles would have shifted the animal's center of mass rearwards, stabilizing the animal as it maneuvered its elongated neck. 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. The horizontal overlap between zygapophyses would have limited lateral movement of the neck, while cervical ribs would have formed a brace along the underside of the neck. The long cervical ribs may have played a similar role to ossified tendons of many large dinosaurs, transmitting the forces from the weight of head and neck down to the pectoral girdle, as well as providing passive support by limiting dorsoventral flexion.Tschanz, K. 1988. Allometry and Heterochrony in the Growth of the Neck of Triassic Prolacertiform Reptiles. Paleontology. 31:997–1011.

Renesto's conclusions were the basis for later investigations of the genus. In 2015, paleoartist Mark Witton estimated that the neck made up only 20% of the entire animal's mass due to its light and hollow vertebrae. By 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 was 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. Witton proposed that ''Tanystropheus'' would have hunted prey from the seashore, akin to a heron. Terrestrial or semi-terrestrial habits are 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.

Aquatic habits and locomotion

In the 1980s, various studies suggested 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.

Renesto and Saller (2018) argued that the hind limbs would have been quite flexible and powerful according to muscle correlations on the legs, pelvis, and tail vertebrae. They argued that ''Tanystropheus'', despite its apparent lack of adaptations for typical swimming styles, utilised a more unusual mode of underwater movement. Namely, a ''Tanystropheus'' could extend its hind limbs forward and then simultaneously retract them, creating a powerful 'jump' forward. Further support for this hypothesis is based on the ichnogenus (trackway fossil) '' Gwyneddichnium'', which was likely created by small tanystropheids such as ''Tanytrachelos''. Some ''Gwyneddichnium'' tracks seem to represent a succession of paired footprints that can be assigned to the hind limbs, without any hand prints. These tracks were almost certainly created by the same form of movement which Renesto and Saller hypothesised was the preferred form of swimming in ''Tanystropheus''.

Under their hypothesis, 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 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 proposal that ''Tanystropheus'' evolved this form of swimming over much more efficient and specialised styles is evidence that it did not live an exclusively aquatic life, in contrast to longer-lasting marine reptiles such as ichthyosaurs or plesiosaurs.

A 2020 digital reconstruction of ''Tanystropheus'' skulls suggested that members of the genus, especially ''Tanystropheus hydroides'', were semiaquatic because of the position of the nostrils. And the poor hydrodynamic profile and limited adaptions to swimming in the limbs suggested it lived in shallow coastal areas even in freshwater.

References

Bibliography

George Olshevsky expands on the history of "P." ''exogyrarum''
on the Dinosaur Mailing List
* Huene, 1902. "Übersicht über die Reptilien der Trias" eview of the Reptilia of the Triassic ''Geologische und Paläontologische Abhandlungen''. 6, 1-84.
* Fritsch, 1905. "Synopsis der Saurier der böhm. Kreideformation" ynopsis of the saurians of the Bohemian Cretaceous formation ''Sitzungsberichte der königlich-böhmischen Gesellschaft der Wissenschaften'', II Classe. 1905(8), 1-7.

{{Taxonbar, from=Q131782

Tanystropheids
Prehistoric reptile genera
Olenekian genus first appearances
Anisian genera
Ladinian genera
Carnian genus extinctions
Triassic reptiles of Europe
Triassic Italy
Fossils of Italy
Triassic Switzerland
Fossils of Switzerland
Fossil taxa described in 1852
Taxa named by Christian Erich Hermann von Meyer