LIFE × EVOLUTION

Dunkleosteus and the Placoderms

~382 to 358 million years ago

Phanerozoic · Paleozoic · Devonian

Placoderms are fish armoured with bone plates over the head and shoulders, which swam the seas between 382 and 358 million years ago, and the largest, Dunkleosteus, sheared with self-sharpening bony blades instead of teeth.

Placoderms are fish whose head and shoulders were covered by bone plates, hinged to the body at the neck. The largest placoderm, Dunkleosteus, had no teeth. It sheared with bony blades that sharpened themselves by grinding against one another. Its skull worked as a four-bar linkage. A four-bar linkage is a mechanism of four rigid elements joined at four pivots, in which moving one element determines the motion of all the others. A single muscular action therefore opened the mouth both widely and rapidly. Many living fishes have the same arrangement of the skull.

Why it matters

Their jaws indicate that placoderms are the first vertebrates equipped to capture and cut up prey capable of evading them.

Dating & uncertainty

Placoderms lived through the whole Devonian. The large predators lived in the second half of the Devonian and disappeared before its end. Researchers estimated the bite force of these predators at over 5,000 newtons, units of force. This estimate rests on a model, and the muscle attachments of the model have since been reconstructed differently. The value is under revision.

Earth history as one day

21:58:502 h 1 min before midnight

if Earth’s 4.54 billion years were compressed into a single day