Stromatoporoids

I aclonal, solitary ^ clonal, modular ^ degree of integration

Figure 11.6 Stratigraphic distribution of reef-building sponges and related parazoans, together with the sderactinian corals.

Late Carboniferous the chaetetid calcified sponges were important reef builders. In the Permian and mid-Triassic, structures involving sphinctozoans were common and the Mid to Late Jurassic was marked by bioherms of lithistid demosponges, while the hexactinel-lids migrated into deeper-water environments. Jurassic sponge reefs dominated by hexacti-nellids and lithistids have been documented throughout the Alpine region. Cup-shaped and discoidal morphotypes dominated hard and soft substrates, respectively, and these developed a substantial topography above the seafloor, and modern analogs of these hexac-tinellid reefs are now known from off the coast of Canada.

As noted earlier, the acquisition of a calcareous skeleton was not confined to any one class; the calcareous skeleton was developed a number of times, convergently, across the phylum, with a few basic plans superimposed on pre-existing sponge morphology. Consequently, various groups have been recognized on the basis of the calcareous skeleton, but components of each group arose independently in different clades. In broad terms, the chaetetids and sphinctozoans, together with the archaeocyaths and stromatoporoids, were the most important calcareous reef builders. However, the decline of the calcareous sponges in reef ecosystems during the Mesozoic is often correlated with the rise of the scleractin-ian corals, equipped with a superior nutrition-gathering system, associated with symbiotic zooxanthellae (see p. 285).

Stromatoporoidea_

The stromatoporoids were mound and sheetlike marine, modular organisms that appeared in the Mid Ordovician. These animals were common components of Late Ordovician, Silurian and Early to Mid Devonian shallow-water marine communities, forming irregular mounds on the seabed, associated with calcareous algae and corals. They have a superficial resemblance to some tabulate corals. The group reached an acme during the Mid Devonian but declined during the later Paleozoic and Mesozoic. Although stromatoporoids have been understandably classified with the cnidarians, their similarity to the modern calcified sponges and the discovery of spicules within the skeleton suggest that these, too, are poriferans and may well be a grade of organization within the Demospongea. In common with a number of other poriferans, the group is polyphyletic, with stromatoporoid taxa showing gross morphological convergence towards a common body plan or grade of organization. Because most stromatoporoids astrorhizal canal astrorhizal canal

Jurassic Stromatoporoids

Figure XX.7 Stromatoporoid morphology.

look like solidified cow pats, and are all superficially very similar, paleontologists must use thin sections to describe the microstructure and classify the species.

Morphology and classification

Typical stromatoporoids have a calcareous skeleton with both horizontal and vertical structures and often a fibrous microstructure (Fig. 11.7). The skeleton is constructed from undulating layers of calcareous laminae punctuated perpendicularly by vertical pillars. The surfaces of some forms are modified by small swellings or mamelons together with astrorhi-zae, radiating stellate canals, which are the traces of the exhalant current canal system. Siliceous spicules have been identified in some Carboniferous and Mesozoic taxa, suggesting that the primary skeleton was in fact spicu-late; the calcareous casing is secondary with probably low magnesium calcite precipitated within a framework of spongin.

Some authors have included the extinct stromatoporoids within the sclerosponges, a small group of enigmatic sponges with siliceous spicules embedded in aragonite, commonly found today in cryptic environments in the tropics. Others have classified them as cyanobacteria, foraminiferans or even as a separate phylum. But these assignments are probably only of historical interest because most morphological evidence places them firmly in the sponges.

Autecology and synecology: stromatoporoid life and times

Stromatoporoids were marine organisms usually associated with shallow-water carbonate sediments often deposited in turbulent environments. Many genera were important constituents of reefs, particularly during the Silurian and Devonian. For example, the spectacular Silurian reefs on the Swedish island of Gotland are characterized by a variety of stro-matoporoid growth forms (Kershaw 1990), whereas throughout North America and northern Europe Devonian reef complexes and bioherms are dominated by stromatopo-roids. These animals had complex water systems and grew in a variety of different ways: columnar, dendroid, encrusting and hemispherical forms were associated with specific energy and turbulence levels

Stromatoporoids were also associated with their own diverse microecosystems; those preserved in the Silurian of Gotland provided habitats for communities with over 30 epibi-ont species (see p. 97) that lived attached to the animals. Boring, encrusting and epifaunal organisms made good use of the cavities and substrates available in and on the stromato-

Stromatoporoid morphotypes laminar low domical low domical low domical low domical low domical low domical laminar

high domical

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extended domical bulbous

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