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Palaeos: CNIDARIA CNIDARIA CNIDARIA Page Back Unit Back Metazoa Metazoa Dendrogram Metazoa References Taxon Index Pieces Page Next Unit Next Unit Home Unit Dendrogram Unit References Glossary Time Cnidaria METAZOA |--PORIFERA (paraphyletic?) `--Radiata |--Ctenophora `--+--Cnidaria | |--Anthozoa | `--+--Hydrozoa | `--+--+--Scyphozoa | | `--Cubozoa | `--Conulata `--Bilateria |--DEUTEROSTOMIA `--PROTOSTOMIA |--Ecdysozoa `--Lophotrochozoa The Cnidaria Symmetry Body Form The deadly Nematocysts Evolutionary history Classification Links Ptilosarcus gurneyi A close-up of the Orange Sea Pen Class Anthozoa - Subclass Octocorallia - Order Pennatulacea - Suborder Subselliflorae - Family Pennatulidae image copyright © Keith Clements and Jon Gross Marine Life of the Northeast Pacific Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us Text by M. Alan Kazlev (Creative Commons Attribution) page uploaded 16 June 2002, last modified 2 May 2003 (originally uploaded on Kheper Site 3 June 1999) checked ATW060205 Palaeos: CNIDARIA CNIDARIA CNIDARIA Page Back Unit Back Metazoa Metazoa Dendrogram Metazoa References Taxon Index Pieces Page Next Unit Next Unit Home Unit Dendrogram Unit References Glossary Time Cnidaria Jellyfishes, Corals, Etc. -- Cambrian to Recent METAZOA |--PORIFERA (paraphyletic?) `--Radiata |--Ctenophora `--+--Cnidaria | |--Anthozoa | `--+--Hydrozoa | `--+--+--Scyphozoa | | `--Cubozoa | `--Conulata `--Bilateria |--DEUTEROSTOMIA `--PROTOSTOMIA |--Ecdysozoa `--Lophotrochozoa The Cnidaria Symmetry Body Form The deadly Nematocysts Evolutionary history Classification Links A zoanthid of the genus Acrozoanthus. Image: Gary Cranitch, © 2008-2010 Australian Institute of Marine Science, Lizard Island 2010, Creative Commons Attribution license The Cnidaria This diverse group of very simple-bodied animals includes corals, sea anemones, hydras, jellyfishes, and their relatives. About 9,000 living species are known. The Cnidaria are the simplest Metazoa, and do not even possess organs. All they have is a stomach and a mouth surrounded by tentacles. Cubozoa Scythozoa Hydrozoa Anthozoa Symmetry All cnidarians are radially symmetrical (the body is symmetrical around a central axis). As with the plant, there is no front or rear, but there is a top and bottom. Body Form Polyp anchored - mouth up Medusa free-floating - mouth down There are two major body forms among the Cnidaria - the polyp and the medusa. Sea anemones and corals are typical of the polyp form, while jellyfish are typical medusae When you look at it, you can see that these are the same body form, except that one is upside down, while the other is not. Lack of Organs Cnidaria have no organs (i.e. groupings of different tissues to perform complex functions e.g. hearts, lungs, kidneys, etc). There is a gastrovascular cavity ("stomach" in the broad sense) with a mouth but no anus, a body wall with outer epidermis and inner gastrodermis, between which is a jelly-like mesogloea. A mouth but no brain and no head The nervous system is arranged as a decentralized network (‘nerve net’), with one or two nets present. There is no brain or ganglia. Note that although there is no head, there is a mouth, surrounded by a crown of tentacles. The tentacles are armed with deadly nematocysts (stinging cells) . A Diffusion based Physiology All the metabolic functions of the body - respiration, digestion, elimination - are carried out by diffusion. Diffusion is only an efficient means of exchange of materials only over short distances (e.g. over about 1 mm for oxygen exchange). This means that all the tissues of a cnidarian which require oxygen (all its living cells) must be within about 1 mm of a surface which is bathed by oxygenated water. This is why cnidarians are either very tiny (e.g. coral polyps) or have very thin or flat bodies or long thin tentacles (both of which increase surface area). And also because of this diffusion based physiology, cnidarians can only live in water (most species are marine, but some Hydrozoa live in fresh water). There are only two surfaces in a cnidarian through which this diffusion can occur - the epidermis (the outer layer or skin, which is bathed in water at all times) and the gastrodermis (the inner layer, representing the stomach lining). The deadly Nematocysts Nematocyst with barb (top) extended copyright © xxxx - source unknown Nematocysts are to cnidaria what choanocysts are to sponges. But whereas the gentle filter-feeding sponge simply sweeps microscopic food particles to it, the predatory cnidarian uses batteries of deadly stinging cells that can paralyze and kill quite large prey. These large cells have a sealed central cavity of poisonous fluid. A small sensory hair is sensitive to vibrations in the water. Any luckless creature passing too close triggers the nematocyst which then shoots out the barb, which penetrates the prey so it can be injected with poison. The prey is then conveyed to the mouth by the tentacle. Evolutionary History Despite this primitive grade of organization they are no more ancient than any other group of animal. The Edicarian biota, supposed jellyfish and soft coral (sea pen) fossils from the latest Proterozoic (Ediacaran) era have turned out to belong to a totally different type of organisms, the enigmatic "Vendobionta". although there is still some argument for diploblastic natuer of these organisms. In any case it is inconceivable that large predatory organisms like jellyfish could have existed at a time when there was nothing else around for them to feed on! The first coral organisms only appear in the Middle Cambrian, (or possibly Cambrian Epoch 2), and only diversified during the Ordovician. the first unambiguous jellyfish are also not known before the Cambrian. Classification There are traditionally four classes of Cnidaria, which represent the three modes of lifestyle: Cubozoa Scythozoa Hydrozoa Anthozoa (medusa) (medusa) (polyp and medusa) (polyps) It is generally agreed that the Anthozoa are different from the others, but there has been disagreement for over a century on whether they branch first or last. The majority of recent work finds that they branched early, and we follow that trend. References and Links Cnidaria (Coelenterata) Cnidaria Cnidaria The Cnidaria Home Page - links to material on the Cnidaria, arranged accordng to topic Phylum Cnidaria - short intro and general information Cnidaria - a good non-technical intro. Part of Keith Davey's Life on Australian Seashores website Systema Naturae 2000 / Classification - Phylum Cnidaria Cnidaria - basic outline Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us Text by M. Alan Kazlev (Creative Commons Attribution) page uploaded 16 June 2002, last modified 2 May 2003 (originally uploaded on Kheper Site 3 June 1999) checked ATW060205 Palaeos: SCYPHOZOA CNIDARIA SCYPHOZOA Page Back Unit Home Page Next Page Down Cladogram Class Scyphozoa (Cambrian - Recent) image © xxxx Members of this class are large free swimming marine organisms. The medusa is the dominant phase in the life cycle and the polyp stage is either absent or it is small and gives rise to medusae by asexual budding. There is a strong four-fold symmetry. <==o SCYPHOZOA |--o STAUROMEDUSAE | |-- Cleistocarpidae | `-- Eleutherocarpidae |--o CORONATAE | |-- Atollidae | |-- Atorellidae | |-- Linuchidae | |-- Nausithoidae | |-- Paraphyllinidae | |-- Periphyllidae | `-- Tetraplatidae |--o SEMAEOSTOMEAE | |-- Cyaneidae | |-- Pelagiidae | `-- Ulmariidae `--o RHIZOSTOMEAE |-- Cassiopeidae |-- Catostylidae |-- Cepheidae |-- Lobonematidae |-- Lychnorhizidae |-- Mastigiidae |-- Rhizostomatidae |-- Stomolophidae |-- Thysanostomatidae `-- Versurigidae Cladogram Reference(s): Parker, S. P. (ed.), 1982: Synopsis and classification of living organisms. Vols. 1 & 2 --McGrew-Hill Book Company images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page uploaded 16 June 2002 checked ATW030411 (originally uploaded on Kheper Site 4 June 1999) dendrogram by Mikko Haaramo Palaeos: CUBOZOA CNIDARIA CUBOZOA Page Back Page Up Unit Home Page Next Page Down Cladogram Class Cubozoa image © xxxx Originally included as an order of the class Scyphozoa (Jellyfish) the Cubozoa are now considered to warrant separate class status. As with the Scyphozoa the medusa is the dominant phase in the life cycle. They differ from Scyphozoa in that they have a velum like structure, the velarium, the bell has four flattened sides and a simple margin, and each polyp produces a single medusa by complete metamorphosis. The name refers to the cubic form of the organism. They occur in tropical and subtropical oceans. There is only one order, the Cubomedusae images not loading? | error messages? | broken links? | suggestions? | criticism? contact us Text by M. Alan Kazlev (Creative Commons Attribution) 1999-2002 page uploaded 16 June 2002 (originally uploaded on Kheper Site 4 June 1999) checked ATW050516 Parent: Cnidaria Palaeos: HYDROZOA CNIDARIA HYDROZOA Sister: Anthozoa Child: Siphonophora Child: Hydroidea Other Children: See below Dendrogram Hydrozoa: Hydroids and Hydromedusae. From the Ordovician Garveia annulata - Golden Hydroid Class Hydrozoa - Order Hydroida - Family Bougainvilliidae image copyright © Jon Gross Marine Life of the Northeast Pacific This class includes hydroids and hydromedusae. $ Medusae small. $ Cnidocysts epidermal. This very diverse class of cnidarians many of which have a true alternation of generation in its reproductive cycle. That is, they passes through both the medusa and the polyp phase. Many groups secrete a chitinous skeleton in the polyp stage. In some groups the polyp stage is colonial and secretes a calcareous skeleton, like a coral. There are seven orders altogether, including: Order Order Order Order Order Order Order Order Trachylina Hydroida Milliporina (Hydrocorallina) Stylasterina Siphonophora Spongiomorphida (extinct) Chondrophora Actinulida Trachylina - medusa stage only. These are perhaps the most primitive Hydrozoa. : Gonionemus Hydra - a small simple freshwater Hydroid, named after a mythical Greek monster image copyright xxxx Hydroida - Ordovician? to Recent - colonial attached polyps. The free-floating medusa stage may be present or absent. Includes most types of Hydrozoans. Freshwater or marine, solitary or colonial, softbodied or with skeleton. There are two suborders: Anthomedusae (Hydra, etc) and Leptomedusae. In the latter the polyps always colonial. e.g. Obelia (left), Sertularia Siphonophora - Large and complex swimming or floating colonies, with division of labour. Jellyfish-like, but with a float or sail-like structure. Mainly tropical. Includes the unusual Portuguese Man-of-War (genus Physalia). Milleporina (should be Milleporida?) (Tertiary - Recent) - Reef builders, massive calcium carbonate skeleton. Marine. Also called Stinging Coral and Fire Coral. The Milleporina and Stylasterina have in the past been included together under the order Hydrocorallida Stylasterina (should be Stylasterida?) (Tertiary - Recent) Reef builders, very like the Milleporina, but with a thick layer of tissue overlying the skeleton Stylaster venustus - Pink (purple) Hydrocoral Class Hydrozoa - Order Stylasterina - Family Stylasteridae image copyright © Jon Gross Marine Life of the Northeast Pacific Spongiomorphida (Triassic - Jurassic). Massive colonies with radial pillars united by horizontal bars. Resemble Stromatoporoids. Chondrophora: Colonial or specialized polypoid individuals, e.g. Velella Alternation of Generations - Ontogenic Metamorphosis image copyright xxxx The above illustration is of Obelia, a Hydroid that passes through both the medusa and the polyp phase. This organism goes through stages where it is in effect a different organism, a true metamorphosis . The philosopher and inventor Arthur M. Young observes that the life cycle of this simple organism passes through a complete series of stages according to process theory: "(1) starts as a single cell [the fertilized egg]; (2) becomes multicellular [3] acquires a shape (identity) [planular stage]; (4)fastens to the ocean floor [hydroid colony]; (5) grows in plant-like fashion; (6) flowers break off into mobile jellyfish (the animal stage); (7) fertilization." [The Reflexive Universe, p.122] an Obelia colony, uncannily resembling a branching plant with flowers image copyright xxxx Division of Labour Hydrozoan colonies frequently exhibit polymorphism, with different polyps taking on different morphologies and performing different functions (reproduction, feeding, etc.). Each colony is a super-organism, like an ant-hive. Part of a colony of the marine hydrocorallid Millepora, showing division of labour. image copyright xxxx images not loading? | error messages? | broken links? | suggestions? | criticism? contact us Text by M. Alan Kazlev (Creative Commons Attribution) page uploaded 16 June 2002, last modified 2 May 2003 (originally uploaded on Kheper Site 4 June 1999) checked ATW050611 Palaeos: CNIDARIA CNIDARIA ANTHOZOA Page Back Unit Back Metazoa Metazoa Dendrogram Metazoa References Taxon Index Pieces Page Next Unit Next Unit Home Unit Dendrogram Unit References Glossary Time Anthozoa Corals, Sea anemones, Sea Pens, Etc. CNIDARIA |--Anthozoa | |--Octocorallia (Alcyonaria) | `--Hexacorallia (Zoantharia) | |--Actiniaria | `--+--Tabulata | `--+--Rugosa | `--Scleractinia `--+--Hydrozoa `--+--+--Scyphozoa | `--Cubozoa `--Conulata The Cnidaria Symmetry Body Form The deadly Nematocysts Evolutionary history Classification Links Cambrian to Recent Epizoanthus scotinus Class Anthozoa - Subclass Ceriantipatharia - Order Ceriantharia - Family Zoanthidae image copyright © Keith Clements and Jon Gross Marine Life of the Northeast Pacific Anthozoans include corals, sea anemones, sea pens, and related organisms. These animals are either solitary or colonial polyps that live attached to a substrate (surface). This is the largest and ecologically the most important group of cnidarians. Anthozoans are unique in their absence of a medusiod phase and polyps much larger than are found in the other two classes. The polyp shows biradial symmetry, with the body cavity divided by septa There are 6,000 known recent species. Phylogeny <==o ANTHOZOA |-- OCTOCORALLIA `--o ZOANTHARIA [Hexacorallia] |--+-- †KILBUCHOPHYLLIDA | `--+?-o CERIANTHARIA | `--+--o CORALLIMORPHARIA | |--+?- Ptychodactiidae* [Ptychodactiatria] | | `-- ACTINIARIA (merivuokot) | `--+-- †NUMIDIAPHYLLIDA | `-- SCLERACTINIA [Madreporaria] `--+-- †TABULOCONIDA `--+-- †COTHONIIDA `--+-- †TABULATA `--+-- †HELIOLITIDA `--+--+?- †HETEROCORALLIA | `-- †RUGOSA `--+?- ANTIPATHARIA `-- ZOANTHIDEA Cladogram Reference(s): Conway Morris, S., 1993: The fossil record and early evolution of the Metazoa. --Nature, vol. 361, 21 January,pp. 219-225 --iNet: The Tree of Life Fautin, D. G., Romano, S. L. & Oliver, W. A. Jr., 1999: Zoantharia - Sea Anemones and corals. --iNet: The Tree of Life Parker, S. P. (ed.), 1982: Synopsis and classification of living organisms. Vols. 1 & 2 --McGrew-Hill Book Company Links Introduction to the Anthozoa Tree of Life - Anthozoa Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page uploaded 16 June 2002 checked ATW050516 (originally uploaded on Kheper Site 3 June 1999) page by M. Alan Kazlev (Creative Commons License) dendrogram by Mikko Haaramo Palaeos: ANTHOZOA CNIDARIA CERANTIPATHARIA Page Back Page Up Unit Home Page Next Page Down Cladogram Subclass Cerantipatharia Horny Corals Pachycerianthus fimbriatus Tube Anemone Class Anthozoa - Subclass Cerantipatharia - Order Ceriantharia - Family Cerianthariidae image copyright © Keith Clements and Jon Gross Marine Life of the Northeast Pacific Included in this subclass are two superficially rather distinct forms. Alternatively they may be included with Rugose and Hexacorals in the subclass Zoantharia. It is possible that the Ceriantipatharia is a polyphyletic taxon Subclass Ceriantipatharia Order Antipatharia: black corals Order Ceriantharia: tube anemones Order Cerantharia - anemone-like forms with elongate bodies adapted for burrowing - the so-called "burrowing sea anemones". These are large solitary Anthozoa adapted for life in soft bottoms. The body is lodged with a secreted mucous tube buried in sand or mud e.g.. Cerianthus (above) Order Antipatharia: Black or Horny Corals. These are gorgonian-like upright plant-like forms. Polyps are arranged around an axial skeleton of black horny material bearing thorns. Most forms are deep water and live in the tropics. images not loading? | error messages? | broken links? | suggestions? | criticism? contact us Text by M. Alan Kazlev (Creative Commons Attribution) 1999-2002 page uploaded 16 June 2002 (originally uploaded on Kheper Site 4 June 1999) checked ATW050516 Palaeos: ANTHOZOA CNIDARIA HEXACORALLIA Parent: Anthozoa Sister: Octocorallia Child: Actiniaria Child: Tabulata Hexacorallia or Zoantharia This very important subclass includes six mesenteries or multiples thereof. Included here are sea anemones, true corals, and two similar groups. Sometimes the subclass Zoantharia is used instead, in which case hexacorals are included with Rugose corals (Rugosa) and Horny Corals (Cerantipitharia). The following four orders have generally been included: Actinaria: Sea anemones Scleractinia: True or stony corals Zoanthidea: Colonial sea-anemone-like animals Corallimorpharia: Coral-like anemones As of this revision (040930), the most recent phylogeny is that of Daly et al. (2003). Unfortunately, these workers were restricted to living taxa, since they used a combined molecular and morphological approach. Their phylogeny of the hexacorals may represented as follows: Root |--Ceriantharia `--Hexacorallia |--Actiniaria `--+--Zoanthidea `--+--Antipatharia `--+--Corallimorpharia `--Scleractinia The Tabulata and Rugosa are usually assigned positions just basal to the Zoanthidea and Antipatharia. For lack of anything more sensible to do, we insert them so as to preserve that relationship. Root |--Ceriantharia `--Hexacorallia |--Actiniaria `--+--Tabulata `--+--Rugosa `--+--Zoanthidea `--+--Antipatharia `--+--Corallimorpharia `--Scleractinia The basic anatomy of a coral is shown in the image. Scleractinian larvae (i.e. the planula stage of polyp development) forms a basal plate on a suitable substrate. It then secretes vertical partitions (septa) which are joined by the outer body wall (theca). The septa may meet at the axis during at least part of their vertical extent. If so, they may form a vertical columella in the center. This is common in scleractinian corals, but rare in rugose corals and absent in the Tabulata. In addition, the polyp forms soft tissue partitions or mesenteries. Ultimately, these control the central pharynx which opens on the dorsal surface. The opening of the pharynx is surrounded by a row of tentacles. Image: from Coral Reefs Biology 200 Lecture Notes and Study Guide by Prof. David A. Krupp of Windward Community College. ATW040930. Text public domain. No rights reserved. Actiniaria: sea anemones From the Cambrian Urticina piscivora: Fish-Eating Urticina. Class Anthozoa Subclass Hexacorallia - Order Actiniaria - Family Actiniidae. Image copyright � Keith Clements and Jon Gross Marine Life of the Northeast Pacific. The soft-bodied sea anemones are large soft-bodied corals. The polyp is usually brightly colored, always solitary, and found in coastal waters world-wide, but are particularly common in the tropics. In size they are usually from 1.5 to 5 cm in diameter, although giant forms like Tealia columbiana (North Pacific USA) and Stoichactus sp. (Great Barrier Reef, Australia) may reach over a meter at the oral (mouth) end. The outer tentacles are much divided, giving a frilly appearance. The mouth occurs in the middle of a flat area called the oral disc; the mouth and pharynx below are oval in shape. There are usually two ciliated grooves called siphonoglyphs, found the length of the pharynx, that create a current of water into the "stomach" (gastrovascular cavity), which maintains a hydrostatic skeleton, against which the muscles can act. Zoanthidea From the Triassic These are small, solitary or colonial, anemone-like hexacorals. Most forms are tropical, some are common reef inhabitants, and some live attached to other invertebrates. The body may be columnar, but is more often short and button-like. A short fringe of tentacles surrounds a the broad oval disc. Corallimorpharia: False Corals, Mushrooms, etc. No fossil record -presumably evolved before the Triassic Amplexidiscus fenestrafer. From CORALLIMORPHARIA (Ordre des Corallimorphaires) These anthozoans resemble stony corals (Scleractinia) but lack skeletons. In fact, they are consistently classified as the sister group of the Scleractinia by a wide variety of taxonomic methods. However, they secrete no skeleton of any kind. The Corallimorpharia have a flattened adherent base, similar to Scleractinia, but without a base plate or basilar muscles. The theca around the column is smooth, sometimes with weak longitudinal muscles. The sphincter is weak or absent. The tentacles around the mouth are disposed in concentric circles, usually forming a series of radial lines rather than being alternately arranged. Both the muscles of the oral disk and the longitudinal muscles of the tentacles are weak. However, the disk is well-supplied with cilia which create a current carrying suspended materials into the pharynx. The polyps can also form the dorsal surface into a sac, lined with nematocysts to trap and subdue larger prey. The mesenteries are often irregular. Corallimorpharia are often solitary, or connected to other polyps by coenenchyme. Corallimorpharia are well known to aquarists because of their striking forms and colors, the latter often due to symbiotic zooxanthellae within the tissues of the polyp. In spite of this trait, Corallimorpharia generally prefer low light levels or indirect light. They feed on a wide variety of food materials. All feed on dissolved and particulate matter, some feed on zooplankton, and some are even known to trap and digest fish. Corallimorpharians usually reproduce vegetatively (e.g., by budding or fission), and often form colonies of cloned individuals. Sexual reproduction is assumed to occur, but has not been well documented. Currently, four families of Corallimorpharia are recognized: Corallimorphidae Discosomatidae Ricordeidae Sideractiidae Corynactis: source unknown Amplexidiscus: from Corallimorphs Ricordea: from Sanjay's Reef Aquariums Sideractis: from Hexacorallians of the World There does not appear to be any good indication of the relative phylogenetic position of these families. Links: classification_path_2.cfm-taxonname=Corallimorpharia, http://web.nhm.ku.edu/inverts/carlgren_1949/corallimorpharia.html, Order Corallimorpharia (ERMS taxonomic hierarchy), Aquarium Invertebrates, Corallimorpharia (looks like great site, but my Icelandic is a bit rusty ...). ATW040930. Text public domain. No rights reserved. Scleractinia From the Triassic All Mesozoic and Cenozoic reef-building corals are members of this order (sometimes, subclass). Reef-building corals are also known as "hard corals" or "stony corals." They are similar to and closely related to sea anemones but unlike those soft-bodied forms they secrete an aragonitic (calcium carbonate) skeleton. It is this skeleton that forms coral reefs. Some forms, like the Indo-Pacific Fungia, are solitary, with single polyps reaching 25 cm in diameter. The majority however are colonial, with very small polyps (about 1 to 3 mm), although the colony as a whole can grow very large. As with all the Hexacorallia, septal insertion occurs in multiples of six, and many scleractinial corals have 6-fold symmetry. Over 60 genera of Scleractinia have a symbiotic relationship with a type of microscopic algae, called zooxanthellae, living within the coral polyp's tissue (within the gastrodermal cells). Although deep water and some cold-water corals lack zooxanthellae, virtually all reef-building possess them. These corals are generally found in clear water at depths of less than 50 meters, the zone where sunlight penetrates. The algae not only provide food but help polyp calcification, and may account for up to 50% of the protein nitrogen of the coral, as well as giving then a yellow-brown or dark brown colour. These so-called hermatypic corals can lay down massive amounts of limestone in the photic zone of shallow tropic seas. Image: Cross section through a coral organism Unless otherwise attributed, text on this page may be used under the terms of a Creative Commons License. images not loading? | error messages? | broken links? | suggestions? | criticism? contact us Text by M. Alan Kazlev (Creative Commons Attribution) 1999-2002 page uploaded 16 June 2002 (originally uploaded on Kheper Site 4 June 1999) checked ATW050611 Palaeos: ANTHOZOA CNIDARIA OCTOCORALLIA Page Back Page Up Unit Home Page Next Page Down Cladogram Octocorallia Calcigorgia spiculifera Class Anthozoa - Subclass Octocorallia - Order Gorgonacea - Suborder Holaxonia - Family Acanthogorgiidae image copyright © Keith Clements and Jon Gross Marine Life of the Northeast Pacific In this group of mostly soft corals, the polyps are almost always formed into colonies, each polyp having eight pinnate (side- branching) tentacles. Octocorallians include a number of diverse forms Subclass Octocorallia: Octocorals. Eight mesenteries. Order Gorgonacea: Sea fans, sea whips Order Telestacea Order Pennatulacea: Sea pens, sea pansies Order Alcyonaea: Soft corals Order Helioporacea Order Stolonifera: Organ pipe corals (among others) Alcyonacea - sometimes known as 'soft corals'. Often inhabit low tidal zone of rocky shores Gorgonacea - members secrete a horny material called gorgonin to act as skeletal support. Abundant on coral reefs, especially in the Caribbean. <==o ALCYONARIA |--o PROTOALCYONARIA | |-- Haimeidae | `-- Taiaroidae |--o STOLONIFERA | |-- Clavulariidae | |-- Cornulariidae | `-- Tubiporidae |--o TELESTACEA | |-- Coelogorgiidae | |-- Pseudocladochonidae | `-- Telestidae |--o GASTRAXONACEA | `-- Pseudogorgiidae* |--o GORGONACEA | |--o HOLAXONIA | | |-- Acanthogorgiidae | | |-- Ainigmaptilidae | | |-- Chrysogorgiidae | | |-- Ellisellidae | | |-- Gorgoniidae | | |-- Ifalukellidae | | |-- Isididae | | |-- Keroeididae | | |-- Paramuriceidae | | |-- Plexauridae | | `-- Primnoidae | `--o SCLERAXONIA | |-- Anthothelidae | |-- Briareidae | |-- Coralliidae | |-- Melithaeidae | |-- Paragorgiidae | |-- Parisididae | `-- Subergorgiidae |--o ALCYONACEA | |-- Alcyoniidae | |-- Astrospiculariidae | |-- Maasellidae | |-- Nephtheidae | |-- Siphonogorgiidae | `-- Xeniidae |?- PENNATULACEA (merisulat) `--o HELIOPORACEA |-- Helioporidae `-- Lithotelestidae Pennatulacea - the sea pens, have a single long main polyp forms a stalk which anchors the colony in its mud substrate. Branches from this bear the feeding and ciliated polyps and generate an inward current of water that keeps the whole colony inflated. It used to be thought that some Edicarian forms were sea-pens, this does not seem to be the case. See also Race Rocks - Ptilosarcus gurneyi - for more on the Sea Pens. <==o PENNATULACEA |--o SESSILIFLORAE | |-- Anthoptilidae | |-- Chunellidae | |-- Echinoptilidae | |-- Funiculinidae | |-- Kophobelemnidae | |-- Protoptilidae | |-- Renillidae | |-- Scleroptilidae | |-- Stachyptilidae | |-- Umbellulidae | `-- Veretillidae `--o SUBSELLIFLORAE |-- Pennatulidae |-- Pteroeididae `-- Virgulariidae images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page uploaded 16 June 2002, last modified 2 May 2003 (originally uploaded on Kheper Site 3 June 1999) checked ATW050611 page by M. Alan Kazlev (Creative Commons License) cladograms © Mikko Haaramo this material may be freely used for non-commercial purposes Palaeos: ANTHOZOA CNIDARIA RUGOSA Parent: Anthozoa Rugosa Horn Corals Middle Ordovician to Late Permian Heliophyllum halli Devonian period image © xxxx The Rugosa or "rugose corals" (referring to their wrinkled appearance), also known as "horn corals" were an important group of Paleozoic organisms. Both solitary and colonial forms are known, but the former are more common. Solitary rugosans usually have a horn shaped (hence the alternative term, "horn corals"), while the colonial types commonly have hexagonal corallites. The skeleton is made of calcite and is generally quite massive. Solitary rugose corals range in size from a few millimeters in diameter and in length to 14 centimeters in diameter and a height of close to one meter. Some colonies may be 4 meters in diameter. Like modern corals (e.g. sea anemones, which can be observed in intertidal rock pools), the coral animal (or polyp) fed by using tentacles to capture and sweep organisms into their mouths As a very general rule, rugose coral have stronger radial septa than they do transverse platforms. The septa radiate out from the center. Rugose corals have both major and minor septa. Rugose corals differ from other corals by the pattern by which they add septa through their ontogeny (development and growth). Both solitary and colonial rugosans have this distinctive septal insertion pattern, which gives most rugose corals bilateral symmetry. The six prosepta are added first, including the cardinal septum and counter septum, which are at 180 degrees from one another. . After this the major septa (metasepta) are inserted serially in four positions; minor septa short and inserted between major septa, probably serially also. It is this four-fold developmental pattern that gives rugose corals their alternative name of tetracorals (tetra meaning four). New corallites may bud asexually, although they also reproduced sexually. The buds have four septa. As the corallite grows, the septa begin to spread further apart, and new septa are added, generally, four septa at a time are added to maintain a rigid structure. The growth lines on the coral span its length from the calice (top) to the base. Rugose corals generally added a new layer of growth each day (a new wrinkle), and the days in the Paleozoic year have been determined through counting growth rings on rugose corals. It is now known for example that the Devonian period had a year of 400 days (in the past the Earth rotated more quickly around it's axis; this rotation is being gradually but continually slowed by the tidal "brake" exerted by the moon's gravity). It is not known whether rugose corals had symbiotic photosymbiotic zooxanthellae (algae) as modern corals do. Some have suggested not, but personally I see no reason why they would not have. They were rarely reef-builders as modern corals are. The reason being they were not able to attach themselves to the bottom the way modern scleractinian corals can. Classification Classification of the Rugose corals is provisional and will no doubt be revised as more detailed investigation of the microstructure of the different groups is made. The present arrangement (from the Treatise of Invertebrate Paleontology) has three suborders: suborder Streptelasmatina (Ordovician to Late Permian) - includes both solitary and colonial types. The tabulae are domed (convex upward). The periphery of the corallum has either a stereozone or a dissepimentarium. Examples include Streptelasma, Heliophyllum. suborder Columnariina (Ordovician—Permian). Usually colonial, rarely solitary forms. Examples: include Spongophyllum and Lonsdaleia. The septa are thin, the tabulae flat, depressed or downwardly convex suborder Cystiphyllina (Ordovician—Devonian). Solitary or colonial forms. The septa are large with complex microstructure. A wide dissepimentarium or a stereozone is present. Examples include Cystiphyllum, Calceola, and Goniophyllum. An alternative classification has instead two orders, and elevates the Rugosa from ordinal to subclass ranking: Subclass Rugosa Order Stauriida (Mid Ordovician - Late Permian) Order Cystiphyllida (Mid Ordovician - Mid Devonian) Evolution The general trend among rugose corals was to evolve a strong skeleton. Several different lineages show convergent trends toward similar morphologies. For example, several lineages developed carinae and columella to strengthen the septa and the central axis of the coral. The simplest and earliest grades of corallite organization appeared during the Middle Ordovician (left). These had only simple walls, septa and tabulae. The polyp lived on top of a tabula in a depression in the top of the coral called the Lambeophyllum - Sandbian (Ordovician) calyx. A little later some types developed a layer (the marginal stereozone) of thickened calcite around the periphery of the corallite. This would have doubtless served a strengthening function. For this whole period, and until the early Silurian, rugose corals remained small and solitary. Streptelasmatina and Columnariina were dominant. The suborders The period from the early Silurian onwards saw the emergence of colonial forms, and an adaptive radiation and exploitation of the reef habitat by both solitary and colonial types. During this time small horizontal internal blisterlike plates called dissepiments appeared in several lineages. These have a strengthening function and also make incremental growth of the corallite possible, as there is no need to lift the whole polyp and create entire new tabula at once. Nevertheless the rugose corals remained subsidiary components of reefs relative totabulate corals and stromatoporoids. The suborder Columnariina dominated. Few Silurian forms survived into the Devonian. A new adaptive radiation of solitary and colonial rugosans occurred in the Middle Devonian. These were all relatively large forms with wide dissepimentaria. above: A Middle Devonian (Eifelian) reef featuring the crinoid Dolatocrinus, a tabulate coral (Favosites) and a trilobite (lower right corner), and a number of species of Rugose corals. The giant one in the center is probably Siphonophrentis gigantea. The squat white ones at the middle right would be Heterophrentis prolifica. The cluster of yellow ones in the middle foreground and right middle background are a colonial corals of the genus Eridophyllum. Coral faunas were seriously affected by the late Devonian mass-extinction events. Then during theearly Carboniferous, a new adaptive radiation occurred in both solitary and colonial forms. At this time a number of types developed a column through the middle of the corallite (through a thickening of the end of the counter septum) called the columella. It is assumed this would also aid in strengthening and growth of the structure. In addition the microstructure of the skeletons became quite complex. The rugose corals of this period were the most advanced types that evolved. The organizational grades would seem to represent a tendency towards greater efficiency, like the three grades of sponge organization. Yet paradoxically the more primitive types continue to co-exist alongside the more advanced forms. The final radiation of rugose corals occurred during the Permian period. Varieties with prominent prosepta appear. These flourished until the end of the period, when the entire group was exterminated by the huge Permo-Triassic extinction event. References & Links Rugose Corals (in notational form, with a few photos) and Rugose Corals (notational form) Introduction to the Rugosa: only a brief mention Unless otherwise attributed, text on this page may be used under the terms of the Creative Commons License. images not loading? | error messages? | broken links? | suggestions? | criticism? contact us Text by M. Alan Kazlev page uploaded 16 June 2002, last modified 2 May2003 (originally uploaded on Kheper Site 3 June 1999) checked ATW040930 Palaeos: ANTHOZOA CNIDARIA TABULATA Page Back Page Up Unit Home Page Next Page Down Cladogram Subclass Tabulata Furongian? / Early Ordovician to Late Permian Tabulates are a Paleozoic group of corals that produce calcite skeletons of varying shapes. Unlike the contemporary Rugose corals, they are always colonial, and never found as solitary forms, and the individual corallites are small. The group takes its name from the organization of the colony. It is built around prominant horizontal "floors" or tabulae. Other skeletal elements, such as septa, are reduced or absent. Because of this it has even been suggested that these are not corals at all, but a type of extinct algae. The larger tabulates were important reef-builders, being found in association with Stromatoporoids. Some species, like the well known Favosites, form mound-like colonies, but there are also sheet-like, branching, and chain-like forms. The usual classification is as follows: Subclass Tabulata Order Order Order Order Order Order Chaetetida Tetradiida Sarcinulida Heliolitida Halysitida Auloporida References and Links Introduction to the Tabulata Corals - Kansas Fossils Tabulata E.N.K. Clarkson, Invertebrate Paleontology and Evolution Unless otherwise attributed, text on this page may be used under the terms of the Creative Commons License. images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page by M. Alan Kazlev page last modified 2 May 2003 ATW020819 Palaeos: CNIDARIA CNIDARIA REFERENCES Unit Home Invertebrates Cnidaria: References Daly, M, DG Fautin & VA Cappola (2003), Systematics of the Hexacorallia (Cnidaria: Anthozoa). Zool. J. Linn. Soc. 139: 419�437. images not loading? | error messages? | broken links? | suggestions? | criticism? contact webmaster page uploaded 7 May 2002 checked ATW030701 page by M. Alan Kazlev (Creative Commons License) Unless otherwise noted, the material on this page may be used under the terms of a Creative Commons License. Palaeos BILATERIA METAZOA OVERVIEW Page Back Bilateria Home Glossary Pieces Page Next Unit Back Metazoa Home Dendrogram Taxon Index Unit Next Bilateria Animals with Bilateral Symmetry Metazoa |--Porifera (paraphyletic?) `--+--Radiata `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa `--Spiralia Introduction Building Bilateria The Problem of Middle Earth Trees in the Entwood "Where Many Paths and Errands Meet" References This is a holding page, pending material being added. Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page uploaded MAK020407, last modified ATW050724 checked ATW050724 this material may be freely used as long as attribution is given Palaeos BILATERIA METAZOA BILATERIA -1 Page Back Bilateria Home Glossary Pieces Page Next Unit Back Metazoa Home Dendrogram Taxon Index Unit Next Bilateria Animals with Bilateral Symmetry Metazoa |--Porifera (paraphyletic?) `--+--Radiata `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa `--Spiralia Introduction Building Bilateria The Problem of Middle Earth Trees in the Entwood "Where Many Paths and Errands Meet" References Introduction This category includes all Metazoa with bilateral symmetry (having a definite front and rear, and left and right body surfaces), either in their adult stage or (in the case of types with secondary radial symmetry, such as echinoderms) in the larval stage. Many bilaterian animals exhibit cephalization, which is the evolutionary trend toward concentration of sensory structures, mouth, and nerve ganglia, at the anterior end of the body. This body type is related to the further elaboration of hox genes All bilaterians are triploblastic, which means they develop three germ layers during embryonic development. The three germ layers are 1. Ectoderm - Covers the surface of the embryo and forms the outer covering of the animal and the central nervous system in some phyla. 2. Endoderm - the innermost germ layer which lines the archenteron (primitive gut). It forms lining of the digestive tract and out-pocketing give rise to the liver and lungs of vertebrates. 3. Mesoderm - located between the ectoderm and endoderm. Forms the muscles and most organs located between the digestive tract and outer covering of the animal. The circulatory and (in vertebrates) the skeletal system stems from this Note that only Bilateria have a mesoderm. Bilateria is defined by reference to a hypothetical organism,Urbilateria, who was the last common ancestor of Deuterostomia (echinoderms and the various chordate phyla) and Protostomia (all other "higher" animals). Classically, these two groups were said to differ in a number of respects. Some of these characteristics have been challenged recently, but they remain generally true: Update (April 2007): A number of papers published in the last few years have made it clear that the whole traditional concept of Bilateria is probably mistaken. Bilateria supposedly have three unique features: (1) bilateral symmetry, (2) hox gene patterning of the anteroposterior axis, and (3) mesoderm. It turns out that developing cnidarians clearly have (1), probably have (2), and may even have (3). It seems even more likely now that the Cnidaria are paraphyletic. That is, all of us bilaterian animals are just jellyfish who happened to use a particular type bilateral symmetry and used the hom genes in a particular way. This may have important phylogenetic implications. We cannot assume that any bilateral organism with a couple of the usual hom genes is inside the crown group of Bilateria. Consequently, the predictions we made last year (see end of this essay) about Metazoan phylogeny are looking good. Most of this is covered in Brooke & Holland (2003), which we cited -- but obviously didn't pay enough attention to. To see how far things have progressed at this point, see Ryan et al. (2007). ATW070404 One important difference involves the cleavage patterns; the division of cells in the early embryo. Protostomes are characterized by a spiral (the planes of cell division are diagonal to the vertical axis of the embryo) and determinant (the goal of each embryonic cell is established very early) cleavage. Deuterostomes undergo radial (parallel or perpendicular to the vertical axis of the embryo) and indeterminate cleavage (each early embryonic cell retains the capacity to develop into a complete embryo if isolated from other cells). But see, Halanych (2004); Hejnol & Schnabel (2004). There is a strange symmetry in the fate of the blastopore (the first opening of the archenteron which forms during gastrulation.). In Protostomes ("mouth first") the blastopore forms the mouth; in Deuterostomes ("mouth second") - blastopore forms the anus. (see following diagram) . Another difference involves how the mesoderm surrounds the body cavity. It can either pouch off or split entirely. In Schizocoela or Protostomia, it splits. In the Enterocoela or Deuterostomia, the mesoderm (or archenteron, the original gut) pouches off This diagram is showing the difference of the two major types of coelomates: the protostomes (molluscs, annelids, arthropods, ...) and deuterostomes (echinoderms, vertebrates, ...). These groups differ in several characteristics of early development; In deuterostomes blastula devisions is called "radial cleavage" because it occurs parallel or perpendicular to the major polar axis. In protostomes the cleavage is called "spirale" because division planes are oriented obliquely to the polar major axis. During gastrulation, protostomes embryos' mouth was given first by the blastopore while the anus was formed later and vis versa for the deuterostomes Diagram and caption by YassineMrabet via Wikipedia. Creative Commons Attribution-Share Alike, GNU Free Documentation License Descriptions Bilateria Hatschek, 1888 : fish > jellyfish Range: Fr Ediacaran Phylogeny: Eumetazoa : Cnidara + * : Protostomia + Deuterostomia Characters: Primary bilateral symmetry, occaisonally secondarily modified to pentameral or radial, organ-system grade of organization, most triploblastic with well-developed mesoderm of endodermal origin, most with body cavity other than the digestive cavity, anus typically present. Comments: For simplicity's sake, minor and problematic phyla have been excluded from the Phylogeny line; these will be added at some later point. The Bilateria evolved from a paraphyletic Radiata; early bilateral phylogeny remains obscure, MAK120423 Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us this page MAK120423 (from bilateria.htm) Palaeos BILATERIA METAZOA INTRODUCTION Page Back Bilateria Home Glossary Pieces Page Next Unit Back Metazoa Home Dendrogram Taxon Index Unit Next Bilateria-1 Animals with Bilateral Symmetry Metazoa |--Porifera (paraphyletic?) `--+--Radiata `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa `--Spiralia Introduction Building Bilateria The Problem of Middle Earth Trees in the Entwood "Where Many Paths and Errands Meet" References Building Bilateria In general terms, it isn't hard to map out the place of the Bilateria in the scheme of things. The first "animals" were probably uninteresting hollow balls of cells closely related to the living choanoflagellate colonies and -- somewhat more distantly -- to the Fungi. These aggregates were clonal. That is, they all derived from a single parent cell, and might be thought of as a single organism. Since maintaining a hollow ball geometry is trickier than it sounds, the first animals became adept at manipulating cellcell interactions, which naturally led to the ability to specialize different parts of the "body." After a bit, some deviant globe of cells developed the ability to create new globes and attach them to the original ball. After a few more million years of experimentation, the result was the Porifera, the sponges, which often have no particular shape, but have a respectable degree of specialization within small subunits. The really difficult part came next -- body-level organization. How exactly this was accomplished we won't even guess, at least not right now, since it would be off-topic. The bottom line is that the ball of cells continued to grow, rather than budding new cell aggregates. Since a soft, hollow sphere is not an easy shape to maintain without collapsing, it promptly collapsed. However, it did so in a very interesting and specific way. The maneuver involved creating an inner pocket, as shown in the image. By geometrical necessity, this creates a population of "inside" cells in addition to the original "outside" cells, and a more or less circular boundary region. This is pretty much all we need to create the hydra body plan of a cnidarian a (sea anemone or jellyfish). This is also exactly what happens in the early development of all animal embryos, during the gastrula phase, which is why we have used embryological terminology to label the parts in the diagram from Technau & Scholz (2003). Ernst Haeckel (1834-1919), whose many accomplishments included looking just exactly like Gandalf the Grey, was the first person to recognize this striking parallel between early animal development and the probable course of early animal evolution. He called his idea the Gastraea Hypothesis, and it is the real source of Haeckel's "Law" that "ontogeny recapitulates phylogeny." This was, and remains, a powerful insight -- true in many cases. It is not a "law" in the same sense as the physical laws of Kepler or Newton. Like many biological "laws," it is, instead, a recurring pattern. It may not apply in any given case, but it happens often enough that it ought to be the default hypothesis until something better comes along. However, in the case of early metazoan evolution, nothing better has come along even now, more than 100 years after Haeckel's time. So, the next time you hear someone belittle Haeckel (as it has become fashionable to do), remind them of this remarkable insight. Ask them if they have a better idea. Actually, there are a number of topologically reasonable ways to create an inner compartment, and most can be found within the embryological diversity of the Cnidaria and Ctenophora. All such methods create endoderm and ectoderm, but the gastrula maneuver creates a critical ring-shaped boundary area where ectoderm and endoderm meet. It isn't clear yet why this should have such importance, but it plainly does. In cnidarians, this is the mouth or hypostome. It is the most specialized region of the body, and around it develop the other specialized structures, particularly the tentacles and a large proportion of the sensory cells. This area is clearly a critical link, but extraordinarily hard to understand, as we will discuss later. The hydra body plan, described above, lacks two important elements which are shared by allBilateria, i.e., Protostomia (bugs, worms, clams, etc.) plus Deuterostomia (sea urchins, people, etc.). These were mentioned in the introduction: (1) mesoderm and (2) bilateral symmetry. Mesoderm is a third embryological "germ layer" (fundamental cell type) which develops in the blastocoel after gastrulation. There are several different ways in which mesoderm is produced, which tend to differ between protostomes and deuterostomes. In general, mesoderm formation appears to be based on a key transcription factor coded by the regulatory gene brachyury. We go into this in excessive detail below. The evolution of bilateral symmetry is actually the more difficult development to explain, and we have no easy answers. The net result of all this reconstruction is a sort of short, worm-like animal with a mouth (but not necessarily an anus), a primitive gut, a specialized area around the mouth with a tendency to develop sensory structures, mesoderm (and, so, probably some internal specializations such as body wall muscles), and bilateral symmetry. That, folks, is Urbilateria. The Problem of Middle Earth Urbilateria the ur-bilaterian, the last common ancestor of Attila the Hun and Atta the ant, is a rather critical player in our story because of the way we have defined our clades. As a reminder, the bare bones of our working phylogeny looks like this: Metazoa (animals) = [toads > toadstools] |--Porifera (sponges) `--+--Cnidaria (anemones, jellyfish, etc.) `--BILATERIA = [men + mollusks] = Urbilateria and all of its descendants |--DEUTEROSTOMIA = [men > mollusks] `--"Middle Earth " (paraphyletic) `--PROTOSTOMIA = [bugs + slugs] |--Ecdysozoa = [bugs > slugs] `--Lophotrochozoa = [slugs > bugs] |--Annelida `--Mollusca Bilateria is a crown group. It is defined as the extended family of the last common ancestor of men and mollusks, or Attila and Atta if you prefer. In short, it consists of Urbilateria and all of its descendants. Since it is a crown group, it has two branches, one leading to men and the other leading to mollusks. Deuterostomia is a stem group, defined to take in everything more closely related to men than mollusks. That is, it takes up all of the branch of Bilateria leading toward humans. However Protostomia is not a stem group. It is another crown group, defined by the intersection of bugs (Ecdysozoa) and slugs (Lophotrochozoa). Therefore, there is a gap of unknown size and composition between Urbilateria and Protostomia containing animals which are bilaterians, but neither deuterostome nor protostome. We have labeled this part of phylospace "Middle Earth," since it may be completely mythical, or quite real and filled with all manner of unfamiliar creatures. In order to get a handle on Bilateria, the challenge is to find out if Middle Earth exists and, if so, who lives there. That, in turn, may depend on what characteristics Urbilateria might have had. We will use two distinct approaches to investigate the population of Middle Earth: phylogeny and embryology. The phylogenetic approach involves the usual business of comparing warring cladograms and methodologies. This approach focuses on the point where bugs and slugs diverge. Anything bilaterian, but basal to that point, is a hobbit. The embryological approach, by contrast, focuses on Urbilateria. It tends to be difficult, theoretical and often quite speculative. However, as we will see, there are excellent reasons for resorting to this sort of thing, as a check on the usual phylogenetic methods. Trees in the Entwood We would love to say that no one could possibly review all of the recent literature on bilaterian phylogeny. Unfortunately several inconsiderate show-offs evidently had nothing better to do, and have published just such reviews, too recently to ignore. Zrzav� (2001); Halanych (2004), Philippe et al. (2005). Confronted with the undeniable evidence of all this industry, we can claim only that no one as shiftless as we are could possibly review all of the recent literature on bilaterian phylogeny. Nonetheless, we have dabbled in the literature in our usual cursory manner -- at least to the extent of casually skimming the said reviews of Zrzav�, Halanych, and Philippe et al. Fortunately, a few examples will suffice to make the relevant point. What we will find is that, while we can obtain some hints, the trees of Middle Earth often turn out to be Ents. That is, they are not fixed in place, but shift their limbs in unexpected ways, with a tendency to ensnare unwary travelers with their long branches. In short, these trees are unstable and exhibit long branch attraction. Consider Steinauer et al. (2005). One of the problems with investigating Middle Earth is that its inhabitants are poorly known. Most candidate species are rare and/or obscure parasites. The Acanthocephala are a case in point. The acanthocephalan Leptorhynchoides is a parasite of vertebrate carnivores which looks a little like a nemertine worm with training wheels. Virtually the only persons with an interest in Leptorhynchoides are a few, notoriously singleminded, public health types whose interest begins and ends with finding new methods for making the Acanthocephela yet rarer and/or more obscure. To the extent that anyone has cared at all, the understanding has been that acanthocephalans are some kind of evil rotifer cousin (Herlyn et al., 2003) or just some kind of evil rotifer (Zrzav�, 2001). Undeterred by considerable morphological evidence, Steinauer et al. sequenced the entire mitochondrial genome of Leptorhynchoides. They found many useful bits of information which, as in most mtDNA studies, they proceeded to ignore completely (see methodological critique at Insectivora). These workers then applied the usual sequence-based techniques to construct a phylogenetic tree. Perversely, they failed to include any sequences from rotifers, gnathostomulids, chaetognaths, or, in fact, anything that might actually be comparable to Acanthocephala. They found that (a) nothing lives in Middle Earth; (b) acanthocephalans are close to Platyhelminthes; (c) both are mollusks; and (d) that long branch attraction might have played a part in the results. We can certainly endorse the last conclusion. If a group of DNA sequences are similar, one can sometimes create a valid phylogenetic tree by comparing the changes at homologous sites. Over time, however, the same site will have changed multiple times. The nucleotide found at the site becomes randomized and carries no valid phylogenetic signal. See discussion at Insectivora. The relevant time frame varies with the site, the species, and by blind luck. Consequently, when one is comparing sequences which diverged long ago and are quite different ("long branches"), the phylogenetic signal is swamped by random noise and by other, nonphylogenetic, effects, giving rise to all kinds of spurious results. This is called "long branch attraction." Unfortunately, long branch attraction actually becomes worse as more data are added to the matrix. Philippe et al. (2005). This effect may be analogous to a subtle problem related to missing data in morphological work. See discussion at Pythonomorpha. However, contrary to a wrong, but oft-repeated, scientific fable, morphological parsimony methods are generally much less subject to this source of error. Siddall & Whiting (1998). For reasons we really ought to discuss one day, small, parasitic organisms tend to mutate a lot faster than others. Most of the possible inhabitants of Middle Earth are just such organisms and are known to mutate faster than a comic book superhero. Lavrov & Lang (2005) (nematode mtDNA "statistically indistinguishable from randomly shuffled genomes with the same gene content") Thus these organisms quickly reach branch lengths at which any phylogenetic signal is overwhelmed by noise. Treehugging In the last few years, a number of groups have attempted various strategies to confine these phylogenetic Ents. While they have had somewhat more success than Sauruman the White, the results have been unsatisfactory. Lavrov & Lang (2005) used the promising technique of comparing mitochondrial gene order (not sequence); but they were forced to eliminate taxa with unacceptably high rates of branch growth. They obtained believable results, but resolution was poor and all of the likely inhabitants of Middle Earth had to be pruned to make their approach work. Much the same might be said of Philippe et al. (2005), who use 18S rRNA sequences. Both recognize the long-branch problem, attempt to deal with it, and recover reasonable trees -- but with very little phylogenetic resolution and no information on the phyla of interest to us. A more productive approach may be that of Ruiz-Trillo et al. (2002), who used one of the myosin genes to map bilaterian phylogeny. Myosin is one of the critical contractile proteins of all metazoan cells -- and almost all eukaryotic cells. The reasoning, which seems to have been sound, was that myosin is under more or less uniform structural constraints and ought to evolve both slowly and regularly. The authors conclude that acoel worms and nemertodermatids (collectively, the Acoelomorpha) are basal to the rest of the former Platyhelminthes which are protostomes. The only difficulty is that the Acoelomorpha also end up basal to the crown group Bilateria! As it turns out, this is entirely possible. See discussion of Cook et al. (2004), toward the end of all this noise. Nevertheless, Ruiz-Trillo et al. (2002) requires no panic-driven redefinition of the Bilateria. We certainly thought about it, but soon stopped screaming (when we ran out of breath). Restored to our customary state of cheerful pomposity, we set about the serious business of making excuses. Fortunately, the branch order of a number of the major clades seems to be slippery in this study. Accordingly, we can't take it as solid authority that Acoelomorpha has fallen right out of Bilateria. Halanych (2004) has attempted to summarize those results in the figure reproduced here. However, it is unclear what definition he is applying to Bilateria. In our scheme, there are only three kinds of Bilateria: deuterostomes, protostomes and hobbits (stem protostomes -- the yellow lines in the figure). Since Halanych supplies no phylogenetic definition, we were forced against our will, and with much gnashing of teeth, rending or garments, etc., to consult the primary sources for the three groups marked with red lines. As it turns out, the Orthonectida are almost certainly metazoans, particularly under our stem group definition; but it's anyone's guess whether they, or any other members of the mysterious Mesozoa, are also bilaterians. Hanelt et al. (1996). At any rate neither Hanelt's study nor any later work exclude the Mesozoa from Middle Earth. Zrzav� (2001). The Myxozoa may well reside in Middle Earth. Monteiro et al. (2002). Zrzav� (2001) asserts that myxozoans are more primitive, but his review was written before Monteiro's report that the morphologically bilaterian Buddenbrockia is a myxozoan. The Acoelomorpha are consist of two different phylum-level groups, the Nemertodermatida and the Acoela. Both of these groups were parts of the classical Phylum Platyhelminthes. However, molecular studies consistently separate the acoelomorphs, and the separation is plausible on morphological grounds. We will return to these candidate hobbits later. For the moment, it is enough to note that both are likely bilaterians, but either or both may be protostomes. Finally, Halanych places the Chaetognatha right in the middle of Middle Earth. This is recent work from Halanych's own lab, and it has not yet attracted much support from elsewhere. Chaetognaths ("arrow worms") were considered to be deuterostomes until quite recently, when they were relocated to the protostomes. Possibly the thought of moving this lot yet again has inspired some resistance. "It is a lovely language, but it takes a very long time to say anything in it." Phylogenetic methods may ultimately be able to sort out the long branch problem by using carefully chosen, highly conserved sequences. But phylogenetic methods, particularly molecular methods, can also suffer from an opposite problem, i.e. not just long branches, but short stems. A branching pattern can best be resolved if the radiation occurs in a relatively slow and orderly way, so that each critical divergence is clearly separate from the previous one. However, Middle Earth was being populated right in the midst of the Cambrian Explosion -- arguably the most rapid series of evolutionary changes which the Animal world has ever experienced. The Molecule Mob have always argued that the Cambrian Explosion did not exist, and that the actual divergences occurred deep in the Neoproterozoic. However, if that were the case, as some of the Moleculons now admit, these tree-building methods ought to work much better than they in fact do. Rokas et al. (2003). Indeed, Rokas et al. (2005) have now turned the argument on its head. They make a fairly convincing case that the failure of sequence-based methods to reconstruct the tree at this point in time tends to prove the existence of a Cambrian Explosion. They point out that, if the Explosion did occur, and if it lasted significantly less than 50 My (which is a reasonable bet on morphological grounds), even the complete sequence of the all living lineages from that event may be mathematically insufficient to reconstruct the branching pattern. In other words, it may not even theoretically possible to construct the correct tree using these methods. The fact that we cannot seem to get consistent results using these methods strongly suggests that the metazoan phyla in fact diverged within a short space of time. Note, however, that Levinton et al. (2004), using the same theoretical framework, claim that that they have recovered the correct phylogeny using sequence methods, so that the Cambrian Explosion cannot have existed. However, the phylogeny they rely on (Wray et al., 1996) actually draws a tree of only 6 metataxa, using individual taxa and methods which are not fully described. We view this dispute with placid ambivalence, suspecting that the truth lies between the positions garrisoned by these contending forces. Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page uploaded MAK020407, last modified ATW050724 checked ATW050724 this material may be freely used as long as attribution is given Palaeos BILATERIA METAZOA BILATERIA -2 Page Back Bilateria Home Glossary Pieces Page Next Unit Back Metazoa Home Dendrogram Taxon Index Unit Next Bilateria-2 Metazoa |--Porifera (paraphyletic?) `--Radiata |--Cnidaria `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa `--Spiralia Introduction Building Bilateria The Problem of Middle Earth Trees in the Entwood "Where Many Paths and Errands Meet" The Fellowship of the Ring References "Where many paths and errands meet" Given the very serious problems with the usual phylogenetic methods, we are forced back onto embryology. Unfortunately, although Haeckel's dictum holds remarkably well in getting us from choanoflagellates to Cnidaria, it makes no robust predictions about Urbilateria or Middle Earth. Nevertheless, by triangulating through the known developmental scripts of existing organisms, it has been possible to say a few useful things about Urbilateria and the inhabitants of Middle Earth. In a few cases, we can look to classical developmental biology, as Haeckel did. However, in most cases, we must depend on the homology of transcription factors, the proteins that control developmental programs by promoting or suppressing the expression of particular genes. Reading the Runes Runic scripts are writing schemes which tend to fall in between pictograms and context-dependent constructs like alphabets. Erwin & Davidson (2002) remind us that transcription factors are somewhat the same way. They argue that just because (1) many protostomes and deuterostomes have eyes and (2) the pax-6 transcription factor is involved in the morphogenesis of eyes in both cases does not mean that the eyes of protostomes and deuterostomes are homologous, or even that Urbilateria had eyes. The lowest common denominator is pax-6 regulation of photosensitive pigments. These pigments may be found in eyes in many organisms, but they may also have been spread throughout the ectoderm of Urbilateria, for all we know, as they are in some echinoderms. On this basis, Erwin & Davidson conclude that Urbilateria was probably a very simple organism, possibly similar to the Ediacaran Kimberella (or the extant, and even simpler, Xenoturbella). It's a good point. The meaning of these runes depends on context and cannot be equated one-to-one with some particular morphological expression. But they aren't a completely arbitrary alphabet, either; and, besides, we know something about their context. Like most questions of homology, it is not entirely a yes-or-no affair. The eyes of protostomes and deuterostomes are homologous -- to the extent that they both use the pax-6 regulatory system in a way which seems to have been unchanged since their last common ancestor (by definition, Urbilateria). But, whether we can make stronger claims to homology depends on what other evidence we have. Consider a few examples. The Underworld of Middle Earth Perhaps the most fundamental issue relates to the formation of mesoderm. The single most basic difference between bilaterians and all other animals is the presence of this third germ layer. If mesoderm is not homologous in all bilaterians, then we may as well pack it in and quit blathering about Bilateria, because Bilateria won't be a monophyletic group. Unfortunately, we aren't going to be able to weasel out of this discussion so easily. Technau & Scholz (2003) have recently reviewed this area. Here, the case for homology is strong, and we may be reasonably certain that all hobbits have mesoderm. To put the matter briefly, mesoderm formation is run by some combination of the following four genes (or gene families): twist, snail, brachyury, and the GATA family. Despite the names, this is probably not a crime syndicate. The GATA family, for example, is supposedly named for part of the sequence to which the GATA gene products bind. On the other hand, the GATA family are elusive, hard to pin down, and appear to operate by "facilitating" the business of higher level inducers, "enforcing" their directives at the level of individual genes. So, it's safer to be ... respectful. The GATAs are unique to Bilateria and may be a synapomorphy of the clade. The other factors play rather interesting roles in the Cnidaria. Twist is responsible for muscle, or, rather it "induces" endodermal cells to form muscle. The endoderm might make muscle anyway, if asked politely, but twist is a very convincing inducer. It hangs around just in case, to keep everyone ... respectful. Snail is also involved in muscle formation, but its role lies in making the right contacts -- regulating cell-to-cell adhesion or mobility, making the right arrangements. Brachyury just sits on the lip of blastopore. It doesn't seem to do much of anything, except to offer little suggestions to cells migrating into the archenteron. Mostly those cells are, as we said, respectful. But, if not, it turns out that brachyury happens to be very close friends with twist and snail. Muscle is one of the basic mesodermal tissues in bilaterians, and it is easy to see how this very efficient arrangement could lead to the formation of an entirely new germ layer. Rieger & Ladurner (2003). All that is needed is to recruit more lieutenants to take on more specialized jobs, which is presumably where the GATA Family came in. While this is an elegant explanation of mesoderm evolution, it seems that not everyone agrees. See, e.g., Lartillot et al. (2002), reporting that Patella orthologs of twist and snail are not involved in mesoderm specification. Perhaps so, but we are unconvinced. Mind you, this has absolutely nothing to do with the alleged little gift we allegedly received from the alleged GATA brothers. They don't even exist. It's all malicious gossip. Honest! We just believe in being ... respectful. Rings of Power Another point which now seems secure is that Urbilateria was not segmented. Seaver (2003). Segmentation is best understood in arthropods and, more generally, in the Ecdysozoa. Almost all ecdysozoans are segmented but, tellingly, their most basal living group, the nematodes, are not segmented. In insects, segmentation involves two types of transcription factors: polarity genes (particularly engrailed) and pair-rule genes. While polarity genes are quite commonly used in the same way in other ecdysozoans, the pair-rule genes are not. They appear to play a secondary role. Seaver (2003). Most lophotrochozoans are unsegmented. Among the segmented annelids, the segmentation pattern is established through stereotyped spiral divisions in early embryogenesis, which is unlike the insect model. The initiating embryological tissue varies a good bit within the Annelida, and even between adult and larval segments. Like vertebrates, but unlike insects, segmentation proceeds in an anterior to posterior sequence; and; unlike both, the number of segments is not even approximately fixed, and segments continue to develop after embryogenesis is complete. Engrailed is involved in segmentation, but it seems to be a secondary messenger. The pattern of engrailed expression differs markedly even among the Polychaeta, and normal development is possible after ablation of engrailed-expressing cells. The heavy lifting is done, instead, by a completely different set of genes, the "gap" genes, which have no role in insect segmentation. Seaver (2003) There is no evidence that any of the basal deuterostomes were segmented. Segmentation was re-invented among early chordates. Unlike any protostome, chordates use a rather delicate timing mechanism based on oscillating expression of the notch gene to produce segments during elongation of the dorsal midline. This creates mesodermal aggregates, somites, which actually create the segmental structures. Pair rule, polarity and gap genes are present, but seem to have little or no role in segmentation. Accordingly, we can be fairly confident that Urbilateria was not segmented and that any hobbits, orcs, elves, etc. are unlikely to exhibit segmentation. Seaver (2003). The Eyes of Sauron As discussed, Erwin & Davidson (2002) use pax-6 and eye development as an example of the crawling evils of naive reliance on crude transcription factor homologies. Actually, a more current version of this story, as reviewed by Arendt & Wittbrodt (2001), is a rather more urbane and polished tale. Arendt and co-workers have further refined the work since that time. Fortunately, it is not necessary for us to summarize this work, since Pharyngula has conveniently predigested the data at Rhabdomeric and ciliary eyes. A lot more than pax6 backs up the claim that eyes have extensive homologies across Bilateria. For those too indifferent even to read Pharyngula's abbreviated account, here's the TV Guide version: Light comedy. Two photosensitive young systems, the ciliary and rhabdomeric, meet in the Proterozoic where they both work as photoperiod detectors. In the deuterostomes, they move in together, but the ciliary system starts seeing others, and the rhabdomeric system is relegated to a supporting role and menial household work. In the protostomes, they break up, and the rhabdomeric system goes on to have a glamorous career in pictures while the ciliary cells have a dead-end job in the brain. Cast of billions. 600 My, mostly B&W or limited Color, PG-13 (adult and/or larval situations, some nudibranchs). "Hobbits ... are inclined to be fat in the stomach" This section could well be called Haeckel's revenge. A number of workers have recently obtained spectacular results with oldfashioned descriptive embryology. In part, the quality of these papers is due to the availability of sophisticated molecular probes for looking at gene expression. However, some of the improved quality simply results from doing better embryology. The tendency in the past has been to compare embryos of vertebrates and Drosophila. But both have very specialized developmental patterns and are unlikely to tell us much about Urbilateria. The tale from the vertebrate-fruit fly comparison is well known. In protostomes, the mouth develops "first." More accurately, the blastopore elongates along the future ventral midline and the mouth and anus develop at opposite ends. In deuterostomes, the gut elongates. The blastopore forms only the anus, and the mouth is formed by punching a new hole in the ectoderm along the future ventral midline. This seems very different only if one bases the comparison on vertebrates, in which mouth formation is delayed and involves some very peculiar behavior by the surrounding ectoderm. See, e.g., The Basisphenoid and Teeth: Overview. Arendt et al. (2001) do the sensible thing and compare the surprisingly similar primary ciliated larvae of polychaete annelids and hemichordates. Despite the fact that these are protostome and deuterostome, respectively, their primary larvae are strikingly similar in morphology and, as it turns out, in the way they express the key transcription factors related to gut formation: otx, brachyury and goosecoid. At this point, we are moved to remind the enthusiastic reader of the teachings of Erwin & Davidson (2002). These factors may not have the same degree of priority in the developmental scheme. They may not be doing the same thing -- but it's a very pretty picture all the same. We lack the space for a full description of what Arendt & Co. did, but the bottom line is a strong case for homology of a three- (actually four-) part gut with a mouth, stomadeum (pharyngeal region in vertebrates), an undifferentiated middle section, and an anus. The Wisdom of the Elves Last, but far from least, we consider the information on the nervous system; and, indeed, the subject makes us nervous. We have readily confessed that the scope of this essay has forced us to rely on secondary sources. Here, we have been schooled by Alain Ghysen (2003) who produces utterly intimidating work at the intersection of molecular, developmental, evolutionary and neurobiology at the Universit� Montpellier II, when he is not raising orchids or studying poetry in an interesting array of archaic languages. Whether Prof. Ghysen is actually an elf, we cannot say. Before rejecting this hypothesis, consider also that he is a dedicated archer. We can say that he, like an elf, is sometimes opinionated and, when asked to give his opinion, he is rarely shy about expressing his views. In Ghysen (2003), he gives fair warning that he will "venture into somewhat speculative grounds." He then proceeds to erect a strongly fortified position around the proposition that Bilateria not only had a reasonably well-organized nervous system, but also the framework of a multi-part brain, brainstem, longitudinal nerve cord, and several types of specialized sensory cells, particularly mechanosesnsory cells. The bit about the proto-brain sounds like a stretch, but others, working from wholly different premises, have come to the same conclusion. Meinhardt (2002) (mainly body patterning in early development). Ghysen, as one might expect, relies on a battery of molecular, developmental, evolutionary, etc. evidence. We won't attempt to summarize it here, since we have gone on too long as it is. At least that's our story, and we're sticking to it. In fact, it is already time to move on to the next section: Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page uploaded MAK020407, last modified ATW050724 checked ATW050724 this material may be freely used as long as attribution is given Palaeos BILATERIA METAZOA BILATERIA -3 Page Back Bilateria Home Glossary Pieces Page Next Unit Back Metazoa Home Dendrogram Taxon Index Unit Next Bilateria-3 Metazoa |--Porifera (paraphyletic?) `--Radiata |--Cnidaria `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa `--Spiralia Introduction Building Bilateria The Problem of Middle Earth Trees in the Entwood "Where Many Paths and Errands Meet" The Fellowship of the Ring References The Fellowship of the Ring In this final episode, we will attempt to do two things: (1) sum up the characteristics of Bilateria and (2) briefly review the candidate Middle Earthlings in view of those findings. Detailed comparison must be left for more detailed consideration of each particular kind of hobbit, orc, elf, etc. The One Ring One of the advantages of really profound ignorance is that it makes neutrality much easier. We began looking into this subject with very few preconceptions about Urbilateria. Having finished for now with this suppositional ancestor of all Bilateria, we find that he is a peculiar mixture of highly derived, and exceedingly primitive. Urbilateria undoubtedly was a triploblast. That is, he had a fairly well-established system of mesoderm induction. Technau & Scholz (2003). Like cnidarians, he had muscles. Since he had a differentiated ectoderm separate from the gut, and also had a longitudinal layer of muscle and bilateral symmetry, he probably had some degree of motility. Indeed, he would almost have to have been mobile, unless he were fixed to the substrate. But a sessile lifestyle seems unlikely. Sessile organisms leave trace fossils in the form of holdfasts, variously reinforced holes in the substrate, and, frequently, mineral remains. Nothing like this is associated with bilaterians until the Small Shelly Fauna of the Terreneuvian. So, Urbilateria was either motile or he arrived extraordinarily late, in addition to being oddly designed for a benthic couch potato. There are several other reasons for believing Urbilateria was not sessile. One is trivial. Sessile aquatic animals are rarely very small. It is simply too easy for them to be buried in sediment. Yet we can be relatively confident that Urbilateria was indeed small, since there are no indications of anything large and bilaterian until the middle of the Early Cambrian. Of course, fossils from any earlier date are extremely rare, so some behemoth proto-planarian might yet show up. However, while the record of Neoproterozoic body fossils is poor, we have a large sample of worm-like trace fossils to examine, beginning in the Ediacaran. These are all quite small. The next proof is important: the complex gut. Arendt et al. (2001). Suspension feeding doesn't require a specialized mouth or stomadeum. In fact, that would be counterproductive. A suspension feeder wants a big surface area to catch potentially edible particles. Filter feeding is a possibility, but this requires rather specialized equipment, none of which seems to have survived as homologous structures in the main Bilaterian clades. The path of high probability is that Urbilateria moved -- if not much -- in search of food. That leaves two possible lifestyles: on the substrate or in the water column. Kimberella seems to have been a preCambrian bilaterian bottom-feeder. If so, at least some bilaterians had this niche. It has recently been shown that some Late Proterozoic bottoms may have experienced extensive, millimeter-scale horizontal bioturbation (Dornbos et al., 2005) -- presumably caused by bilaterians not yet able to penetrate the substrate. In any case, it is nearly impossible to imagine something this small and ungainly as a swimmer. Doubtless there were planktonic forms, but Urbilateria seems rather complex for life as a floater, and there would be little point in its various physical specializations. So, we conclude that Urbilateria was part of the benthic epifauna -- a bottom crawler. We have two sorts of physical models for Urbilateria. The trace fossils and Acoela suggest something long and generally worm-like. Kimberella and Xenoturbella look more like very primitive mollusks. Fortuitously, the living Nemertodermatida are, morphologically, more or less in the middle, which is probably our best bet. Finally, The evidence indicates that Urbilateria had a nervous system with one or (more likely) two main longitudinal nerve cords and an anteriorly-placed nerve plexus, sensory cells for the perception of light, and some kind of mechanosensory system. The Fellowship of the Ring All this is excellent news, since it constitutes a reasonably good fit with some of the candidates for Middle Earth citizenship, particularly the Acoelomorpha. This is the name given to a supposed clade made up of Nemertodermatida and Acoela. Four other groups are possibilities. Two seem too primitive, and may well turn out to be weird Radiata: the Myxozoa and Mesozoa. Two others are probably too specialized and may turn out to be protostomes (and ecdysozoans in particular): the Chaetognatha and Gnathostomulida. Nevertheless, we will include them all in this section for now, although we'll probably change our minds before it's finished. Acoela: The Acoela were classically shelved with the Platyhelminthes, those exceedingly dull worms which you quickly forgot after the first quiz in Introductory Zoology. You heard on good authority that the instructor never asks about them on the Final. This turned out to be true. In fact, it is always true. No instructor ever asks about the Platyhelminthes on the final. It is simply one of those unspoken laws of academic nature. So, how could we possibly prevail on you to stay awake long enough to absorb serious data on the Acoela -- a group of worms so cloddish and uncouth that they have been demoted from the platyhelminths to an even lower plane of phylospace? We will perform this impossible task by applying a time-honored academic maxim. "The difficult, we do immediately. The impossible, by pawning it off on someone else". See the excellent essay on this topic by Prof. Seth Tyler (U. Maine), or pages 9-11 of Tekle (2006). Tentatively, we disagree with almost all of Prof. Tyler's phylogenetic conclusions. The Acoela are quite specialized and weird, but this is only to be expected of a surviving twig at the base of an enormous radiation 600 My ago, or more. Further, the closely related Nemertodermatida provide a connecting link to more conventional bilaterian designs. The acoel gut is not a conventional 3-part bilaterian gut, but the longitudinal body wall muscle net, nerves, brain, mode of life, and body form are correct. For that matter, even the syncytial "gut" departs further from the cnidarian or poriferan system than from the bilaterian archenteron. Development proceeds in a recognizably bilaterian fashion, with a spiral cleavage pattern (albeit a spiral pattern different from that of annelids). Morphologically, the Acoela are simply well-behaved hobbits with full stomachs. The one real hitch comes from Cook et al. (2004), who looked at acoel hox genes. To make a long and somewhat uncertain story shorter and more certain than it really is, at least one acoel has only three hox genes, rather than the usual bilaterian minimum of 7-8. Then again, as these authors note, nematodes also have a reduced hox set. In that case, it is generally thought that the number is due to gene loss. So, the number in Nemertodermatida may be due to a similar effect of small body size. Nemertodermatida: The Nemertodermatida are a small group of acoels in rehab. They're trying to conform. They have guts. Sure, they've gained a little weight, but their nerves are better. Certainly, their sex lives are simpler and more conventional. Their developmental pattern is similar to that of acoels. Larsson (2003). They're taking it one day at a time. Myxozoa: The Myxozoa are odd parasites, originally thought to be protists (Brooke & Holland, 2003), with almost nothing to suggest that they are bilaterians -- usually. The Myxozoa are generally found as intracellular parasites with very complicated lives, including two hosts; for example, annelid and fish. Recently, Monteiro et al. (2002) realized that the mystery worm and putative bilaterian Buddenbrockia was actually an unrecognized form of the myxozoan, Tetracapsula. In this avatar, the myxozoan has body wall muscles in four regular muscle blocks and vaguely bilateral symmetry. Id. Initial molecular and morphological studies placed the myxozoans as close cousins of the parasitic cnidarian Polypodium. Later molecular work put them in the Bilateria. Halanych (2004). Nevertheless, outside the noisy clan of molecular enthusiasts, those who have actually studied Myxozoa seem to be convinced that myxozoans are, in fact, cnidarians. Some are polite enough not to say so directly. Kent et al. (2001). Others are less polite, possibly with good reason. Siddall & Whiting (1998). As Siddall & Whiting point out, it is hard to accept a bilaterian with nematocysts, and the polar capsules of Myxozoa are undoubtedly nematocysts modified as crowbars to break into host cells. Zrzav� (2001) (citing much other evidence). One obvious problem is, as we mentioned at the beginning, that few participants in these debates ever define their terms. It is not clear that any of these folks would assert that Myxozoa are bilaterians if they actually bothered to give Bilateria a phylogenetic definition. It is entirely possible that this entire dispute is, quite literally, about nothing. Nevertheless, we will treat the Myxozoa as presumptive Bilateria for now, simply for lack of a better place to deal with them. Mesozoa: The Mesozoa are another group of intracellular parasites -- unlikely candidates, you'd think, for bilaterian status. They are now believed to be made up of two, possibly unrelated, groups: the Dicyemida (Rhombozoa) and Orthonectida. They are known to have hox-like genes, and one of the dicyemid genes has a lophotrochozoan signature. Some, generally conservative, workers have found that the Mesozoa are in fact monophyletic and that they branch near the Acoelomorpha. Siddall & Whiting (1998); Zrzav� (2001). This creates difficulties, since the Mesozoa have very little other than molecular traits to tie them to the Bilateria. However, so far as we are aware, the members of this taxon are all obligate intracellular parasites. So, extreme simplification is only to be expected. Chaetognatha: As mentioned, the chaetognaths ("arrow worms") are clearly orthodox bilaterians. The question to be answered is, "why aren't they protostomes?" Classically, they were regarded as deuterostomes, but that now seems extremely unlikely. The chaetognaths were subsequently moved to the ecdysozoan region, but this became less likely based on subsequent detailed study of chaetognath development and neuroanatomy. The pattern of early development looks "spiralian," i.e. spiral cleavage with each of the early cells having a fixed role in the organism. (This is seems to be the primitive condition. Extavour & Akam, 2003.) A full complement of hox genes are present, but their expression is restricted to the central nervous system, as in certain (other?) lophotrochozoans. Papillon et al. (2005). Thus, the current best bet is that the chaetognaths are either lophotrochozoans or basal to Protostomia. Chances are that they are lophotrochozoans, but mitochondrial gene order phylogenies place them in a more basal position. Gnathostomulida: This is, as you may have guessed, one more gutless group of worms. They differ from the others in having jaws of a sort. As far as we can make out, no one has seriously proposed that they are basal to Protostomia. Like the Acanthocephala, they are probably evil rotifers. However, no one seems particularly comfortable with them either. Consequently, we will add them here as candidate orcs. And, while we're at it, perhaps we'll do the Acanthocephala, too. Perhaps we ought to be a bit more candid. Our reasons are actually not quite so frivolous -- almost as frivolous, but not quite. At least since the publication of an influential paper by Giribet et al. (2000), there has been a vague suspicion that the Ecdysozoa-Lophotrochozoa dichotomy masks a third, possibly rather large, clade. This clade -admittedly of variable composition -- tends to show up in a polytomy with the other two. It might be basal to Protostomia (bugs + slugs). It might not. Consequently, we have picked two likely members of the group, just to keep an eye on this possibility. So. There we have it. Middle Earth, an imaginary land, has been populated with a hypothetical population of odd creatures. Just for fun, we now add our completely baseless and intuitive guess about how things will turn out in the next five to ten years. Radiata = Cnidaria (i.e. Bilaterians are Cnidarians!) |--Myxozoa `--+--Acoelomorpha | |--Nemertodermatida | `--Acoela `--Bilateria |--Deuterostomia (by definition) `--+--Gnathostomulida `--Protostomia (bugs + slugs) |--Ecdysozoa (bugs > slugs) | |--Acanthocephala? | `--bugs `--Lophotrochozoa (slugs > bugs) |--Chaetognatha `--+--Mesozoa `--slugs ATW060218 References Arendt, D, U Technau & J Wittbrodt (2001), Evolution of the bilaterian larval foregut. Nature 409: 81-85. Arendt, D & J Wittbrodt (2001), Reconstructing the eyes of Urbilateria. Phil. Trans. Roy. Soc. B 356: 1545-1563. Brooke, NM & PWH Holland (2003), The evolution of multicellularity and early animal genomes. Curr. Op. Genet. Devel. 13: 599-603. Cook, CE, E Jim�nez, M Akam & Emili Sal� (2004), The Hox gene complement of acoel flatworms, a basal bilaterian clade. Evol. Devel. 6: 154-163. 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Page Back Page Top Unit Home Page Next images not loading? | error messages? | broken links? | suggestions? | criticism? contact us page uploaded MAK020407, last modified ATW050724 checked ATW050724 this material may be freely used as long as attribution is given Palaeos BILATERIA METAZOA PROTOSTOMIA Page Back Bilateria Home Glossary Pieces Page Next Unit Back Metazoa Home Dendrogram Taxon Index Unit Next Protostomia Metazoa |--Porifera (paraphyletic?) `--+--Radiata `--Bilateria |--Deuterostomia `--Protostomia |--Ecdysozoa | |--Tardigrada | `--Arthropoda | |--Trilobita | `--Pancrustacea | |--Crustacea | `--Insecta `--Spiralia (Lophotrochozoa) |--Bryozoa |--Mollusca `--+--Annelida `--Brachiopoda The Bilateral (and Coelomate) phyla can be divided into two groups, variously called the Schizocoela and Enterocoela, or the Protostomia and Deuterostomia. These are distinguished by embryonic cleavage patterns, the fate of the blastopore and coelom formation. The Deuterostomes are a relatively small (in terms of species and abundance) group that includes vertebrates, echinoderms, and a few assorted minor phyla. We probably consider them (and specifically the vertebrates) important mainly for anthropocentric reasons (we are deuterostomes). The Protostomes in contrast include the great majority of animal species, such as arthropods, molluscs, annelids, and many less wellknown taxa. The protostome condition is defined by a spiral and determinant cleavage in the early stages of embryo development, schizocoelous coelom formation (as the archenteron (embryonic gut) forms the coelom begins as splits within the solid mesodermal mass, and the formation of the blastopore (the original opening) into the mouth Beginning with Aguinaldo et. al. 1997, molecular studies in metazoan phylogeny have divided the Protostomia can into two further groups, one called Ecdysozoa or moulting animals, the other the Lophotrochozoa or Spiralia (the first name is more widely used, but the second is preferable, both being the senior synonym and because it and describes the spiral larval development of these invertebrates). Although there was originally some molecular phylogeneticists and morphologists did not accept this approach, it has bene increasingly confirmed by later studies and is now the mainstream position. Indeed, over the last decade or so the emphasis has shifted from the distinction between protostomes and deuterostomes and determining which phyla go where, to that of the phylogenetic relationships of teh various phyla that make up the Lophotrochozoa/Spiralia and Ecdysozoa (along with a few problematica which may belong to either group), with the now well resolved Deuterostomes the third party, on the sidelines. Descriptions Protostomia Grobben, 1908 : slugs > bugs Range: Fr Ediacaran Phylogeny: Bilateria : Deuterostomia + * : Ecdysozoa or moulting animals, the other the Spiralia Characters: Blastopore becomes mouth, typically schizocoelous with spiral cleavage. Comments: Various microscopic animals and others with simple body plans like the flatworms (platyhelminthes) are sometimes included in a third clade, Platyzoa, but this seems to be an artificial assemblage of mostly miniaturised spiralians. 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