The Devonian period lasted 57 million years and was characterized by diverse fish species, including the first known vertebrates with jaws (acanthodians) and heavily armored placoderms. Vascular plants enabled the first true forests and soils, and lobe-finned fishes began to develop legs towards the end of the period.
The Devonian period is a geological period extending from about 416 to 359 million years ago, with a total duration of 57 million years, slightly longer than the average for a geological division. Part of the longer Paleozoic Era, the Devonian was preceded by the Silurian and followed by the Carboniferous. Its onset is defined as the appearance of graptolite fossils called Monograptus uniformis. Graptolites were colonial marine worms related to modern acorn worms. The end of the Devonian is defined as the appearance of the conodont (eel-like vertebrate) Siphonodella sulcata.
The Devonian Period was characterized by deep seas inhabited by increasingly diverse fish species. For this reason, the Devonian is often called “The Age of Pisces”. Many families of fish evolved during this period, including the ancestors of modern bony fish, class Osteichthyes.
The most primitive jawless fish, the ostracoderms, had become extinct during the early Devonian. The first jawed fish, the acanthodians, also called spiny sharks (although they looked more like fish than modern sharks) appeared in the Late Silurian/Early Devonian and became one of the most abundant freshwater fish species for the rest of the Paleozoic. These “spiny sharks” were the first known vertebrates with jaws and are known for the stiff spines that extend from their vertebrae to support numerous fins, sometimes as many as eight. Some species of spiny sharks even had tiny spines all over their bodies. This class of fish predates modern sharks by about 50 million years. Both acanthodians and early sharks had cartilaginous skeletons.
Another fascinating group of fish that lived during the Devonian were the placoderms, fish with heavily armored heads and no modern analogues. Placoderms were the dominant vertebrate group during the Devonian Period, growing larger than other groups of fish. Placoderms were a relatively short-lived group (~50 million years) sometimes considered “an early experiment on jawed fish”. Unlike all other vertebrates with jaws, placoderms lacked true teeth. Instead, their “teeth” were just self-sharpening extensions of their jaws. The largest known placoderm, Dunkleosteus, reached a length of 6 m (20 ft), is considered to be among the earliest vertebrate apex predators.
Full colonization of land by plants and animals occurred throughout the Devonian. Vascular plants had first appeared in the late Silurian, accompanied by primitive terrestrial arthropods and other invertebrates such as molluscs. Vascular plants enabled the first true forests and soils, in stark contrast to the tiny “forests” of moss and liverworts of the Silurian. Towards the end of the Devonian, lobe-finned fishes began to develop uncertain legs, first using them to navigate swamps, then to travel on land. In just 57 million years, the earth had gone from being almost completely barren to something beginning to look like today’s world.
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