Fukangichthys: CT scan data and surface files from middle Triassic fossil scanilepiform fish

Most scanilepiform fossils are heavily compressed, limiting investigations to external anatomy. The Middle Triassic Fukangichthys represents an important exception. High-resolution micro computed tomography (μCT) of three-dimensionally preserved skulls illuminates internal cranial anatomy of scanilepiforms.

Article abstract: Modern ray-finned fishes (Actinopterygii) comprise half of extant vertebrate species and are widely thought to have originated during or before the Middle Devonian (~380 million years (Myr) ago). 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. By contrast, the earliest material assigned to the polypterid lineage is mid-Cretaceous (ca. 100 Mya) in age, implying a quarter-of-a-billion-year palaeontological gap. We show that scanilepiforms, a widely distributed Triassic (ca. 251-200 Mya) radiation, are stem polypterids. Significantly, 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 Myr. Recalibration of molecular clocks to exclude phylogenetically reassigned Palaeozoic taxa results in age estimates for the actinopterygian crown lineage ~20–40 million years younger than previous molecular analyses. These new dates are broadly consistent with our revised palaeontological timescale and coincident with an interval of conspicuous morphological and taxonomic diversification among ray-fins centred on the Devonian Carboniferous boundary. A shifting timescale, combined with ambiguity in the relationships of late Palaeozoic actinopterygians, highlights this part of the fossil record as a major frontier in understanding the evolutionary assembly of modern vertebrate diversity.