Protosqualus: a small-toothed Cretaceous dogfish

Protosqualus was a Cretaceous dogfish shark whose fossil record consists mainly of isolated teeth only a few millimetres across. The type species, P. sigei, was described from Albian deposits at Wissant in northern France. Its low, broad root and slanted cutting crown distinguish it from the living genus Squalus, although later species identifications have required revision. The tooth-based lineage is one of the contrasting records in the ancient fish catalogue.

Quick facts

A genus named from a tiny tooth

Henri Cappetta established Protosqualus in 1977 while describing shark remains from the upper Albian of Wissant, in northern France. The type species is P. sigei, named in honour of palaeontologist Bernard Sigé. Its type specimen, USTL STR-8, is a tooth rather than a skeleton. The name refers to resemblance with the living dogfish genus Squalus, but similarity does not establish a direct ancestor-descendant relationship.

The teeth are small, typically measured in millimetres. In P. sigei, the crown’s main cusp leans and is relatively broad where it meets the root. The labial side bears a rounded apron-like projection. A thick, low root has separated nutrient foramina. Researchers distinguish the genus by comparing the cusp, lower crown edge, apron and root together. A worn crown or a broken root can erase the characters needed for confident identification.

Different teeth in one jaw

Sharks continually replace their teeth, and the forms can differ from the front to the sides of a jaw. The Albian type material provides the original reference, while P. barringtonensis from Cenomanian deposits of England and France preserves a broader sample of dental positions. Its teeth vary along the jaw and between upper and lower positions. Researchers have also discussed possible sexual heterodonty, but isolated teeth do not record the sex of the animal. That explanation remains a hypothesis rather than an established pattern.

The variation creates two opposite risks. One assemblage may be split into several species because front and lateral teeth look different; alternatively, unrelated sharks may be lumped together because one tooth resembles another. A useful revision examines collections and new specimens together, and treats tooth position as part of the identification rather than as noise.

Species and changing assignments

Several named forms have been included in the genus, among them P. sigei, P. albertsi, P. pachyrhiza, P. glickmani and P. barringtonensis. Their crowns and roots differ in proportions and in the development of the labial apron. A particularly thick root is part of the diagnosis of P. pachyrhiza. The list is taxonomic, not a single group of equally complete animals: almost all are known from teeth.

Material from Lithuania shows how identifications can change. Teeth once classified as Acanthias were reassigned to Protosqualus glickmani after the original collection was re-examined alongside new Albian and Cenomanian material. That kind of revision tests the visible fossils rather than inheriting an old label uncritically.

A separate case concerns the alleged latest record. Protosqualus argentinensis was named from Maastrichtian teeth in Argentina, but subsequent work transferred it to Squalus. The genus therefore should not automatically be described as surviving until the end of the Cretaceous. Its secure fossil record lies within the Cretaceous, with younger range claims dependent on the classification of particular teeth.

Where the teeth occur

Records come from marine deposits in western Europe, eastern Europe and Russia, India, East Asia and other regions. The type species is Albian; several better-supported species or samples extend into the Cenomanian and later Cretaceous. A Campanian occurrence in Hokkaido has been reported from a marine succession in Japan. These scattered teeth show a broad distribution of the genus or close relatives, but they do not demonstrate that one species moved between every region.

All secure fossils are from marine sediments. Some associated faunas suggest that early squaliform sharks occupied a range of shelf and deeper-water settings. A single tooth cannot reveal the depth at which its owner normally swam. Nor does its small size mean the whole shark was tiny: the tooth provides no reliable body-length estimate without a verified jaw and a carefully chosen comparison.

Feeding and the unseen body

Overlapping blade-like crowns could slice or separate animal tissue. That is a functional reading of the tooth row, not a direct record of prey. No stomach contents, confidently linked bite marks or coprolites identify a specific meal for Protosqualus. The genus cannot be assigned a measured diet from tooth form alone.

No complete skeleton is known. A small dogfish-like body is a reasonable comparative illustration, but the fossils do not establish total length, fin proportions, dorsal spines, skin texture or colour. Deep-water habits, bioluminescence and other traits of some living squaliform sharks cannot simply be transferred to a Cretaceous relative. The associated profiles of Paraorthacodus and Acrodus show how different shark lineages can be when the evidence comes from different teeth and skeletons.

Explore related evidence in the ancient fish catalogue.