Platysomus: a deep-bodied fish with an ancient tongue bite

Platysomus was a deep-bodied ray-finned fish known from Carboniferous and Permian rocks. Its compact, laterally compressed shape contrasts with the long bodies of many Palaeozoic fishes, but the genus has a complicated species history because similar outlines once carried too much taxonomic weight. A three-dimensionally preserved skull of P. parvulus revealed an unusual combination of jaw teeth and an opposing tooth plate on the gill skeleton. That evidence adds a distinctive feeding mechanism to the ancient fish catalogue.

Quick facts

A long-lived name for a changing group

Louis Agassiz established Platysomus in the nineteenth century. The name became attached to numerous high-bodied Palaeozoic fishes from Europe and North America. Their common outline was striking, but it did not guarantee that all belonged to one natural group. Researchers now rely on skull bones, jaws, fin supports, scales and other characters to assess each species. The traditional family name Platysomidae has also been used with changing membership.

Several fossils illustrate why locality and specimen quality matter. A nearly complete P. superbus from the Viséan of Glencartholm in Scotland is held as NHMUK PV P 4060. The skull and front trunk of P. parvulus, NHMUK PV P 11697, came from Middle Pennsylvanian coal-measure ironstone at Fenton, Staffordshire. A Tournaisian platysomid report from Nova Scotia is an older and more tentative occurrence, not a described articulated Platysomus skeleton.

Why the body was so deep

In profile, the body was short and high, with a narrow side-to-side depth. Dorsal and anal fins lay toward the rear, around the caudal region. Overlapping rhomboid scales formed a hard covering. This general anatomy can be read from flattened specimens, although a slab may compress the body, separate bones or obscure fin margins.

A deep body can resist rapid straight-line acceleration while helping a fish turn within a compact space. That is a biomechanical interpretation, not evidence that every species lived among plants or reefs. The sediments at a particular site are needed to describe whether the fish lived in marine water, a coastal basin or another environment. It is safer to keep habitat claims tied to the fossil bed rather than assume one ecology for a genus with a broad and revisable record.

A feeding mechanism preserved in three dimensions

In 2025, Giles, Kolmann and Friedman examined the skull of P. parvulus with micro-computed tomography. The fossil preserves a large, multipartite tooth plate on the basibranchial part of the gill skeleton. It faced an upper tooth field involving the vomer and paired entopterygoid bones. The lower jaw also carried teeth. In this animal, food could therefore be bitten by the jaws and further manipulated between upper and lower tooth-bearing surfaces deeper in the mouth.

The authors described this as the oldest known actinopterygian tongue bite, at roughly 310 million years old. This does not mean it was the first mechanism of its kind ever to evolve; only its earliest currently documented fossil example. Nor does the structure identify the exact food. Crushing or processing resistant prey is consistent with the plates, but no associated meal records a shellfish or other specific item.

The skull was preserved in several pieces, and the researchers used scans to separate bones that overlap in the rock. Some structures were digitally repositioned to account for post-mortem displacement and cracks. These reconstructions clarify anatomy but need to be distinguished from the untouched fossil surfaces. The authors also argued that P. parvulus retained toothed jaws, unlike later bobasatraniid fishes whose jaws were reduced as their internal plates became more specialised.

Species, comparison and unresolved questions

The best-known high-bodied forms should not be combined just because they share an outline. A 2021 paper discussed a putative early platysomid occurrence in Nova Scotia, while the 2025 feeding study used two named species and a separate fossil traditionally called “Platysomus” schultzei. Quotation marks in that usage signal that its assignment to the genus is not settled. This variation shows why a genus-wide body size, diet or ecological role is not yet defensible.

Older writers compared Platysomus with later pycnodont fishes such as Pycnodus, partly because both could have deep bodies and resistant-food adaptations. They lived at very different times, and the 2025 skull documents a specific branchial mechanism in P. parvulus. Similar feeding structures can evolve independently. A useful comparison asks which bones and teeth are shared, rather than treating shape as proof of close relationship.

Reconstruction can show a compact fish with a high body, rear-set fins and scales. The precise colour, soft-fin edges, preferred swimming speed and detailed diet are not preserved. The particular tongue-bite apparatus belongs to the species whose skull was scanned; it is an important discovery, but not a template to place into every illustration of Platysomus.

Explore related evidence in the ancient fish catalogue.