Lower Palaeozoic plankton

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Abstract
The graptolite biocoenosis is accepted as the epipelagic or oceanic plankton fauna of the Lower Palaeozoic, although the geosynclinal deposits in which graptolite remains occur are not typically oceanic. Local variation within geosynclines implies a measure of permanence and stability in their faunas, but the general similarity in graptolite successions calls for frequent intercommunication, and epipelagic plankton probably drifted into geosynclinal seas from outlying oceanic areas. The location of these oceans introduces the problem of continental drift. Reconsideration of morphological evidence suggests that nema (or virgula) attachment was the exception in the adult graptoloid but may have been more prevalent in juvenile stages; the occurrence of a general ‘size limit’ for stages with prominently developed nema could prove functionally significant. Some form of vacuolated extrathecal tissue seems the most plausible alternative buoyancy mechanism for adult forms. There is some evidence of attachment in (adult) Tremadocian dendroids, and the origin of the Graptoloidea may be causally related to a phase in the development of Sargasso-like seas. Other constituents of the plankton are briefly reviewed; the few organisms that appear acceptable are principally algae (including filamentous blue-green algae and hystrichosphaeres), radiolaria, coelenterates (various medusae, siphonophores, and ? ctenophores), small inarticulate brachiopods, conodonts, possibly certain small trilobites, and small orthoconic nautiloids. The suggestion that graptolites lived at various depths is not accepted. Temperature, however, might well have affected their geographical distribution. Nemagraptus gracilis and Monograptus turriculatus range from nearly 40° s to about 75° n, but if related to latitudes indicated by palaeomagnetic evidence, both approximate more closely to a warm-temperate or tropical zone. Relation to ‘pre-continental drift’ positioning of the Americas introduces anomalies. Further work on the distribution-patterns of individual graptolite species may provide evidence on the presence of an early Atlantic Ocean; failing this, the central parts of various geosynclinal seas would provide the only indication of epipelagic regions (other than the Pacific) throughout the Lower Palaeozoic.

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