Devonian

The Devonian is a major geologic period within the Paleozoic era. It spans 60.7 million years of Earth's history. This period began 419.2 million years ago at the end of the Silurian. It concluded 358.5 million years ago with the start of the Carboniferous. The Devonian is the fourth period of both the Paleozoic and Phanerozoic eons. Scientists named this system after Devon, a county in South West England. This is where researchers first studied rocks from this era.

During this time, life underwent a massive transformation on land. This process is called the Silurian-Devonian Terrestrial Revolution. Initially, free-sporing land plants, or pteridophytes, began to spread across dry land. These plants eventually formed massive coal forests that covered entire continents. By the middle of the period, many vascular plants evolved true roots and leaves. Toward the end of the Devonian, the first seed-bearing plants, known as pteridospermatophytes, appeared.

Small animals also moved onto land during this revolution. Arthropods like myriapods, arachnids, and hexapods became well-established early on.

In the oceans, the Devonian is often called the "Age of Fishes." This name reflects the incredible diversity of fish that evolved. Armored fish called placoderms dominated almost every aquatic environment. Meanwhile, cartilaginous fishes like primitive sharks became more numerous than in previous periods. A very important group called tetrapodomorphs also began to diverge. These were lobe-finned fish with robust, muscled pectoral and pelvic fins. Over time, these fins gradually evolved into forelimbs and hindlimbs. These fish were the ancestors of all four-limbed vertebrates, or tetrapods.

Marine ecosystems were complex and filled with various life forms. The first ammonites appeared, which are a subclass of cephalopod molluscs. Trilobites, brachiopods, and massive coral reefs remained very common. However, the period ended with two devastating extinction events. The Late Devonian extinction began approximately 375 million years ago. It severely affected marine life by killing most reef systems and jawless fishes. It also wiped out the placoderms and nearly all trilobites, except for the order Proetida.

A second extinction occurred around 359 million years ago. This event completed the destruction of calcite sponge reefs and all placoderms.

The geography of the Devonian world was dominated by large landmasses. The supercontinent Gondwana sat in the far south. It included modern South America, Africa, Australia, Antarctica, and India. To the north was the small continent of Siberia. A medium-sized continent called Laurussia sat to the east. Laurussia formed from the collision of Laurentia and Baltica. This collision caused major mountain-building events called orogenies. In North America, the Acadian Orogeny helped build the Appalachian Mountains. In Europe, the Variscan Orogeny was also beginning.

Climate during the Devonian was generally warm but varied by region. Some evidence suggests significant glaciers may have existed during the Early and Middle Devonian. In the Early Devonian, many areas like Siberia and North America were quite arid. However, Africa and South America experienced a warm temperate climate. The average surface temperature in the Early Devonian was about 16 °C. As new forests grew, they drew carbon dioxide out of the atmosphere. This process may have caused a cooling of about 5 °C during the Mid-Devonian.

The movement of continents and changing climates shaped the entire planet. Tectonic activity caused the Rheic Ocean to close. It also caused the separation of South China from Gondwana. This movement helped expand the Paleo-Tethys Ocean. These massive changes set the stage for the later formation of Pangaea. The Devonian shows how life, land, and atmosphere are all connected. The evolution of plants and fish changed the very chemistry of the Earth. This period remains a vital chapter in the story of our living world.