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Environmental controls on the taphonomy and distribution of Carboniferous malacostracan crustaceans

ABSTRACT Shrimp-like malacostracan crustaceans first appeared in the late Devonian and underwent a substantial adaptive radiation in the Carboniferous. They are rarely found in rocks of fully marine origin but are well represented in sediments laid down in brackish water and marginal marine conditions; such transitional environments provide the exceptional circumstances required for the preservation of unmineralised shrimps. The best examples are in the Dinantian of Scotland, the Namurian of Montana, and the Westphalian of Illinois. It is probable that shrimps were widespread in contemporaneous marine environments, but are not preserved. Any approach to understanding the physiology of fossil organisms is necessarily indirect. The diversity of crustacean communities, and the nature of associated taxa and trace fossils, are the most useful biotic factors for interpreting the habitat and tolerance of fossil examples. All known Carboniferous crustacean communities lived in brackish conditions; none is fully marine. Malacostracan assemblages in the Dinantian of Britain show a general trend of increasing diversity with salinity, from a single taxon at Gullane (stratified freshwater lake or brackish lagoon) to ten at Glencartholm (approaching normal marine). Tealliocaris is associated with low salinities. Crangopsis socialis is confined to the brackish water interdistributary bay environment, but Bairdops and Belotelson display a broader environmental tolerance. Crangopsis eskdalensis, Sairocaris, and Perimecturus occur only at Glencartholm, indicating a requirement for a strong marine influence. Those taxa confined to a limited environmental range all occur where a marine influence is pronounced; none occurs solely in areas of lower salinity. While salinity was apparently the dominant influence on distribution, a complex of independently varying environmental factors was involved. The range of habitats colonised early in the Carboniferous indicates that the preserved taxa had already developed advanced osmoregulatory mechanisms.