Tanystropheus: a long-necked Triassic archosauromorph

Tanystropheus was a Middle Triassic archosauromorph whose neck was longer than its trunk and tail combined in the largest individuals. It was not a dinosaur, a plesiosaur or a lizard. Tanystropheids formed a separate branch of early archosauromorphs, and the most complete skeletons come from Monte San Giorgio on the border between Switzerland and Italy.

The extraordinary neck was built from only thirteen cervical vertebrae. Each middle vertebra was stretched to an extreme length and reinforced below by overlapping cervical ribs. This construction produced a long but relatively stiff structure, not the freely coiling neck shown in some older restorations.

The skeleton establishes the neck proportions, skull form and aquatic specialisations. The exact strike, skin pattern, colour and social behaviour remain reconstructions.

Quick facts

Name and history of study

The history of the name began with isolated elongated bones found in the nineteenth century. They were first interpreted as wing phalanges of a pterosaur. Better skeletons later showed that the supposed wing bones were cervical vertebrae. The name Tanystropheus refers to their unusually stretched form.

Material from the Monte San Giorgio shales was long combined under T. longobardicus. The skeletons differed greatly in size and tooth shape, but the small animals were often treated as juveniles of the large form. A 2019 revision showed that the differences could separate species, although crushed skulls and uncertainty about individual age initially prevented a firm conclusion.

Computed tomography of the flattened skulls and examination of bone tissue resolved much of the dispute. In 2020 the large form was named Tanystropheus hydroides, while small skeletally mature specimens remained in T. longobardicus. The name of the large species refers to the trap-like teeth of an aquatic predator.

Type specimens and collections

The modern revision designated PIMUZ T 2791 as the neotype of T. longobardicus. This is an almost complete articulated skeleton of the smaller form, lacking the rear part of the tail. It is held by the Palaeontological Institute and Museum of the University of Zurich. A neotype was needed to anchor the scientific name to a diagnostic specimen.

The large T. hydroides is based on material from the same Monte San Giorgio collections. Its skulls were strongly compressed by sediment. Synchrotron microtomography allowed researchers to separate the elements virtually and assemble a three-dimensional model without destroying the fossil.

Isolated elongated vertebrae from other parts of Europe, Asia and North America show that tanystropheids were widespread. A single neck bone does not always identify a species, however, because vertebral shape varies along the neck and between individuals.

Age and the Monte San Giorgio environment

The best skeletons come from the Besano Formation and Meride Limestone of the Middle Triassic Period, around 242 million years ago. The region then lay near the western margin of the Tethys Ocean. Warm lagoons and coastal basins alternated with carbonate platforms and islands.

Fish, cephalopods, crustaceans and several marine reptile lineages occupied these waters. Nothosaurus had a very different body plan and limbs better suited to paddling. Finding the animals in the same regional deposits does not mean that they ate the same prey or moved in the same way.

Thinly layered sediment preserved articulated skeletons but often flattened them. Overall body proportions can therefore be read directly, while the original three-dimensional form of the skull and joints requires digital reconstruction.

A neck made from thirteen vertebrae

The extreme neck length came from elongating each vertebra rather than increasing their number. In the middle of the series, the centrum of a vertebra was many times longer than it was high. Long cervical ribs ran along the underside and overlapped several neighbouring bones.

Those ribs limited bending. Images in which the neck forms tight coils or snake-like curves conflict with its construction. The base of the neck and the region near the head retained more movement, allowing the small head to change direction without flexing every long vertebra.

A small head at the end of a long lever could approach prey while the trunk remained farther away. The exact path of a feeding strike is not preserved. A sideways movement is mechanically plausible, but it is an inference from anatomy rather than an observed behaviour.

Skulls and teeth of two species

The large T. hydroides had a flattened snout, nostrils on the upper surface and long curved teeth that formed a trap for slippery prey. These features fit feeding in water better than the older picture of an animal standing on a cliff and lowering its neck into the sea.

T. longobardicus had a different dental arrangement, including crowns with several points. The contrast indicates that the two species could exploit different food resources. Niche separation would reduce direct competition even when both lived in the same ecosystem.

Teeth demonstrate the capture of animal food, but they cannot reveal hunting frequency, seasonal changes or the depth of every dive. Abdominal contents provide the more direct evidence.

Size and methods of estimation

Adult T. longobardicus reached roughly 1.5 metres. The large T. hydroides grew to about 5–6 metres. Much of that total was neck, so body length alone should not be used to compare it with heavier marine predators.

An estimate changes with the completeness of the tail, compression of the slab and the restored gaps between displaced bones. A range is more honest than one exact figure. Body mass is less secure because the outline of soft tissue and the volume of the trunk must be modelled.

Movement in water and on land

The large species was predominantly aquatic. Skull construction, nostril position, teeth and overall proportions all support that conclusion. Its limbs did not become rigid flippers like those of many animals in the marine reptile catalogue. It probably swam with movements of the trunk and tail while the feet assisted with steering.

A long, heavy neck created drag and does not suit constant high-speed pursuit. Comparatively slow approach followed by a short movement of the head is more plausible. This remains a functional interpretation, not a measured swimming speed.

Limited travel on land cannot be ruled out. The feet retained digits and could support the body, but a large animal with these proportions would have been awkward during sustained terrestrial movement. The smaller species may have used the shore differently, although its exact routine is also unknown.

Diet and direct stomach evidence

Fish remains and parts of cephalopods occur in the abdominal region of individual skeletons. These are direct dietary observations and show that tanystropheids really captured aquatic prey rather than merely possessing fish-catching teeth.

The dental differences support separate feeding niches. Large T. hydroides could hold relatively large slippery prey, while smaller T. longobardicus took smaller animals. The evidence does not justify a rule that either species ate only one kind of food.

Growth, injury and predators

Bone tissue in small specimens records skeletal maturity. They were therefore not simply young stages of a six-metre animal. Histology can also reveal growth patterns, but the sampled bones are too few to build a precise year-by-year mass curve or maximum lifespan.

Two specimens preserve breaks and damage to cervical vertebrae consistent with a powerful bite. The position of the injuries suggests that an attacker may have struck from above or behind and severed the neck from the trunk. The identity of the predator is unknown. Large fish and reptiles lived in the same sea, but matching a general tooth size is not enough to name the attacker.

Two injuries also cannot show that the neck was always a fatal weakness or caused the disappearance of the genus. They record particular events in the lives of particular animals.

Relationships and limits of reconstruction

Tanystropheus belongs to Archosauromorpha but lies outside Archosauria and Dinosauria. It was a distant relative of the broader line that later included crocodilians, pterosaurs and dinosaurs. The old assemblage called “protorosaurs” proved to combine several branches; Tanystropheidae is now treated as a distinct lineage.

Some names based mainly on vertebrae are doubtful. T. meridensis has been placed in synonymy with T. longobardicus, while material once called T. fossai was transferred to the separate genus Sclerostropheus. Fragmentary remains may be safest as Tanystropheus sp. without forcing a species identification.

Colour, skin pattern and the soft outline of the neck are unknown. Courtship, nests, number of young and social organisation are also unrecorded. Any permanent herd or family group in an illustration is an artistic choice rather than fossil evidence.