The Devonian Period (416-360 Million Years Ago)

Prehistoric Life During the Devonian Period

acanthostega
Acanthostega was one of the first tetrapods of the Devonian period (Gunter Bechly).

From a human perspective, the Devonian period was a crucial time for the evolution of vertebrate life: this was the period in geological history when the first 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 average ocean temperatures of "only" 80 to 85 degrees Fahrenheit (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 gradually merged to form Euramerica, while the giant Gondwana (which was destined to break apart millions of years later 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 evolutionary event in the history of life 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 marine 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 employ 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 colonized dry land. There was 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 to gradually spread inland (though still not too far away from bodies of water). During this time, though, the vast bulk of life on earth lived deep in the water.

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 populous family of fish on earth today. Relatively small 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 (invertebrate sea scorpions) successfully competed with vertebrate sharks for prey.

Plant Life During the Devonian Period

It was during the Devonian period that the temperate regions of the earth's evolving continents first 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 in harvesting energy 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 helped to counteract the force of gravity.

The End-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 by the End-Devonian Extinction: reef-dwelling placoderms and trilobites were especially vulnerable, but deep-sea 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