Stromatoporoids Water Flow

Extended domical: V/B > 1 ' ^--'

enveloping non-enveloping

Figure 11.8 Stromatoporoid growth modes. (Based on Kershaw, S. 1984. Palaeontology 27.

Figure 11.8 Stromatoporoid growth modes. (Based on Kershaw, S. 1984. Palaeontology 27.

poroid skeleton; both bivalves and brachio-pods have been seen in borings within stromatoporoids that may have provided some of the first cryptic habitats for Phanero-zoic biotas.

Animals with a stromatoporoid grade of organization have been identified from rocks of Botomian age; however these forms were apparently short lived. Pseudostylodictyon from the Middle Ordovician of New York and Vermont may be the oldest true stromato-poroid, derived from a soft-bodied, spongelike ancestor in the Early Ordovician. Stromatoporoids formed the basis for reef ecosystems during the Silurian and Devonian, becoming largely extinct during the end-Frasnian (Late Devonian) event. The group revived in the Mid and Late Jurassic when stromatoporoids again participated in reef frameworks. Nevertheless, most groups disappeared at the end-Cretaceous mass extinction. However, some living sponges have a stromatoporoid grade of organization; Astrosclera and Calcifimbrospongia are both calcified demosponges with a stromatoporoid architecture.

Archaeocyatha

The Archaeocyatha or "ancient cups" are one of only a few major animal groups that are entirely extinct. They appear to have been an evolutionary dead end. The group exploited calcium carbonate during the early part of the Cambrian radiation to construct porous cup-or cone-like skeletons, usually growing together in clumps and often living with stromatolites to form reefs. The Archaeocyatha dominated shallow-water marine environments, usually in tropical paleolatitudes. From an Early Cambrian origin on the Siberian Platform, the group spread throughout the tropics, forming the first Paleozoic reefs. However, by the end of the Early Cambrian and the start of the Middle Cambrian, archaeocyaths are known only from Australia, the Urals and Siberia. They disappeared at the end of the Cambrian.

Current studies suggest that the Archaeo-cyatha have a grade of organization similar to poriferans; in fact most authorities would place the group firmly within the sponges as a separate class. Because no living intervallum septum outer wall intervallum septum outer wall

aporous septum inner wall porous septum sole

aporous septum inner wall porous septum sole

Ajacicyathida /v

Monocyathida solitary branching solitary

Archaeocyathida Tabulacyathida solitary branching

catenulate

pseudo-cerioid

catenulate

Archaeocyathida Tabulacyathida solitary

Inferred soft tissue distribution branching

Figure 11.9 The Archaeocyatha: (a) morphology and (b) classification, function and growth modes of the main groups. (Based on Wood et al. 1992.)

Direction of water flow: —Inhalent Exhalent

Inferred soft tissue distribution

branching

Kazachsetanicyathida

Kazachsetanicyathida encrusting

encrusting

Figure 11.9 The Archaeocyatha: (a) morphology and (b) classification, function and growth modes of the main groups. (Based on Wood et al. 1992.)

representatives of the group exist there has been, in the past, considerable speculation about the taxonomic affinities of the archaeo-cyaths: they have been classified with algae, calcified protozoans, poriferan-grade metazo-ans, animals with a grade of organization intermediate between protozoans and meta-zoans, and cnidarians - none of which now seems likely.

Morphology and classification: archaeocyath individuals and modules

Archaeocyaths are most commonly found in carbonates, and details of their morphology are usually reconstructed from thin sections.

Unfortunately many Cambrian carbonates have been recrystalized, often destroying the details of skeletal morphology. The exoskele-ton of the archaeocyathan animal is aspiculate and usually composed of a very porous, inverted cone composed of two nested concentric walls separated from each other by radially arranged, vertical septa (Fig. 11.9). Both the inner and outer walls are densely perforated and together define the intervallum, or central cavity, partitioned into a number of segments (loculi) by the radial septa, which are often less porous than the walls or sometimes aporous. The inner wall circumscribes the central cavity, open at the top and closed at its base to form a tip. The apex of the

Archaeocyaths

Figure 11.10 Some archaeocyaths from the Lower Cambrian of Western Mongolia, in thin section: (a) cryptic, solitary individual of Cambrocyathellus showing holdfast structures (x7.5), and (b) branching Cambrocyathellus tuberculatus with skeletal thickening between individuals associated with transverse sections of Rotundocyathus lavigatus (x5). (Courtesy of Rachel Wood.)

Figure 11.10 Some archaeocyaths from the Lower Cambrian of Western Mongolia, in thin section: (a) cryptic, solitary individual of Cambrocyathellus showing holdfast structures (x7.5), and (b) branching Cambrocyathellus tuberculatus with skeletal thickening between individuals associated with transverse sections of Rotundocyathus lavigatus (x5). (Courtesy of Rachel Wood.)

skeleton is usually buried in the sediment with a basal flange and roots or holdfasts adding anchorage and stability. In some taxa, the intervallum is partitioned horizontally firstly by porous shelves or tabulae or secondly with aporous, convex dissepiments, often extending into the adjacent central cavity.

Two main subdivisions have been defined within the group: the "Regulares" and the "Irregulares". The regular forms have an initial aporous, single-walled stage lacking dissepiments; soft tissue filled the entire body. The inner and outer walls are punctuated by septa and tabulae developed either singly or together. The irregular forms have initial aporous, single-walled stages with dissepiments. The twin walls have irregular pore structures, always dissepiments, and the skeleton is asymmetric; soft tissue was restricted by the development of secondary skeletal material. These groupings have now been shown to have little taxonomic value, reflecting rather ecological preferences (Debrenne 2007). Most archaeocyaths are "Regulares", including the orders Ajacicy-athida and Coscinocyathida; however the apparent abundance of regular genera may be due to excessive taxonomic splitting. There are fewer "Irregulares" but this ecogroup includes the orders Archaeocyathida and Kazachsetanicyathida.

Synecology: archaeocyath reefs

The archaeocyaths were exclusively marine, probably living at depths of 20-30 m on carbonate substrates. The phylum developed an innovative style of growth based on modular organization (Fig. 11.10). Such modularity permitted encrusting abilities and the possi

Figure 11.11 Archaeocyathan reef structures which, when preserved, become (a) boundstones, (b) bafflestones, (c) bindstones or (d) bioherms. (Based on Wood et al. 1992.)

Figure 11.11 Archaeocyathan reef structures which, when preserved, become (a) boundstones, (b) bafflestones, (c) bindstones or (d) bioherms. (Based on Wood et al. 1992.)

bilities of secure attachment on a soft substrate; moreover growth to large size was enabled, together with a greater facility for regeneration (Wood et al. 1992). The archaeo-cyaths were thus key elements of the first reef-type structures of the Early Cambrian (Fig. 11.11), in intervals of high turbulence and rates of sedimentation. However, although archaeocyathan reefs were probably not particularly impressive, usually up to 3 m thick and between 10 and 30 m in diameter, they were nevertheless amongst the first animals to establish complex biological frameworks, processing large amounts of seawater through their bodies (Box 11.2). Archaeocyathan reefs were always associated with calcimicrobes that may have been the main frame builders. There are also some examples of cryptic organisms living within the reef cavities, including other sponges.

Distribution: Cambrian world of the archaeocyaths

The first archaeocyaths are known from the lowest Cambrian (Tommotian) rocks of the Siberian Platform and are represented by mainly solitary regulars. During the Early Cambrian, the phylum diversified, migrating into areas of North Africa, the Altai Moun tains of the former Soviet Union, North America and South Australia (Fig. 11.13). Archaeocyaths were most common in the Mid to Early Cambrian (Botomian) when a number of distinct biogeographic provinces can be defined, but by the Lenian Stage the group was very much in decline. Few genera have been recorded from the Middle Cambrian and only one is known from Upper Cambrian strata. Archaeocyath history demonstrates a progressive move towards a more modular architecture in response to conditions of high turbulence. In general, solitary taxa dominated the Early Cambrian; but following the late Botomian, modular morphotypes continued after the extinction of most solitary forms (Fig. 11.14; Box 11.3). One advantage is that the abundance and diversity of the group in some parts of the world, particularly in Lower Cambrian rocks, has allowed its effective use in biostratigraphic correlation when there were few other organisms around that could act as zone fossils (see p. 28).

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