THE FABULOUS FOSSILS OF JOHN DENNISON'S DEVONIAN

John Dennison’s lasting legacy will be his contributions to understanding the Paleozoic stratigraphy of the Appalachian basin. While John focused on the environmental changes that occurred in the basin, most notably the transgressions and regressions within the basin, he allowed his students to pursue their own research so a more complete story of the basin could be told. When John revised the rocks of the Devonian “Chemung Formation” to include his Foreknobs Formation he presented an opportunity for biostratigraphic analysis of the Frasnian-Famennian (F/F) extinction event which occurs within the Foreknobs. The fossils recovered during this investigation not only shed light on the extinction itself but many were fascinating in their own right.

One interesting aspect is examples of gigantism among certain brachiopods. Specimens of Cyrtospirifer chemungensis attain widths exceeding 116 mm, almost double that of typical forms. Very large individuals of Orbiculoidea measuring 52 mm long are especially intriguing as the genus is typically represented by small individuals. Forms of Spinatrypa diverge from their typical morphology by developing a large triangular shape and a well pronounced fold. A second aspect of the Foreknobs fossils is the detailed preservation of some forms in the sandstones and siltstones such as the posterior spine on the pelecypod Leptodesma spinigerum and the “wings” on Pterinopecten suborbicularis. Also found were the rarely preserved muscle scars on the brachiopod Ambocoelia umbonata and teeth of Tropidileptus carinatus Perhaps most interesting is the occurrence of the nominally European Famennian brachiopod Steinhagella in Frasnian units of the Foreknobs which implies an Appalachian origin for the genus. Biostratigraphic analysis of all the fossils suggest that either the lithostratigraphic marker for the F/F extinction (the Pound Member) is time transgressive within the basin or that certain areas acted as refugia where some Frasnian species survived the extinction event into the Famennian.