EARLY VISEAN (OSAGEAN) VERTEBRATE FAUNA OF THE “BURLINGTON-KEOKUK BONE BED”-UPPERMOST BURLINGTON FORMATION OF SOUTHEASTERN IOWA

Results are presented of an investigation of the marine vertebrate fauna of the Burlington-Keokuk bone bed from the Cedar Fork Member of the Burlington Formation at the Nelson Quarry in Des Moines County, Iowa. Twelve kilograms of bone-bed echinoderm grainstone with as much as 10 weight % skeletal phosphate was digested in a 10% formic acid solution. Fish teeth, dermal scales, and conodonts were recovered from insoluble residues. Fish teeth were identified through comparisons with coeval taxa recently described from China and Australia. Associated invertebrates include silicified solitary rugose corals and shells of the brachiopods Spirifer grimesi and Toryinifer sp. Associated Cedar Fork conodonts are assigned to the Eotaphrus burlingtonensis Subzone of the Early Visean.

The Burlington-Keokuk bone bed fauna features distinctive teeth and tooth families of 13 fish species, representing the classes Chondrichthyes or Acanthodii. One tooth specimen is an acanthodian (Acanthodii Ischnacanthiformes). The majority of specimens are chondrichthyians of the Subclass Holocephali (~65%), with smaller numbers of specimens assignable to the Subclass Elasmobranchii. The three most abundant teeth/tooth families in the fauna belong to elasmobranch Stethacanthus symmoriiformes, the orodontidid Orodontidae orodus, and elasmobranch Phoebodontiformes diplodus. The first two comprise 41% of all identifiable vertebrate bone in the assemblage. Other taxa present include: the elasmobranchs Ctenacanthiformes saivodus striatus and Protacrodontioidea protacrodus; the holocephalids Petalodontiformes antliodus, P. cenustodus, P. chomatodus, P. petalodus, P. pholyrhizodus, P. venustodu, and Cochliodontidae deltodus. The Burlington-Keokuk bone bed assemblage is dominated by cosmopolitan species known from the tropical shelves of Larussia and Gondwana during the Visean as described in recent studies by Ginter, Duncan, and Sequeira.