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Devonian Period—419.2 to 358.9 MYA

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fossil brachiopod
Devonian age fossil brachiopod, Death Valley National Park, California and Nevada.

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Introduction

Before their Cambrian/Silurian naming controversy, Adam Sedgwick and Roderick Impey Murchison jointly named the Devonian System for the rocks of Devonshire in southern England, having done the actual work prior to their misunderstanding. Actually Devonshire is a poor type area because the rocks are intensely deformed and the base of the system is not exposed. Nevertheless, the rocks are fossiliferous, and their distinctive faunas—intermediate between those of the Silurian and the Carboniferous—led to their identification as the Devonian. Murchison and Sedgwick showed that fossils could be used to recognize the Devonian System in the Rhineland in Germany, where this age of rocks is much better exposed and much more fossiliferous (Eicher 1976).

Significant Devonian events

Along with the Silurian, the Devonian Period is called the “Age of Fishes.” However, plant, invertebrate, and other vertebrate lifeforms also experienced major changes in the Devonian. For instance, land plants began to show great diversification. Additionally, the first forests appeared, and the first seeds and soils developed, allowing plants to reproduce on dry land. The oldest fossils of insects and spiders date to this time. Early tetrapods, ancestors of land-dwelling vertebrates, first appeared during the Devonian. The Devonian ended with a mass extinction.

Learn more about events in the Devonian

The marine group showing the most diversification during the Devonian was fish. Two prominent groups were agnathans (jawless fish) and placoderms (the first jawed fish). Some placoderms reached very large sizes; for example, paleontologists have measured fossils of Dunkleosteus (“terror fish”) at 26 feet (8 m) in length with a skull of 4 feet (1.2 m) across. This animal did not have teeth, but sharp bone on the top surface of the jaws made it a fearsome predator. Other groups of fish that appeared in the Devonian were sharks, lobed-finned fish (e.g., coelacanths and lungfish), and the ancestors of the ray-finned fish, the most abundant type of fish today (e.g., trout, salmon, and tuna).
As plants moved out of their watery environments onto continents, they needed stronger structures to support their own weight in air, a much less dense fluid than water and therefore less supportive. This and the need to conserve water favored species with thick stems and eventually wood. The evolution of a system to transport water and food through these stems—the so-called vascular system—became necessary for survival.

At the beginning of the Devonian, terrestrial plants were generally small (an inch or so tall) and did not have roots, seeds, leaves, or woody tissue. Plant height was restricted because the organisms did not have tissues capable of dealing with the stresses associated with extensive vertical growth. The development of roots, seeds, leaves, and woody tissues provided the means for species diversification, growth into large trees, and finally the development of the first forests.

Roots improved water and nutrient absorption and the ability of plants to “anchor” into soil, which also appeared during the Devonian. Soil as we know it, with a high content of organic matter from decaying vegetation (and other organisms), developed as plants colonized the continents. Seeds allowed dissemination over longer distances and protected reproductive tissue during dry periods. Leaves provided more photosynthetic surface area for more efficient food production. Finally, the development of woody tissues (and efficient vascular systems) provided a mechanism to allow significant vertical growth.
Insects appeared in the fossil record not too long (geologically speaking) after the early land plants developed (Ordovician) but before amphibians began to populate the continents (Mississippian). Some paleontologists consider collembolans (springtails), whose fossils have been found in Devonian rocks, to be the oldest known insects.

The earliest insects were wingless. The habits and habitats of insects were widely diversified even in the Paleozoic, and their evolution must have been closely intertwined with the development of terrestrial plants and animals (Macdougall 1996). Today the insects are the most numerous terrestrial animals on Earth, both in terms of number of species and individuals.
The Devonian ended with a mass extinction, during which 22% of all marine families disappeared. Little is known about the extinction of land organisms at the time. Although various investigators have suggested causes (e.g., global cooling tied to glaciations on Gondwana or an extraterrestrial impact), neither glaciation nor the impact hypothesis is unequivocally supported by the available data. The development of forests may have also been a factor, by changing the atmosphere and releasing an overabundance of nutrients into the water.

Visit—Devonian Parks

Every park contains some slice of geologic time. Below, we highlight selected parks associated with the Devonian Period. This is not to say that a particular park has only rocks from the specified period. Rather, rocks in selected parks exemplify a certain event or preserve fossils or rocks from a certain geologic age.

More about the Paleozoic

Last updated: September 28, 2020