Habitat and diversity of the Bear Gulch fish: Life in a 318 million year old marine Mississippian bay

The Bear Gulch Limestone of Montana preserves a Late Mississippian (318 Mya) tropical marine bay in its entirety and provides a rare picture of vertebrate community structure and diversity in deep time. Decades of quarrying has produced over 5700 fish of 149 species from nine habitat zones within the bay. The fish fauna differs from that of modern faunas in that Coelacanthiformes form the most abundant guild and Chondrichthyes comprise the group with the most species and genera and exhibit the greatest spectrum of adaptive suites.Within the bay ecosystem, strong differences in habitats were reflected in equally strong differences in composition and distribution of the fishes on the generic and suprageneric levels. Genus richness within each habitat was high. The upper bay assemblages were the most distinct in terms of both their supported fish fauna and environments; Acanthodes comprised a significant fraction of the fauna in these zones. Across the bay, Chondrichthyes and Osteichthyes showed markedly different responses to environmental variation at both local and regional scales. For both clades, dominant genera occurred ubiquitously across the bay, but showed habitat-related trends in their distribution. Rare genera comprised an unusually large proportion of total genus richness and exhibited a high degree of habitat preference. This research also includes an initial assessment of the impact of the predominance of ecologically rare taxa on the quantitative estimation of biodiversity.

► The Bear Gulch is a lagerstätte that preserves an entire 318 Mya tropical marine bay. ► The species-rich fish fauna differs significantly from that of modern faunas. ► Despite high within-habitat alpha diversity, the beta diversity of the fauna is low. ► Differences in habitats were reflected in differences in fish distributions. ► Sharks and bony-fish responded differently to local and regional habitat variation.