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The Devonian Period, part of the Paleozoic Era, saw the evolution of fish with legs, the emergence of insects and spiders on land, and the first true forests. Calcareous algae and coral-like stromatoporoids built large coral reefs, but a mass extinction wiped them out. The cause of the extinction is unknown.
The Devonian Period is the fourth of six geological periods that make up the Paleozoic Era, the oldest era of multicellular life on Earth, spanning from approximately 542 to 251 million years ago. The Devonian period itself spans from about 416 to 359 million years ago. The Devonian is sometimes called the “Age of Pisces” due to the abundance and diversity of fish genera that evolved during this period. Ammonites also emerged during the Devonian period. These nautilus-like organisms continued to thrive right up until the extinction of the dinosaurs just 65 million years ago.
The Devonian period was particularly important in the evolution of earthly life. Fish first developed legs and began walking on land as tetrapods, and the first insects and spiders also colonized the land. Centipede ancestors had already achieved this several tens of millions of years earlier, during the Silurian, but the Devonian period represented the first serious diversity of life on land. Fish legs evolved from muscular fins that fish would have used to propel themselves across small land bridges that separate bodies of water.
The Devonian Period also saw the first vascular carrier plants, which produced the first true forests, leading to a virtuous cycle of soil accumulation and plants exploiting that soil. These forests contained various early insects, including the earliest spiders in the fossil record, spider mites, springtails, and extinct mite-like arachnids called trigonotarbids. Trigonotarbids were among the first terrestrial predators, while the other organisms lived on leaf litter and tree sap, as evidenced by tiny punctures in well-preserved plant fossils from the Devonian period.
Calcareous algae and coral-like stromatoporoids built large coral reefs, thousands of kilometers long, around the edges of the Devonian continents, but were wiped out in a mass extinction towards the end of the period. Reef building did not pick up again for over a hundred million years later when different organisms took up this activity.
The extinctions that hit during the late Devonian period affected organisms that lived in shallow, warm waters the most, and cold-water and terrestrial organisms least. About 364 million years ago, jawless fish suddenly disappeared from the fossil record. 57% of marine genera have become extinct. Today, the reasoning behind the cause of the Devonian extinction is largely speculative, although the usual suspects have been proposed: asteroid impact, climate change, methane hydrate release, etc.
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