The Ordovician is a geologic period and system, the second of six periods of the Paleozoic Era, and the second of twelve periods of the Phanerozoic Eon. The Ordovician spans 41.6 million years from the end of the Cambrian Period 485.4 Ma to the start of the Silurian Period 443.8 Ma.
Brown algae are a large group of multicellular algae comprising the class Phaeophyceae. They include many seaweeds located in colder waters of the Northern Hemisphere. Brown algae are the major seaweeds of the temperate and polar regions. Many brown algae, such as members of the order Fucales, commonly grow along rocky seashores. Most brown algae live in marine environments, where they play an important role both as food and as a potential habitat. For instance, Macrocystis, a kelp of the order Laminariales, may reach 60 m (200 ft) in length and forms prominent underwater kelp forests that contain a high level of biodiversity. Another example is Sargassum, which creates unique floating mats of seaweed in the tropical waters of the Sargasso Sea that serve as the habitats for many species. Some members of the class, such as kelps, are used by humans as food.
The Early Ordovician is the first epoch of the Ordovician period, corresponding to the Lower Ordovician series of the Ordovician system. It began after the Age 10 of the Furongian epoch of the Cambrian and lasted from 485.4 ± 1.9 to 470 ± 1.4 million years ago, until the Dapingian age of the Middle Ordovician. It includes Tremadocian and Floian ages.
Coralline algae are red algae in the order Corallinales. They are characterized by a thallus that is hard because of calcareous deposits contained within the cell walls. The colors of these algae are most typically pink, or some other shade of red, but some species can be purple, yellow, blue, white, or gray-green. Coralline algae play an important role in the ecology of coral reefs. Sea urchins, parrot fish, and limpets and chitons feed on coralline algae. In the temperate Mediterranean Sea, coralline algae are the main builders of a typical algal reef, the Coralligène ("coralligenous"). Many are typically encrusting and rock-like, found in marine waters all over the world. Only one species lives in freshwater. Unattached specimens may form relatively smooth compact balls to warty or fruticose thalli.
Bioerosion describes the breakdown of hard ocean substrates – and less often terrestrial substrates – by living organisms. Marine bioerosion can be caused by mollusks, polychaete worms, phoronids, sponges, crustaceans, echinoids, and fish; it can occur on coastlines, on coral reefs, and on ships; its mechanisms include biotic boring, drilling, rasping, and scraping. On dry land, bioerosion is typically performed by pioneer plants or plant-like organisms such as lichen, and mostly chemical or mechanical in nature.
Halimeda is a genus of green macroalgae. The algal body (thallus) is composed of calcified green segments. Calcium carbonate is deposited in its tissues, making it inedible to most herbivores. However one species, Halimeda tuna, was described as pleasant to eat with oil, vinegar, and salt.
Stromatoporoidea is an extinct clade of sea sponges common in the fossil record from the Middle Ordovician to the Late Devonian. They can be characterized by their densely layered calcite skeletons lacking spicules. Stromatoporoids were among the most abundant and important reef-builders of their time, living close together in flat biostromes or elevated bioherms on soft tropical carbonate platforms.
The Floian is the second stage of the Ordovician Period. It succeeds the Tremadocian with which it forms the Lower Ordovician series. It precedes the Dapingian Stage of the Middle Ordovician. The Floian extended from 477.7 to 470 million years ago. The lower boundary is defined as the first appearance of the graptolite species Tetragraptus approximatus.
The Great Ordovician Biodiversification Event (GOBE), was an evolutionary radiation of animal life throughout the Ordovician period, 40 million years after the Cambrian explosion, whereby the distinctive Cambrian fauna fizzled out to be replaced with a Paleozoic fauna rich in suspension feeder and pelagic animals.
Girvanella is a fossil thought to represent the calcified sheath of a filamentous cyanobacterium known from the Burgess Shale and other Cambrian fossil deposits. Specimens are also known from the Early Ordovician San Juan Formation, Argentina.
The order Microconchida is a group of small, spirally-coiled, encrusting fossil "worm" tubes from the class Tentaculita found from the Upper Ordovician to the Middle Jurassic (Bathonian) around the world. They have lamellar calcitic shells, usually with pseudopunctae or punctae and a bulb-like origin. Many were long misidentified as the polychaete annelid Spirorbis until studies of shell microstructure and formation showed significant differences. All pre-Cretaceous "Spirorbis" fossils are now known to be microconchids. Their classification at the phylum level is still debated. Most likely they are some form of lophophorate, a group which includes phoronids, bryozoans and brachiopods. Microconchids may be closely related to the other encrusting tentaculitoid tubeworms, such as Anticalyptraea, trypanoporids and cornulitids.
The Sporolithaceae is the only known family of algae in the Sporolithales order.
The extinct genus Hedstroemia was once thought to be a rivulariacean cyanobacterium. It forms oval blobs composed of sinuous tubes that have rounded outlines and occurs in conjunction with Solenopora.
Rothpletzella is a genus of calcimicrobe known from the Silurian of Gotland, the Devonian of France, as well as the Ordovician of China. It has been hypothesised to be a cyanobacterium, and shares morphological similarities with extant cyanobacteria. The genus is named in honor of August Rothpletz.
Wetheredella is a genus of calcimicrobes initially described from the Silurian of England, and subsequently reported from the Upper Ordovician to the end of the Carboniferous periods; its reefs are stated as being characteristic of the Ordovician-Silurian periods. Its taxonomic position is uncertain; it has been suggested to be a foraminiferan, a cyanobacterium or simply treated as a microproblematicum; Vachard & Cózar (2010) refer it to the Algospongia, a similarly controversial group that they assigned to the Protista but later, per Vachard, 2021, to Algae incertae sedis, in its own family and suborder (Wetheredellina) in the order Moravamminida. The genus is named in honor of the geologist Edward Wethered.
Sphaerocodium is a fossil that represents the remains of bacteria in the phylum Cyanobacteria, often called blue-green algae.
Proaulopora is a Cambrian–Ordovician fossil genus of calcareous algae. It has been variously thought to belong to the green algae, red algae or cyanobacteria. It was originally established by the Russian paleontologist Aleksandr Grigoryevich Vologdin in 1937, for species known from the Lower Cambrian of the western Altai Mountains.
The Winneshiek Shale is a Middle Ordovician (Darriwilian-age) geological formation in Iowa. The formation is restricted to the Decorah crater, an impact crater near Decorah, Iowa. Despite only being discovered in 2005, the Winneshiek Shale is already renowned for the exceptional preservation of its fossils. The shale preserves a unique ecosystem, the Winneshiek biota, which is among the most remarkable Ordovician lagerstätten in the United States. Fossils include the oldest known eurypterid, Pentecopterus, as well as giant conodonts such as Iowagnathus and Archeognathus.
Heliolites is a large and heterogenous genus of extinct tabulate corals in the family Heliolitidae. Specimens have been found in Ordovician to Devonian beds in North America, Europe, Africa, Asia, and Australia. The genus is particularly abundant in the Wellin Member of the Hanonet Formation of Belgium.
Cyclocrinitids are an early (mid-Ordovician–early-Silurian) Dasycladalean algae, resembling but probably not closely related to the Receptaculitids.
