The evolution and ecology of Mississippian Fishes and a view of life in a marine Mississippian Bay
Dr. Eileen D. Grogan and Dr. Dick Lund
Saint Joseph’s University & Carnegie Museum of Natural History
Ordinarily, sharks do not fossilize well. As a result the early history of sharks has been documented primarily by isolated teeth, spines and scales and, so, remains the subject of much speculation. Yet, one Paleozoic bay entirely preserved in stone has proven to be an exception, allowing us to “capture” a large number of chondrichthyans that include life stages from fetuses and larvae, up to pregnant females and mating adults. The high quality of preservation at this site also extends to other inhabitants (plant, vertebrate and invertebrate), thereby providing a glimpse of the highly diverse community in which these early chondrichthyans existed. Excavations for fossils in the rock unit identified as the Bear Gulch Limestone of Montana started in 1968 and continue to the present.
Sampling over 100 sites within the bay has provided the data to construct a model of the climate and hydrology that explains the quality of preservation while also providing details of soft as well as hard tissue anatomy in a suite of fishes. As a result of this unique preservation, we have been granted a rare view of Mississippian chondrichthyan diversity and the various habitats in which these and other fishes lived and died. The chondrichthyans of this time (318 million years ago) were the dominant marine fishes both in terms of species richness and in their suites of adaptations. They had very little resemblance or relationship to any cartilaginous fishes alive today. Our studies thus start with deciphering the anatomical variations of these Paleozoic chondrichthyans, which challenge classical views or theories of the chondrichthyan form. These variations, in turn, require re-examination of the structure and development of the other early gnathostomes. To add further intrigue and complication, although we can recognize members of the subclasses Elasmobranchii and Holocephali, the majority of our “sharks” fit into neither category. Some only fit into the Class Chondrichthyes by strict definition, based on skeletal mineralization and claspers in mature males.
Our lab projects extend beyond the identification, classification, and phylogeny of these fishes. With over 5700 fish collected from around the bay, we are currently involved in detailed studies of the their habitat preferences, distribution, and diversity. The prevalence of extreme secondary sexual dimorphism reflects the great diversity of chondrichthyan reproductive adaptations in this small shallow tropical embayment. Other lab research examines the evolution and function of chondrichthyan mineralized tissues and relatedness of these tissues to that of other vertebrates, including modern chondrichthyans. As for the field, our excavations continue to yield new taxa, including those with dramatic surprises in morphology and form and, despite 40 plus years of efforts, our retrieval rate of new taxa has yet to show signs of diminishing. So, data from fossil and field observations are expected to fuel the Bear Gulch Project for years to come and to continue providing opportunities for graduate and collaborative studies.
Visit our web site, http://sju.edu/research/bear_gulch for a summary of published information resulting from our research. PDFs of our published work are available from egrogan@sju.edu. The Bear Gulch Project would not have been possible without the labors of many volunteers through the years.