Why did the placoderms go extinct?

Placoderms appeared in the fossil register during the Silurian period, and all placoderms (including the nightmarish Dunkleosteus terrelli) went extinct on the Late Devonian extinction.

On the Carboniferous period there were already no placoderm fishes in the oceans.

This Paleozoic era extinction(s) is not yet fully understood.

However, the extinction of many marine life families in the Devonian/Carboniferous mass extinction, during the Frasnian/Famennian stages and specially the Kellwasser event, was due to the widespread oceanic anoxia.

Few other reliable paleontologic data is known.