Chondrichthyan life history styles as revealed by the 320 million years old Mississippian of Montana
Chondrichthyans comprise 55 of the 94 vertebrate species found to date in the Mississippian marine Bear Gulch Limestone 6 km by 19 km tropical bay. The chondrichthyan-osteichthyan faunal proportions differ strongly from those of modern marine fish faunas. Secondary sexual dimorphism, size partitioning, life period segregation, and sexual segregation, reinforce the considerable morphological specializations among the chondrichthyans that were involved in the maintenance of this diversity. The large number of rare species suggests that many species are represented only by occasional recruitment, reflecting differential life history styles as well as habitat limitations for these species. Repeated fluctuations in environmental parameters through time, producing fine-scale spatial and temporal non-equilibrium, were probably ultimately responsible for the maintenance of this broad-scale high diversity community.