Palaeontology: Plenty of fish in the tree.

Abstract

Polypterid fish were considered to be archaic outliers of the bony-fish grouping. Fossil analysis now places them at the heart of early ray-finned fishes, a radical change that transforms the timing of their evolution. See Letter p.265 The bichirs (Polypterus), also known as ropefish, are a relic group of very primitive fishes now confined to freshwater habitats in Africa. With their combination of lobe fins, lungs and thick scales, bichirs have been allied with Devonian lobefins and even amphibians, but it is now generally accepted that they are the living sister group of all other ray-finned fishes (Actinopterygii). However, their fossil record is suspiciously meagre for such a well-armoured fish. It goes back to only the Cretaceous, leaving a very long ghost lineage back to the Devonian, when crown actinopterygians are thought to have evolved. Some have compared bichirs with scanilepiforms, a group of primitive actinopterygians from the Triassic, but the resemblances have been superficial. Now a computed tomography (CT) scan of one of these scanilepiforms, Fukangichthys, and comparisons with related forms, shows that bichirs do indeed belong to this group. A revised actinopterygian phylogeny bumps the origins of bichirs upwards from the Devonian to the Triassic, with the implication that crown actinopterygians also evolved later, in the Carboniferous rather than the Devonian.