The Silurian period spans 443.7 million years to 416 million years ago. Sir Roderick Impey Murchison (1792-1871), a Scottish geologist, worked to map the greywacke rocks underlying the Old Red Sandstone in Wales. Many of these layers contained fossils very different from those of the Cambrian. Murchison published his work as The Silurian System in 1839. This period was named for the Silures, a Celtic tribe living in Whales during the Roman conquest (USGS).
Primary Producers & Reefs
Evidence at the Ordovician/Silurian boundary indicates glaciation, lowering of sea levels and decreased temperatures. At the beginning of the Silurian reefs were virtually absent and the deposition of marine limestones were reduced. An important warm water calcareous alga also disappeared (Stanley, 1987, p. 75). Most phyla avoided extinction and their surviving representatives would expand to create new ecosystems during the Silurian period. These adaptive radiations would not only occur in the marine environments but also now on the land.
The dominant primary producers in the oceans continue to be cyanobacteria, green and red algae (Knoll, Summons, Waldbauer, and Zumberge, 2007, p. 148). As the Silurian unfolded, reef construction would be re-established. In some locations, reefs made primarily of tabulate corals make their first appearance in the Silurian (Kiessling, 2001, p. 44). Overall, stromatoporoids and tabulate corals would be the primary builders of reef systems in the Silurian. Rugose corals and bryozoans would play only minor roles in reef building (Stanley, 1987, p. 75). In some environments algae and stromatolites may have been important reef builders (Webb, 2001, p. 175).
Crinoids, and brachiopods continue to spread and diversify. Trilobites, graptolites, stromatolites, and mollusks continue to be present in large numbers. Sponges, corals, and bryozoans play important roles in reef building. Calymene celebra is a bottom-feeding trilobite that lived during the Silurian and is the state fossil for Wisconsin.Eurypterids became major predators in shallow marine/estuary environments during the Silurian (Johnson and Stucky, 1995, p. 36). The eurypterid is a chelicerate arthropod and looks like a cross between a scorpion and lobster.
The Bertie Waterlime is a fossil lagerstatten that provides excellent examples of eurypterids and other rare arthropod fossils. Dolostones in the Bertie Group can be used to make cement that cures while under water, thus giving it the name waterlime. Fossils are collected from formations associated with the Bertie and Roundabout groups throughout New York and into Ontario, Canada. The Bertie Waterlime eurypterid containing dolostones represent a hypersaline lagoon. The majority of eurypterids found represent molts. Several species of eurypterids are found in these deposits. Eurypterus remipes is the state fossil for New York. A variety of marine life is preserved in the Bertie Waterlime dolostones. Primitive horseshoe crabs, aquatic scorpians, phyllocarid crustaceans, trilobite-like arthropods, gastropods, orthocone cephalopods, brachiopods, and stromatolites have been collected. A few examples of the land plant Cooksonia have also been recovered. A variety of eurypterid species have been found in deposits around the word representing a time span of 100 million years (Ordovician to Permian). One species of eurypterid reached a length of 2 meters, making it the largest known arthropod to have existed on Earth (Nudds & Selden, 2008, pp. 73-92).
Fish continue to diversify into new forms. Freshwater fish and fish with jaws make their first appearance during the Silurian (Fisher, 1998).
A new group of Ostracoderms (jawless fish), the cephalaspids appear. Cephalaspids were more advanced than heterostracans. Cephalaspids or osteostracans (order Osteostraci) had a head shield made of a single bone, which did not grow during the life of the fish. Osteostracans had scale-covered extensions that acted as pectoral fins, which improved their swimming ability. They also had a thin layer of bone over their cartilage along with sensory organs on the sides and tops of their heads.
The first vertebrates to evolve jaws are the acanthodians or spiny sharks. Although these fish have a shark-shaped body with paired fins and an upturned tail, they are not sharks. There is good reason to believe that the jaws of acanthodians evolved from the first gill arch found in their ancestral jawless fish (Dixon, D., 1988, p 32). Acanthodians had skeletons made of cartilage, close fitting scales in their skin made of bone, bone protecting their head and girdle, and fins with a broad bony base that extended along the anterior edge as a dentine spine. All of the fins had a reinforcing spine except for the tail. Some acanthodians developed a bony operculum covering their gill openings. Acanthodians lacked teeth, but instead had gill-rakers used for suspension feeding. Acanthodians first appear in the Silurian as marine fish, but evolve into many species including freshwater forms. The class Acanthodii goes extinct in the Permian. Placodermii, a class of armored jawed fish also makes it first appearance in the mid-Silurian; only fragments of placoderm fossils have been found in Silurian deposits.
Land Plants & Terrestrial Ecosystems
Evidence for land plants and primitive terrestrial ecosystems becomes more common in the Silurian. The first land plants belonging to the genus Cooksonia were stick-like with bifurcating stems and no leaves or roots (Kenrick & Davis, p. 22). The stems may have grown from rhizomes. The leafless, forked stems of Cooksonia stood around 3 inches tall and where photoshythetic. Wind carried spores were produced in rounded structures at the tips of stems (Jonshon & Stucky, 1995, p. 44). Cooksonia is a representative of the most primitive division of vascular plants, the Rhyniophyta. Rhyniophytes are the first plants that possessed the three evolutionary innovations for living on land, vascular tissue, cuticles and stomata (Cleal & Thomas, 2009, p. 63). Primitive land plants had no competition and attained a worldwide distribution during the Silurian. The flora at this time was low in diversity. Primitive terrestrial environments containing millipedes, centipedes, arachnids, the first fungi, bacteria, worms, and simple plants become established (Kazlev, Silurian Page). The Silurian period would unfold without any large-scale biological disturbance (Stanley, 1987, p. 75).
Cleal C.J. & Thomas,
B.A. (2009). Introduction to Plant Fossils. United Kingdom:
Cambridge University Press.
Falkowski, P.G. Knoll, A.H. (2007). Evolution of Primary Producers in the Sea. China: Elsevier Academic Press.
Fisher D., Liu T., Yip E., & Yu K. (1998). Silurian Life page in The Web Geological Time Machine by UCMP Berkley. See: http://www.ucmp.berkeley.edu/help/timeform.html
Johnson, K.R. & Stucky R.K. (1995). Prehistoric Journey: A History of Life on Earth. Boulder, Colorado: Roberts Rinehart Publishers.
Kazlev, M.A. (2002). Palaeos Website. see: http://www.palaeos.com/Timescale/default.htm
Kenrick, P. & Davis, P. (2004). Fossil Plants. Washington: Smithsonian Books.
Kiessling, W. (2001). Phanerozoic Reef Trends Based on the Paleoreef Database. In Stanley, G.D. Jr. [Ed] The History and Sedimentology of Ancient Reef Systems (41-88). New York: Kluwer Academic/Plenum Publishers.
Nudds J.R. & Selden P.A. (2008). Fossil Ecosystems of North America: A Guide to the Sites and Their Extraordinary Biotas. Chicago: The University of Chicago Press.
Stanley, S.M., (1987). Extinction. New York: Scientific American Books.
USGS Publication: Major Division of Geologic Time see: http://pubs.usgs.gov/gip/geotime/divisions.html
G.E. (2001). Biologically Induced Carbonate Precipitation in
Reefs through Time. In Stanley, G.D. Jr. [Ed] The History
and Sedimentology of Ancient Reef Systems (159-203). New York:
Kluwer Academic/Plenum Publishers.