G8

Fig. 5 Orthambonites-crinoid Community a Orthambonites (Brachiopoda: Orthida)

b Pleurorthis (Brachiopoda: Orthida)

c Praenucula (Mollusca: Bivalvia: nuculoid)

d Actinodonta (Mollusca: Bivalvia: actinodont)

e Ramseyocrinus (Echinodermata: Crinozoa)

f Neseuretus (Arthropoda: Trilobita)

g Merlinia (Arthropoda: Trilobita)

Lower Ordovician 5 Orthambonites-crinoid Community

In the Lower Ordovician most bottom-dwelling invertebrates, with the exception of the trilobites, lived in shallow water. Since these were mostly suspension feeders, like brachiopods, molluscs and echinoderms, they preferred the turbulent environments where the food supply was most abundant. The trilobites, being chiefly deposit feeders, were not so restricted. Unlike later benthic communities, Ordovician communities were thus more diverse in shallow water than in the deeper parts of the sea. They also differed from those of the Cambrian, where suspension feeders made up a smaller proportion of the total fauna. But trilobite diversities were different: in clastic sediments the diversity of trilobites generally increased with depth during the Lower Palaeozoic, but the most diverse of all communities were in certain shallow water carbonate environments.

A typical example of a rich Lower Ordovician (Arenig) shallow water fauna occurs in south-west Wales (Bates, 1969). At this time, the brachiopods were dominated by impunctate orthids like Orth-ambonites and Pleurorthis, but before the end of the Lower Ordovician many other groups of articulate brachiopods made their first appearance. Both these genera have strong radial ribs which may have served to strengthen the shell, but Rudwick (1964, 1970) has shown that strong ribs also help to keep sand grains out of the brachiopod interior while admitting a sufficient flow of water for feeding and respiration. Ribbed brachiopods are thus especially common in turbulent sandy environments. These genera also have large pedicle openings and were probably anchored by strong pedicles to shell fragments on the sea floor.

The bivalves include both suspension feeders and deposit feeders; unlike most modern bivalves in this environment they had not developed ribs. Praenucula (a deposit feeder) and Actinodonta (probably a filter feeder) are the commonest genera of the five which have been recorded in this community; they have moderately strong teeth which serve to keep the two valves interlocked.

In addition to the brachiopods and bivalves, the echinoderms, gastropods and bryozoans are also present in the epifauna. The echinoderms include crinoids like Ramseyocrinus which can occur in large numbers, cystoids, and asterozoans.

The trilobites were only represented by a few genera in this shallow environment where sands with little organic matter were the dominant sediments; most trilobites, being deposit feeders, were associated with shales which contain a higher proportion of organic matter. Neseuretus and Merlinia are two genera which were apparently able to feed in sand and silt: they are the only trilobites present at many localities.

6 Asaphid Community

In deeper water, provided it was not stagnant, the trilobites were more abundant and diverse while the suspension feeders decreased greatly in importance. The trilobites included such varied forms as the large Asaphus, Ogygiocaris and nileids and the small blind tri-nucleids. Many asaphaceans have flattened margins to their shell which probably assisted them on soft muddy substrates. Asaphus is distinguished from Ogygiocaris by having a wider axial region.

Fig. 6 Asaphid Community a Asaphus (Arthropoda: Trilobita) b Ogygiocaris (Arthropoda: Trilobita) c Homalopteon (Arthropoda: Trilobita) d trinucleid (Arthropoda: Trilobita)

Fig. 6 Asaphid Community a Asaphus (Arthropoda: Trilobita) b Ogygiocaris (Arthropoda: Trilobita) c Homalopteon (Arthropoda: Trilobita) d trinucleid (Arthropoda: Trilobita)

Ogygiocaris

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