Dinichthys herzeri is an extinct, giant, marine arthrodire placoderm from the Late Devonian of Ohio and Tennessee. For much of the 20th century, many unrelated large arthrodires were classified together within this genus, including species now assigned to Dunkleosteus, Eastmanosteus, and Titanichthys. Today, Dinichthys is considered a monotypic genus, containing only the type species, D. herzeri. Devonian is the fourth period of the Paleozoic Era, encompassing an interval of geologic time between 418 and 362 million years before present based on radiometric data. The great diversification of fish in the Devonian has led to the term "Age of Fishes" for the period. The first ray-finned and lobe-finned bony fish appeared, while the placoderms began dominating almost every known aquatic environment. In the oceans, primitive sharks became more numerous than in the Silurian and the late Ordovician. The first ammonite mollusks appeared. Trilobites, the mollusk-like brachiopods and the great coral reefs, were still common. The Late Devonian extinction which started about 375 million years ago, severely affected marine life, killing off all placoderms, and all trilobites, save for a few species of the order Proetida. No artist was credited in the caption.