Early members of ‘living fossil’ lineage imply later origin of modern ray-finned fishes

• Published in: Nature (London) 2017-09, Vol.549 (7671), p.265-268
• Main Authors: Giles, Sam, Xu, Guang-Hui, Near, Thomas J., Friedman, Matt
• Format: Article
• Language: English
• Subjects: 631/181/414; 631/181/757; 631/601/2722; Actinopterygians; Carboniferous; Cretaceous; Devonian; Fins; Fish; Fossils; Humanities and Social Sciences; letter; Living fossils; Morphology; multidisciplinary; Natural history; Paleozoic; Phylogenetics; Phylogeny; Science; Taxa; Triassic; Vertebrates
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature23654

High-resolution scans of fossilized fish skulls suggest that modern ray-finned fishes originated later than previously thought and necessitate reconsideration of the evolution of this major vertebrate group. Untying ropefish origins 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. Modern ray-finned fishes (Actinopterygii) comprise half of extant vertebrate species and are widely thought to have originated before or near the end of the Middle Devonian epoch (around 385 million years ago) 1 , 2 , 3 , 4 . Polypterids (bichirs and ropefish) represent the earliest-diverging lineage of living actinopterygians, with almost all Palaeozoic taxa interpreted as more closely related to other extant actinopterygians than to polypterids 5 , 6 , 7 , 8 , 9 , 10 . By contrast, the earliest material assigned to the polypterid lineage is mid-Cretaceous in age (around 100 million years old) 11 , implying a quarter-of-a-billion-year palaeontological gap. Here we show that scanilepiforms, a widely distributed radiation from the Triassic period (around 252–201 million years ago), are stem polypterids. Importantly, these fossils break the long polypterid branch and expose many supposedly primitive features of extant polypterids as reversals. This shifts numerous Palaeozoic ray-fins to the actinopterygian stem, reducing the minimum age for the crown lineage by roughly 45 million yea