Abstract

Based on abundant topotype specimens from the Oil Creek Formation at the type locality in southern Oklahoma, Histiodella labiosa is described and revised. The morphological variation of the Pa, Pb, and M elements of this previously little-known species is also systematically documented and illustrated. The revised species definition, with formal designation of a lectotype and a new diagnosis supported by morphological comparison with its ancestral and descendent species, has enhanced its value for correlation of the lower Darriwilian. The inferred phylogeny of Histiodella that resulted from this study is based on both cladistic and morphological analyses. It supports the monophyletic grouping of Histiodella species and their evolutionary relationships. This has wide biostratigraphic implications because many Histiodella species are valuable for detailed global correlation of upper Dapingian to middle Darriwilian strata.