Devonian Period

Devonian Period, fourth division of the Palaeozoic Era of the geological timescale, spanning an interval from about 422 to 359 million years ago, and named after Devon, England, where the sedimentary rocks of that period were first studied in the 1830s. It is preceded by the Silurian Period and succeeded by the Carboniferous Period.

The main geological event of the Devonian Period was the gradual closure of the Rheic Ocean, which had separated the vast southern supercontinent Gondwana (comprising present-day Africa, South America, India, Australia, and Antarctica) from the somewhat smaller equatorial continent Laurussia (comprising North America and Europe). By the end of the Devonian this seaway had closed, allowing land animals and plants to move freely between Gondwana and Laurussia. Meanwhile, the mountain ranges that had been created when North America collided with Europe to form Laurussia, during the preceding Silurian Period, were eroding rapidly and shedding large amounts of reddish sands and gravels on to the lowlands and margins of the continent. Today, these sediments are known collectively as Old Red Sandstone, and can be seen for example in many parts of Scotland and in the Baltic States. The eroded roots of the mountains still survive, in the form of the northern Appalachians and the mountain ranges of Scotland and Norway. The climate of the Devonian was warm, with no evidence of extensive glaciation.

Devonian marine invertebrate life did not differ greatly from that of the preceding Silurian Period. Brachiopods (lampshells), corals, and crinoids (sea lilies) remained abundant, while the trilobites slowly declined in numbers and diversity. Ammonoids, coiled shellfish related to the modern squid and octopus, became very abundant and diverse. However, the most important evolutionary events of the Devonian were the rapid diversification of the vertebrates (backboned animals) and the emergence of complex ecosystems on land.

At the beginning of the Devonian, the most advanced land plants were only some 10 to 20 cm (4 to 8 in) tall, and most of them were very simple forms consisting of branching stems without leaves. By the middle of the Devonian, plants had become far more diverse and complex, and also much larger, reaching several metres in height. Primitive clubmosses (lycopods), ferns, horsetails and ancestral seed plants (progymnosperms) had appeared. The evolution of woody stems in club mosses and progymnosperms led rapidly to the appearance of the first real trees, and thus forests. By the end of the Devonian true seed plants (gymnosperms) had evolved from the progymnosperms, and complex, extensive forest ecosystems had come into existence. There were as yet no winged insects, but scorpions, centipedes, and many other arthropods had moved into this new environment.

Jawless armoured fishes (ostracoderms) were still prominent during the Early Devonian, as they had been during the Silurian, but the jawed fishes were diversifying rapidly and had completely eclipsed the ostracoderms before the end of the period. Some of these jawed fishes belonged to the archaic, now extinct, groups called placoderms and acanthodians, but others represented the beginnings of modern lineages—sharks, ray-finned fishes, lungfishes, and coelacanths. The lungfishes and coelacanths belong to a group called sarcopterygians or lobe-finned fishes, which were much more prominent during the Devonian than today. During the Late Devonian, one sarcopterygian lineage (the “osteolepiforms”) gave rise to the first land vertebrates. The best-known examples of these are Ichthyostega and Acanthostega, both fossils from Late Devonian Old Red Sandstone deposits in Greenland. They combine traits of land animals (limbs, ribcage, pelvis attached to backbone) and fishes (gills, tail fins) with some unique features (feet with seven or eight toes, rather than five), and are probably very close to the common ancestry of all later amphibians, reptiles, birds, and mammals.