Mollusc trace fossils Ptychoplasma Fenton and Fenton, 1937 and Oravaichnium Plička and Uhrová, 1990: Their type material and ichnospecies

Abstract

Ichnogenera belonging to cleft-foot and wedge-foot bivalves can in most cases be distinguished. This paper addresses the taxonomy of locomotion traces made by wedge-foot bivalves. Ichnogenera Ptychoplasma Fenton and Fenton and Oravaichnium Plička and Uhrová are reviewed together with related ichnotaxa. Both are interpreted as the locomotion traces of non-cleft, wedge-foot bivalves. Both are usually preserved as hypichnial ridges that include a distinctive bioprint of amygdaloid segments. Ptychoplasma is dominated by amygdaloid segments whereas Oravaichnium is wall-like with only sporadic amygdaloid segments. Ichnospecies of Ptychoplasma are distinguished on the basis of their continuity: P. excelsum Fenton and Fenton (the type) is a relatively continuous trace fossil consisting largely of welded, imbricated amygdaloid segments; P. conica Pieńkowski and Uchman is less welded and consists of conical mounds; and P. vagans (Książkiewicz) consists of a largely discontinuous series of such segments. Only one ichnospecies of Oravaichnium is yet recognized, the type O. hrabei Plička and Uhrová. Tuberculichnus Książkiewicz is a valid ichnogenus, based on its type species T. meandrinus Książkiewicz, but another ichnospecies, T. vagans Książkiewicz, belongs to Ptychoplasma. Hostynichnium Plička and Siráňová is regarded as a junior synonym of Ptychoplasma; its only ichnospecies, H. duplex Plička and Siráňová, is a junior synonym of P. vagans. Lockeia serialis Seilacher and Seilacher, 1994 is a nomen nudum and belongs to Ptychoplasma.