Recent work reviewing the jaw dimensions of living sharks reveals limits in using their mouth size to infer the overall body length of ancient placoderms. The new findings, reported in PeerJ, urge a cautious approach to size reconstruction in these extinct armored fishes.
Placoderms form an extinct group of marine vertebrates once known as the armored fishes. They inhabited Earth’s oceans during roughly 435 to 360 million years ago, and some individuals could reach lengths around 9 to 10 meters. Their heads and much of their bodies were coated in bony plating, while the internal skeleton was primarily cartilaginous, resembling the condition seen in sharks. This cartilaginous skeleton, combined with the heavy armor, makes placoderms rare to pin down in the fossil record. As a result, estimates of body size for certain species are imprecise, and investigators have sometimes derived body length for the arthrodires from jaw size by seeking functional analogies with modern sharks.
To test this approach, Russell Engelman and colleagues from Case Western Reserve University compiled data from living sharks and from exceptionally well-preserved placoderm fossils where body size measurements could be made directly. The team aimed to determine whether jaw dimensions reliably scale with total length in these ancient armored fishes.
The researchers note a long-standing practice of assigning a Dunkleosteus length typically in the 5 to 10 meter range. Yet they emphasize that this range has rarely undergone rigorous statistical testing against known specimens within the arthrodire group. The question was whether methods that link jaw size to overall body length would hold up when applied to arthrodires as a group or when tested across different species with verified body sizes.
Analysis showed that the diameter and gape of a shark’s mouth do not provide a dependable proxy for the full body size of arthrodires. In many cases, more robustly built arthropods have proportionally larger mouths than sharks of the same overall length. Consequently, estimating total length from jaw measurements tends to exaggerate the true size by roughly two to two and a half times. Dunkleosteus, noted for its especially formidable maw, demonstrates this limitation: its massive jaws do not correspond to the body length expected if skull size alone were used as a guide to the total dimensions of the animal.
These results help explain an earlier puzzle in placoderm reconstructions. Past estimates suggested that dunkleosteus possessed an elongated body with a relatively small head, a pattern that would imply gill areas too small to support such a form and could make respiration problematic. The new evidence shows that jaw-based inferences can misrepresent the overall proportions of these ancient predators, underscoring the need to lean on multiple lines of evidence when reconstructing their biology.
Further work in paleontology continues to refine how researchers translate fragmentary fossil data into living biology. The study showcases the value of directly measured body sizes in fossil groups and cautions against relying solely on jaw dimensions for broad body-size reconstructions in extinct armored fishes. In the broader context of evolutionary biology, these insights contribute to a clearer understanding of the diversity and constraints shaping early jawed vertebrates. Authentically reconstructing how these ancient predators lived demands a careful synthesis of anatomy, functional morphology, and fossil preservation data, rather than dependence on a single skeletal feature. For those tracing the lineage of bony armor from the Devonian to later seas, the takeaway is simple: context matters, and jaw scalars alone rarely capture the whole animal. Citations to related work and independent verification help readers gauge the robustness of such reconstructions. Attribution for the findings is provided by peer-reviewed publication records and supplementary data from collaborating researchers.