Tanystropheus
Tanystropheus | |
---|---|
Modelled Tanystropheus skeleton | |
Scientific 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 | |
| |
Synonyms | |
Genus synonymy
Species synonymy
|
Tanystropheus (
Tanystropheus contains at least two valid species as well as fossils which cannot be referred to a specific species. The
Most Tanystropheus fossils hail from marine or coastal deposits of the Middle Triassic epoch (Anisian and Ladinian stages), with some exceptions. For example, a vertebra from Nova Scotia was recovered from primarily freshwater sediments. The youngest fossils in the genus are a pair of well-preserved skeletons from the Zhuganpo Formation, a geological unit in China which dates to the earliest part of the Late Triassic (early Carnian stage).[2] The oldest putative fossils belong to "T. antiquus", a European species from the latest part of the Early Triassic (late Olenekian stage). T. antiquus had a proportionally shorter neck than other Tanystropheus species, so some paleontologists consider that T. antiquus deserves a separate genus, Protanystropheus.
The lifestyle of Tanystropheus has been the subject of much debate.[3] Tanystropheus is 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, reconstructing Tanystropheus as an animal equally capable on land and in the water. Despite its length, the neck was lightweight and stabilized by tendons, 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. Tanystropheus was most likely a piscivorous ambush predator: the narrow subtriangular skull of T. longobardicus is supplied with three-cusped teeth suited for holding onto slippery prey, while the broader skull of T. hydroides bears an interlocking set of large curved fangs similar to the fully aquatic plesiosaurs.
History and species
Monte San Giorgio species
19th century excavations at Monte San Giorgio, a UNESCO world heritage site on the Italy-Switzerland border, revealed a fragmentary fossil of an animal with three-cusped (tricuspid) teeth and elongated bones. Monte San Giorgio preserves the Besano Formation (also known as the Grenzbitumenzone), a late Anisian-early Ladinian lagerstätte recognised for its spectacular fossils.[4] In 1886, Francesco Bassani interpreted the unusual tricuspid fossil as a pterosaur, which he named Tribelesodon longobardicus.[5][6] The holotype specimen of Tribelesodon longobardicus was stored in the Museo Civico di Storia Naturale di Milano (Natural History Museum of Milan), and was destroyed by allied bombing of Milan in World War II.[6]
Excavations by
Well-preserved T. longobardicus fossils 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), the Paleontological Museum of Zürich (PIMUZ), and the Museo Cantonale di Scienze Naturali di Lugano (MCSN).[6] Rupert Wild reviewed and redescribed all specimens known at the time via several large monographs in 1973/4 and 1980. In 2005, Silvio Renesto described a T. longobardicus specimen from Switzerland which preserved the impressions of skin and other soft tissue.[8] Five new MSNM specimens of T. longobardicus were 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 younger
Tanystropheus specimens from Monte San Giorgio have long been segregated into two morphotypes based on their tooth structure.
Polish and German species
The first Tanystropheus specimens to be described were found in the mid-19th century. They included eight large vertebrae from the Upper
Over 500 "Tanystropheus conspicuus" specimens have been recovered from a Lower Keuper bonebed near the Silesian village of Miedary. This is the largest known concentration of Tanystropheus fossils, 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 of Tanystropheus fossils suggests that it was an animal well-suited for this kind of habitat.[14]
In the late 1900s, Friedrich von Huene named several dubious Tanystropheus species from Germany and Poland. T. posthumus, from the Norian of Germany, was later reevaluated as an indeterminate theropod vertebra and a nomen dubium. Several more von Huene species, including "Procerosaurus cruralis", "Thecodontosaurus latespinatus", and "Thecodontosaurus primus", have been reconsidered as indeterminate material of Tanystropheus or other archosauromorphs.[15][6]
One of Von Huene's species appears to be valid: 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. The proportions of T. antiquus fossils are easily distinguishable, and it is currently considered a valid species of archosauromorph,[6] though its referral to the genus Tanystropheus has been questioned.[16][17] The Gogolin Formation ranges from the upper Olenekian (latest part of the Early Triassic) to the lower Anisian in age. Assuming they belong within Tanystropheus, the fossils of T. antiquus may be the oldest in the genus. Specimens likely referable to T. antiquus are also known from throughout Germany and the fossiliferous Winterswijk site in the Netherlands.[18][6]
Other Tanystropheus fossils
In the 1880s, E.D. Cope named three supposed new Tanystropheus species (T. bauri, T. willistoni, and T. longicollis) from the Late Triassic Chinle Formation in New Mexico. However, these fossils were later determined to be tail vertebrae belonging to theropod dinosaurs, which were named under the new genus Coelophysis.[6] Authentic Tanystropheus specimens from the Makhtesh Ramon in Israel were described as a new species, T. haasi, in 2001.[19] 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.[20] It has also been considered possibly synonymous with T. longobardicus. A Tanystropheus-like vertebra from the middle Ladinian Erfurt Formation (Lettenkeuper) of Germany was described in 1846 as one of several fossils gathered under the name "Zanclodon laevis". Though likely the first Tanystropheus fossil to be discovered, the vertebra is now lost, and surviving jaw fragments and other fossil scraps of "Zanclodon laevis" represent indeterminate archosauriforms with no relation to Tanystropheus.[21][6] Tanystropheus vertebrae have also been found in the Villány Mountains of Hungary.[22]
The most well-preserved Tanystropheus fossils outside of Monte San Giorgio come from the Guizhou province of China, 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 lower Carnian Zhuganpo 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 any Tanystropheus species. The Chinese material includes a large morphotype (T. hydroides?) specimen, GMPKU-P-1527, and an indeterminate juvenile skeleton, IVPP V 14472.[2]
Indeterminate Tanystropheus remains are also known from the Jilh Formation of Saudi Arabia and various Anisian-Ladinian sites in Spain, France, Italy, and Switzerland.[6] The youngest Tanystropheus fossil in Europe is a vertebra from the lower Carnian Fusea site in Friuli, Italy.[24][6] In 2015, a large Tanystropheus cervical vertebra was described from the Economy Member of the Wolfville Formation, in the Bay of Fundy of Nova 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 preserving Tanystropheus fossils.[26] Tanystropheus-like tanystropheid fossils are known from another freshwater formation in North America: the Anisian-age Moenkopi Formation of Arizona and New Mexico.[27]
Several new
Anatomy
Tanystropheus was 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. hydroides was around the same size, with the largest specimens at an estimated length of 5.25 meters (17.2 feet).[11] T. longobardicus was significantly smaller, with an absolute maximum size of two meters (6.6 feet).[13][17] Despite the large size of some Tanystropheus species, 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 a Tanystropheus skeleton. For a Tanystropheus individual 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 of Tanystropheus longobardicus
The skull of Tanystropheus longobardicus is roughly triangular when seen from the side and top, narrowing towards the snout.
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. longobardicus is a reptile with heterodont dentition, 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 the crown split into three stout triangular cusps (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, early pterosaurs such as Eudimorphodon developed an equivalent tooth shape, and tricuspid teeth can also be found in a few modern lizard species.[30][31] Some individuals of T. longobardicus have 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 each
The paired
The eye was supported by more than 10 rectangular ossicles (tiny plate-like bones) connecting into a
The
Skull of Tanystropheus hydroides
The skull of Tanystropheus hydroides is broader and flatter than that of T. longobardicus. The first five of six teeth in the premaxilla are very large and fang-like, forming an interlocking "fish trap" similar to
The palate of T. hydroides has 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 elongated choanae (internal openings of the nasal cavity).[6][11][13] Most other archosauromorphs, T. longobardicus included, have restricted vomers with rows of minuscule teeth. The rest of the palate is completely toothless in T. 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 the ectopterygoids (linking bones between the pterygoid and maxilla). The epipterygoids (vertical bones in front of the braincase) are tall and flattened from the side.[13]
T. hydroides is a rare example of an early archosauromorph with a three-dimensionally preserved braincase.
In the lower jaw, the dentaries meet each other at a robust symphysis with 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 in T. longobardicus, but some details differ in T. hydroides.[13] For example, the splenial is plate-like and covers a larger portion of the internal dentary than in T. 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's adductor (vertical biting) muscles, and a prominent surangular foramen is positioned in front of the jaw joint.[13]
Neck
The most recognisable feature of Tanystropheus is its hyperelongate neck, equivalent to the combined length of the body and tail.
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.[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 ribs via 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]
Torso and tail
There are 12
The two
Shoulder and forelimbs
The pectoral girdle (shoulder girdle) has a fairly standard form shared with other tanystropheids. The clavicles (collarbones) were curved and slightly twisted rods.[9][2] They lie along the front edge of the interclavicle, 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 from Macrocnemus.[33] The scapula (upper shoulder blade) has the form of a large semicircular plate on a short, broad stalk. It lies above the coracoid (lower shoulder blade), which is a large oval-shaped plate with a broad glenoid facet (shoulder socket).[8][9][2]
The
Hip and hindlimbs
The components of the pelvis (hip) are proportionally small, though their shape is unremarkable relative to other tanystropheids.[9] The ilium (upper hip blade) is low and extends to a tapered point at the rear. The pubis (lower front hip blade) is vertically oriented, with a small but distinct obturator foramen and a concave rear edge. The lower front tip of the large, fan-shaped ischium (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 the thyroid foramen.[8][2]
Two pairs of large, curved bones, known as heterotopic ossifications[8][2][6] or postcloacal bones,[34] 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 possibly sexually dimorphic, and have also been reported in the small American tanystropheid Tanytrachelos. 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).[35][8]
The hindlimbs are significantly larger than the forelimbs, though similar in overall structure and proportions. The femur (thigh bone) is long, slender, and sigmoid (curved at both ends). It has a longitudinal muscle crest for muscle attachment (the internal trochanter) on its underside, and it contacts the acetabulum (hip socket) at a broad smooth joint. The tibia and fibula (shin bones) are straight, with the former much thicker and more expanded at the knee. The large proximal tarsals (ankle or heel bones contacting the shin) consist of a rounded calcaneum and a blocky astragalus, 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. Tanystropheus has a reduced number of distal tarsals: only a small fourth distal tarsal and a minuscule third distal tarsal.[9][6] There are five closely appressed metatarsals (foot bones), 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.[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
Historical interpretations (1920s-1980s)
Knowledge on the anatomy of Tanystropheus was transformed by Bernhard Peyer's discoveries in the 1920s and 1930s, but its relationship to other reptiles remained enigmatic for much of the 20th century. Most paleontologists (including modern authorities) agree that Tanystropheus was closely related to Macrocnemus, a smaller and less specialized reptile found in the same geological strata.[36][37][17] Beyond this conclusion, Peyer initially suggested that Tanystropheus was related to other long-necked Triassic reptiles. Sauropterygians such as plesiosaurs and nothosaurs were one possibility, and another was the fragmentary German reptile Trachelosaurus.[7] Later, Peyer classified Tanystropheus and Macrocnemus closer to "protorosaurs", a term initially used for Permian reptiles such as Protorosaurus and Araeoscelis.[36]
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 "
As the century progressed, two competing hypotheses for the affinities of Tanystropheus developed 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 that Tanystropheus and kin (particularly Macrocnemus and Prolacerta) were ancestral to "
In 1975, a paper by South African paleontologist C.E. Gow argued that none of these hypotheses were entirely correct.[49] He proposed that Prolacerta, and by extension Macrocnemus and Tanystropheus, occupied an extinct spur on the reptile family tree near the ancestry of archosaurs, a diverse group of reptiles with lightweight skulls and serrated teeth set in deep sockets.[49] Dinosaurs are among the most famous subset of archosaurs, as are modern crocodilians and their prehistoric ancestors.[37] Several newly discovered "prolacertiforms", including Tanystropheus-,[50] Protorosaurus-,[51] and Prolacerta-like species,[52] 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)
In the 1980s, the advent of
A series of phylogenetic analyses in the late 1980s and 1990s strongly supported the proposal of Gow (1975).[53][56][52][57][58] Tanystropheus, Macrocnemus, Protorosaurus, and Prolacerta were 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 its priority.[59] Most others used "Prolacertiformes" arguing that "Protorosauria" was a name that carried too much historical baggage, since it had previously encompassed non-archosauromorph "euryapsids" like Araeoscelis.[56]
As a "prolacertiform", Tanystropheus is typically considered the
The following cladogram is from Dilkes (1998), a study with a small sample of "prolacertiforms" but closer resemblance to most analyses of the 2000s and 2010s:[58]
Sauria |
| ||||||||||||||||||||||||||||||||||||||||||||||||
Recent studies and the rejection of "prolacertiform" monophyly (2000s-present)
Starting with Dilkes (1998), many phylogenetic analyses began to recover Prolacerta in a position close to archosauriforms and away from other "prolacertiforms".[58] In addition, a 2009 redescription of Protorosaurus shifted it away from Tanystropheus and close to the base of Archosauromorpha.[63] 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 a paraphyletic grade or polyphyletic category of archosauromorphs united by "primitive" characteristics (such as a slender neck and lizard-like body) rather than a shared evolutionary history.[64][63][37]
The family Tanystropheidae has come to succeed those older names, acting as a monophyletic clade oriented around Tanystropheus. Tanystropheidae hosts a growing list of former "protorosaurs" with closer affinities to Tanystropheus than to Prolacerta, Protorosaurus, or other major archosauromorph groups. Tanystropheus is well-nested within Tanystropheidae, sometimes as the sister taxon to Amotosaurus.[65][60][37] Macrocnemus is most commonly the basal-most (first diverging) tanystropheid.[65][60][37][17]
The following cladogram is from Pritchard et al. (2015), a study focused specifically on tanystropheids:[65]
|
The following cladogram is from Ezcurra (2016), a study focused generally on archosauromorphs and early archosauriforms:[37]
|
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. Tanystropheus was split into five taxonomic units in this study: T. longobardicus, T. hydroides, T. "conspicuus", "T. antiquus" (
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:
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 (
Archosauromorpha |
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||