Box Classification of Cnidaria

The phylum is characterized by radial symmetry, with the ectoderm and endoderm separated by the mesoglea; the enteron has a mouth surrounded by tentacles with stinging cells. The phylum ranges from Upper Precambrian to Recent. The putative medusoid Brooksella, which predates the Ediacara fauna may, in fact, be a trace fossil. The group has a wide range of body plans (Fig. 11.18).

Class HYDROZOA

• This includes six main orders of small, usually polymorphic forms. Each has an undivided enteron and solid tentacles, and may form colonies. There are six main orders; the Chondrophora contains some of the oldest cnidarians

• Ediacaran to Recent

Class SCYPHOZOA

• Mainly jellyfish, contained in the Scyphomedusae, which are only preserved in Lagerstätten. The extinct Conulata is often included here since the group has a tetrameral symmetry and apparently has tentacles. Their long conical shells, for example Conularia, are composed of chitinophos-phate; conulates appeared in the Cambrian and were extinct by the Mid Triassic

• Ediacaran to Recent

Class ANTHOZOA

• These are exclusively marine, and most are sessile, colonial forms (though they have mobile planula larvae). The three subclasses, Ceriantipatharia, Octocorallia and Zoantharia (including the orders Rugosa, Tabulata and Scleractinia), all lack medusoid stages, possess hollow tentacles and have the enteron divided, longitudinally, by vertical septa. Both solitary and colonial forms occur. The class includes corals, sea anemones and sea pens. Octocorals often produce spicules that occur as microfossils

• Ediacaran to Recent

Class CUBOZOA

• The sea wasps and box jellyfish have both medusae and polyps and are mainly restricted to tropical and subtropical latitudes

• Carboniferous to Recent

Cnidarian Calcification
Figure 11.18 Main cnidarian body plans: (a) generalized scleractinian polyp, (b) generalized part of scleractinian coral colony, (c) living anemone, and (d) living jellyfish. (From various sources.)

include freshwater and colonial forms together with the fire corals and most kinds of "jellyfish". There are over 3000 living species inhabiting water depths up to 8000 m, mainly in marine environments. Supposed hydrozo-ans have been recorded from the Late Precam-brian Ediacara fauna (see p. 242), where genera such as Eoporpita and Ovatoscutum may be the oldest sessile members of the phylum. Hydrozoans reproduce either sexually or by asexual budding; the polyp stage is asexual and the medusoid normally sexual. The scyphozoans are mainly free-swimming medusae or jellyfish often inhabiting open-ocean environments. Some elements of the Ediacara fauna may be scyphozoans, for example Conomedusites and Corumbella; however many of the best-preserved fossil

Table 11.1 Features of the main coral groups.

Table 11.1 Features of the main coral groups.

Growth mode Septa

Colonial and solitary 6 prosepta; later septa in only 4 spaces Usual Calcite Poor

Ordovician to Permian

Colonial

Septa weak or absent

Colonial and solitary 6 prosepta; later septa in all 6 spaces

Growth mode Septa

Tabulae

Skeletal material

Stability

Range

Colonial and solitary 6 prosepta; later septa in only 4 spaces Usual Calcite Poor

Ordovician to Permian

Colonial

Septa weak or absent

Well developed Calcite Poor

Ordovician to Permian Triassic to Recent

Colonial and solitary 6 prosepta; later septa in all 6 spaces

Absent Aragonite

Good with basal plate forms have been collected from the Late Jurassic Solnhofen Limestone of Bavaria. Living members of the group include Aurelia, the moon jellyfish, and the compass jellyfish, Chrysaora. Although the anthozoans include the sea anemones, sea fans, sea pens and sea pansies, the class also includes the soft and stony corals. Following a short, mobile, planula larval phase, all members of the group pursue a sessile life strategy as polyps.

Corals_

Corals are probably best known for their place in one of the planet's most diverse but most threatened ecosystems, the coral reef. Shallow-water coral reefs form only in a zone extending 30° degrees north and south of the equator and reef-forming corals generally do not grow at depths over 30 m or where the water temperature falls below 18°C, although certain groups of corals can also form structures in deep-water environments. Corals are not the only reef-forming organisms but throughout geological time they have constructed three main types of reefs: fringing reefs, barrier reefs and atolls. These structures formed the basis for Charles Darwin's then cutting-edge analysis Coral Reefs published in 1842. Unfortunately, such structures are under current threat, including damage from increased bleaching, coastal development, temperature change of seawater, tourism, runoff containing agricultural chemicals, abrasion by ships' hulls and anchors, smothering by sediment, poisoning or dynamiting during fishing, overfishing of important herbivores and predators, and even harvesting for jewelry. There seems little hope for this spectacular habitat unless more attention is paid to conservation.

The anthozoans are the most abundant fossil cnidarians, pursuing a polypoid lifestyle. The class Anthozoa contains two subclasses with calcareous skeletons. Whereas the Octocorallia have calcified spicules and axes, the Zoantharia include the more familiar fossil coral groups, the orders Rugosa, Tabulata and Scleractinia (Table 11.1). The Octocorallia, including the Alcyonaria, have eight complete mesenteries and a ring of eight hollow tentacles; the skeleton lacks calcified septa but calcareous or gorgonin spicules and axes comprise solid structures in the skeleton. Although the group is only sporadically represented in Silurian, Permian, Cretaceous and Tertiary rocks, the octocorals are important reef dwellers today. Some familiar genera include Alcyonium (dead men's fingers), Gor-gonia (sea pen) and Tubipora (organ-pipe coral).

Morphology: general architecture

There are four main elements to the zoanthar-ian coral skeleton: radial and longitudinal structures, together with horizontal and axial elements. Corals have planula larvae. Following the planula larval stage the coral polyp initially rests on a basal plate or disk termed the holotheca and begins the secretion of a series of vertical partitions or septa in a radial

Brooksella Images

turbinate ceratoid cylmdncal calceoloid pyramidal

Figure 11.19 Terminology for the main modes of solitary growth in corals. (From Treatise on Invertebrate Paleontology, Part F. Geol. Soc. Am. and Univ. Kansas Press.)

turbinate ceratoid cylmdncal calceoloid pyramidal

Figure 11.19 Terminology for the main modes of solitary growth in corals. (From Treatise on Invertebrate Paleontology, Part F. Geol. Soc. Am. and Univ. Kansas Press.)

arrangement. At the circumference, the septa are joined to the theca or skeletal wall, which extends longitudinally from the apex of the corallum to the calice where the polyp is attached. During growth the polyp may secrete a series of horizontal sheets, or tabulae, together with smaller curved or angled plates or dissepiments. The columella, usually arising from the fusion of the axial edges of the septa, occupies the core region of the corallum. The vertical walls or septa radiate outwards from the columella and divide the corallite. Despite the apparent simplicity of the coral skeleton, there is a great deal of variation in both solitary and colonial growth programs and the end result is a remarkable array of shapes and sizes of corals.

The three main subclasses of stony corals have colonial or compound growth modes whereas only the Scleractinia and Rugosa have solitary skeletons. The solitary growth forms include conical, ceratoid or horn-shaped, calceoloid, cylindrical, discoid, patel-late, scolecoid, trochoid and turbinate skeletons (Fig. 11.19). Colonial corals with corallites have adopted either fasciculate or massive growth modes. Fasciculate styles exhibit either dendroid or phaceloid strategies with either no or poor integration. The hal-ysitid or cateniform chain-like growth strategy is a further variation on this pattern. The massive colonies are much more varied, with cerioid, astraeoid, aphroid, thamnasteroid, meandroid and hydnophoroid together with coenenchymal or coenostoid growth pro grams (Fig. 11.20). Moreover colonies with imperforate walls may exhibit phaceloid, cateniform, cerioid and meandroid forms, whereas those with perforate walls have only phaceloid and cerioid growth modes together with coenenchymal structures in some taxa, such as the sarcinulids. These growth modes are variably developed across the rugosans, tabulates and scleractinians - but meandroid and hydnophoroid modes were developed during the Mesozoic and are thus restricted to the scleractinians.

Colonial integration usually involves a loss of individuality. Many organisms display a transition from solitary growth modes, through morphologies with asexually budded modules, to a fully integrated colony with the growth or astogeny of the compound structure showing little variation across the individual corallites. The degree of integration of a colony is usually measured by the amount of cohesion between the individual skeletal parts and soft tissues and by the range of form observed between individual components. Clearly there is a spectrum from phaceloid modes with little or no integration to thamn-asteroid and meandroid (and coenenchymal) modes with high levels of integration. Individual polyps are no longer separated by cor-allite walls and may share a common enteron and nervous system. This suggests a high degree of integration where the colony approaches the body plan of a typical meta-zoan. These modes have varied through time (Fig. 11.21).

-mm^nmm siigai iiîïsi-'1ïïSZ'-Si&rvir¡^^W.Í^;,.^î'•»

Aphroid CoralsAphroid Coral

aphroid meandroid hydnophoroid aphroid meandroid hydnophoroid

coenostoid phaceloid halysitid

Figure 11.20 Terminology for the main modes of colonial growth in corals. (Redrawn from various sources.)

coenostoid phaceloid halysitid

Figure 11.20 Terminology for the main modes of colonial growth in corals. (Redrawn from various sources.)

Scleractinian corals may be highly integrated because they have symbiotic zooxanthellae (see p. 285). The relatively low levels of integration seen in the Rugosa and some Tabulata colonies perhaps suggests a lack of algal sym-bionts. There has been a great deal of argument about this. Some rugosans are in fact quite highly integrated, and it is questionable whether high integration should only be associated with the presence of zooxanthellae.

Coral experts also use quantitative approaches in describing colony shapes. Key measurements are made on the colony and these are plotted on a ternary diagram. A series of fields can be mapped out within the triangle - for example, bulbous, columnar, domal, tabular and branching colonies are discriminated (Fig. 11.22). These different growth strategies may be ecophenotypic (see p. 123), commonly reflecting ambient environmental conditions.

Rugose corals

Rugose corals are generally robust, calcitic forms with both colonial and solitary life modes, more varied than those of tabulates. Rugosans have well-organized septal arrangements with six cardinal or primary septa. Secondary septa are inserted in four spaces around the corallum - between the cardinal septa and the two alar septa and also between the two counterlateral septa and lateral septa (Fig. 11.23a). Horizontal structures such as the tabulae, dissepiments and dissepimentaria are also well developed across the order. Undoubted rugosans, such as Streptelasma, with short secondary septa and lacking a dis-sepimentarium, are not recorded until the Mid Ordovician. By the Late Ordovician, rugose faunas were well established with the development of a wide variety of morphologies (Fig. 11.23b; Box 11.5). For example, the

Figure 11.21 Schematic graph of the distribution of colonial growth modes through the Phanerozoic. (Based on data in Coates, A.G. & Oliver, W.A. Jr. 1973. In Animal Colonies: Development and function through time. Dowden, Hutchinson and Ross.)

Silurian Goniophyllum was pyramidal with a deep calyx, whereas the Devonian Calceola was a slipper-shaped form with a semicircular lid and the compound Phillipsastrea had a massive, astraeoid growth mode (Fig. 11.25).

Diverse rugosan faunas occurred during the Carboniferous Period. Solitary forms such as the large horn-shaped to cylindrical Caninia, the cylindrical Dibunophyllum with a marked dissepimentarium, the long cylindrical Palaeosmilia, and the smaller horn-shaped Zaphrentis are often conspicuous members of Carboniferous coral assemblages. The fasciculate, phaceloid Lonsdaleia and Lithostrotion with usually massive, cerioid growth modes are locally common. The order declined during the Permian until there were only 10 families left, and these disappeared by the end-Permian mass extinction (see p. 170).

Tabulate corals

As the name suggests, tabulate corals have well-developed tabulae (Fig. 11.26). The septa are usually very much reduced to short spines or are absent, and dissepiments are variably developed (Fig. 11.27). The group is varied, with erect, massive, sheet-like and chain-like colonies and branching forms; some authors have suggested that some tabulates, such as

Rugosan Horn Coral Parts
Figure 11.22 Ternary plot of colonial growth modes based on the shape of the colonial coral. (Based on data in Scrutton, C.T. 1993. Cour. Forsch. Inst. Senckenberg 164.)
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  • azelio zetticci
    When coral lack water?
    6 years ago

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