The bizarre anvil-backed shark that might actually be a cousin to the chimaera

Nature · Published 3 October 2026 Swipe more like this →

In short: Stethacanthus was a prehistoric fish defined by a strange, anvil-shaped dorsal crest covered in spikes. While it looked like a shark, its unique anatomy—including mysterious fin whips and a spine-brush complex—has led researchers to question its place in the evolutionary tree, suggesting it may be closer to modern ratfish.

Living from the Late Devonian to the Late Carboniferous, Stethacanthus was a cartilaginous fish that reached lengths of up to 3 meters. Its most striking feature was the spine-brush complex, an anvil-shaped structure found only on sexually mature males. This crest was covered in forward-facing denticles, while the top of the head featured a separate collection of backward-pointing denticles. Theories regarding this complex range from it being a mating display or a tool for attachment to a mechanism used to frighten predators by mimicking the mouth of a larger fish.

The internal structure of this complex is equally curious. It contains parallel, membranous tubules made of globular calcified cartilage, which researcher Zangerl suggested might have been inflatable, similar to erectile tissues in humans. The presence of such large masses of globular calcified cartilage in both the crest and the pelvic claspers is significant, as it may indicate that the prismatic cartilage defining modern chondrichthyans evolved from these earlier, globular forms. Furthermore, the crest is coated in a thin, acellular bone layer, potentially marking one of the earliest records of endoskeletal bone in primitive chondrichthyans.

Classifying Stethacanthus has proven notoriously difficult. Early paleontologists like John Strong Newberry initially mistook isolated spines for pectoral fins. Even today, the relationship between stethacanthids and symmoriids remains debated, with some suggesting they are the same group or that symmoriids are simply females. Recent reclassifications have even placed the order Symmoriiformes within the subclass Holocephali. If this holds true, Stethacanthus was not a true shark, but a distant relative of the modern-day chimaera, or ratfish. Regardless of its exact lineage, the creature remains a testament to the experimental diversity of early marine life, likely living as a slow-moving, bottom-dwelling carnivore that migrated between breeding grounds like the Bear Gulch Limestone in Montana.

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