Early cephalopod evolution: lnsights from new methods and old fossils

Abstract

Classical taxonomy divides cephalopods into three major subclasses: Coleoidea, Nautiloidea and Ammonoidea. However, it has long been known that this system does not accurately reflect the evolution of the group, because the Nautiloidea is paraphyletic. Numerous attempts to divide nautiloids into further subgroups have been complicated by difficulties in deciphering the evolution of the earliest forms in the Cambrian and Ordovician, the time of the first major radiation of cephalopods. We show here how new methods, such as Bayesian phylogenetic inference applying the fossilized birth-death model can help to resolve longstanding questions of phylogenetic relationships within “nautiloid” cephalopods. Our results reveal a paraphyletic group at the base of the cephalopod tree and a rapid diversification into three major clades near the Cambrian-Ordovician boundary: Endoceratoidea, Multiceratoidea and Orthoceratoidea. While the Orthoceratoidea contains the probable ancestors of the Ammonoidea and Coleoidea, the ancestral group of the Nautilida is to be found within either Orthoceratoidea or Multiceratoidea, with profound consequences for the extent of the cephalopod crown and stem group. Despite the potential of these methods, further taxonomic work remains fundamental. We demonstrate this with the example of a rich fossil collection from the Cambrian-Ordovician boundary of Queensland, Australia that has been known at least since the 1970’s, but remained undescribed to this day. Specimens from this collection provide valuable insights into early cephalopod evolution, as it contains the first three-dimensionally presented “protactinocerids”, confirming their re-interpretation as plectronocerids. In addition, we report the oldest member of the Endoceratoidea from the earliest Ordovician.