A complete primitive rhizodont from Australia

摘要

Studies of the origin and developmental genetics of tetrapod limbs have focused attention on the need to identify the precise type of sarcopterygian (lobe-finned fish) fin from which limbs evolved. This can only be achieved through a phylogenetic analysis of sarcopterygians. Sarcopterygian fin skeletons vary in structure; use of an inappropriate fin skeleton as a model limb precursor will lead to erroneous inferences about the evolution of morphology and the developmental pathways at the fish-tetrapod transition. The pectoral fin of the rhizodont sarcopterygian Sauripteris is strikingly limb-like and features prominently in discussions about the origin of limbs. It is thus important to establish the phylogenetic position of rhizodonts. However, their anatomy is incompletely known. Published phylogenetic analyses are based on poorly substantiated characters, such as the alleged presence of two external nostrils in the Australian genus Barameda. Here we present, from the Upper Devonian period of Canowindra, Australia, the most primitive and by far the most complete rhizodont discovered so far. It has a single external nostril but possesses no other derived tetrapod-like features. Our new evidence shows that rhizodonts are more remote from tetrapods than are osteolepiform and elpistostegid lobe-fin fishes. Similarities between rhizodont fins and tetrapod limbs are thus probably convergent, and the pectoral fin of Sauripteris should not be used as a model limb precursor.