Evolution of Paleozoic benthic marine invertebrate communities

Abstract

Three marine benthic communities are recognized in the Upper Ordovician clastic sequences of the central Appalachians. These communities can be traced throughout part or all of Paleozoic time and, when compared with other previously designated Paleozoic communities, emphasize some evolutionary trends at the community level. Although environmental and geographic coverage is limited, all well known Paleozoic marine invertebrate communities are grouped into five major associations. These associations and their approximate duration are: (1) linguloid-molluscan (Ordovician-Permian); (2) strophomenid-trilobite (Ordovician-Devonian); (3) atrypid-bryozoan (Ordovician-Devonian); (4) productid-chonetid (Devonian-Permian);(5) fusulinid-palaeotextulariid (Pennsylvanian-Permian). Throughout Paleozoic time nearshore communities were consistently dominated by nuculoid bivalves, bellerophontid gastropods and linguloid brachiopods (association 1), whereas offshore articulate brachiopod-dominated communities (associations 2–4) underwent much more rapid changes in faunal composition; these changes were especially evident near the end of the Devonian Period. A modified form of the Moore-Newell emergence hypothesis does not appear to explain the greater instability or evolutionary plasticity of the offshore communities relative to that of the nearshore ones. An Early Paleozoic selection for species tolerant of fluctuating nearshore environmental conditions firmly established the dominant eurytypic molluscan-inarticulate brachiopod communities, whereas invertebrate adaptation to the monotonous offshore environment placed severe limitations on the potential variability of the brachiopod-dominated communities. Thus relatively slight changes in physical or biotic selective pressures could effectively distrupt the offshore stenotypic communities.