From a human perspective, the Devonian period was a crucial time for the evolution of life: this was when the first vertebrate tetrapods climbed out of the primordial seas and began to colonize dry land. The Devonian occupied the middle part of the Paleozoic Era (542-250 million years ago), preceded by the Cambrian, Ordovician and Silurian periods and followed by the Carboniferous and Permian periods.
Climate and geography. The global climate during the Devonian period was surprisingly mild, with ocean temperatures of only 80 to 85 degrees (compared to as high as 120 degrees during the preceding Ordovician and Silurian periods). The North and South Poles were only marginally cooler than the areas closer to the equator, and there were no ice caps; the only glaciers were to be found atop high mountain ranges. The smallish continents of Laurentia and Baltica merged to form Euramerica, while the giant Gondwana (which was destined to break apart into Africa, South America, Antarctica and Australia) continued its slow southward drift.
Terrestrial Life During the Devonian Period
Vertebrates. It was during the Devonian period that the archetypal event of evolution took place: the adaptation of lobe-finned fish to life on dry land. The two best candidates for the earliest tetrapods (four-footed vertebrates) are Acanthostega and Ichthyostega, which themselves evolved from earlier, exclusively aquatic vertebrates like Tiktaalik and Panderichthys. Surprisingly, many of these early tetrapods possessed seven or eight digits on each of their feet, meaning they represented "dead ends" in evolution--since all terrestrial vertebrates on earth today use the five-finger, five-toe body plan.
Invertebrates. Although tetrapods were certainly the biggest news of the Devonian period, they weren't the only animals that lived on dry land. There were also a wide array of small arthropods, worms, flightless insects and other pesky invertebrates, which took advantage of the complex terrestrial plant ecosystems that started to develop at this time.
Marine Life During the Devonian Period
The Devonian period marked both the apex and the extinction of the placoderms, prehistoric fish characterized by their tough armor plating (some placoderms, such as the enormous Dunkleosteus, attained weights of three or four tons). As noted above, the Devonian also teemed with lobe-finned fish, from which the first tetrapods evolved, as well as relatively new ray-finned fish, the most familiar type of fish on earth today. Sharks--such as the bizarrely ornamented Stethacanthus and the weirdly scaleless Cladoselache--were an increasingly common sight in the Devonian seas. Invertebrates like sponges and corals continued to flourish, but the ranks of the trilobites were thinned out, and only the giant eurypterids (sea scorpions) successfully competed with vertebrate sharks.
Plant Life During the Devonian Period
It was during the Devonian period that the temperate regions of the earth's evolving continents became truly green. The Devonian witnessed the first significant jungles and forests, the spread of which was aided by the evolutionary competition among plants to gather as much sunlight as possible (in a dense forest canopy, a tall tree has a significant advantage over a tiny shrub). The trees of the late Devonian period were the first to evolve rudimentary bark (to support their weight and protect their trunks), as well as robust internal water-conduction mechanisms that counteracted the force of gravity.
The Devonian Extinction
The end of the Devonian period ushered in the second great extinction of prehistoric life on earth, the first being the mass extinction event at the end of the Ordovician period. Not all animal groups were affected equally: reef-dwelling placoderms and trilobites were especially vulnerable, but deep-ocean organisms escaped relatively unscathed. The evidence is sketchy, but many paleontologists believe that the Devonian extinction was caused by multiple meteor impacts, debris from which may have poisoned the surfaces of lakes, oceans and rivers.
Next: the Carboniferous Period