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113-632: Phoronids (scientific name Phoronida , sometimes called horseshoe worms ) are a small phylum of marine animals that filter-feed with a lophophore (a "crown" of tentacles), and build upright tubes of chitin to support and protect their soft bodies. They live in most of the oceans and seas, including the Arctic Ocean but excluding the Antarctic Ocean , and between the intertidal zone and about 400 meters down. Most adult phoronids are 2 cm long and about 1.5 mm wide, although

226-600: A dense rock equivalent volume of as much as 1,140 cubic kilometres (270 cu mi). Remarkably, this appears to have had little impact on life. There was vigorous tectonic activity along northwest margin of Gondwana during the Floian, 478 Ma, recorded in the Central Iberian Zone of Spain. The activity reached as far as Turkey by the end of Ordovician. The opposite margin of Gondwana, in Australia, faced

339-484: A phylum ( / ˈ f aɪ l əm / ; pl. : phyla ) is a level of classification or taxonomic rank below kingdom and above class . Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature for algae, fungi, and plants accepts the terms as equivalent. Depending on definitions, the animal kingdom Animalia contains about 31 phyla,

452-543: A sister group to the deuterostomes , a super-phylum which includes chordates and echinoderms . While a minority adhere to this view, most researchers now regard phoronids as members of the protostome super-phylum Lophotrochozoa . Although analysts using molecular phylogeny are confident that members of Lophotrochozoa are more closely related to each other than of non-members, the relationships between members are mostly unclear. Some analyses regard phoronids and brachiopods as sister-groups , while others place phoronids as

565-464: A U-shaped gut, and in 2004 Chen interpreted Eophoronis as a phoronid. However, in 2006 Conway Morris regarded Iotuba and Eophoronis as synonyms for the same genus, which in his opinion looked like the priapulid Louisella . In 2009 Balthasar and Butterfield found in western Canada two specimens from about 505 million years ago of a new fossil, Lingulosacculus nuda , which had two shells like those of brachiopods but not mineralized . In

678-603: A biogeographic affinity with Gondwana, and the Alborz margin of Gondwana was linked biogeographically to South China. Southeast Asia's fauna also maintained strong affinities to Gondwana's. North China was biogeographically connected to Laurentia and the Argentinian margin of Gondwana. A Celtic biogeographic province also existed, separate from the Laurentian and Baltican ones. However, tropical articulate brachiopods had

791-468: A burst of volcanic activity that deposited new silicate rocks, which draw CO 2 out of the air as they erode. Another possibility is that bryophytes and lichens, which colonized land in the middle to late Ordovician, may have increased weathering enough to draw down CO 2 levels. The drop in CO 2 selectively affected the shallow seas where most organisms lived. It has also been suggested that shielding of

904-556: A certain degree of morphological or developmental similarity (the phenetic definition), or a group of organisms with a certain degree of evolutionary relatedness (the phylogenetic definition). Attempting to define a level of the Linnean hierarchy without referring to (evolutionary) relatedness is unsatisfactory, but a phenetic definition is useful when addressing questions of a morphological nature—such as how successful different body plans were. The most important objective measure in

1017-407: A character unique to a sub-set of the crown group. Furthermore, organisms in the stem group of a phylum can possess the "body plan" of the phylum without all the characteristics necessary to fall within it. This weakens the idea that each of the phyla represents a distinct body plan. A classification using this definition may be strongly affected by the chance survival of rare groups, which can make

1130-590: A deep embayment between Siberia and the Central Mongolian terranes . Most of the terranes of central Asia were part of an equatorial archipelago whose geometry is poorly constrained by the available evidence. The period was one of extensive, widespread tectonism and volcanism. However, orogenesis (mountain-building) was not primarily due to continent-continent collisions. Instead, mountains arose along active continental margins during accretion of arc terranes or ribbon microcontinents. Accretion of new crust

1243-607: A distinct band around the Earth, and that the breakup of the parent body may have formed a ring system for a period of about 40 million years, with frequent falling debris causing these craters. The Ordovician was a time of calcite sea geochemistry in which low-magnesium calcite was the primary inorganic marine precipitate of calcium carbonate . Carbonate hardgrounds were thus very common, along with calcitic ooids , calcitic cements, and invertebrate faunas with dominantly calcitic skeletons. Biogenic aragonite , like that composing

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1356-464: A given landmass were severely affected. Tropical lifeforms were hit particularly hard in the first wave of extinction, while cool-water species were hit worst in the second pulse. Those species able to adapt to the changing conditions survived to fill the ecological niches left by the extinctions. For example, there is evidence the oceans became more deeply oxygenated during the glaciation, allowing unusual benthic organisms (Hirnantian fauna) to colonize

1469-402: A group ("a self-contained unity"): "perhaps such a real and completely self-contained unity is the aggregate of all species which have gradually evolved from one and the same common original form, as, for example, all vertebrates. We name this aggregate [a] Stamm [i.e., stock] ( Phylon )." In plant taxonomy , August W. Eichler (1883) classified plants into five groups named divisions,

1582-602: A group containing Viridiplantae and the algal Rhodophyta and Glaucophyta divisions. The definition and classification of plants at the division level also varies from source to source, and has changed progressively in recent years. Thus some sources place horsetails in division Arthrophyta and ferns in division Monilophyta, while others place them both in Monilophyta, as shown below. The division Pinophyta may be used for all gymnosperms (i.e. including cycads, ginkgos and gnetophytes), or for conifers alone as below. Since

1695-693: A more cosmopolitan distribution , with less diversity on different continents. During the Middle Ordovician, beta diversity began a significant decline as marine taxa began to disperse widely across space. Faunas become less provincial later in the Ordovician, partly due to the narrowing of the Iapetus Ocean, though they were still distinguishable into the late Ordovician. Trilobites in particular were rich and diverse, and experienced rapid diversification in many regions. Trilobites in

1808-728: A period known as the Ordovician meteor event . It has been theorized that this increase in impacts may originate from a ring system that formed around Earth at the time. In 2008, the ICS erected a formal international system of subdivisions for the Ordovician Period and System. Pre-existing Baltoscandic, British, Siberian, North American, Australian, Chinese, Mediterranean and North- Gondwanan regional stratigraphic schemes are also used locally. The Ordovician Period in Britain

1921-400: A phylum based on body plan has been proposed by paleontologists Graham Budd and Sören Jensen (as Haeckel had done a century earlier). The definition was posited because extinct organisms are hardest to classify: they can be offshoots that diverged from a phylum's line before the characters that define the modern phylum were all acquired. By Budd and Jensen's definition, a phylum is defined by

2034-471: A phylum much more diverse than it would be otherwise. Total numbers are estimates; figures from different authors vary wildly, not least because some are based on described species, some on extrapolations to numbers of undescribed species. For instance, around 25,000–27,000 species of nematodes have been described, while published estimates of the total number of nematode species include 10,000–20,000; 500,000; 10 million; and 100 million. The kingdom Plantae

2147-706: A phylum, other phylum-level ranks appear, such as the case of Bacillariophyta (diatoms) within Ochrophyta . These differences became irrelevant after the adoption of a cladistic approach by the ISP, where taxonomic ranks are excluded from the classifications after being considered superfluous and unstable. Many authors prefer this usage, which lead to the Chromista-Protozoa scheme becoming obsolete. Currently there are 40 bacterial phyla (not including " Cyanobacteria ") that have been validly published according to

2260-401: A set of characters shared by all its living representatives. This approach brings some small problems—for instance, ancestral characters common to most members of a phylum may have been lost by some members. Also, this definition is based on an arbitrary point of time: the present. However, as it is character based, it is easy to apply to the fossil record. A greater problem is that it relies on

2373-541: A set of island arcs. The accretion of these arcs to the eastern margin of Gondwana was responsible for the Benambran Orogeny of eastern Australia. Subduction also took place along what is now Argentina (Famatinian Orogeny) at 450 Ma. This involved significant back arc rifting. The interior of Gondwana was tectonically quiet until the Triassic . Towards the end of the period, Gondwana began to drift across

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2486-484: A significant proportion of the zooplankton biomass. Phoronis australis bores into the wall of the tube of a cerianthid anemone , Ceriantheomorphe brasiliensis , and uses this as a foundation for building its own tube. One cerianthid can house up to 100 phoronids. In this unequal relationship , the anemone experiences no significant benefits nor harm, while the phoronid benefits from: a foundation for its tube; food (both animals are filter-feeders); and protection, as

2599-535: A significant proportion of the zooplankton biomass. Predators include fish, gastropods (snails), and nematodes (tiny roundworms). One phoronid species is unpalatable to many epibenthic predators. Various parasites infest phoronids' body cavities, digestive tract and tentacles. It is unknown whether phoronids have any significance for humans. The International Union for Conservation of Nature (IUCN) has not listed any phoronid species as endangered. As of 2010 there are no indisputable body fossils of phoronids. There

2712-438: A stack of circles). The process is regulative (the fate of each cell depends on interaction with other cells, not on a rigid program in each cell), and experiments that divided early embryos produced complete larvae. Mesoderm is formed from mesenchyme originating from the archenteron . The coelom is formed by schizocoely , and the blastopore (a dent in the embryo) becomes the mouth. The slug-like larva of Phoronis ovalis ,

2825-526: A sub-group within brachiopoda. Most adult phoronids are 2 to 20 cm long and about 1.5 mm wide, although the largest are 50 cm long. Their skins have no cuticle but secrete rigid tubes of chitin , similar to the material used in arthropods ' exoskeletons , and sometimes reinforced with sediment particles and other debris. Most species' tubes are erect, but those of Phoronis vancouverensis are horizontal and tangled. Phoronids can move within their tubes but never leave them. The bottom end of

2938-401: A subjective decision about which groups of organisms should be considered as phyla. The approach is useful because it makes it easy to classify extinct organisms as " stem groups " to the phyla with which they bear the most resemblance, based only on the taxonomically important similarities. However, proving that a fossil belongs to the crown group of a phylum is difficult, as it must display

3051-424: A term that remains in use today for groups of plants, algae and fungi. The definitions of zoological phyla have changed from their origins in the six Linnaean classes and the four embranchements of Georges Cuvier . Informally, phyla can be thought of as groupings of organisms based on general specialization of body plan . At its most basic, a phylum can be defined in two ways: as a group of organisms with

3164-474: Is a paraphyletic taxon, which is less acceptable to present-day biologists than in the past. Proposals have been made to divide it among several new kingdoms, such as Protozoa and Chromista in the Cavalier-Smith system . Protist taxonomy has long been unstable, with different approaches and definitions resulting in many competing classification schemes. Many of the phyla listed below are used by

3277-405: Is defined in various ways by different biologists (see Current definitions of Plantae ). All definitions include the living embryophytes (land plants), to which may be added the two green algae divisions, Chlorophyta and Charophyta , to form the clade Viridiplantae . The table below follows the influential (though contentious) Cavalier-Smith system in equating "Plantae" with Archaeplastida ,

3390-481: Is generally included in kingdom Fungi, though its exact relations remain uncertain, and it is considered a protozoan by the International Society of Protistologists (see Protista , below). Molecular analysis of Zygomycota has found it to be polyphyletic (its members do not share an immediate ancestor), which is considered undesirable by many biologists. Accordingly, there is a proposal to abolish

3503-521: Is good evidence that phoronids created trace fossils found in the Silurian , Devonian , Permian , Jurassic and Cretaceous periods , and possibly in the Ordovician and Triassic . Phoronids, brachiopods and bryozoans (ectoprocts) have collectively been called lophophorates , because all use lophophores to feed. From about the 1940s to the 1990s, family trees based on embryological and morphological features placed lophophorates among or as

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3616-411: Is inside the base of the crown of tentacles but to one side. The gut runs from the mouth to one side of the stomach , in the bottom of the ampulla. The intestine runs from the stomach, up the other side of the body, and exits at the anus, outside and a little below the crown of tentacles. The gut and intestine are both supported by two mesenteries (partitions that run the length of the body) connected to

3729-489: Is marked by a sudden abundance of hard substrate trace fossils such as Trypanites , Palaeosabella , Petroxestes and Osprioneides . Bioerosion became an important process, particularly in the thick calcitic skeletons of corals, bryozoans and brachiopods, and on the extensive carbonate hardgrounds that appear in abundance at this time. Green algae were common in the Late Cambrian (perhaps earlier) and in

3842-690: Is strongest in the top section, including the lophophore, which is exposed to predators when phoronids feed. When the lophophores were removed in an experiment, the phoronids were more palatable, but this effect reduced over 12 days as the lophophores regenerated. These broadly effective defenses, which appear unusual among invertebrates inhabiting soft sediment, may be important in allowing Phoronopsis viridis to reach high densities. Some parasites infest phoronids: progenetic metacercariae and cysts of trematodes in phoronids' coelomic cavities; unidentified gregarines in phoronids' digestive tract; and an ancistrocomid ciliate parasite, Heterocineta , in

3955-703: The Bacteriological Code Currently there are 2 phyla that have been validly published according to the Bacteriological Code Other phyla that have been proposed, but not validly named, include: Ordovician The Ordovician, named after the Welsh tribe of the Ordovices , was defined by Charles Lapworth in 1879 to resolve a dispute between followers of Adam Sedgwick and Roderick Murchison , who were placing

4068-518: The Cambrian , reef -forming corals appeared in the early Ordovician, including the earliest known octocorals , corresponding to an increase in the stability of carbonate and thus a new abundance of calcifying animals. Brachiopods surged in diversity, adapting to almost every type of marine environment. Even after GOBE, there is evidence suggesting that Ordovician brachiopods maintained elevated rates of speciation. Molluscs , which appeared during

4181-565: The Catalogue of Life , and correspond to the Protozoa-Chromista scheme, with updates from the latest (2022) publication by Cavalier-Smith . Other phyla are used commonly by other authors, and are adapted from the system used by the International Society of Protistologists (ISP). Some of the descriptions are based on the 2019 revision of eukaryotes by the ISP. The number of protist phyla varies greatly from one classification to

4294-455: The Devonian . The first land plants are known from this period. The Great Ordovician Biodiversification Event considerably increased the diversity of life. Fish , the world's first true vertebrates , continued to evolve, and those with jaws may have first appeared late in the period. About 100 times as many meteorites struck the Earth per year during the Ordovician compared with today in

4407-766: The Rheic Ocean between Gondwana and Avalonia. Avalonia collided with Baltica towards the end of Ordovician. Other geographic features of the Ordovician world included the Tornquist Sea , which separated Avalonia from Baltica; the Aegir Ocean, which separated Baltica from Siberia; and an oceanic area between Siberia, Baltica, and Gondwana which expanded to become the Paleoasian Ocean in Carboniferous time. The Mongol-Okhotsk Ocean formed

4520-471: The endocerid cephalopods died out completely, except for possible rare Silurian forms. The Ordovician–Silurian extinction events may have been caused by an ice age that occurred at the end of the Ordovician Period, due to the expansion of the first terrestrial plants , as the end of the Late Ordovician was one of the coldest times in the last 600 million years of Earth's history. On the whole,

4633-422: The linguliform brachiopods. Another alternative is that Eccentrotheca lies somewhere in the phoronid stem lineage. There is good evidence that species of Phoronis created the trace fossils of the ichnogenus Talpina , which have been found in the Devonian , Jurassic and Cretaceous periods. The Talpina animal bored into calcareous algae , corals , echinoid tests (shells), mollusc shells and

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4746-484: The rostra of belemnites . Hederellids or Hederelloids are fossilized tubes, usually curved and between 0.1 and 1.8 mm wide, found from the Silurian to the Permian , and possibly in the Ordovician and Triassic . Their branching colonies may have been made by phoronids. Phoronids, brachiopods and bryozoans (ectoprocts) are collectively called lophophorates , because all feed using lophophores. From about

4859-432: The 1940s to the 1990s, family trees based on embryological and morphological features placed lophophorates among or as a sister group to the deuterostomes , a super-phylum that includes chordates and echinoderms . In the early development of their embryos, deuterostomes form the anus before the mouth, while protostomes form the mouth first. Nielsen (2002) views the phoronids and brachiopods as affiliated with

4972-516: The Cambrian or even the Ediacaran , became common and varied, especially bivalves , gastropods , and nautiloid cephalopods. Cephalopods diversified from shallow marine tropical environments to dominate almost all marine environments. Graptolites, which evolved in the preceding Cambrian period, thrived in the oceans. This includes the distinctive Nemagraptus gracilis graptolite fauna, which

5085-472: The Cambrian were succeeded by those that dominated the rest of the Paleozoic, such as articulate brachiopods, cephalopods , and crinoids . Articulate brachiopods, in particular, largely replaced trilobites in shelf communities. Their success epitomizes the greatly increased diversity of carbonate shell-secreting organisms in the Ordovician compared to the Cambrian. Ordovician geography had its effect on

5198-967: The Dapingian and the early Darriwilian. The Llanvirn corresponds to the late Darriwilian. The Caradoc covers the Sandbian and the first half of the Katian. The Ashgill represents the second half of the Katian, plus the Hirnantian . The Ashgill Epoch, the last epoch of the British Ordovician, is made of four ages: the Hirnantian Age, the Rawtheyan Age, the Cautleyan Age, and the Pusgillian Age. These ages make up

5311-848: The Early Eocene Climatic Optimum. Carbon dioxide levels were very high at the Ordovician period's beginning. By the late Early Ordovician, the Earth cooled, giving way to a more temperate climate in the Middle Ordovician, with the Earth likely entering the Early Palaeozoic Ice Age during the Sandbian, and possibly as early as the Darriwilian or even the Floian. The Dapingian and Sandbian saw major humidification events evidenced by trace metal concentrations in Baltoscandia from this time. Evidence suggests that global temperatures rose briefly in

5424-532: The Early Ordovician, leveling off somewhat during the middle of the period. Locally, some regressions occurred, but the sea level rise continued in the beginning of the Late Ordovician. Sea levels fell steadily due to the cooling temperatures for about 3 million years leading up to the Hirnantian glaciation. During this icy stage, sea level seems to have risen and dropped somewhat. Despite much study,

5537-559: The Great Ordovician Biodiversification Event) was no less remarkable; marine faunal genera increased fourfold, resulting in 12% of all known Phanerozoic marine fauna. Several animals also went through a miniaturization process, becoming much smaller than their Cambrian counterparts. Another change in the fauna was the strong increase in filter-feeding organisms. The trilobite, inarticulate brachiopod, archaeocyathid , and eocrinoid faunas of

5650-507: The Hirnantian glaciation. As with North America and Europe , Gondwana was largely covered with shallow seas during the Ordovician. Shallow clear waters over continental shelves encouraged the growth of organisms that deposit calcium carbonates in their shells and hard parts. The Panthalassic Ocean covered much of the Northern Hemisphere , and other minor oceans included Proto-Tethys , Paleo-Tethys , Khanty Ocean , which

5763-469: The Ordovician of Wisconsin have been found with an age of about 460 million years ago, a time when the land flora most likely only consisted of plants similar to non-vascular bryophytes . Though stromatolites had declined from their peak in the Proterozoic, they continued to exist in localised settings. The Ordovician came to a close in a series of extinction events that, taken together, comprise

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5876-421: The Ordovician were very different from their predecessors in the Cambrian. Many trilobites developed bizarre spines and nodules to defend against predators such as primitive eurypterids and nautiloids while other trilobites such as Aeglina prisca evolved to become swimming forms. Some trilobites even developed shovel-like snouts for ploughing through muddy sea bottoms. Another unusual clade of trilobites known as

5989-582: The Ordovician, when at least two volcanic island arcs collided with Laurentia to form the Appalachian Mountains . Laurentia was otherwise tectonically stable. An island arc accreted to South China during the period, while subduction along north China (Sulinheer) resulted in the emplacement of ophiolites. The ash fall of the Millburg/Big Bentonite bed, at about 454 Ma, was the largest in the last 590 million years. This had

6102-428: The Ordovician. The ice age was possibly not long-lasting. Oxygen isotopes in fossil brachiopods show its duration may have been only 0.5 to 1.5 million years. Other researchers (Page et al.) estimate more temperate conditions did not return until the late Silurian. The late Ordovician glaciation event was preceded by a fall in atmospheric carbon dioxide (from 7000 ppm to 4400 ppm). The dip may have been caused by

6215-557: The Ordovician. Terrestrial plants probably evolved from green algae, first appearing as tiny non- vascular forms resembling liverworts , in the middle to late Ordovician. Fossil spores found in Ordovician sedimentary rock are typical of bryophytes. Among the first land fungi may have been arbuscular mycorrhiza fungi ( Glomerales ), playing a crucial role in facilitating the colonization of land by plants through mycorrhizal symbiosis , which makes mineral nutrients available to plant cells; such fossilized fungal hyphae and spores from

6328-656: The Paleozoic Era by the International Geological Congress . Life continued to flourish during the Ordovician as it had in the earlier Cambrian Period, although the end of the period was marked by the Ordovician–Silurian extinction events . Invertebrates, namely molluscs and arthropods , dominated the oceans, with members of the latter group probably starting their establishment on land during this time, becoming fully established by

6441-613: The South Pole. This contributed to the Hibernian glaciation and the associated extinction event. The Ordovician meteor event is a proposed shower of meteors that occurred during the Middle Ordovician Epoch, about 467.5 ± 0.28 million years ago, due to the break-up of the L chondrite parent body. It is not associated with any major extinction event. A 2024 study found that craters from this event cluster in

6554-564: The Wuliuan, exploded in diversity during the Tremadocian, quickly becoming globally widespread. Several groups of endobiotic symbionts appeared in the Ordovician. In the Early Ordovician, trilobites were joined by many new types of organisms, including tabulate corals, strophomenid , rhynchonellid , and many new orthid brachiopods, bryozoans, planktonic graptolites and conodonts, and many types of molluscs and echinoderms, including

6667-497: The Zygomycota phylum. Its members would be divided between phylum Glomeromycota and four new subphyla incertae sedis (of uncertain placement): Entomophthoromycotina , Kickxellomycotina , Mucoromycotina , and Zoopagomycotina . Kingdom Protista (or Protoctista) is included in the traditional five- or six-kingdom model, where it can be defined as containing all eukaryotes that are not plants, animals, or fungi. Protista

6780-458: The above definitions is the "certain degree" that defines how different organisms need to be members of different phyla. The minimal requirement is that all organisms in a phylum should be clearly more closely related to one another than to any other group. Even this is problematic because the requirement depends on knowledge of organisms' relationships: as more data become available, particularly from molecular studies, we are better able to determine

6893-470: The authors' opinion, the U-shaped gut extended beyond the hinge line and outside the smaller shell. This would have precluded the attachment of muscles to close and open the shells, and the 50% of the animal's length beyond the hinge line would have needed longitudinal muscles and also a cuticle for protection. Hence they suggest that Lingulosacculus may have been a member of a phoronid stem group within

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7006-399: The blood vessels perform first-stage filtration of soluble wastes into the main coelom's fluid. Two metanephridia , each with a funnel-like intake, filter the fluid a second time, returning any useful products to the coelom and dumping the remaining wastes through a pair of nephridiopores beside the anus. There is a nervous center between the mouth and anus, and a nerve ring at the base of

7119-407: The body is an ampulla (a flask-like swelling in a tube-like structure), which anchors the animal in the tube and enables it to retract its body when threatened, reducing the body to 20 percent of its maximum length. Longitudinal muscles retract the body very quickly, while circular muscles slowly extend the body by compressing the internal fluid. For feeding and respiration each phoronid has at

7232-459: The body wall lead downward from the lophophore ring, and in most species these are combined into one a little below the lophophore ring. The downward vessel(s) leads back to the peritoneum, and also to blind branches throughout the body. There is no heart, but muscles in the major vessels contract in waves to move the blood. Unlike many animals that live in tubes, phoronids do not ventilate their trunks with oxygenated water, but rely on respiration by

7345-405: The body wall, and another mesentery connects the gut to the intestine. The body is divided into coeloms , compartments lined with mesothelium . The main body cavity, under the crown of tentacles, is called the metacoelom , and the tentacles and their base share the mesocoelom . Above the mouth is the epistome, a hollow lid which can close the mouth. The cavity in the epistome is sometimes called

7458-514: The cerianthid withdraws into its tube when danger threatens, and this alerts the phoronid to retract into its own tube. Although predators of phoronids are not well known, they include fish, gastropods (snails), and nematodes (tiny roundworms). Phoronopsis viridis , which reaches densities of 26,500 per square meter on tidal flats in California (USA), is unpalatable to many epibenthic predators, including fish and crabs. The unpalatability

7571-512: The depths. These organisms were cosmopolitan in distribution and present at most latitudes. At the end of the second event, melting glaciers caused the sea level to rise and stabilise once more. The rebound of life's diversity with the permanent re-flooding of continental shelves at the onset of the Silurian saw increased biodiversity within the surviving Orders. Recovery was characterized by an unusual number of "Lazarus taxa", disappearing during

7684-473: The details remain unresolved. In particular, some researches interpret the fluctuations in sea level as pre-Hibernian glaciation, but sedimentary evidence of glaciation is lacking until the end of the period. There is evidence of glaciers during the Hirnantian on the land we now know as Africa and South America, which were near the South Pole at the time, facilitating the formation of the ice caps of

7797-788: The deuterostome pterobranchs , which also filter-feed by tentacles, because the current-driving cells of the lophophores of all three have one cilium per cell, while lophophores of bryozoans , which he regards as protostomes, have multiple cilia per cell. Helmkampf, Bruchhaus and Hausdorf (2008) summarise several authors' embryological and morphological analyses which doubt or disagree that phoronids and brachiopods are deuterostomes: Loricifera Nematoda Nematomorpha Arthropoda Onychophora Tardigrada Kinorhyncha Priapulida Dicyemida Orthonectida Gnathostomulida Chaetognatha Limnognathia Rotifera Gastrotricha Platyhelminthes Symbion Annelida Mollusca Nemertea Phylum In biology ,

7910-422: The diversity of fauna; Ordovician invertebrates displayed a very high degree of provincialism. The widely separated continents of Laurentia and Baltica, then positioned close to the tropics and boasting many shallow seas rich in life, developed distinct trilobite faunas from the trilobite fauna of Gondwana, and Gondwana developed distinct fauna in its tropical and temperature zones. The Tien Shan terrane maintained

8023-559: The early Katian (Boda Event), depositing bioherms and radiating fauna across Europe. The early Katian also witnessed yet another humidification event. Further cooling during the Hirnantian, at the end of the Ordovician, led to the Late Ordovician glaciation . The Ordovician saw the highest sea levels of the Paleozoic, and the low relief of the continents led to many shelf deposits being formed under hundreds of metres of water. The sea level rose more or less continuously throughout

8136-399: The eggs of other members), while others are dioecious (have separate sexes). The gametes ( sperms and ova ) are produced in the swollen gonads, around the stomach. The gametes swim through the metacoelom to the metanephridia. Sperm exit by the nephridiopores and some are captured by the lophophores of individuals of the same species. Species that lay small fertilized eggs release them into

8249-408: The epistome (lid above the mouth) or be rejected by the tentacles, whose cilia can switch into reverse. The gut uses cilia and muscles to move food towards the stomach and secretes enzymes that digest some of the food, but the stomach digests the majority of the food. Phoronids also absorb amino acids (the building blocks of proteins ) through their skins, mainly in summer. Solid wastes are moved up

8362-425: The extinction and reappearing well into the Silurian, which suggests that the taxa survived in small numbers in refugia . An alternate extinction hypothesis suggested that a ten-second gamma-ray burst could have destroyed the ozone layer and exposed terrestrial and marine surface-dwelling life to deadly ultraviolet radiation and initiated global cooling. Recent work considering the sequence stratigraphy of

8475-485: The fauna that emerged in the Ordovician were the template for the remainder of the Palaeozoic. The fauna was dominated by tiered communities of suspension feeders, mainly with short food chains. The ecological system reached a new grade of complexity far beyond that of the Cambrian fauna, which has persisted until the present day. Though less famous than the Cambrian explosion , the Ordovician radiation (also known as

8588-578: The first rugose corals appeared. The planktonic graptolites remained diverse, with the Diplograptina making their appearance. One of the earliest known armoured agnathan (" ostracoderm ") vertebrates, Arandaspis , dates from the Middle Ordovician. During the Middle Ordovician there was a large increase in the intensity and diversity of bioeroding organisms. This is known as the Ordovician Bioerosion Revolution. It

8701-507: The first publication of the APG system in 1998, which proposed a classification of angiosperms up to the level of orders , many sources have preferred to treat ranks higher than orders as informal clades. Where formal ranks have been provided, the traditional divisions listed below have been reduced to a very much lower level, e.g. subclasses . Wolf plants Hepatophyta Liver plants Coniferophyta Cone-bearing plant Phylum Microsporidia

8814-502: The globe. At the start of the period, the continents of Laurentia (in present-day North America ), Siberia , and Baltica (present-day northern Europe) were separated from Gondwana by over 5,000 kilometres (3,100 mi) of ocean. These smaller continents were also sufficiently widely separated from each other to develop distinct communities of benthic organisms. The small continent of Avalonia had just rifted from Gondwana and began to move north towards Baltica and Laurentia, opening

8927-423: The hood and larval tentacles are absorbed and the juvenile body forms from the larva's metasomal sack. The adult lophophore is created around the mouth, and by growing a ventral side that is extremely long compared to the dorsal side, the gut develops a U-bend so that the anus is just under and outside the lophophore. Finally the adult phoronid builds a tube. Phoronids live for about one year. Phoronids live in all

9040-468: The inner sides of the tentacles flick food particles into a groove in a circle under and just inside the tentacles, and cilia in the groove push the particles into the mouth. Phoronids direct their lophophores into the water current, and quickly reorient to maximize the food-catching area when currents change. Their diet includes algae , diatoms , flagellates , peridinians , small invertebrate larvae, and detritus. Unwanted material can be excluded by closing

9153-429: The intestine and out through the anus , which is outside and slightly below the lophophore. A blood vessel starts from the peritoneum (the membrane that loosely encloses the stomach), with blind capillaries supplying the stomach. The blood vessel leads up the middle of the body to a circular vessel at the base of the lophophore, and from there a single blind vessel runs up each tentacle. A pair of blood vessels near

9266-401: The largest are 50 cm long. The name of the group comes from its type genus : Phoronis . The bottom end of the body is an ampulla (a flask-like swelling), which anchors the animal in the tube and enables it to retract its body very quickly when threatened. When the lophophore is extended at the top of the body, cilia (little hairs) on the sides of the tentacles draw food particles to

9379-409: The lophophore, which extends above hypoxic sediments. The blood has hemocytes containing hemoglobin , which unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments. The blood of Phoronis architecta carries as much oxygen per cm as that of most vertebrates ; the blood's volume in cm per gm of body weight is twice that of a human. Podocytes on the walls of

9492-672: The lophophore. The blood contains hemoglobin , which is unusual in such small animals and seems to be an adaptation to anoxic and hypoxic environments. The blood of Phoronis architecta carries twice as much oxygen as a human of the same weight. Two metanephridia filter the body fluid, returning any useful products and dumping the remaining soluble wastes through a pair of pores beside the anus. One species builds colonies by budding or by splitting into top and bottom sections, and all phoronids reproduce sexually from spring to autumn. The eggs of most species form free-swimming actinotroch larvae, which feed on plankton. An actinotroch settles to

9605-451: The lophophore. The ring supplies nerves to the tentacles and, just under the skin, to the body-wall muscles. Phoronis ovalis has two nerve trunks under the skin, whereas other species have one. The trunk(s) have giant axons (nerves that transmit signals very fast) which co-ordinate the retraction of the body when danger threatens. Except for retracting the body into the tube, phoronids have limited and slow movement: partial emerging from

9718-475: The middle of the body from the stomach to a circular vessel at the base of the lophophore, and from there a single blind vessel runs up each tentacle. A pair of blood vessels near the body wall lead downward from the lophophore ring to the stomach and also to blind branches throughout the body. There is no heart, but the major vessels can contract in waves to move the blood. Phoronids do not ventilate their trunks with oxygenated water, but rely on respiration through

9831-431: The mouth, which is inside and slightly to one side of the base of the lophophore. Unwanted material can be excluded by closing a lid above the mouth or be rejected by the tentacles, whose cilia can switch into reverse. The food then moves down to the stomach, which is in the ampulla. Solid wastes are moved up the intestine and out through the anus , which is outside and slightly below the lophophore. A blood vessel leads up

9944-411: The next. The Catalogue of Life includes Rhodophyta and Glaucophyta in kingdom Plantae, but other systems consider these phyla part of Protista. In addition, less popular classification schemes unite Ochrophyta and Pseudofungi under one phylum, Gyrista , and all alveolates except ciliates in one phylum Myzozoa , later lowered in rank and included in a paraphyletic phylum Miozoa . Even within

10057-573: The oceans and seas including the Arctic and excepting the Antarctic Ocean , and appear between the intertidal zone and about 400 meters down. Some occur separately, in vertical tubes embedded in soft sediment such as sand, mud, or fine gravel. Others form tangled masses of many individuals buried in or encrusting rocks and shells. In some habitats populations of phoronids reach tens of thousand of individuals per square meter. The actinotroch larvae are familiar among plankton , and sometimes account for

10170-404: The only known species with a lecithotrophic (non-feeding) larvae, lack tentacles and swims for about 4 days, creeps on the seabed for 3 to 4 days, then bores into a carbonate floor. Nothing is known about three species. The remaining species develop free-swimming actinotroch larvae, which feed on plankton. The actinotroch is an upright cylinder with the anus at the bottom and fringed with cilia. At

10283-548: The ophiuroids ("brittle stars") and the first sea stars . Nevertheless, the arthropods remained abundant; all the Late Cambrian orders continued, and were joined by the new group Phacopida . The first evidence of land plants also appeared (see evolutionary history of life ). In the Middle Ordovician, the trilobite-dominated Early Ordovician communities were replaced by generally more mixed ecosystems, in which brachiopods, bryozoans, molluscs, cornulitids , tentaculitids and echinoderms all flourished, tabulate corals diversified and

10396-539: The other hand, the highly parasitic phylum Mesozoa was divided into two phyla ( Orthonectida and Rhombozoa ) when it was discovered the Orthonectida are probably deuterostomes and the Rhombozoa protostomes . This changeability of phyla has led some biologists to call for the concept of a phylum to be abandoned in favour of placing taxa in clades without any formal ranking of group size. A definition of

10509-640: The plant kingdom Plantae contains about 14 phyla, and the fungus kingdom Fungi contains about 8 phyla. Current research in phylogenetics is uncovering the relationships among phyla within larger clades like Ecdysozoa and Embryophyta . The term phylum was coined in 1866 by Ernst Haeckel from the Greek phylon ( φῦλον , "race, stock"), related to phyle ( φυλή , "tribe, clan"). Haeckel noted that species constantly evolved into new species that seemed to retain few consistent features among themselves and therefore few features that distinguished them as

10622-417: The protocoelom, although other authors disagree that it is a coelom and Ruppert, Fox and Barnes think it is built by a different process. The tube comprises a three-layered organic inner cylinder, and an agglutinated external layer. When the lophophore is extended, cilia (little hairs) on the sides of the tentacles draw water down between the tentacles and out at the base of the lophophore. Shorter cilia on

10735-475: The relationships between groups. So phyla can be merged or split if it becomes apparent that they are related to one another or not. For example, the bearded worms were described as a new phylum (the Pogonophora) in the middle of the 20th century, but molecular work almost half a century later found them to be a group of annelids , so the phyla were merged (the bearded worms are now an annelid family ). On

10848-474: The same rock beds in North Wales in the Cambrian and Silurian systems, respectively. Lapworth recognized that the fossil fauna in the disputed strata were different from those of either the Cambrian or the Silurian systems, and placed them in a system of their own. The Ordovician received international approval in 1960 (forty years after Lapworth's death), when it was adopted as an official period of

10961-400: The sea level dropped, and the vast shallow intra-continental Ordovician seas withdrew, which eliminated many ecological niches. When they returned, they carried diminished founder populations that lacked many whole families of organisms. They then withdrew again with the next pulse of glaciation, eliminating biological diversity with each change. Species limited to a single epicontinental sea on

11074-426: The sea, and about 49% of genera of fauna disappeared forever; brachiopods and bryozoans were greatly reduced, along with many trilobite , conodont and graptolite families. The most commonly accepted theory is that these events were triggered by the onset of cold conditions in the late Katian, followed by an ice age , in the Hirnantian faunal stage, that ended the long, stable greenhouse conditions typical of

11187-853: The seabed after about 20 days and then undergoes a radical change in 30 minutes: the larval tentacles are replaced by the adult lophophore; the anus moves from the bottom to just outside the lophophore; and this changes the gut from upright to a U-bend, with the stomach at the bottom of the body. One species forms a "slug-like" larva, and the larvae of a few species are not known. Phoronids live for about one year. Some species live separately, in vertical tubes embedded in soft sediment , while others form tangled masses buried in or encrusting rocks and shells. Species able to bore into materials like limestone and dead corals do so by chemical secretions. In some habitats populations of phoronids reach tens of thousand of individuals per square meter. The actinotroch larvae are familiar among plankton , and sometimes account for

11300-512: The second largest of the five major extinction events in Earth's history in terms of percentage of genera that became extinct. The only larger one was the Permian–Triassic extinction event . The extinctions occurred approximately 447–444 million years ago and mark the boundary between the Ordovician and the following Silurian Period. At that time all complex multicellular organisms lived in

11413-425: The shells of most molluscs , dissolved rapidly on the sea floor after death. Unlike Cambrian times, when calcite production was dominated by microbial and non-biological processes, animals (and macroalgae) became a dominant source of calcareous material in Ordovician deposits. The Early Ordovician climate was very hot, with intense greenhouse conditions and sea surface temperatures comparable to those during

11526-492: The sun's rays from the proposed Ordovician ring system, which also caused the Ordovician meteor event , may have also led to the glaciation. As the southern supercontinent Gondwana drifted over the South Pole, ice caps formed on it, which have been detected in Upper Ordovician rock strata of North Africa and then-adjacent northeastern South America, which were south-polar locations at the time. As glaciers grew,

11639-495: The tentacles. It is unknown whether phoronids have any significance for humans. The International Union for Conservation of Nature (IUCN) has not listed any phoronid species as endangered. As of 2016 there are no indisputable body fossils of phoronids. Researching the Lower Cambrian Chengjiang fossils , in 1997 Chen and Zhou interpreted Iotuba chengjiangensis as a phoronid since it had tentacles and

11752-577: The time period from c. 450 Ma to c. 443 Ma. The Rawtheyan, the second last of the Ashgill ages, was from c. 449 Ma to c. 445 Ma. It is in the Katian Age of the ICS's Geologic Time Scale . During the Ordovician, the southern continents were assembled into Gondwana , which reached from north of the equator to the South Pole . The Panthalassic Ocean, centered in the northern hemisphere, covered over half

11865-527: The top end a lophophore , a "crown" of tentacles with which the animal filter-feeds . In small species the "crown" is a simple circle, in medium-size species it is bent into the shape of a horseshoe with tentacles on the outer and inner sides, and in the largest species the ends of the horseshoe wind into complex spirals. These more elaborate shapes increase the area available for feeding and respiration . The tentacles are hollow, held upright by fluid pressure, and can be moved individually by muscles. The mouth

11978-413: The top is a lobe or hood, under which are: a ganglion, connected to a patch of cilia outside the apex of the hood; a pair of protonephridia (smaller and simpler than the metanephridia in the adult); the mouth; and feeding tentacles that encircle the mouth. After swimming for about 20 days, the actinotroch settles on the seabed and undergoes a catastrophic metamorphosis (radical change) in 30 minutes:

12091-399: The trinucleids developed a broad pitted margin around their head shields. Some trilobites such as Asaphus kowalewski evolved long eyestalks to assist in detecting predators whereas other trilobite eyes in contrast disappeared completely. Molecular clock analyses suggest that early arachnids started living on land by the end of the Ordovician. Although solitary corals date back to at least

12204-576: The tube; bending the body when extended; and the lophophore's flicking of food into the mouth. Only the smallest species of horseshoe worms, Phoronis ovalis , naturally builds colonies by budding or by splitting into top and bottom sections which then grow into full bodies. In experiments, other species have split successfully, but only when both parts have enough gonadal (reproductive) tissue. All phoronids breed sexually from spring to autumn. Some species are hermaphroditic (have both male and female reproductive organs) but cross-fertilize (fertilize

12317-423: The water as plankton, while species with larger eggs brood them either in the body's tube or stuck in the center of the lophophore by adhesive. The brooded eggs are released to feed on plankton when they develop into larvae. Development of the eggs is a mixture of deuterostome and protostome characteristics. Early divisions of the egg are holoblastic (the cells divide completely) and radial (they gradually form

12430-537: Was closed off by the Late Ordovician, Iapetus Ocean , and the new Rheic Ocean . For most of the Late Ordovician life continued to flourish, but at and near the end of the period there were mass-extinction events that seriously affected conodonts and planktonic forms like graptolites . The trilobites Agnostida and Ptychopariida completely died out, and the Asaphida were much reduced. Brachiopods , bryozoans and echinoderms were also heavily affected, and

12543-554: Was distributed widely during peak sea levels in the Sandbian. Some new cystoids and crinoids appeared. It was long thought that the first true vertebrates (fish — Ostracoderms ) appeared in the Ordovician, but recent discoveries in China reveal that they probably originated in the Early Cambrian . The first gnathostome (jawed fish) may have appeared in the Late Ordovician epoch. Chitinozoans, which first appeared late in

12656-495: Was limited to the Iapetus margin of Laurentia; elsewhere, the pattern was of rifting in back-arc basins followed by remerger. This reflected episodic switching from extension to compression. The initiation of new subduction reflected a global reorganization of tectonic plates centered on the amalgamation of Gondwana. The Taconic orogeny , a major mountain-building episode, was well under way in Cambrian times. This continued into

12769-532: Was traditionally broken into Early (Tremadocian and Arenig ), Middle ( Llanvirn (subdivided into Abereiddian and Llandeilian) and Llandeilo ) and Late ( Caradoc and Ashgill) epochs. The corresponding rocks of the Ordovician System are referred to as coming from the Lower, Middle, or Upper part of the column. The Tremadoc corresponds to the ICS's Tremadocian. The Arenig corresponds to the Floian, all of

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