Nemacanthus: a Triassic shark known from spines and teeth

Nemacanthus is a Late Triassic shark-like fish best recognised by long dorsal-fin spines with a smooth anterior ridge and rows of lateral tubercles. A 2025 review of the Rhaetian bone beds at Bonenburg also synonymised the tooth genus Rhomphaiodon with Nemacanthus, so the animal is now known from more than spines alone. The teeth and spines are not preserved as one articulated skeleton, but their repeated association and dental anatomy refine the case. This changing record belongs in the ancient fish catalogue.

Quick facts

A spine genus with a revised tooth record

Louis Agassiz introduced Nemacanthus monilifer in the 1830s from Late Triassic material. Its name became associated with long fin spines whose ornament is distinctive among the remains recovered from European bone beds. For much of the history of the genus, researchers treated it as a spine-based form because no complete skeleton joined the spine to a jaw or tooth row.

That picture changed with the 2025 taxonomic and palaeoecological review of the Rhaetian chondrichthyan fauna at Bonenburg in Germany. The authors treated Rhomphaiodon as a junior synonym of Nemacanthus and recognised two species, N. monilifer and N. nicolensis. The teeth long assigned to Rhomphaiodon occur consistently with the spine material and have anatomical and histological features that distinguish them from superficially similar hybodont teeth. This is a reasoned taxonomic association, not a fossil in which spine and jaws remain joined.

What the spines and teeth preserve

The fin spine has a smooth enameloid ridge along its front. Its flattened lateral sides carry small, rounded tubercles arranged in vertical rows. The posterior face lacks the same enameloid cover; tiny rear denticles or barbs may occur, although they are often broken or absent from the available fragments. A cross-section can be oval, with a possible central nutrient canal in some specimens.

The teeth assigned to N. monilifer are small, ranging from about 1.5 to 12 millimetres across in the Bonenburg sample. They typically have a tall, broad central cusp flanked by two pairs of lateral cusplets, separated from the main cusp by a notch or gap. Fine striations run over the cusps and may divide near their bases. Smaller teeth near the jaw symphysis can have only one pair of lateral cusplets, so tooth position contributes to variation in a single dentition.

Histology adds evidence beyond outer shape. The 2025 review notes triple-layered enameloid and pseudopolyaulacorhize root vascularisation in the teeth, contrasting with the simpler root condition commonly described for Hybodus sensu lato. These characters support the authors' placement of Nemacanthus in Palaeospinacidae among neoselachian sharks. Older discussions connected the spine to several different shark groups, which illustrates how little a spine alone can settle about relationships.

Rhaetian bone beds and preservation

At Bonenburg, N. monilifer occurs in multiple Rhaetian bone beds and is unusually common compared with other shark remains from the same interval. The review records its spines and teeth alongside a diverse chondrichthyan and ray-finned fish fauna. Such concentrations formed through the accumulation and preservation of hard parts in marine settings; they can combine animals from different times and microhabitats.

The repeated association of the two kinds of remains is informative, but it does not make every nearby fossil part of the same animal. A named tooth or spine still has to match diagnostic anatomy. Nor does an abundance in a bone bed demonstrate that the live fish was the most numerous shark in the sea. The deposit is a sample shaped by which elements survived and how the site was collected.

What remains unknown

Neither a complete body outline nor a direct record of colour, swimming depth or reproduction is known. A long dorsal spine suggests a prominent fin structure, but its mechanical role cannot be observed from a fossil. The small cusped teeth are consistent with grasping or handling animal prey; there is no stomach content that identifies a meal or proves a particular hunting strategy.

In 1977, J. G. Maisey compared Nemacanthus with the better-preserved Early Jurassic shark Palaeospinax and proposed a close relationship. That historical hypothesis helped frame the anatomy, but it does not establish that the Triassic genus was a direct ancestor. The recent family placement is based on comparative characters and remains open to testing as more articulated material is found.

The safest reconstruction combines the known spine and tooth characters with a general shark-like body plan. Details borrowed from living sharks should remain provisional. The key advance is narrower and more secure: the genus now has a better-supported dental record, while the missing connections between isolated fossils remain visible rather than being filled in by assumption.

Explore related evidence in the ancient fish catalogue.