A re-examination of sarcopterygian interrelationships, with special reference to the Porolepiformes

Abstract

The anatomy of the axial skeleton, median fins and skull of the fossil group Porolepiformes (Osteichthyes: Sarcopterygii) is reviewed, and descriptions by previous authors are modified in the light of new evidence. The first dorsal fin support lacks radials, whereas the posterior radials in the second dorsal fin support form a branching structure. In the snout, porolepiforms primitively have two lateral rostral bones on each side, the posterior one corresponds to the “nariodal” of Jarvik, and the infraorbital canal runs along the suture between this bone and the premaxilla. A cladistic analysis based on 54 characters places the Porolepiformes as sister-group of a clade Powichthys + Youngolepis + Diabolepis + Dipnoi. The postcranial skeletons of porolepiforms and early lungfishes are very similar, and the skulls of Powichthys, Youngolepis and Diabolepis can be interpreted as forming a transformation series between porolepiform and dipnoan cranial morphologies. Lungfishes thus appear to be derived from ‘rhipidistian’ ancestors. The porolepiform-lungfish clade is the sister group of another clade containing tetrapods, panderichthyids, osteolepiforms and rhizodonts. Onychodonts and coelacanths are joing unresolved sister groups of the other sarcopterygians. This cladogram is compared with those of other authors, and possible explanations for the differences are discussed. An outline classification of the Sarcopterygii is proposed.