Ecomorphology of the Mississippian fishes of the Bear Gulch Limestone (Heath formation, Montana, USA)

The 323 million year old Bear Gulch Lens is recognized for the preservational quality and diversity of its fauna. Here we capitalize on these factors and 45 years of field data collection to test and evaluate the relationship between functional morphology and habitat in this Upper Mississippian marine fish fauna. The fishes are members of the Coelacanthiformes, Actinopterygii, Chondrichthyes, and Acanthodii. Fish were scored for 12 size-free morphological characters of functional significance that reflect habitat preference and resource utilization among extant fishes. Based on similarities in eco-coding, taxa are grouped into 13 ecomorphotypes, only one of which contained both actinopterygian and chondrichthyan taxa. Chondrichthyans are partitioned into nine highly diverse ecomorphotypes, spanning a spectrum of grossly different feeding, propulsive, and sexually dimorphic adaptations. These ecomorphotypes exhibit strong distributional trends relative to bay habitats. In contrast, Bear Gulch actinopterygians display few ecologically sensitive morphological deviations from a fusiform body plan and generalized feeding mechanism at this scale of analysis. The relative distribution of actinopterygian ecomorphotypes varies little across the bay habitats. The coelacanth ecomorphotype is restricted to lower bay habitats. These analyses lend new insight into the adaptive radiations of Chondrichthyes and Osteichthyes subsequent to the Upper Devonian mass extinctions.