Palæos:		Paleozoic Ecosystems
PALEOZOIC ERA		Benthic Marine Ecosystems

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note: the following passage is from Patrick J. Benchley and by D. A. T. Harper, Palaeoecology, pp. 128-137

The low-level Paleozoic benthos was dominated by fixed suspension feeders, mainly the brachiopods supported by the bryozoans and microcarnivores such as the rugose and tabulate corals. Mollusks were generally rare, although some bivalve-dominated associations occur throughout the Paleozoic and may have inhabited specialized habitats. The sponge-like stromatoporoids were responsible for carbonate buildups, particularly during the mid-Paleozoic, when they developed a range of growth modes including columnar, dendroid, encrusting and hemispherical forms (Kershaw, 1990).

By the mid-Ordovician, two major groups of anthozoans, the rugose and tabulate corals, were firmly established as microphages, although neither ever built substantial reef structures, lacking many of the adaptations, such as a basal plate, that helped the scleractinians to develop as the dominant reef-builders of the Modern fauna.

Paleozoic corals also advanced the development of colonial or compound life strategies. Tabulate corals developed only colonial forms, many fine-tuned for life on a soft substrate; the location, orientation. spacing and development of offsets in the corallites have been developed in a tool-kit of adaptations for different substrates.

Brachiopods dominated a range of nearshore to shelf-edge environments, suited to a variety of substrates at a range of water depths in marine conditions. The group was the principal epifauna of the Paleozoic. Certain basic aspects of the brachiopod morphology, such as the presence or absence of a pedicle, had a major control on the brachiopod life strategy. More subtle features of the animal, for example the size, shell shape and ornament, also had clear adaptive significance.

Crinoids, together with brachiopods, dominated the Paleozoic sessile benthos. The crinoids developed a range of articulatory structures allowing the stem considerable flexibility.  Although most crinoids are and were fixed, rheophile organisms, orientating their crowns into the current when feeding, some such as the Recent comatulid Antedon are mobile or attached by small roots or cirri, whereas the Jurassic Saccocoma may have been either epiplanktonic or part of the free-living benthos

Evolution, initially, in the crinoids of discoidal holdfasts permitted attachment to hard substrates; however, the development of more versatile root-like holdfasts allowed the group to target soft, muddy substrates commonly located in more offshore environments (Sprinkle and Guensburg, 1995). This transition between these two types of attachment structures may have driven a large-scale diversification of the group and its expansion into deeper-water environments during the early Paleozoic.

Bryozoan diversified during the Ordovician radiation, building larger and more complex colony types. Initially colonies were low with few zooids; multi-story complexes were developed to occupy available space and evolve efficient feeding strategies.

Only a shallow infauna was well developed in the Paleozoic fauna, although there were exceptions; it included bivalves, scaphopods, trilobites and crustaceans occupying depths of up to about 100 mm.  Nevertheless, by the early Carboniferous, trace fossil data suggests that depths of up to one meter of sediment were penetrated by bivalves. Bivalves first appeared during the early Cambrian as part of a shallow infauna.  Throughout the Phanerozoic the group has demonstrated a remarkable spectrum of adaptive morphologies for life above, at and within the sediment-water interface. Aspects of the Paleozoic bivalve fauna heralded the intense infaunal radiation of the group during the Mesozoic Marine Revolution, when the group became much more dominant.

Tiered profiles evolved during the Paleozoic (Ausich and Bottjer, 1982). The intermediate-level benthos (50-200 mm) was dominated by sponges, corals, giant bivalves, giant brachiopods, stalked echinoderms and fixed dendroid graptolites. High-level sessile benthos (200-500 mm) contained mainly crinoids and blastoids. The stalked echinoderms concentrated on improving the efficiency of their feeding techniques with the development of more elaborate calices.

Trilobites continued to dominate the mobile benthos, although bivalves, gastropods and echinoids locally moved across and through the sediment. Cephalopod mollusks such as the orthoconic nautiloids patrolled the benthos, in a role as the main macrophagous predator.

The following sequence of bottom-living biotas, defined according to major extinctions, adaptive radiations, and community reorganizations, is suggested for this period by Arthur Boucot (Phanerozoic Extinctions: How similar are they to each other?", Lecture Notes in Earth Sciences 30, Springer-Verlag Berlin Heidelberg 1990). Note that these do not always follow or equate with the Terrestrial or Freshwater succession:

• Early Ordovician
• Mid to Late Ordovician
• Llandovery (Early Silurian)
• Siluro-Devonian
• Famennian (Late Devonian)
• Carboniferous-Permian

	(Image not available)
Rugose Coral	Conulariid	Brachiopod	Nautiloid	Trilobite	Placoderm

 

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