Fish diversity of the Bear Gulch Limestone, Namurian, Lower Carboniferous of Montana, USA

Abstract

Quarrying operations for fossil fish from the Bear Gulch Limestone (Heath Formation, Namurian,Montana, USA) have occurred from 1968 to 1997 and continue. The Bear Gulch Limestone lens (14×9 km) thins to a shore line on all edges except the buried eastern boundary, and contains an entirely marine flora and fauna. Eighty outcrops have been sampled, and 4 547 identifiable fish of 113 taxa have been recorded. This study details aspects of the diversity and distribution of the fishes of the Bear Gulch Bay analyzed on the basis of geographic areas, following facies and sedimentary criteria. The total diversity of the fish fauna is 21.08 (Simpson index) and 12.07 (Margalef index). Diversity decreases from east to west and towards the southern margin. These trends mask dramatic differences in species occurrences and dominant species in each area. The high diversity of the “Depocenter” area can be accounted for by its proximity to the mouth of the bay and access to the epicontinental seaway to the east. The southeast region is comprised of the Blacktail, Allen, and Buchek areas. Diversity declines greatly from the algae - and sponge - rich Blacktail area southwestward to the marginal Buchek area. This region contains several dominant species that are adapted to lurk in or maneuver among plants, feed from the bottom or burrow. High numbers of larval paleoniscoids and the coelacanth Caridosuctor lower the diversity of the near-shore North Shore area. The western end of the bay is dominated by Acanthodes,, scales of Listracanthus and Strepsodus,, and large cladodont sharks. Fish distributions, preliminary assessments of morphological adaptations, and facies all support the concept of a heterogeneous set of habitats within this fossil bay.