Devonian Period

Devonian Period, fourth division of the Paleozoic Era of geologic time, spanning an interval from about 416 million to 359 million years before present, and named for Devonshire, England, where the sedimentary rocks of that period were first studied in the 1830s. The Devonian is preceded by the Silurian Period and followed by the Carboniferous Period.

The ancient landmasses of North America and Europe straddled the equator at this time, while the African and South American portions of Gondwanaland, the supercontinent that would eventually form the modern southern continents, were centered over the South Pole. The principal geologic event of the Devonian Period was a three-way collision between the landmasses of North America, Eurasia, and Gondwanaland. Compression generated by this collision made mountains by folding the thick sections of sedimentary strata that had accumulated in troughs of crustal weakness called geosynclines. See also Plate Tectonics.

This Devonian episode of mountain building is called the Acadian orogeny in North America, and the Caledonian orogeny in Britain and Norway. The eroded roots of the mountains produced by the Acadian orogeny form the northern section of today's Appalachians, extending from New England to Newfoundland and Labrador. Great volumes of coarse red sand and gravel eroded from these new Acadian mountains and deposited on the stable platform of the continental interior. The continental interior was occupied intermittently by warm, shallow seas in which reefs of coral and sponge grew. Many of these reef rocks and Devonian sandstones became locally saturated with petroleum.

Great thicknesses of the red sand accumulated in a vast Catskill delta that buried and preserved the world's first forests: tall, slender scale trees; primitive, leafy evergreens; and ferns. Iron oxide cemented the sand grains and colored them red, forming the Old Red Sandstone of the British Isles and the Catskill red beds of southeastern New York. These iron oxides, preserved forests, and coral reefs are evidence of a warm, moist climate.

Under these climatic conditions, and with a newly formed atmospheric ozone layer for protection from ultraviolet radiation, the first air-breathing arthropods—spiders and mites—appeared on dry land. In the seas, coiled shellfish called ammonoids (distantly related to modern squid) were the major form of invertebrate life. Devonian fish, considerably evolved from the armor-plated species of the preceding Ordovician Period, now had fins, scales, and jaws. One group, the lobe-fins, developed into the first air-breathing vertebrates, the amphibians, which invaded the land at the end of the Devonian Period and set the stage for the advent of the reptiles in the Carboniferous Period that followed.

In 2006 paleontologists reported the discovery of an intermediate link between fish and land animals that lived during the Devonian Period. Several well-preserved fossil specimens from Devonian rock on Ellesmere Island in the Canadian territory of Nunavut were clearly derived from a group that included ancient lungfish. Scientists said the animals ranged from 1.2 m (4 ft) to 3 m (9 ft) in length. The scientists named the newly discovered species Tiktaalik roseae. (Tiktaalik is an Inuit word for large freshwater fish.) The fossils showed anatomical features characteristic of land animals, such as wrist and elbow bones and parts of a primitive hand embedded in the pectoral fins.

This transitional creature was found by scientists in rock dating from 375 million years ago. The scientific team, led by paleontologist Neil H. Shubin of the University of Chicago, learned in 1999 of an abundance of exposed Devonian rock on Ellesmere Island that had never been explored for fossils. In 2004 they discovered the first fossil specimens of Tiktaalik roseae. Subsequent analysis showed that the creature had a flat skull with eye sockets on top resembling the skull of a crocodile, a neck, ribs, and other features similar to four-limbed animals known as tetrapods. Tiktaalik roseae preceded amphibians, reptiles and dinosaurs, and mammals. Scientists compared the significance of its discovery to that of Archaeopteryx, the feathered dinosaur that was transitional between reptiles and birds.