The problem of vast numbers of cladodont shark denticles in the Pennsylvanian Excello Shale of Pike County, Indiana

Abstract

While studying X-ray films of new iniopterygians (Subterbranchialia, Chondrichthyes) in the Excello Shale from the Field Museum of Natural History's Bethel Quarry locality in southern Pike County, Indiana, a moderately dense spread of tiny cladodont shark denticles was noticed on the radiographs, confined to micro-horizons of shale about 7.5 mm thick, which also contained the iniopterygians.These denticle spreads, which are entirely unrelated to the iniopterygians, but are seen on X-ray films of most of the chondrichthyan skeletons collected from a sheet of Excello Shale of about 500 m2 in extent, and mostly from two very fossiliferous levels, have an average density of about nine denticles per 1 cm2. Because the chondrichthyan skeletons were collected from all parts of the quarry, it is probable that the denticle spreads in several micro-horizons extended over the entire quarry area and perhaps beyond. The number of denticles in each of the micro-horizons in 500 m2 of shale thus amounts to about 4.6 million.A discussion of the origin of these vast numbers of cladodont denticles, given a variety of taphonomic and depositional constraints, results in the conclusion that these large numbers of denticles could not have resulted from the two species of sharks, Denaea meccaensis and Stethacanthulus longipeniculus, that are members of the burial assemblage and bear dentition teeth indistinguishable from those in the areal spread. Evidence suggesting very rapid deposition of organic muds that produced the characteristic Mecca Quarry type carbonaceous, sheety shales rules out the possibility of accumulation of the denticle spreads over extended periods of time. Because sharks do not possess nearly enough dentition teeth to account for this occurrence, one must entertain the possibility that the denticles are not all dentition teeth, but perhaps for the most part mucous membrane (or even dermal) denticles, though none such are presently known to display cladodont design.