Ancient braincase connects the dots on jaw evolution

The first detailed description of the braincase of an extinct fossil fish, Ptomacanthus anglicus, which lived some 415 million years ago, is providing new clues on the evolution of jawed vertebrates — the so-called modern gnathostomes.

Collected from the Wayne Herbert Quarry Lagerstätte in Herefordshire in the UK, the Early Devonian fossil belongs to the Acanthodians, which are a long-extinct group of fossil fishes that stand close to the divergence of cartilaginous and bony fishes. Acanthodian morphology has the potential to reveal much about the early evolution of jawed vertebrates, but their anatomy is poorly known and largely confined to the genus Acanthodes, which lived late in Acanthodian history.

Martin Brazeau from Uppsala University, Sweden, reports in Nature this week that the structure of Ptomacanthus is very different to that of Acanthodes, and shares features with ancient armored fish and early cartilaginous fish. Brazeau suggests that Ptomacanthus was either a very early relative of sharks, or close to the common ancestry of all modern jawed vertebrates.

“These new data alter earlier conceptions of basal gnathostome phylogeny,” writes Brazeau, “and thus help to provide a more detailed picture of the acquisition of early gnathostome characters.”