Scotland’s Megalichthys pygmaeus fossil fish skull reveals new clues

A new study has reconstructed the skull of a small predatory fossil fish in greater detail using remains found in Scotland’s old coal waste.

The findings give the Victorian-era species Megalichthys pygmaeus a clearer form, although a longstanding question about its family relationships in Scotland’s coalfields remains unanswered.

Finding Megalichthys pygmaeus

Scattered skull bones found across five coal waste tips in Lanarkshire, a Scottish mining region, can now be assembled into a more coherent picture of the fish’s head.

Francis M. Elliott of the Scottish Universities Environmental Research Centre (SUERC) compared the fragments and linked them to a single fish.

The resulting pattern clarifies material that had previously depended on isolated jaws, scales and plates in old mining collections.

Although the reconstruction gives the species a more secure identity, it does not resolve whether the fish was distinct or a juvenile of a larger relative, Megalichthys hibberti.

Rebuilding a name

Since the 1800s, the name had been based on scattered jaws, scales and plates dating from the Carboniferous Period, the coal-forming age more than 315 million years ago.

Because the original specimens had been lost, Elliott designated a neotype, or replacement reference specimen, from a corresponding Scottish coal layer.

The selected lower jaw measured approximately 0.9 centimetres, leaving the question of whether it represented a separate species unresolved.

This replacement provided a fixed specimen for subsequent comparisons, rather than another unattached name in an old mining collection.

Clues in the skull

Beneath the reconstructed skull roof, the fish’s outer skull had small surface bones with sharply defined patterns.

These dermal bones, the hard pieces that form the skull surface, bore minute pits and grooves marking sensory pathways.

A covering known as cosmine, a hard tissue resembling enamel, often obscured the sutures, so an absent seam did not necessarily indicate an absent structure.

Those features allowed Elliott to compare skull shape as well as bone size with that of closely related fossil fish that may have shared ancestors.

A nursery in shale

Much of the most useful material came from thin shale near Wester Bracco and Dewshill, two coal sites in North Lanarkshire, Scotland.

At those sites, a fossil-rich strip only about 4.1 centimetres thick preserved plentiful fish bones between fine layers.

A 1.5-centimetre band of hard, shiny coal marked the relevant layer and helped collectors locate the richest fossils.

These shallow deposits support the idea that young or small fish congregated where deeper-water predators were less common than in open channels.

Small jaws and a big debate

Four lower jaws ranged in length from approximately 0.9 to 4.3 centimetres.

With increasing jaw size, their outlines became straighter and more closely resembled those of the larger related fish, Megalichthys hibberti.

The pattern supports a juvenile connection, although one small jaw could simply have come from a tiny Megalichthys hibberti.

Size cannot settle the question by itself, since growth stages from the same deposits are seldom fossilised in neat, complete sequences.

Hints about breathing

A cheek bone fitted close to a small opening behind the jaw, providing another clue to the skull’s evolutionary history.

The opening, called the spiracle, is a passage associated with the history of breathing and hearing and has a complex record among early vertebrates.

In some fish close to the land-vertebrate line, comparable openings probably helped move air into the throat. In this Scottish fossil, the bones only hint at that role because the soft tissues were not preserved.

A family problem

The skull also offered a test of whether the fish belongs within Megalichthyidae, a family of extinct fishes close to the land-vertebrate line.

Earlier research recognised the family, but defining its precise membership has remained difficult because many characteristics change as animals grow.

A later fossil from Pennsylvania brought the issue into sharper focus by narrowing the skull traits that define Megalichthys, the naming group above species.

Judged against those criteria, Megalichthys pygmaeus appears close to the family but cannot be placed securely in that naming group.

Museum specimens help

Elliott’s comparison at SUERC was possible because the new fragments could be matched against Megalichthys hibberti bones at the Hunterian Museum, a university museum in Glasgow, Scotland.

Lower jaws, shields, cheek bones and shoulder bones each contributed part of the missing plan of the head.

Some fragments were common, but upper-jaw and cheek bones remained scarce, leaving gaps in the reconstruction.

This uneven fossil record made the reconstructed head useful, though its cheek and upper-jaw regions remain partly tentative.

What remains uncertain

Despite the reconstruction, the fish remains close to a phylogenetic boundary: a line on the family tree that is difficult to define through species naming.

Shared skull characteristics link it to Megalichthys hibberti, but those features may indicate family kinship rather than a juvenile stage.

“Until now, no other cranial material has been assigned to Megalichthys pygmaeus and, consequently, the phylogenetic position of the taxon has remained uncertain,” Elliott wrote.

For that reason, Megalichthys pygmaeus remains the safest name until stronger evidence of growth emerges from connected fossils.

Why it matters

Reconstructed from waste-tip fragments, the fish now gives scientists a clearer picture of a small predator from Scotland’s coal-forming wetlands, where shallow water once lay.

The question of whether it was a juvenile or a separate species will require future finds that link age, growth and skull shape.