The Silurian is a geologic period and system spanning 24.6 million years from the end of the Ordovician Period, at 443.8 million years ago (Mya), to the beginning of the Devonian Period, 419.2 Mya. The Silurian is the shortest period of the Paleozoic Era. As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by a few million years. The base of the Silurian is set at a series of major Ordovician–Silurian extinction events when up to 60% of marine genera were wiped out.
Tentaculites is an extinct genus of conical fossils of uncertain affinity, class Tentaculita, although it is not the only member of the class. It is known from Lower Ordovician to Upper Devonian deposits both as calcitic shells with a brachiopod-like microstructure and carbonaceous 'linings'. The "tentaculites" are also referred to as the styliolinids.
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.
Tabulata, commonly known as tabulate corals, are an order of extinct forms of coral. They are almost always colonial, forming colonies of individual hexagonal cells known as corallites defined by a skeleton of calcite, similar in appearance to a honeycomb. Adjacent cells are joined by small pores. Their distinguishing feature is their well-developed horizontal internal partitions (tabulae) within each cell, but reduced or absent vertical internal partitions. They are usually smaller than rugose corals, but vary considerably in shape, from flat to conical to spherical.
Sabellidae, or feather duster worms, are a family of marine polychaete tube worms characterized by protruding feathery branchiae. Sabellids build tubes out of a tough, parchment-like exudate, strengthened with sand and bits of shell. Unlike the other sabellids, the genus Glomerula secretes a tube of calcium carbonate instead. Sabellidae can be found in subtidal habitats around the world. Their oldest fossils are known from the Early Jurassic.
Carbonate hardgrounds are surfaces of synsedimentarily cemented carbonate layers that have been exposed on the seafloor. A hardground is essentially, then, a lithified seafloor. Ancient hardgrounds are found in limestone sequences and distinguished from later-lithified sediments by evidence of exposure to normal marine waters. This evidence can consist of encrusting marine organisms, borings of organisms produced through bioerosion, early marine calcite cements, or extensive surfaces mineralized by iron oxides or calcium phosphates. Modern hardgrounds are usually detected by sounding in shallow water or through remote sensing techniques like side-scan sonar.
Cornulitida is an extinct order of encrusting animals from class Tentaculita, which were common around the globe in the Ordovician to Devonian oceans, and survived until the Carboniferous. Organisms that may be the oldest cornulitids have been found in Cambrian sediments of Jordan.
Conichnus is an ichnogenus of trace fossil.
Gastrochaenolites is a trace fossil formed as a clavate (club-shaped) boring in a hard substrate such as a shell, rock or carbonate hardground. The aperture of the boring is narrower than the main chamber and may be circular, oval, or dumb-bell shaped. Gastrochaenolites is most commonly attributed to bioeroding bivalves such as Lithophaga and Gastrochaena. The fossil ranges from the Ordovician to the Recent. The first Lower Jurassic Gastrochaenolites ichnospecies is Gastrochaenolites messisbugi Bassi, Posenato, Nebelsick, 2017. This is the first record of boreholes and their producers in one of the larger bivalves of the globally occurring Lithiotis fauna which is a unique facies in the Lower Jurassic Tethys and Panthalassa.
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.
Tentaculita is an extinct class of uncertain placement ranging from the Early Ordovician to the Middle Jurassic. They were suspension feeders with a near worldwide distribution. For a more thorough discussion, see Tentaculites.
Trypanoporida is an extinct order of encrusting animals within Class Tentaculita, which were common in Devonian oceans. Their affinity is unknown; they have been placed among worms and corals. They appear to be closely related to other taxa of uncertain affinity, including the microconchids, cornulitids, and tentaculitids. Spirally coiled trypanoporids (Devonian) were most likely derived from the geologically older microconchids.
Anticalyptraea is a fossil genus of encrusting tentaculitoid tubeworms from the Silurian to Devonian of Europe and North America . Anticalyptraea commonly encrust various invertebrate fossils such as stromatoporoids, rugose corals, bryozoans, brachiopods and crinoids, but they can also be common on the hardgrounds.
Chaetosalpinx is an ichnogenus of bioclaustrations. Chaetosalpinx includes straight to sinuous cavities that are parallel to the host's axis of growth. The cavity is circular to oval in cross-section and it lacks a wall lining or floor-like tabulae. They are common in tabulate and rugose corals from Late Ordovician to Devonian of Europe and North America. They may have been parasites.
Biomineralising polychaetes are polychaetes that produce minerals to harden or stiffen their own tissues (biomineralize).
Cornulites is a genus of cornulitid tubeworms. Their shells have vesicular wall structure, and are both externally and internally annulated. They usually occur as encrusters on various shelly fossils. Their fossils are known from the Middle Ordovician to the Carboniferous.
Oikobesalon is an ichnogenus of unbranched, elongate burrows (a type of trace fossil) in originally soft substrate. The burrows are unbranched and straight, single-entrance with circular to elliptical cross-section. They are covered with thin mineralized lining. The burrow lining has a transverse ornamentation in the form of fusiform annulation. The earliest Oikobesalon traces are known from the Cambrian.
Tymbochoos is an extinct genus of encrusting tentaculitoid tubeworms. Tymbochoos has a laminar tube structure and pseudopuncta similar to those of the tentaculitoids. It has previously been interpreted as a Palaeozoic polychaete. The world's oldest build-ups with tube-supported frameworks belong to Tymbochoos sinclairi. They occur in the Ordovician limestones of the Ottawa Valley.
Tremichnus is an ichnogenus or trace fossil. It is an embedment structure formed by an organism that inhibited growth of the crinoid host stereom. The most common endobiotic symbiont in Paleozoic crinoids is Tremichnus
Olev Vinn is Estonian paleobiologist and paleontologist.
