The Devonian Period, 419 mya - 359 mya

The Devonian Period is the fourth of the six Paleozoic (542 mya - 251 mya) periods, coming after the Silurian and before the Carboniferous. The Devonian began around 419.2 million years ago (give or take a few million years), and lasted until 359 million years ago. It is divided into the older (“lower”) period, the middle period, and the later (“upper”) period. The upper period is itself divided into the Frasnian Age (from about 383 million years ago to 372 million years ago), and the final Famennian era.

The Devonian era was an important transitional period in early earth history, and during that age, Life made many important innovations. At the start of the Devonian, there were already mosses, liverworts, lichens, and the famous little leafless heart Cooksonia, which had all been developing on land through the Silurian, but by the end of the Devonian there were forests with actual woody trees with leaves, roots, and seeds. At the beginning of the Devonian, various worms, millipedes, centipedes, and perhaps some slugs were crawling around in the moss and algae mats on land, but in the forests of the late Devonian there were primitive amphibians, many different varieties of insects, and spider-like creatures (trigonotarbids) and arachnids (spiders) living among the ferns and trees. Yes, the famous movement of vertebrate animals from sea to land occurred in the Devonian (and, possibly, we have the fossil of the Devonian creature that made the transition: the Tiktaalik roseae).

The Devonian is also sometimes known as the “age of fishes” as the vertebrates in the sea diversified and multiplied. Bony fishes evolved right at the end of the Silurian or the start of the Devonian, so in the early Devonian most fish were armored with plates, and jawless fishes were still very common, but by the end of the Devonian the currently living classes and sub-classes of fishes (jawless fish, cartilaginous fish, ray-finned fish, and lobe-finned fish) as well as the various sub-classes of Osteichthyes (bony fishes) were all present and flourishing by the middle of the Devonian age. Two additional classes of fishes that had appeared in the late Ordovician and diversified in Silurian thrived in the Devonian; in fact, one of these, the placoderms, was the dominant class of fish through most of the Devonian, displacing the jawless fishes in diversity and quantity by the end of the early Devonian. Another class, the Acanthodians (fish with jaws and bones, but also some features of cartilaginous fishes) also flourished in the Devonian. The placoderms, however, died out by the end of the Devonian, and the Acanthodians declined around the same time, although they didn’t go extinct until the Permian.

Toward the end of the Devonian there was a mass die-off, or more likely a series of mass die-offs, that wiped out much of the diversity in the warm and shallow seas. The most likely explanation is major volcanic activity that changed the atmosphere and climate, subjecting the planet to unstable temperatures and eventually some sort of ice age that lowered ocean levels (exposing to the air the warm, shallow seas). Late in the Devonian there were also a series of significant meteor impacts, although their role is questioned, because they seem to have struck after most of the cataclysmic mass extinctions. The evolution of the ferns and primitive trees and the subsequent appearance of forests on the two major continents (EuroAmerica and Gondwanaland) may have also sequestered much of the carbon dioxide in the atmosphere, further contributing to climate changes that triggered ice ages that caused a mass extinction for marine species that thrived in warmer, shallow seas.

Crinoids like this fossil one were prevalent in the Devonian and later eras.

Some models of placoderms and a cast of a placoderm fossil, all fishes of the Devonian.

A very-well preserved fish from the Miguasha estuary (Escuminac Formation) in Quebec, 380 mya.

Fish of the Escuminac Formation about 380 million years ago (mya) in the Frasnian age of the Devonian.

A model of a Devonian reef of the sort that would have been near what is now Chicago.

A model placoderm of the lower (earlier) Devonian, a jawless Drepanaspis gemuendenensis.

Many of larger natural history museums display the predatory placoderm Dunkleosteus.

A well-preserved 380-million-year-old fossil lobe fish of the Devonian at Miguasha National Park.

Placoderm fish armored head and fins (Bothriolepis canadensis). From Quebec, Canada. This fish may very well have climbed up into the mud and moved around on land a bit (it seems to have had lungs).

This is an ostracoderm, a fish with a shell. This particular jawless fish with a shell is Protaspis transversa, which lived in the Devonian in seas that covered what is now Wyoming.

This is a Pteraspis from the early Devonian, about 405 million years ago (mya). Its spike nose is odd.

Links about the Devonian Period:

1. There is an entire website devoted to the Devonian era known as the Devonian Times.

2. The University of California at Berkeley has a good summary of the Devonian.

3. Miguasha National Park in Quebec has a rich fossil bed from the Devonian, and their website offers a fine overview of the era.

4. The Falls of the Ohio State Park in Indiana (across the Ohio River from Louisville, Kentucky) has an immense bed of Devonian fossils you can explore (when the Ohio River isn’t in flood stage).

This work by Eric & Chun-Chih Hadley-Ives is licensed under a

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