Pycnodus: growth and crushing teeth at Monte Bolca

Pycnodus apodus was a deep-bodied ray-finned fish from the Eocene fossil beds of Monte Bolca in northern Italy. Its palate and lower jaws carried rounded tooth plates suited to crushing hard objects, while the articulated skeletons preserve its changing shape as it grew. A study of 52 specimens helped distinguish growth-related variation from separate species. Many older fossils once called Pycnodus have since been removed from the restricted genus, a useful reminder when browsing the ancient fish catalogue.

Quick facts

A name narrowed by revision

Giovanni Serafino Volta named the Bolca fish Coryphaena apoda in the early nineteenth century. Henri de Blainville later used Zeus platessus, and Louis Agassiz established the genus Pycnodus while retaining the epithet platessus. A second name, P. gibbus, was applied to specimens with a more strongly arched back. These names accumulated before researchers could compare large museum series.

Cawley and colleagues measured 52 specimens held in nine collections. Their biometric and geometric analyses linked much of the shape variation to size. Small fish had deeper bodies, larger-looking eyes and a more convex forehead; larger individuals were less disc-shaped. The study did not identify a set of discrete body forms that would support several Bolca species. Because Volta’s original name apoda has priority for the holotype, the revision used Pycnodus apodus. Older labels reading P. platessus often refer to the same Bolca species under an earlier name.

The genus had also become a catch-all for isolated pycnodont teeth and plates from Jurassic, Cretaceous and Paleogene rocks. Those records are not equivalent to the articulated Bolca skeletons. A rounded tooth can resemble another pycnodont’s tooth without demonstrating that the whole animal belongs to Pycnodus. The restricted genus is therefore much more useful for comparison than a broad list assembled from isolated pieces.

Body and fins in the Bolca fossils

The body was short, deep and compressed from side to side. The dorsal and anal fins lay far back, forming long steering surfaces around the caudal region, while the tail supplied forward thrust. Scales, skull bones and fin positions are visible in flattened slabs. The fish was not closely related to modern angelfishes simply because both have a tall silhouette. Deep bodies evolved independently in several fish lineages.

The growth pattern matters for reconstruction. A juvenile outline should not be treated as a miniature adult with identical proportions, and the more arched back once used to name P. gibbus can be explained by the measured size-related change. The fossils document a series of body forms, but the proposed division between young fish sheltering in reef crevices and adults swimming above the bottom remains an ecological interpretation rather than a directly observed behaviour.

Two kinds of teeth and a likely hard-prey diet

Small teeth at the front of the mouth could grasp or position food. Broader rounded teeth on the vomer and prearticular bones formed opposing crushing surfaces. Wear on these plates is consistent with repeated contact under load. Together, the different tooth types provide a stronger functional picture than a single isolated crown.

Shell-bearing molluscs, crustaceans and other resistant food are reasonable possibilities. Yet a tooth’s mechanical capacity is not a complete menu: the fossils do not preserve a diagnostic collection of shells in the gut. Soft-bodied animals, algae or mixed bottom food cannot be excluded. Unlike the spiral tooth whorl of Helicoprion, the pycnodont’s crushing apparatus was built from separate tooth plates.

Monte Bolca and the reach of the genus

Monte Bolca preserves fishes in thin-bedded limestone from a warm, shallow marine setting. Fine sediment and local conditions allowed whole bodies to remain on slabs, making the locality unusually informative about the fish community. The term “reef” describes the broader setting; the precise burial site may have been a lagoon or nearby part of the shelf with restricted water exchange.

Secure articulated P. apodus material is associated with Eocene Italy. Partial specimens from elsewhere may be left as Pycnodus sp. until their anatomy can be compared. Older reports from much earlier rocks often rest on tooth plates alone and should not be used to claim that one species persisted unchanged across the Mesozoic. For another contrast in fish body plans, the Jurassic Leptolepis was much more elongate and belonged to a different ray-finned lineage.

What an illustration can and cannot show

The body outline, scale pattern, fin positions and tooth batteries are grounded in articulated fossils. Soft fin margins can be approximated from their impressions, but original colour, stripes, group behaviour and a particular coral neighbour were not preserved. A reconstruction can place the fish in a Bolca-like marine scene; exact colour and behaviour remain artistic choices.

Explore related evidence in the ancient fish catalogue.