STRONTIUM ISOTOPE ANALYSIS OF PETALODUS OHIOENSIS DENTARY AND THE EFFECT ON HABITAT INTERPRETATION

This research was undertaken to determine the amount of time between transgressive and regressive sequences. One of the methods used to determine the amount of missing time in an unconformity is the ⁸⁷Sr / ⁸⁶Sr ratio. The ⁸⁷Sr / ⁸⁶Sr ratio will be high around brackish and freshwater environments while it will be lower in open marine conditions. Limestones and their constituent parts are susceptible to diagenetic processes that can contaminate or obliterate isotope values contained within the rocks. The purpose of this research was to discover if reliable isotope information could be pulled from Pennsylvanian aged shark teeth.

Nine teeth from the shark, Petalodus ohioensis, were analyzed to find the value of the ⁸⁷Sr / ⁸⁶Sr ratio contained within the teeth. The values of the ratio from the teeth were found to be much higher than what was expected. Contamination was thought to be the cause behind the high ratio values. From the literature, P. ohioensis was thought to be an open marine dwelling shark. The remains of P. ohioensis have been found in rocks interpreted to be of marine origin.

The high values found in teeth were similar to the values found in brackish and freshwater environments. The limestones the teeth were found in, however, showed a ⁸⁷Sr / ⁸⁶Sr ratio that matched open marine conditions. The Calcite in the limestones is more susceptible to alteration than the Apatite that composes the teeth. So contamination of the ratio in the teeth does not appear to be the cause of the higher than expected values. If the limestones had not been altered then the teeth should have not been altered.

Research was then conducted into shark evolutionary history to see if the high values could be explained by brackish / freshwater tolerant sharks. If P. ohioensis lived in these conditions rather than open ocean, it would explain the values found in its teeth. This researcher found that several modern species of shark could tolerate freshwater habitats and that some of the ancestors of these sharks were freshwater dwelling as well. It was then found that P. ohioensis was related to the lineage of sharks that also contained the brackish / freshwater tolerant sharks. The conclusions reached by this research indicate that the literature has the habitat of P. ohioensis wrong and that it was indeed a brackish / freshwater tolerant species of shark.