Wikipedia

Tanystropheus

Tanystropheus(Ancient Greek:τανυ~ 'long' +στροφευς'hinged') is an extinctgenusofarchosauromorphreptilewhich lived during theTriassic Periodin Europe, Asia, and North America. It is recognisable by its extremely elongated neck, longer than the torso and tail combined. The neck was composed of 13vertebraestrengthened by extensivecervical ribs.Tanystropheusis one of the most well-described non-archosauriformarchosauromorphs, known from numerous fossils, including nearly complete skeletons. Some species within the genus may have reached a total length of 6 meters (20 ft), makingTanystropheusthe longest non-archosauriform archosauromorph as well.[1]Tanystropheusis the namesake of the familyTanystropheidae,acladecollecting many long-necked Triassic archosauromorphs previously described as "protorosaurs"or"prolacertiforms".

Tanystropheus
Temporal range:Anisian– earlyCarnianPossible lateOlenekianrecord
ModelledTanystropheusskeleton
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Class: Reptilia
Clade: Archosauromorpha
Family: †Tanystropheidae
Genus: †Tanystropheus
Meyer,1852
Type species
†Tanystropheus conspicuus
nomen dubium
Von Meyer, 1855
Other species
  • †T. antiquus?Von Huene, 1907-1908
  • †T. longobardicus(Bassani,1886)
  • †T. hydroidesSpiekmanet al.,2020
Synonyms
Genus synonymy
Species synonymy
  • T. biharicusJurcsák, 1975 (synonym ofT. longobardicus?)
  • T. haasiRieppel, 2001 (nomen dubium)
  • T. meridensisWild, 1980 (synonym ofT. longobardicus)

Tanystropheuscontains at least two valid species as well as fossils which cannot be referred to a specific species. Thetype speciesofTanystropheusisT. conspicuus,adubious nameapplied to particularly large fossils fromGermanyandPoland.Complete skeletons are common in theBesano FormationatMonte San Giorgio,on the border ofItalyandSwitzerland.Monte San Giorgio fossils belong to two species: the smallerT. longobardicusand the largerT. hydroides.These two species were formally differentiated in 2020 primarily on the basis of their strongly divergent skull anatomy. WhenT. longobardicuswas first described in 1886, it was initially mistaken for apterosaurand given the name "Tribelesodon".Starting in the 1920s, systematic excavations at Monte San Giorgio unearthed many moreTanystropheusfossils, revealing that the putative wing bones of"Tribelesodon"were actually neck vertebrae.

MostTanystropheusfossils hail from marine or coastal deposits of theMiddle Triassicepoch (AnisianandLadinianstages), with some exceptions. For example, a vertebra fromNova Scotiawas recovered from primarily freshwater sediments. The youngest fossils in the genus are a pair of well-preserved skeletons from theZhuganpo Formation,a geological unit in China which dates to the earliest part of theLate Triassic(earlyCarnianstage).[2]The oldest putative fossils belong to"T. antiquus",a European species from the latest part of theEarly Triassic(lateOlenekianstage).T. antiquushad a proportionally shorter neck than otherTanystropheusspecies, so some paleontologists consider thatT. antiquusdeserves a separate genus,Protanystropheus.

The lifestyle ofTanystropheushas been the subject of much debate.[3]Tanystropheusis unknown from drier environments and its neck is rather stiff and ungainly, suggesting a reliance on water. Conversely, the limbs and tail lack most adaptations for swimming and closely resemble their equivalents in terrestrial reptiles. Recent studies have supported an intermediate position, reconstructingTanystropheusas an animal equally capable on land and in the water. Despite its length, the neck was lightweight and stabilized bytendons,so it would not been a fatal hindrance to terrestrial locomotion. The hindlimbs and the base of the tail were large and muscular, capable of short bursts of active swimming in shallow water.Tanystropheuswas most likely apiscivorousambush predator:the narrow subtriangular skull ofT. longobardicusis supplied with three-cusped teeth suited for holding onto slippery prey, while the broader skull ofT. hydroidesbears an interlocking set of large curved fangs similar to the fully aquaticplesiosaurs.

History and species

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Monte San Giorgio species

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The destroyedholotypeofTanystropheus longobardicus,misinterpreted as a long-tailed pterosaur ("Tribelesodon") until the late 1920s. Fossil (left) and 1923 restoration byFranz Nopcsa(right).

19th century excavations atMonte San Giorgio,aUNESCO world heritage siteon theItaly-Switzerlandborder, revealed a fragmentary fossil of an animal with three-cusped (tricuspid) teeth and elongated bones. Monte San Giorgio preserves theBesano Formation(also known as the Grenzbitumenzone), a lateAnisian-earlyLadinianlagerstätterecognised for its spectacular fossils.[4]In 1886,Francesco Bassaniinterpreted the unusual tricuspid fossil as apterosaur,which he namedTribelesodon longobardicus.[5][6]Theholotypespecimen ofTribelesodon longobardicuswas stored in theMuseo Civico di Storia Naturale di Milano(Natural History Museum ofMilan), and was destroyed byallied bombing of MilaninWorld War II.[6]

Excavations byUniversity of ZürichpaleontologistBernhard Peyerin the late 1920s and 1930s revealed many more complete fossils of the species from Monte San Giorgio.[6]Peyer's discoveries allowedTribelesodon longobardicusto be recognised as a non-flying reptile, more than 40 years after its original description.[7]Its supposed elongated finger bones were recognized as neck vertebrae, which compared favorably with those previously described asTanystropheusfrom Germany and Poland. Thus,Tribelesodon longobardicuswas renamed toTanystropheus longobardicusand its anatomy was revised into a long-necked, non-pterosaur reptile. Specimen PIMUZ T 2791, which was discovered in 1929, has been designated as theneotypeof the species.[6]

Well-preservedT. longobardicusfossils continue to be recovered from Monte San Giorgio up to the present day. Fossils from the mountain are primarily stored at the rebuilt Museo Civico di Storia Naturale di Milano (MSNM), thePaleontological Museum of Zürich(PIMUZ), and the Museo Cantonale di Scienze Naturali diLugano(MCSN).[6]Rupert Wildreviewed and redescribed all specimens known at the time via several large monographs in 1973/4 and 1980. In 2005, Silvio Renesto described aT. longobardicusspecimen from Switzerland which preserved the impressions of skin and other soft tissue.[8]Five new MSNM specimens ofT. longobardicuswere described by Stefania Nosotti in 2007, allowing for a more comprehensive view of the species' anatomy.[9]

A small but well-preserved skull and neck, specimen PIMUZ T 3901, was found in the slightly youngerMeride Limestoneat Monte San Giorgio. Wild (1980) gave it a new species,T. meridensis,based on a set of skull and vertebral traits proposed to differ fromT. longobardicus.Later reinvestigations failed to confirm the validity of these differences, renderingT. meridensisajunior synonymofT. longobardicus.[10][6]A 2019 revision ofTanystropheusfound thatT. longobardicusandT. antiquuswere the only valid species in thegenus.[6]

Tanystropheusspecimens from Monte San Giorgio have long been segregated into two morphotypes based on their tooth structure.[9]Smaller specimens bear tricuspid teeth at the back of the jaw while larger specimens have a set of single-pointed fangs. The two morphotypes were originally considered to represent juvenile and adult specimens ofT. longobardicus,though many studies have supported the hypothesis that they represent separate species.[6]A 2020 study found numerous differences between the skulls of large and small specimens, formalizing the proposal to divide the two into separate species. Moreover, ahistologicalinvestigation revealed that one small specimen, PIMUZ T 1277, was a skeletally mature adult at a length of only 1.5 meters (4.9 ft). The larger one-cusped morphotype was named as a new species,Tanystropheus hydroides(referencing theHydraofGreek mythology), while the smaller tricuspid morphotype retains the nameT. longobardicus.[11]

Polish and German species

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Typevertebrae of "Tanystropheus conspicuus ",described in 1855

The firstTanystropheusspecimens to be described were found in the mid-19th century. They included eight large vertebrae from the UpperMuschelkalkofGermany,and a partial skeleton from theLower KeuperofPoland.These geological units occupy part of theMiddle Triassic,from the latestAnisianto middleLadinianstages.[6]Though the fossils were initially given the nameMacroscelosaurusbyCount Georg Zu Münster,the publication containing this name is lost and its genus is considered anomen oblitum.In 1855,Hermann von Meyersupplied the nameTanystropheus conspicuus,thetype speciesofTanystropheus,to the fossils.[12]They were later regarded asTanystropheusfossils undiagnostic relative to other species, renderingT. conspicuusanomen dubiumpossibly synonymous withT. hydroides.[6][13]

Over 500 "Tanystropheus conspicuus"specimens have been recovered from a Lower Keuper bonebed near theSilesianvillage ofMiedary.This is the largest known concentration ofTanystropheusfossils, more than double the number collected from Monte San Giorgio. Though the Miedary specimens are individually limited to isolated postcranial bones, they are preserved in three dimensions and show great potential for elucidating the morphology of the genus. The Miedary locality represents an isolated brackish body of water close to the coast, and the abundance ofTanystropheusfossils suggests that it was an animal well-suited for this kind of habitat.[14]

In the late 1900s,Friedrich von Huenenamed several dubiousTanystropheusspecies from Germany and Poland.T. posthumus,from the Norian of Germany, was later reevaluated as an indeterminate theropod vertebra and anomen dubium.Several more von Huene species, including "Procerosaurus cruralis","Thecodontosauruslatespinatus",and"Thecodontosaurus primus",have been reconsidered as indeterminate material ofTanystropheusor otherarchosauromorphs.[15][6]

One of Von Huene's species appears to be valid:T. antiquus,from theGogolin Formationof Poland, was based on cervical vertebrae which were proportionally shorter than those of otherTanystropheusspecies. Long considered destroyed in World War II, severalT. antiquusfossils were rediscovered in the late 2010s. The proportions ofT. antiquusfossils are easily distinguishable, and it is currently considered a valid species of archosauromorph,[6]though its referral to the genusTanystropheushas been questioned.[16][17]The Gogolin Formation ranges from the upperOlenekian(latest part of theEarly Triassic) to the lower Anisian in age. Assuming they belong withinTanystropheus,the fossils ofT. antiquusmay be the oldest in the genus. Specimens likely referable toT. antiquusare also known from throughout Germany and the fossiliferous Winterswijk site inthe Netherlands.[18][6]

OtherTanystropheusfossils

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In the 1880s,E.D. Copenamed three supposed newTanystropheusspecies (T. bauri,T. willistoni,andT. longicollis) from the Late TriassicChinle FormationinNew Mexico.However, these fossils were later determined to be tail vertebrae belonging to theropod dinosaurs, which were named under the new genusCoelophysis.[6]AuthenticTanystropheusspecimens from theMakhtesh RamoninIsraelwere described as a new species,T. haasi,in 2001.[19]However, this species may be dubious due to the difficulty of distinguishing its vertebrae fromT. conspicuusorT. longobardicus.Another new species,T. biharicus,was described fromRomaniain 1975.[20]It has also been considered possibly synonymous withT. longobardicus.ATanystropheus-like vertebra from the middle LadinianErfurt Formation(Lettenkeuper) of Germany was described in 1846 as one of several fossils gathered under the name "Zanclodonlaevis".Though likely the firstTanystropheusfossil to be discovered, the vertebra is now lost, and surviving jaw fragments and other fossil scraps of "Zanclodon laevis "represent indeterminatearchosauriformswith no relation toTanystropheus.[21][6]Tanystropheusvertebrae have also been found in theVillány MountainsofHungary.[22]

The most well-preservedTanystropheusfossils outside of Monte San Giorgio come from theGuizhouprovince ofChina,as described by Li (2007)[23]and Rieppel (2010).[2]They are also among the youngest and easternmost fossils in the genus, hailing from the upper Ladinian or lowerCarnianZhuganpo Formation.Although the postcrania is complete and indistinguishable from the fossils of Monte San Giorgio, no skull material is preserved, and their younger age precludes unambiguous placement into anyTanystropheusspecies. The Chinese material includes a large morphotype (T. hydroides?) specimen, GMPKU-P-1527, and an indeterminate juvenile skeleton, IVPP V 14472.[2]

IndeterminateTanystropheusremains are also known from theJilh FormationofSaudi Arabiaand various Anisian-Ladinian sites inSpain,France,Italy, and Switzerland.[6]The youngestTanystropheusfossil in Europe is a vertebra from the lower Carnian Fusea site inFriuli, Italy.[24][6]In 2015, a largeTanystropheuscervical vertebra was described from the Economy Member of theWolfville Formation,in theBay of FundyofNova Scotia, Canada.[25][6]The Wolfville Formation spans the Anisian to Carnian stages, and the Economy Member is likely Middle Triassic (Anisian-Ladinian) in age. It is a rare example of predominantly freshwater strata preservingTanystropheusfossils.[26]Tanystropheus-like tanystropheid fossils are known from another freshwater formation in North America: the Anisian-ageMoenkopi FormationofArizonaand New Mexico.[27]

Several newtanystropheidgenera have been named from formerTanystropheusfossils. Fossils from the AnisianRöt Formationin Germany, previously referred toTanystropheus antiquus,were named as a new genus and species in 2006:Amotosaurus rotfeldensis.[28]In 2011, fossils from theLipovskaya FormationofRussiawere given the new genus and speciesAugustaburiania vataginiby A.G. Sennikov. He also named the new genusProtanystropheusforT. antiquus,[16]though a few studies continued to retain that species withinTanystropheus.[6]Tanystropheus fossai,from theNorian-ageArgillite di Riva di Soltoin Italy, was given its own genusSclerostropheusin 2019.[6]

Anatomy

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Size comparison betweenT. 'conspicuus',T. hydroides(PIMUZ T 2793), andT. longobardicus(MSNM V 3730)

Tanystropheuswas one of the longest known non-archosauriform archosauromorphs. Vertebrae referred to "T. conspicuus"may correspond to an animal up to five or six meters (16.4 to 20 feet)in length.[6]T. hydroideswas around the same size, with the largest specimens at an estimated length of 5.25 meters (17.2 feet).[11]T. longobardicuswas significantly smaller, with an absolute maximum size of two meters (6.6 feet).[13][17]Despite the large size of someTanystropheusspecies, the animal was lightly built. One mass estimate used crocodiles as a density guideline for a 3.6 meter (11.8 feet)-long model of aTanystropheusskeleton. For aTanystropheusindividual of that length, the weight estimate varied between 32.9 kg (72.5 lbs) and 74.8 kg (164.9 lbs), depending on the volume estimation method. This was significantly lighter than crocodiles of the same length, and more similar to large lizards.[29]

Skull ofTanystropheus longobardicus

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PIMUZ T 3901, a specimen ofTanystropheus longobardicusformerly described as "Tanystropheus meridensis".Skull and partial neck (top) and a closer view of the skull (bottom).

The skull ofTanystropheus longobardicusis roughly triangular when seen from the side and top, narrowing towards the snout.[9]Eachpremaxilla(the toothed bone at the tip of the snout) has a long tooth row, with six teeth. The premaxillary teeth are conical, fluted by longitudinal ridges, and havesubthecodontimplantation, meaning that the inner wall of each tooth socket is lower than the outer wall. The premaxilla meets themaxilla(the succeeding toothed bone) along a long, slanted contact. This shape is produced by an elongated postnarial process (rear prong) of the premaxilla, which extends below and behind thenares(nostril holes).[9][6][13]Thenasals(bones at the top edge of the snout) are poorly known, but were likely narrow and flat.[9]A 2020 reinvestigation revealed that the front part of the nasals and the inner spur of the premaxillae are too short to keep the nares divided. This leaves a single central narial opening for the nostrils, opening upwards. An undivided naris is seen in few otherarchosauromorphs,namelyrhynchosaurs,mostallokotosaurs,moderncrocodilians,andTeyujagua.[11][13]

The maxilla is triangular, reaching its maximum height at mid-length and tapering to the front and rear.[9]There are up to 14[9]or 15[6]teeth in the maxilla, though some individuals have fewer.[9]T. longobardicusis a reptile withheterodontdentition, meaning that it had more than one type of tooth shape. In contrast to the simple fang-like premaxillary teeth, most or all of the maxillary teeth have a distinctive tricuspid shape, with thecrownsplit into three stout triangularcusps(points). The cusps are arranged in a line from front-to-back, with the central cusp larger than the other two cusps.[9]Among Triassic reptiles, earlypterosaurssuch asEudimorphodondeveloped an equivalent tooth shape, and tricuspid teeth can also be found in a few modernlizardspecies.[30][31]Some individuals ofT. longobardicushave tricuspid teeth along their entire maxilla, while in others up to seven maxillary teeth are single-cusped fangs similar to the premaxillary teeth.[9][6]

The front edge of eachorbit(eye socket) is marked by two bones: theprefrontalandlacrimal.The prefrontal is tall and projects a low vertical ridge in front of the orbit. The small, sliver-shaped lacrimal is nestled further down along the maxilla.[9]The lower edge of the orbit is formed by thejugal,a bone with a slender anterior process (front branch) and a somewhat broader dorsal process (upper branch). There is also a very short pointed posterior process (rear branch) which ends freely and fails to contact any other bone.[9]The shape of the jugal inTanystropheusis typical for early archosauromorphs; the underdeveloped posterior process indicates that the margin of theinfratemporal fenestra(lower skull hole behind the eye) was incomplete and open from below.[13]Thepostorbital bone,which links the jugal to the top of the skull, was tall and roughly boomerang-shaped, though poor preservation obscures some details.[11]Thesquamosal bone,which extends behind the postorbital, is also poorly known inT. longobardicus,and many supposed squamosal fossils in the species have been reinterpreted as displaced postorbitals.[11][13]Thequadrate bone,which forms the rear edge of the skull and upper half of the jaw joint, is wide and tall. It has a strong lateral crest and a low pterygoid ramus (a vertical internal plate, articulating with thepterygoid bonein the roof of the mouth).[9]No fossils ofT. longobardicuspreserve aquadratojugal,a bone which normally lies along the quadrate at the rear lower corner of the skull. Nevertheless, a quadratojugal was likely present in the species, since it occurs inT. hydroidesand nearly every other early archosauromorph.[11][13]

Casts of complete skeletons ofTanystropheus longobardicuspreserving skulls: theneotypePIMUZ T 2791 (top) and MSNM BES SC 1018 (bottom).

The pairedfrontals(skull roof bones above the orbits) have been described as "axe-shaped flanges", projecting broad curved plates above each orbit.[9]Together, the frontals are narrowest at the front, terminating at a three-lobed contact with the nasals. The sutures between the frontals and their neighboring bones are coarse and interdigitating (interlocking). A small triangular bone, thepostfrontal,wedges behind the rear outer corner of each frontal. A pair of larger plate-like bones, theparietals,sit directly behind the frontals on the skull roof. InT. longobardicus,the parietals are fairly broad and flat, with a shallowly concave outer edge.[9][6]Like the frontals, the paired parietals are seemingly separate bones, unfused to each other in every member of the species.[6]A large hole, thepineal foramen[6][13](sometimes called the parietal foramen),[9]is present at the midline of the skull between the front part of each parietal. When seen from below, a pair of curved crests along the frontals and parietals mark the edge of theforebrain,as defined by a bulbous central hollow.[9]

The eye was supported by more than 10 rectangular ossicles (tiny plate-like bones) connecting into ascleral ring,though a full reconstruction of the ring, with 18 ossicles, is conjectural.[9]Few details of thebraincaseandpalate(bony roof of the mouth) are known forT. longobardicus.The scant available evidence suggests that these regions of the skull are rather unspecialized in this species.[13]Thevomers(front components of the palate) are narrow and dotted with at least nine tiny teeth. The succeedingpalatineandpterygoidbones are also supplied with rows of teeth: up to six relatively large teeth in the former and at least 12 small teeth in the latter.[9][6]Teeth on the vomers, palatines, and pterygoids are the norm for early archosauromorphs and reptiles as a whole.[6][13]

Thelower jawis slender, and most of its length is devoted to the tootheddentarybone. The dentary is downturned at its tip and its outer surface is dotted with a row of prominent foramina (blood vessel pits). There are up to 19 teeth in the dentary.[9]Most commonly, the first six teeth are prominent conical fangs, akin to the premaxilla, while the remainder are small and tricuspid, akin to the maxilla. There is some variation in the number of each tooth shape, and some individuals may have up to 11 conical teeth.[9]The inner surface of the dentary is joined by a splint-shaped bone, thesplenial,at its lower edge.[9]The splenial was most likely not visible in lateral view.[13]At its rear, the dentary seems to be partially overlapped by thesurangular,a bone which comprises much of the rear part of the jaw.[9][13]Although it is plausible that a smallcoronoidbone could be present in front of the surangular, evidence is ambiguous at best for allTanystropheusspecies.[9][13]A sheathe-like bone, theangular,is well-exposed under the dentary and surangular, though sutures between these bones are difficult to interpret with certainty.[13]The joint at the back of the jaw lies on thearticular,a lumpy rectangular bone which is floored and reinforced by a similar bone: theprearticular.InTanystropheusspecies with known skull material, both the articular and prearticular contribute equally to a segment of the jaw extending back beyond the level of the jaw joint. This projection, known as a retroarticular process, is enlarged[6]to a similar degree to that of early rhynchosaurs.[13]

Skull ofTanystropheus hydroides

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The skull of PIMUZ T 2790, the holotype ofTanystropheus hydroides.Digital reconstruction in lateral views (top); dorsal, ventral, and occipital views (left middle); line diagram in left lateral, dorsal, and ventral views (right middle); 3D printed model (bottom).

The skull ofTanystropheus hydroidesis broader and flatter than that ofT. longobardicus.The first five of six teeth in the premaxilla are very large and fang-like, forming an interlocking "fish trap" similar toDinocephalosaurusand manysauropterygianssuch asplesiosaursandnothosaurs.[11][13]All teeth in the skull have a single cusp which is sharp, curved, and unserrated.[6][11][13]They have an oval-shaped cross section and shallow subthecodont implantation. LikeT. longobardicus,T. hydroideshas a single central narial opening. UnlikeT. longobardicus,T. hydroideshas a nearly vertical rear edge of the premaxilla, without a postnarial process.[6][11][13]The maxilla is low, with a large and rectangular front portion. There is a perforation near the front of the bone, which would have been penetrated by the tenth dentary tooth when the mouth was closed.[11][13]Towards the rear, the maxilla develops a concave edge overlooking a long and slender posterior process (rear branch) that projects under the rounded orbit. There are 15 teeth in the maxilla, increasing in size up to the eighth tooth, which is about as large as the premaxillary fangs.[13]T.hydroidesis not known to possess aseptomaxilla,a neomorphic bone at the rear tip of the naris in some reptiles. The nasals are broad and plate-like, with a depressed central portion.[11][13]The lacrimal and prefrontal, though incompletely known, were likely similar to those ofT. longobardicus.T. hydroideshas a particularly largenasolacrimal duct,a tubular channel opening out of the rear of the lacrimal.[13]The frontals are quite wide and form much of the upper edge of the orbit, a condition akin toT. longobardicus.However, the paired frontals meet along a straight suture with a low ridge on the lower (internal) surface, in contrast toT. longobardicus,where the frontals meet at an interdigitating suture with a broad furrow on the underside.[11][13]

The flattened single-cusped skull of PIMUZ T 2819, a large morphotype (T. hydroides) specimen

The parietals are strongly modified inT. hydroides.[6][11]They are fused into a single X-shaped bone, somewhat resembling the parietals oferythrosuchids.[13]This shape may have resulted from fusion between the parietals' anterolateral processes (front branches) and the postfrontals, which are separate bones inT. longobardicusbut not apparent inT. hydroides.A prominent pineal foramen is positioned near the straight contact with the frontals, one of the few similarities withT. longobardicus.[13]Strongsupratemporal fossaeexcavate into the outer edge of the parietal and define a lowsagittal crestalong the midline of the skull. This trend is shared with other large archosauromorphs, likeDinocephalosaurusandAzendohsaurus.[13]Thesupratemporal fenestrae(upper skull holes behind the eye) are wide and semi-triangular, exposed almost entirely from above.[13]The postorbital has large and blocky ventral and medial processes (lower and inward branches), which meet at a sharper angle than in any other early archosauromorph. The jugal, conversely, is basically indistinguishable from that ofT. longobardicus.The squamosal is deep and rectangular when viewed from the side, with little differentiation between the tall suture with the postorbital and the small suture with the quadratojugal. As a result, most of the posterior skull is clustered together, and the infratemporal fenestra is reduced to a small diagonal hole. The quadratojugal is a curved sliver of bone which twists back alongside the quadrate. Relative toT. longobardicus,the quadrate has a larger pterygoid ramus and a strongly hooked projection at its upper extent.[11][13]

The palate ofT. hydroideshas several unique traits.[6][11][13]The vomers are wide and tongue-shaped, each hosting a single row of 15 relatively large curved teeth along the outer edge of the bone, adjacent to the elongatedchoanae(internal openings of thenasal cavity).[6][11][13]Most other archosauromorphs,T. longobardicusincluded, have restricted vomers with rows of minuscule teeth. The rest of the palate is completely toothless inT. hydroides,even the palatines and pterygoids, which bear tooth rows in most early archosauromorphs.[6][11][13]The pterygoids are also unusual for their broad palatal ramus (front plate) and a loose, strongly overlapping connection to theectopterygoids(linking bones between the pterygoid and maxilla). Theepipterygoids(vertical bones in front of the braincase) are tall and flattened from the side.[13]

Digital reconstruction of the braincase of PIMUZ T 2790. Basioccipital (blue), parabasisphenoid (red), and composite bone (yellow).

T. hydroidesis a rare example of an early archosauromorph with a three-dimensionally preserved braincase.[13]Thebasioccipital(rear lower component of the braincase) was small, with inset basitubera (vertical plates connecting to neck muscles) linked by a transverse ridge, similar to allokotosaurs andarchosauriforms.Theparabasisphenoid(front lower component) is less specialized; it lies flat and tapers forwards to a blade-like cultriform process. The rear part of the bone has a deep triangular excavation (known as a median pharyngeal recess) on its underside, flanked by low crests and a pair of small basipterygoid processes (knobs connecting to the pterygoid).[13]The remainder of the braincase is fully fused together into a strongly ossified composite bone, and its constituents must be estimated by comparison to other reptiles. Theexoccipitals,which mostly encompass theforamen magnum(spinal cord hole), are perforated with nerve foramina. Each exoccipital merges outwards into theopisthotic,which sends out a straight, elongatedparoccipital process(thick outer branch) to the edge of the cranium.[13]InT. longobardicus,the paroccipital processes are shorter and narrower at their base.[6][13]Thestapes,a bone which transmits vibrations from the ear to the braincase, is slender and splits into two small prongs where it contacts the opisthotic. The opisthotic merges forwards into theprootic,which extensively contacts the parabasisphenoid and hosts a range of larger nerve foramina. The prootic forms much of the front edge of the paroccipital process, akin to the condition in archosauriforms.[13]Another archosauriform-like feature is the presence of alaterosphenoid,an additional braincase component in front of the prootic and above the exit hole for thetrigeminal nerve(also known as cranial nerve V).[11]The laterosphenoid is small, similar to that ofAzendohsaurus.[13]The upper rear part of the braincase is formed by thesupraoccipitals,which were presumably fused together as a continuous surface sloping smoothly down to the foramen magnum.[13]

PIMUZ T 2793, a lower jaw and associated vertebrae ofT. hydroides

In the lower jaw, the dentaries meet each other at a robustsymphysiswith an interdigitating suture.[13]The front end of the dentary hosts a prominent keel on its lower edge, a unique trait of the species.[6][11][13]There are at least 18 dentary teeth; the first three are by far the largest teeth in the skull, forming the lower half of the interlocking "fish trap" with the premaxilla. Most other teeth in the dentary are small, with the exception of the tenth tooth, which juts up to pierce the maxilla. The remainder of the jaw contains the same set of bones as inT. longobardicus,but some details differ inT. hydroides.[13]For example, the splenial is plate-like and covers a larger portion of the internal dentary than inT. longobardicus.In addition, the rear of the dentary overlaps a large portion of the surangular, rather than the surangular acting as the overlapping bone where they meet. The surangular internally bears a large fossa for the jaw'sadductor(vertical biting) muscles, and a prominent surangular foramen is positioned in front of the jaw joint.[13]

Neck

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Atlas-axis complex of PIMUZ T 2790

The most recognisable feature ofTanystropheusis its hyperelongate neck, equivalent to the combined length of the body and tail.[8]Tanystropheushas 13cervical (neck) vertebrae,most of which are massive, though the two closest to the head are smaller and less strongly developed.[2][6]Theatlas(first cervical), which connects to the skull, is a small, four-part bone complex. It consists of an atlantalintercentrum(small lower component) andpleurocentrum(large lower component), and a pair of atlantal neural arches (prong-like upper components). There does not appear to be aproatlas,which slots between the atlas and skull in some other reptiles. The intercentrum and pleurocentrum are not fused to each other, unlike the single-part atlas ofallokotosaurs.The tiny crescent-shaped intercentrum is overlain by a semicircular pleurocentrum, which acts as a base to the backswept neural arches. Theaxis(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 lowneural spineset forwards, and smallprezygapophyses(front articular plates). The largepostzygophyses(rear articular plates) are separated by a broad trough and support pointedepipophyses(additional projections).[13]

Skeletal diagram ofT. hydroides(top) and mounted model skeleton (bottom) ofTanystropheus.The modeled skeleton is reconstructed fromT. longobardicusandT. hydroidesmaterial

The third to eleventh cervicals are hyperelongate inT. longobardicusandT. hydroides,ranging from three to 15 times longer than tall. They are somewhat less elongated inT. 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.[6]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.[13]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.[9][6]

All cervicals, except potentially the atlas, connected to holocephalous (single-headed)cervical ribsvia facets at their front lower corner. Each cervical rib bears a short stalk connecting to two spurs running under and parallel to the 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.[8][9][2]

The 12th cervical and its corresponding ribs, though still longer than tall, are notably shorter (from front-to-back) 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 has a single wide articulation and an unconnected forward branch, more similar to the cervical ribs than the dorsal ribs.[2]

The elongation ofTanystropheus's neck is mostly a consequence of particular vertebrae lengthening. This is a contrast to trachelosaurids such asDinocephalosaurus,which achieve a long neck by the addition of numerous cervicals, for a total cervical count exceeding 30. Nevertheless,Tanystropheusdoes have more vertebrae in its neck than typical archosauromorphs.Protorosaurus,for example, has only seven cervicals, whileMacrocnemusandProlacertahave eight. To achieve a cervical count of 13,Tanystropheusacquired four additional elongated cervicals in the front half of the neck, in addition to a stout vertebra which shifted from the dorsal series into the base of the neck, transforming into the 13th cervical. Tanystropheids are unusual among reptiles in that they acquire their long necks without prolongedsomitogenesis(an increase in the overall number of presacral vertebrae during early development). Instead, their overall number of presacral vertebrae remains at a constant count of 25, the same as their shorter-necked ancestors. This would require a shift in regionalization, encouraging the development of new cervical vertebrae rather than dorsals.[32]

Torso and tail

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PIMUZ T 2817, a large morphotype(T. hydroides)specimen missing only the skull and a portion of the neck

There are 12dorsal (torso) vertebrae.[2]This count is very low among early archosauromorphs:Protorosaurushas up to 19,Prolacertahas 18, andMacrocnemushas 17.Tanystropheus's dorsals are smaller and less specialized than the cervicals.[32]Though their neural spines are taller than those of the cervicals, they are still rather short. Thedorsal ribsare double-headed close to the shoulder and single-headed in the rest of the torso, sitting on stouttransverse processesprojecting outwards from the front half of each vertebra.[8][9][2]More than 20 angled rows ofgastraliaextend along the belly, each gastral element represented by a pair of segmented rods which intermingle at the midline.[9][2]

The twosacral (hip) vertebraeare low but robust, bridging over to the hip with expanded sacral ribs.[2]The latter sacral rib is a single unit without a bifurcated structure.[8][6][33]The tail is long, with at least 30 and possible up to 50caudal vertebrae.[9]The first few caudals are large, with closely interlinked zygapophyses and widely projecting pleurapophyses (a term for transverse processes lacking 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.[8][9][2]A row of longchevronsis present under a short portion of the tail, though not immediately behind the hips.[2]

Shoulder and forelimbs

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Shoulder region and forearm of PIMUZ T 2817

The pectoral girdle (shoulder girdle) has a fairly standard form shared with othertanystropheids.Theclavicles(collarbones) were curved and slightly twisted rods.[9][2]They lie along the front edge of theinterclavicle,a plate-like bone at the center of the chest with a rhombic (broad, diamond-shaped) front region followed by a long stalk at the rear.[6]The interclavicle is rarely preserved and its connections to the rest of the pectoral girdle are mostly inferred fromMacrocnemus.[34]Thescapula(upper shoulder blade) has the form of a large semicircular plate on a short, broad stalk. It lies above thecoracoid(lower shoulder blade), which is a large oval-shaped plate with a broadglenoid facet(shoulder socket).[8][9][2]

Thehumerus(upper arm bone) is straight and slightly constricted at the middle. Near the elbow it is expanded and twisted, with an ectepicondylar groove on its outer edge. Theradius(outer forearm bone) is slender and somewhat curved, while theulna(inner forearm bone) is similar in shape to the humerus and lacks a distinctolecranon(elbow projection). There are fourcarpals(wrist bones): theulnare,radiale,and two distal carpals. The ulnare and radiale are large and cuboid, enclosing a small foramen (gap) between them. The larger outer distal carpal connects tometacarpalsIII and IV, while the much smaller inner distal carpal 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'sphalangealformula (joints per finger) is 2-3-4-4-3. The terminal phalanges (fingertips) may have formed thick, blunt claws.[9][2][6]

Hip and hindlimbs

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Hind foot of PIMUZ T 2817

The components of thepelvis(hip) are proportionally small, though their shape is unremarkable relative to other tanystropheids.[9]Theilium(upper hip blade) is low and extends to a tapered point at the rear. Thepubis(lower front hip blade) is vertically oriented, with a small but distinctobturator foramenand a concave rear edge. The lower front tip of the large, fan-shapedischium(lower rear hip blade) converges towards the pubis, but does not contact it. The large oval-shaped gap between the pubis and ischium is known as thethyroid foramen.[8][2]

Two pairs of large, curved bones, known as heterotopic ossifications[8][2][6]or postcloacal bones,[35]sit behind the hips in about half of known specimens preserving the area. They occupy the base of the tail, a region which lacks chevrons.[8][2][6]These bones are possiblysexually dimorphic,and have also been reported in the small American tanystropheidTanytrachelos.Heterotopic ossifications may be linked to reproductive biology, supporting reproductive organs (if they belong to males) or an egg pouch (if they belong to females).[36][8]

The hindlimbs are significantly larger than the forelimbs, though similar in overall structure and proportions. Thefemur(thigh bone) is long, slender, and sigmoid (curved at both ends). It has a longitudinal muscle crest for muscle attachment (theinternal trochanter) on its underside, and it contacts theacetabulum(hip socket) at a broad smooth joint. Thetibiaandfibula(shin bones) are straight, with the former much thicker and more expanded at the knee. The large proximaltarsals(ankle or heel bones contacting the shin) consist of a roundedcalcaneumand a blockyastragalus,which meet each other along a straight or shallowly indented contact in most specimens.[9][2]Like most non-aquatic reptiles, a set of small pebble-shaped distal tarsals are present between the proximal tarsals and the foot bones.Tanystropheushas a reduced number of distal tarsals: only a small fourth distal tarsal and a minuscule third distal tarsal.[9][6]There are five closely appressedmetatarsals(foot bones), with thefourthandthirdbeing the longest. Though the first four metatarsals are slender and similar in length, thefifth(outermost) is very stout and subtly hooked, slotting into the ankle along a smooth joint.[8][9][2]The estimated phalangeal formula (joints per toe) is 2-3-4-5-4. The first phalange of the fifth toe was very long, filling a metatarsal-like role as seen in other tanystropheids.[8][6]

Classification

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Historical interpretations (1920s-1980s)

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Macrocnemus,a Triassic contemporary ofTanystropheus.Since the 1930s, the two genera have been nearly universally regarded as close relatives
Protorosaurus,aLate Permian"protorosaur".Some 20th-century paleontologists classifiedTanystropheusandMacronemusas Triassic protorosaurs.

Knowledge on the anatomy ofTanystropheuswas transformed byBernhard Peyer's discoveries in the 1920s and 1930s, but its relationship to other reptiles remained Enigma tic for much of the 20th century. Most paleontologists (including modern authorities) agree thatTanystropheuswas closely related toMacrocnemus,a smaller and less specialized reptile found in the same geological strata.[37][38][17]Beyond this conclusion, Peyer initially suggested thatTanystropheuswas related to other long-necked Triassic reptiles.Sauropterygianssuch asplesiosaursandnothosaurswere one possibility, and another was the fragmentary German reptileTrachelosaurus.[7]Later, Peyer classifiedTanystropheusandMacrocnemuscloser to "protorosaurs", a term initially used forPermianreptiles such asProtorosaurusandAraeoscelis.[37]

In the early and mid-20th century, it was commonplace for Permian and Triassic reptiles of uncertain affinity to intermingle together in classification schemes. Names such as "Eosuchia","Euryapsida","Younginiformes","Protorosauria",and others were all applied by different authors with little consistency.[39][40][41]TheEarly TriassicreptileProlacerta,fromSouth Africa,also became involved upon its discovery.[42]Prolacertawas the namesake of yet another term introduced into the convoluted space of reptile taxonomy: "Prolacertiformes".[43]

As the century progressed, two competing hypotheses for the affinities ofTanystropheusdeveloped from the groundwork set by Peyer. Both hypotheses were justified by patterns of skull fenestration (the shape of holes in the skull behind the eye) and cranial kinesis (the flexibility of joints within the skull). One idea was thatTanystropheusand kin (particularlyMacrocnemusandProlacerta) were ancestral to "lacertilians",an antequated term forlizards.This hypothesis was supported up until the 1980s by German and Swiss paleontologists, including Rupert Wild,[44][18]and Peyer's successor at Zürich,Emil Kuhn-Schnyder.[45][46][40]The other idea maintained thatTanystropheuswas a "protorosaur", closer toProtorosaurusandAraeoscelisand unrelated toProlacerta.This was popular among American paleontologists likeAlfred Romer.[47]Some publications from the mid-20th century argued that "protorosaurs" were "euryapsids" (reptiles with only anupper temporal fenestra) related to sauropterygians,[48][39]though later accounts admitted that Euryapsida was likelypolyphyletic,with its members lacking a common ancestor.[41][49]

In 1975, a paper by South African paleontologist C.E. Gow argued that none of these hypotheses were entirely correct.[50]He proposed thatProlacerta,and by extensionMacrocnemusandTanystropheus,occupied an extinct spur on the reptile family tree near the ancestry ofarchosaurs,a diverse group of reptiles with lightweight skulls and serrated teeth set in deep sockets.[50]Dinosaursare among the most famous subset of archosaurs, as are moderncrocodiliansand their prehistoric ancestors.[38]Several newly discovered "prolacertiforms", includingTanystropheus-,[51]Protorosaurus-,[52]andProlacerta-like species,[53]were described in the 1970s, not long after the field of paleontology was reinvigorated by the "dinosaur renaissance"in the 1960s and beyond.

Cladistics and Archosauromorpha (1980s-1990s)

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Cladistic analyses agree thatTanystropheusbelongs within acladeorgradeofbasalarchosauromorphs. Many studies from the 1970s to 1990s referred to long-necked basal archosauromorphs as "prolacertiforms" (namesakeProlacertapictured)

In the 1980s, the advent ofcladisticssaw a paradigm shift in the field oftaxonomy,emphasizingmonophyleticclades(all-encompassing groups defined by shared ancestry) over other categorization styles.Phylogenetic analyseswere invented to evaluate reptile evolution in a quantitative manner, by collecting a set of characteristics in sampled species and then using computational models to find the simplest (most parsimonious) path evolution could take to produce that character distribution. Cladistics stabilized and defined a fundamental split in the family tree of reptiles: one side of the family tree,Lepidosauromorpha,leads tolepidosaurssuch assquamates(lizards and snakes) and thetuatara.The other side,Archosauromorpha,leads to archosaurs.[54][55]Cladistics was one of many lines of evidence that helped to demonstrate the dinosaurian origin of birds. This left crocodilians and birds as the two surviving archosaur groups.[56]

A series of phylogenetic analyses in the late 1980s and 1990s strongly supported the proposal of Gow (1975).[54][57][53][58][59]Tanystropheus,Macrocnemus,Protorosaurus,andProlacertawere always placed as members of Archosauromorpha, closer to archosaurs than to squamates. "Protorosauria" and "Prolacertiformes" were used interchangeably for the archosauromorph subgroup encompassing these superficially lizard-like reptiles. Some authors preferred "Protorosauria" for itspriority.[60]Most others used "Prolacertiformes" arguing that "Protorosauria" was a name that carried too much historical baggage, since it had previously encompassed non-archosauromorph "euryapsids" likeAraeoscelis.[57]

As a "prolacertiform",Tanystropheusis typically considered thesister taxontoTanytrachelos,a much smaller tanystropheid fromVirginia.Another small tanystropheid,CosesaurusfromSpain,is allied with theTanystropheus+Tanytrachelosclade in many analyses of the 1980s and 1990s.[57][58][53]Within Archosauromorpha, "prolacertiforms" are joined by several other groups.[38]The cladeArchosauriformesis a diverse archosauromorph subset includingcrown grouparchosaurs and their predatory close relatives such asEuparkeriaandProterosuchus.Stocky Triassic herbivores likerhynchosaurs,Trilophosaurus,andazendohsaurids[61]additionally qualify as archosauromorphs.[38]The bizarrechameleon-likedrepanosaurswere also included by many analyses,[57][62][59]though more recently they have been reinterpreted as a morebasaltype of reptile unrelated to Archosauromorpha.[63]

The followingcladogramis from Dilkes (1998), a study with a small sample of "prolacertiforms" but closer resemblance to most analyses of the 2000s and 2010s:[59]

Sauria

Recent studies and the rejection of "prolacertiform" monophyly (2000s-present)

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Starting with Dilkes (1998), many phylogenetic analyses began to recoverProlacertain a position close to archosauriforms and away from other "prolacertiforms".[59]In addition, a 2009 redescription ofProtorosaurusshifted it away fromTanystropheusand close to the base of Archosauromorpha.[64]These results have driven paleontologists to the conclusion that "Protorosauria" / "Prolacertiformes" is not a natural monophyletic clade and fails to adequately describe the structure of Archosauromorpha. In the modern cladistic framework, it could be considered aparaphyleticgradeorpolyphyleticcategory of archosauromorphs united by "primitive" characteristics (such as a slender neck and lizard-like body) rather than a shared evolutionary history.[65][64][38]

The familyTanystropheidaehas come to succeed those older names, acting as a monophyletic clade oriented aroundTanystropheus.Tanystropheidae hosts a growing list of former "protorosaurs" with closer affinities toTanystropheusthan toProlacerta,Protorosaurus,or other major archosauromorph groups.Tanystropheusis well-nested within Tanystropheidae, sometimes as the sister taxon toAmotosaurus.[66][61][38]Macrocnemusis most commonly the basal-most (first diverging) tanystropheid.[66][61][38][17]

A set of phylogenetic analyses by Spiekman et al. (2021) attempted to tackle the question of "protorosaur" relationships using an expanded and updated sample of archosauromorph species described over the past few decades.Tanystropheuswas split into five taxonomic units in this study:T. longobardicus, T. hydroides, T. "conspicuus", "T. antiquus" (Protanystropheus), and GMPKU P1527 (the large ChineseTanystropheusspecimen). Two types of analyses were designed to test for bias: one disregarded non-discretecharacters and character state ordering, while the other included these settings. In some analyses, "wildcard" taxa with inconsistent positions were excluded to improve resolution.[17]

Regardless of the setting,T. longobardicus, T. hydroides, T. "conspicuus",and GMPKU P1527 always formed a clade, though the latter two were excluded from some analyses as "wildcards". Under some settings (but not the most stable analysis), another tanystropheid was added to this clade:Raibliania calligarisi,from the Carnian of Italy. The mainTanystropheusclade was well-nested within Tanystropheidae."Tanystropheus antiquus",whenever included in an analysis, was never found to clade with the otherTanystropheustaxa. Instead, it was consistently allied withDinocephalosaurusandPectodens,forming the newly named cladeDinocephalosauridae,outside of Tanystropheidae.Sclerostropheus fossai,another species formerly referred toTanystropheus,was an unpredictable "wildcard", sometimes placed within Dinocephalosauridae and other times within Tanystropheidae.[17]

A 2024 study recognizedTrachelosaurusas a close relative ofDinocephalosaurus,with their family as the sister taxon to Tanystropheidae. Dinocephalosauridae was renamed toTrachelosauridae,and the Trachelosauridae + Tanystropheidae clade was given the nameTanysauria.[67]

The following cladogram is a simplified representation of the most stable analysis preferred by Spiekman et al. (2021), analysis 4. In this particular analysis, ratio (continuous) characters are included, certain characters are ordered, and five wildcard taxa are excluded before running the analysis:Czatkowiella harae,Tanystropheus "conspicuus","Tanystropheus antiquus",Orovenator mayorumandElessaurus gondwanoccidens.[17]

Archosauromorpha

Paleoecology

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Diet

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Tanystropheus(11) with other marine predators from theEarly Triassic(Griesbachiansubstage) to theMiddle Triassic(Anisian stage)[68]

The diet ofTanystropheushas been strongly debated in the past, though most recent studies consider it apiscivorous(fish-eating) reptile.[9][11]The teeth at the front of the snout are long, conical, and interlocking, similar to those ofnothosaursandplesiosaurs.This was likely an adaptation for catching aquatic prey. Additionally, fish scales and hooklets fromcephalopodtentacles have been found in the stomach region of some specimens, further support for a piscivorous diet.[9][11]

Small specimens from Monte San Giorgio (T. longobardicus) are noted to possess tricuspid teeth at the back of the jaw. This shape is unorthodox and uncommon among extinct or living reptiles. Wild (1973/1974) considered these three-cusped teeth to be an adaptation for gripping insects. Cox (1985) noted thatmarine iguanas,which feed onalgae,also have three-cusped teeth. As a result, he attributed the same preferences toTanystropheus.Taylor (1989) rejected both of these hypotheses, as he interpreted the neck ofTanystropheusto be too inflexible for the animal to be successful at either lifestyle.[9]

The most likely function of tricuspid 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 ofseals,such as thehooded sealandcrabeater seal,also have multi-cusped teeth which assist their diet to a similar effect.[9]Similar teeth have also been found in thepterosaurEudimorphodonand the fellow tanystropheidLangobardisaurus,both of which are considered piscivores.Crustaceansand other soft invertebrates are also plausible food items forTanystropheus longobardicus.Larger individuals (Tanystropheus hydroides) lack three-cusped teeth, instead possessing typical conical fangs along the entire rim of the mouth. This difference in dentition indicates a degree of niche partitioning, withT. hydroidespreferring larger and more active prey thanT. longobardicus.[11]

Predation

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PIMUZ T 2819, aT. hydroidesspecimen which was decapitated by a larger predator

While long necks were a successful evolutionary strategy for many marine reptile clades during the Mesozoic, they also increased the animals' vulnerability to predation. Spiekman and Mujal (2023) investigated twoTanystropheusfossils (PIMUZ T 2819 and PIMUZ T 3901), each consisting solely of a skull attached to an articulated partial neck. PIMUZ T 2819 (a large specimen ofT. hydroides) is preserved up to cervical vertebra 10, which is splintered by punctures and scoring. The shape of the marks indicate that the neck was severed in two rapid bites by a predator attacking from above and behind. A similar predation attempt occurred against PIMUZ T 3901 (the Meride Limestone specimen ofT. longobardicus), which was bitten at cervical 5 and severed at cervical 7. The authors further suggested that since the decapitation occurred in the mid-section of the neck, this was likely an optimal target due to its distance from the head and the muscular base of the neck. While many contemporary marine reptiles were capable of attacking PIMUZ T 3901, only the largest predators of the Besano Formation could have attacked PIMUZ T 2819.Paranothosaurus giganteus,Cymbospondylus buchseri,andHelveticosaurus zollingeriare all candidates for the latter case.[69]

Paleobiology

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Skull biomechanics

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InT. hydroides,the connection between the quadrate and squamosal is loose, with the upper extremity of the quadrate hooking into a deep concavity on the squamosal. This would have enabled a degree of flexibility along the quadrate-squamosal contact, allowing the quadrate to swivel around anotic joint.This a condition is a form ofcranial kinesis(movement among bones in the cranium) known asstreptostyly,which is found in some living lizards. The quadrate is also loosely connected to the pterygoid, and the quadratojugal fails to contact the jugal, two qualities which allow movement of the quadrate without hindrance. While streptostyly is possible in the reconstructed skull, it cannot be demonstrated whether it was actively used by the living animal.[13]

Fragments of rod-like hyobranchial elements (throat bones) have been found in fossils of bothT. hydroidesandT. longobardicus.These hyobranchials are very slender and disarticulated, without a bony corpus (thickened "body" of thehyoid apparatus) to connect elements from either side of the throat. These traits indicate thatTanystropheusrelied on biting and enlarged teeth to capture prey.Suction feedingis rejected, since it is correlated with a more robust and integrated hyoid apparatus.[13]

Growth and development

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Histologicalsampling has demonstrated thatTanystropheushad a fairly slow growth rate. The femur, cervical vertebrae, cervical ribs, and postcloacal bones all have alamellaror parallel-fiberedcortex.This corresponds to slow and sturdy bone accumulation. Lamellar deposition is characteristic of the cervical ribs and the upper part of the vertebra, andsharpey's fibersare abundant in the cervical ribs and postcloacal bones. The upper part of the vertebra is subject to remodeling by secondaryosteons,smoothing out and strengthening that part of the bone as the animal grows. There is no evidence for woven-fibered bone, a type of uneven fast-developing texture apparent in many archosauromorphs, including other "protorosaurs" likeA Enigma stropheusandProlacerta.This suggests thatTanystropheus(and its relativeMacrocnemus) retained an ancestrally lowmetabolic ratemore similar to lizards than to archosauriforms.[35]

Respiration

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As neck length increases, so doestrachealvolume, which imposes a biological limitation on breathing. Every time the animal inhales, a significant portion of oxygenated air (so-called dead space volume) fails to pass fully through the trachea and reach the lungs. Many long-necked animals have adaptations meant to overcome this limitation. For example,giraffeshave a narrow trachea and infrequent breathing, which reduces the dead space volume.Sauropoddinosaurs supplement their trachea with air sacs that allow for greater air movement through therespiratory system.Birds utilize both air sacs and infrequent breathing.Tanystropheuswould need to rely on exceptionally specialized lungs which exceed anyallometricpredictions based on modern reptiles. In a compromise between energy usage and minimizing dead space volume, the ideal trachea width forTanystropheusis around 1 cm (0.4 inches), for a neck 1.7 meters (5.6 feet) in length. During periods of high activity, the only lung structure capable of meeting oxygen needs is a multicameral lung (partitioned into multiple smaller chambers) with unidirectional air flow and infrequent breathing. This type of respiratory system is seen in modern archosaurs and turtles. In any case,Tanystropheus's lung capacity was too small for frequent activity or life at higher altitudes. This supports its proposed ecology as coastal ambush predator.[29]

Soft tissue

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A specimen described by Renesto in 2005 displayed an unusual "black material" around the rear part of the body, with smaller patches at the middle of the back and tail. Although most of the material was amorphous, 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 juvenileMacrocnemusdescribed in 2002.[70]The portion of the material at the base of the tail is particularly thick and rich inphosphate.Many small spherical structures are also present in this portion, which upon further preparation were revealed to be composed ofcalcium 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 inMacrocnemus,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.[8]

Brain and inner ear

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Digital endocast of PIMUZ T 2790, showing the brain cavity (blue), inner ear (red), and nerve canals (yellow)

Impressions on the frontal bones ofTanystropheus longobardicusfossils indicate that that species at least had a bulbousforebrainwith pairedolfactory bulbs.[9]The complete braincase ofTanystropheus hydroidesspecimen PIMUZ T 2790 allowed for a partial reconstruction of the brain cavity andinner earvia a digitalendocast.Theflocculusis large and broad and leads forward to the rest of thecerebellum,which is narrowest between theendosseus labyrinth(inner ear canals). A large flocculus may relate to greater head and eye stabilization, though evidence is inconclusive. Long-necked sauropods show a reduction of the flocculus and there is no clear correlation between flocculus size and function in modern mammals and birds. Like other reptiles,Tanystropheushas threesemicircular canalsringing out of the inner ear.Tanystropheuslikely stayed in shallow waters or on land, since its semicircular canals are much thinner than those of deep-diving seabirds. The anterior semicircular canal, which curves up and around the flocculus, is enlarged. The posterior semicircular canal (which slopes backwards and outwards from the brain) is smaller, as is the lateral semicircular canal (which arches outwards). The lateral semicircular canal is nearly horizontal in orientation, which possibly relates to a horizontal head posture. There is also a long straightcochlear ductextending outwards, and a long cochlear duct typically indicates good hearing ability in living reptiles.[13]

Terrestrial capabilities

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Life reconstruction ofT. longobardicushunting from the seashore

The lifestyle ofTanystropheusis controversial, with different studies favoring a terrestrial or aquatic lifestyle for the animal. Major studies onTanystropheusanatomy and ecology byRupert Wild(1973/1974, 1980) argued that it was an active terrestrial predator, keeping its head held high with an S-shaped flexion.[44]Though this interpretation is not wholly consistent with its proposed neck biomechanics, more recent arguments have supported the idea thatTanystropheuswas fully capable of movement on land.[8][71][72][73]

Renesto (2005) argued that the neck ofTanystropheuswas lighter than previously suggested, and that the entire front half of the body was more lightly built than the more robust and muscular rear half.[8]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. The neck ofTanystropheushas low neural spines, a condition which posits that its epaxial musculature was underdeveloped. This would suggest that intrinsic back muscles (such as them. longus cervicis) were the driving force behind neck movement instead. The zygapophyses of the neck overlap horizontally, which would have limited lateral movement. The elongated cervical ribs would have formed a brace along the underside of the neck. They may have played a similar role to theossified tendonsof many large dinosaurs, transmitting forces from the weight of the head and neck down to the pectoral girdle, as well as providing passive support by limiting dorsoventral (vertical) flexion.[74][8]Unlike ossified tendons, the cervical ribs ofTanystropheusare dense and fully ossified throughout the animal's lifetime, so its neck was even more inflexible than that of dinosaurs.[35]

A pair of 2015 blog posts by paleoartistMark Wittonestimated that the neck made up only 20% of the entire animal's mass, due to its light and hollow vertebrae. By comparison, inpterosaursof the familyAzhdarchidae,which were clearly large terrestrial predators, the neck and head made up almost 50% of their mass. Witton proposed thatTanystropheuswould have hunted prey from the seashore, akin to aheron.[71][72]Renesto (2005) supported this type of lifestyle as well.[8]A later published estimate argued that the neck comprised about 30 to 43% of the body mass.[29]Terrestrial or semi-terrestrial habits are supported bytaphonomicevidence:Tanystropheusspecimens preserved at Monte San Giorgio have high completeness (most bones are present in an average fossil) but variable articulation (bones are not always preserved in life position). This is similar toMacrocnemus(which was terrestrial) and opposite the pattern seen inSerpianosaurus(which was fully aquatic).[73]Renesto and Franco Saller's 2018 follow-up to Renesto (2005) offered more information on the reconstructed musculature ofTanystropheus.This study determined that the first few tail vertebrae ofTanystropheuswould 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.[75]

Aquatic capabilities

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Tschanz (1986, 1988) suggested thatTanystropheuslacked the musculature to raise its neck above the ground, and that it was probably completely aquatic, swimming by undulating its body and tail side-to-side like a snake or crocodile.[74]This interpretation has been contradicted by later studies,[8]althoughTanystropheusmay have still spent a large portion of its life in shallow water.[75][11][13]

Renesto (2005) argued thatTanystropheuslacked clear adaptations for underwater swimming to the same degree as most other aquatic reptiles. The tail ofTanystropheuswas compressed vertically (from top-to-bottom) at the base and thinned towards the tip, so it would not have been useful as a fin for lateral (side-to-side) movement. The long neck and short front limbs shifted the center of mass back to the long hind limbs, which would have made four-limbed swimming inefficient and unstable if that was the preferred form of locomotion. He additionally claimed that thrusting with only the hind limbs, as in swimming frogs, was an inefficient form of locomotion for a large animal such asTanystropheus.[8]

Life restoration ofTanystropheusin the water, from Renesto and Saller (2018)
Reconstruction of the major muscles between the legs, hip, and tail inTanystropheus,from Renesto and Saller (2018)

Contrary to earlier arguments, Renesto and Saller (2018) found some evidence thatTanstropheuswas adapted for an unusual style of swimming.[75]They noted that, based on reconstructions of muscle mass, the hind limbs would have been quite flexible and powerful according to muscle correlations on the legs, pelvis, and tail vertebrae. Their proposal was thatTanystropheusmade use of a specialized mode of underwater movement: extending the hind limbs forward and then simultaneously retracting them, creating a powerful 'jump' forward. Further support for this hypothesis is based on theichnogenus(trackway fossil)Gwyneddichnium,which was likely created by small tanystropheids such asTanytrachelos.SomeGwyneddichniumtracks seem to represent a succession of paired sprawling footprints from the hind limbs, without any hand prints. These tracks may have been created by the same form of movement which Renesto and Saller (2018) hypothesized as the preferred method of swimming inTanystropheus.[75]

Nevertheless,lateral undulationcannot be disregarded as a potential swimming style; vertebrae near the hips have extended transverse processes, which are associated with powerful undulating tail muscles in reptiles such as crocodilians. Tail movements may be more effective for swimming than paddling or thrusting with the hindlimbs, since the foot bones ofTanystropheusare narrowly bundled together with little room for webbing.[13]

The skull ofTanystropheusshows additional support for a semiaquatic habits: bothT. hydroidesandT. longobardicushave large undivided nares positioned on the upper surface of the snout, a location consistent with this lifestyle in other animals.[11]In addition, the femur density approaches that ofLariosaurus,an aquaticnothosaur.[35]When hunting underwater,Tanystropheusmay have acted as anambush predator,using its long neck to stealthily approach schools of fish or squid while keeping its large body undetected. Upon selecting a suitable prey item, it would dash forwards[75]or snap to the side.T. hydroideswas particularly well-suited for lateral biting, thanks to its low skull and procumbent fangs.[11]A methodical and intermittent approach to underwater hunting would be appropriate forTanystropheus,considering its lack of adaptations for an exclusively aquatic life. It was likely incapable ofpursuit predation,in contrast to more persistent and specialized marine reptiles such asichthyosaursorplesiosaurs.[75][11][13]

References

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Bibliography

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  • George Olshevsky expands on the history of "P."exogyrarumArchived2016-04-12 at theWayback Machine,on the Dinosaur Mailing List
  • Huene, 1902. "Übersicht über die Reptilien der Trias" [Review of the Reptilia of the Triassic].Geologische und Paläontologische Abhandlungen.6, 1-84.
  • Fritsch, 1905. "Synopsis der Saurier der böhm. Kreideformation" [Synopsis of the saurians of the Bohemian Cretaceous formation].Sitzungsberichte der königlich-böhmischen Gesellschaft der Wissenschaften,II Classe.1905(8), 1–7.
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