The diet of Tanystropheus has been controversial in the past, although most recent studies consider it a piscivorous (fish-eating) reptile.
The teeth at the front of the narrow snout were long, conical, and interlocking, similar to those of nothosaurs and plesiosaurs.
This was likely an adaptation for catching aquatic prey.
Additionally, hooklets from cephalopod tentacles and what may be fish scales have been found near the belly regions of some specimens, further support for a piscivorous lifestyle.

However, small specimens of the genus possess an additional, more unusual form of teeth.
This form of teeth, which occurred in the rear part of the jaws behind the interlocking front teeth, were tricuspid (three-pronged), with a long and pointed central cusp and smaller cusps in front of and behind the central cusp.
Wild (1974) considered these three-cusped teeth to be an adaptation for gripping insects.
Cox (1985) noted that marine iguanas also had three-cusped teeth, and that Tanystropheus likely fed on marine algae like that species of lizard.
Taylor (1989) rejected both of these hypotheses, as he considered the neck of Tanystropheus to be too inflexible for the animal to be successful at either diet.

The most likely function of these teeth, as explained by Nosotti (2007), was that they assisted the piscivorous diet of the reptile by helping to grip slippery prey such as fish or squid.
Several modern species of seals, such as the hooded seal and crabeater seal, also have multi-cusped teeth which assist their diet to a similar effect.
Similar teeth patterns have also been found in the pterosaur Eudimorphodon and the fellow tanystropheid Langobardisaurus, both of whom are considered piscivores.
Large individuals of Tanystropheus, over 2 meters (6.6 ft) in length, lack these three-cusped teeth, instead possessing typical conical teeth at the back of the mouth.
They also lack teeth on the pterygoid and palatine bones on the roof of the mouth, which possess teeth in smaller specimens.
The two morphotypes were originally considered to represent juvenile and adult specimens of T. longobardicus.
However, histology of the small specimens and restudy of the large specimens has shown that they each represent adult forms of two different species.
The larger one-cusped morphotype was given a new species, T. hydroides, while the smaller tricuspid morphotype retained the name T. longobardicus.
The diet of Tanystropheus has been controversial in the past, although most recent studies consider it a piscivorous (fish-eating) reptile. The teeth at the front of the narrow snout were long, conical, and interlocking, similar to those of nothosaurs and plesiosaurs. This was likely an adaptation for catching aquatic prey. Additionally, hooklets from cephalopod tentacles and what may be fish scales have been found near the belly regions of some specimens, further support for a piscivorous lifestyle.
However, small specimens of the genus possess an additional, more unusual form of teeth. This form of teeth, which occurred in the rear part of the jaws behind the interlocking front teeth, were tricuspid (three-pronged), with a long and pointed central cusp and smaller cusps in front of and behind the central cusp. Wild (1974) considered these three-cusped teeth to be an adaptation for gripping insects. Cox (1985) noted that marine iguanas also had three-cusped teeth, and that Tanystropheus likely fed on marine algae like that species of lizard. Taylor (1989) rejected both of these hypotheses, as he considered the neck of Tanystropheus to be too inflexible for the animal to be successful at either diet.
The most likely function of these teeth, as explained by Nosotti (2007), was that they assisted the piscivorous diet of the reptile by helping to grip slippery prey such as fish or squid. Several modern species of seals, such as the hooded seal and crabeater seal, also have multi-cusped teeth which assist their diet to a similar effect. Similar teeth patterns have also been found in the pterosaur Eudimorphodon and the fellow tanystropheid Langobardisaurus, both of whom are considered piscivores. Large individuals of Tanystropheus, over 2 meters (6.6 ft) in length, lack these three-cusped teeth, instead possessing typical conical teeth at the back of the mouth. They also lack teeth on the pterygoid and palatine bones on the roof of the mouth, which possess teeth in smaller specimens. The two morphotypes were originally considered to represent juvenile and adult specimens of T. longobardicus. However, histology of the small specimens and restudy of the large specimens has shown that they each represent adult forms of two different species. The larger one-cusped morphotype was given a new species, T. hydroides, while the smaller tricuspid morphotype retained the name T. longobardicus.
Tanystropheus