The Silurian Period is a time in Earth's history that lasted about 23.5 million years. It began at the end of the Ordovician Period, around 443.1 million years ago, and ended when the Devonian Period started, about 419.62 million years ago. The Silurian is the third and shortest period of the Paleozoic Era and the third of twelve periods in the Phanerozoic Eon. Like other geologic periods, the rock layers that mark the start and end of the Silurian are clearly identified, but the exact dates are not known precisely and may differ by a few million years. The beginning of the Silurian is linked to a series of major extinction events that occurred during the transition from the Ordovician to the Silurian, during which up to 60% of marine species disappeared.
A major event during the Silurian was the first appearance of life on land, known as the Silurian-Devonian Terrestrial Revolution. During this time, vascular plants evolved from simpler land plants, and two types of fungi—dikaryan and glomeromycotan—began to spread and become more diverse. Additionally, three groups of arthropods (myriapods, arachnids, and hexapods) fully adapted to living on land.
Another important development during the Silurian was the increase in the variety of jawed fish. These included placoderms, acanthodians (which later led to cartilaginous fish), and osteichthyans (bony fish, which are further divided into lobe-finned and ray-finned fish). At the same time, jawless fish such as conodonts and ostracoderms declined in number.
History of study
The Silurian system was first discovered by the Scottish geologist Roderick Murchison in the early 1830s. He studied fossil-rich sedimentary rock layers in south Wales and named the rock sequences after the Silures, a Celtic tribe from Wales. This name was inspired by his friend Adam Sedgwick, who had previously named the Cambrian system after a Latin word for Wales. Although the Silurian rock layers and the lands once inhabited by the Silures show little connection (as seen in the geologic map of Wales and the map of pre-Roman tribes of Wales), Murchison believed the Silures’ territory included areas like Caer Caradoc and Wenlock Edge. He also argued that even if these areas were not part of the Silures’ homeland, Silurian rocks were found elsewhere and could support the name. In 1835, Murchison and Sedgwick published a joint paper titled On the Silurian and Cambrian Systems, Exhibiting the Order in which the Older Sedimentary Strata Succeed each other in England and Wales. This work helped shape the modern geological time scale. However, as the Silurian layers were studied further, they overlapped with Sedgwick’s Cambrian layers, causing strong disagreements that ended their friendship.
The English geologist Charles Lapworth solved this conflict by creating a new system called the Ordovician, which included the disputed rock layers. Another name for the Silurian system was "Gotlandian," named after the rock layers on the Baltic island of Gotland.
The French geologist Joachim Barrande expanded on Murchison’s work and used the term "Silurian" more broadly than later studies supported. He divided Silurian rocks in Bohemia into eight stages. However, in 1854, Edward Forbes challenged Barrande’s interpretation, and later stages (F, G, and H) were later found to belong to the Devonian period. Although some of Barrande’s groupings were revised, his work helped establish Bohemia as an important location for studying the earliest Silurian fossils.
Paleogeography
During the Silurian period, the supercontinent Gondwana was positioned over the equator and much of the southern hemisphere. A large ocean covered most of the northern part of Earth. High sea levels and flat land areas with few mountains created many island chains, leading to a wide variety of environments.
Gondwana slowly moved southward toward high southern latitudes during the Silurian. Evidence shows that Silurian icecaps were smaller than those from the late-Ordovician glaciation. The southern continents remained connected during this time. Melting icecaps and glaciers caused sea levels to rise. This is seen in Silurian sediments that lie above eroded Ordovician sediments, forming an unconformity. The continents of Avalonia, Baltica, and Laurentia moved together near the equator, beginning the formation of a new supercontinent called Euramerica.
When proto-Europe collided with North America, coastal sediments that had formed since the Cambrian period along the east coast of North America and the west coast of Europe were folded. This event, known as the Caledonian orogeny, created mountain ranges stretching from New York State through Europe, Greenland, and Norway. At the end of the Silurian, sea levels dropped again, leaving evaporite basins from Michigan to West Virginia. The newly formed mountains were quickly eroded. The Teays River flowed into a shallow mid-continental sea, eroding Ordovician rocks and forming Silurian rock deposits in northern Ohio and Indiana.
The vast ocean of Panthalassa covered most of the northern hemisphere. Other smaller oceans included two phases of the Tethys—Proto-Tethys and Paleo-Tethys—the Rheic Ocean, the Iapetus Ocean (a narrow seaway between Avalonia and Laurentia), and the newly formed Ural Ocean.
Climate and sea level
The Silurian period was once thought to have had steady and warm temperatures, different from the very cold times of the Ordovician before it and the very hot times of the Devonian that followed. However, scientists now know that the Earth’s climate changed many times during the Silurian, as shown by large changes in carbon and oxygen levels recorded in rocks from this time. Sea levels rose from their lowest point during the Hirnantian stage in the early Silurian, then dropped later in the period. Smaller changes in sea levels happened throughout this time. Fifteen times, sea levels were higher than the edge of the continental shelf, and the highest level was likely about 140 meters (459 feet) above the lowest level.
During the Silurian, Earth’s climate became warm, with high levels of carbon dioxide (about 4,500 ppm), leading to warm, shallow seas covering much of the land near the equator. Early in the Silurian, glaciers moved back toward the South Pole and nearly disappeared by the middle of the period. Layers of broken shells, called coquina, show that strong storms, similar to those today, were common due to warm ocean surfaces.
The Silurian climate and carbon cycle were unstable, with more frequent changes in carbon and oxygen levels than in any other period. Events like the Ireviken, Mulde, and Lau events each show sudden changes in these levels, following small mass extinctions and linked to quick changes in sea levels. These events left similar marks in the geological record, both in chemical and biological evidence. Free-swimming animals, brachiopods, corals, and trilobites were especially affected. Extinctions usually happened in short, rapid bursts. While climate changes are often linked to repeated glaciations, the lack of glacial deposits called tillites in the middle to late Silurian makes this explanation unclear.
Flora and fauna
The Silurian period is often seen as a time of recovery after the Late Ordovician mass extinction (LOME), which stopped the steady increase in the variety of life that had been happening since the Cambrian and much of the Ordovician periods.
During the Silurian, the first large fossils of plants and other life on land appeared. These included moss-like small forests near lakes and streams, as well as networks of large plants called mycorrhizal nematophytes. This marked the start of the Silurian-Devonian Terrestrial Revolution. However, animals on land did not have a major effect on Earth until they became more diverse in the Devonian period.
The first fossils of vascular plants, which are land plants with special tissues to move water and food, appeared in the second half of the Silurian period. The earliest known examples are Cooksonia. Most fossils of Cooksonia were found in marine sediments, and they likely lived near rivers and streams. Another early plant, Baragwanathia, is about the same age and lived around 420 million years ago. It had branching stems and needle-like leaves that were 10–20 centimeters long. Fossils of Baragwanathia have been found in Australia, Canada, and China. Eohostimella heathana is another early plant from the Silurian, found in compression fossils. Its chemical makeup is similar to vascular plants, not algae.
Fossils of animals that lived on land are also found in Silurian rocks. The oldest known millipede is Kampecaris obanensis and Archidesmus sp., which lived about 425 million years ago in what is now Scotland. Other millipedes, centipedes, and early spiders were found from around 420 million years ago. These fossils suggest that simple food webs existed, with non-predatory animals as prey. Researchers have suggested that early food webs included animals that ate dead matter or microscopic organisms. Some millipedes, like Cowiedesmus and Pneumodesmus, were thought to be from the middle Silurian, but later studies suggest they may be from the Devonian period. Regardless, Pneumodesmus is important because it shows the oldest known evidence of openings for breathing air.
The first bony fish, called Osteichthyes, appeared during the Silurian. These fish had bony scales and developed movable jaws from the supports of their gill arches. Sea scorpions, called eurypterids, were common in Silurian seas and lakes in North America, with many fossils found in New York. Brachiopods, a type of marine animal, were abundant and diverse during the Silurian, similar to those in the Ordovician period. After the LOME, surviving brachiopods adapted to environmental stress and were often found on specific ancient landmasses, but later expanded their range. Atrypids and pentamerides were the most common brachiopods. Bryozoans, a group of colonial animals, lived in specific areas and formed relationships with corals and other organisms. Many bivalve fossils, including the first deep-boring bivalves, are found in Silurian rocks. Chitons, a type of shellfish, were most diverse during the middle Silurian. Hederelloids, a group of marine organisms, thrived in the Silurian and sometimes lived with coral. Tentaculitoids, a group of marine animals, diversified mainly in Baltoscandia and expanded their range during the Silurian. Trilobites, which were common in the Ordovician, began to recover after the LOME and remained successful in the Silurian. Crinoids, a type of marine animal, experienced changes in diversity during the Early Silurian. Flexibilia, a group of crinoids, became more important in Silurian seas. Monobathrid camerates diversified in the Llandovery, while other groups like cyathocrinids and dendrocrinids diversified later. Scyphocrinoid loboliths suddenly appeared near the end of the Silurian and disappeared quickly. Corals and stromatoporoids often had symbiotic relationships with other organisms. Rugose corals were covered by a variety of epibionts, including hederelloids. Photosymbiotic scleractinians, a type of coral, first appeared in the Middle Silurian. Reef fossils were sometimes common but often missing from the rock record.