Starting in the 1920s, systematic excavations at Monte San Giorgio unearthed many more Tanystropheus fossils, revealing that the putative wing bones of "Tribelesodon" were actually neck vertebrae.

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).

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.

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.

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".

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.

[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.

[9] The shape of the jugal in Tanystropheus is typical for early archosauromorphs; the underdeveloped posterior process indicates that the margin of the infratemporal fenestra (lower skull hole behind the eye) was incomplete and open from below.

[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).

[13] The basioccipital (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 and archosauriforms.

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).

The dorsal ribs are double-headed close to the shoulder and single-headed in the rest of the torso, sitting on stout transverse processes projecting outwards from the front half of each vertebra.

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.

[47] Some publications from the mid-20th century argued that "protorosaurs" were "euryapsids" (reptiles with only an upper temporal fenestra) related to sauropterygians,[48][39] though later accounts admitted that Euryapsida was likely polyphyletic, with its members lacking a common ancestor.

[38] Several newly discovered "prolacertiforms", including Tanystropheus-,[51] Protorosaurus-,[52] and Prolacerta-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.

In the 1980s, the advent of cladistics saw a paradigm shift in the field of taxonomy, emphasizing monophyletic clades (all-encompassing groups defined by shared ancestry) over other categorization styles.

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.

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.

[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.

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.

In a compromise between energy usage and minimizing dead space volume, the ideal trachea width for Tanystropheus is around 1 cm (0.4 inches), for a neck 1.7 meters (5.6 feet) in length.

[29] 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.

Major studies on Tanystropheus anatomy and ecology by Rupert 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 that Tanystropheus was fully capable of movement on land.

They may have played a similar role to the ossified tendons of 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.

[35] A pair of 2015 blog posts by paleoartist Mark Witton estimated that the neck made up only 20% of the entire animal's mass, due to its light and hollow vertebrae.

[75] Tschanz (1986, 1988) suggested that Tanystropheus lacked 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.

[13] The skull of Tanystropheus shows additional support for a semiaquatic habits: both T. hydroides and T. longobardicus have large undivided nares positioned on the upper surface of the snout, a location consistent with this lifestyle in other animals.

[35] When hunting underwater, Tanystropheus may have acted as an ambush predator, using its long neck to stealthily approach schools of fish or squid while keeping its large body undetected.