Squatirhina: a Cretaceous ray with an uncertain family tree

Squatirhina is a Cretaceous cartilaginous fish genus described from teeth. Its flattened crowns and distinctive roots have prompted different interpretations of its relationships, from shark-like groups to batoid rays. The type species, S. lonzeensis, was named from Late Cretaceous material in Belgium; later species and isolated teeth have expanded the record but not resolved the family tree. It is a useful example of the limits of dental taxonomy in the ancient fish catalogue.

Quick facts

A genus named from a small tooth

Edgar Casier established Squatirhina in 1947 for S. lonzeensis, a fossil from the Late Cretaceous of Belgium. The original description focused on dental anatomy. As with many fossil cartilaginous fishes, the mineralised tooth is far more likely to survive than the animal's cartilage skeleton. This makes the genus recognisable through repeated tooth characters while leaving its whole-body form unknown.

The teeth are small and compressed from front to back. Their crowns can be low, with a short cusp and a broad basal apron, while the root and ornament on the crown faces provide additional diagnostic details. A tooth described from the Maastrichtian at Stevns Klint, Denmark, measures about 3.5 millimetres across and preserves a rhomboid labial surface, a short cusp, and a deep groove dividing the root lobes. These details can distinguish a specimen, but they do not directly reveal the animal's body proportions.

Why its position in the fish tree is uncertain

Casier regarded the tooth as combining features that seemed intermediate among several shark-like and ray-like groups. Later authors proposed different placements. Some comparisons emphasised crown shape and suggested orectolobiform affinities; others focused on the root and argued for a relationship with rhinobatoid rays. Cappetta treated the genus as an enigmatic rhinobatoid, but subsequent studies continue to describe its family placement cautiously.

These disagreements are not unusual when only teeth are available. Tooth crowns are functional structures that can converge in unrelated animals, while roots and internal tissues may preserve more specific evolutionary information. Even so, one isolated tooth provides far fewer characters than an articulated skull or skeleton. A stable family assignment would require a wider set of evidence than the present material supplies.

Species lists need revision

Several species have been assigned to Squatirhina across European and North American deposits. Taxonomic revisions have challenged some placements. For example, teeth once called S. americana from the Lance Formation have been reassigned to the separate genus Restesia. Other records are identified tentatively from a single tooth, sometimes with a question mark, because the characters are incomplete or the specimen's horizon is uncertain.

That history matters when describing the genus's range. An Albian French species, a Cenomanian German form, and later Santonian or Maastrichtian records do not automatically document one continuous lineage. Each occurrence should be judged by its type material, tooth anatomy, and geological context. A broad map may combine well-founded species with identifications that remain provisional.

What can be reconstructed

The fossils support a Cretaceous cartilaginous fish with distinctive small teeth. They do not establish its exact size, diet, swimming behaviour, or body shape. A ray-like outline may be used as a cautious visual comparison if the uncertainty is stated, but an isolated tooth is not a complete skeleton. Likewise, an ecological scene with a particular prey item is illustrative rather than direct evidence.

Squatirhina is most informative as a case in how palaeontologists classify incomplete material. Its name is anchored by a type tooth, its diagnostic features are real, and its wider relationships remain unresolved. New associated teeth or skeletal remains could change both its classification and the list of species assigned to it.

Explore related evidence in the ancient fish catalogue.