A New Silurian Fish Illuminates the Early Evolution of Gnathostome Appendages and Dentition

Abstract

The Silurian Period occupies a pivotal stage in the unfolding of key evolutionary events, including the rise of jawed vertebrates. However, the understanding of this early diversification is often hampered by the patchy nature of the Silurian fossil record, with the articulated specimens of jawed vertebrates only known in isolated localities, most notably Qujing, Yunnan, China. Here we report a new Silurian maxillate placoderm, Bianchengichthys gracilis, from the Ludlow of Chongqing, with a complete dermal skeleton preserved in articulation. Although geographically departed, the new taxon resembles the previously reported Qilinyu in possessing a unique combination of dermatoskeletal characters, suggesting a close systematic relationship. However, the dentition carried by the preserved mandible is unexpectedly different from that in both Qilinyu and Entelognathus in having a row of large tooth-like denticles reminiscent of the arthrodire dentition, in addition to the toothless flange similar to the marginal jaw plate of Entelognathus . The preserved paired fin squamation reveals a long base and an extensively scale-covered lobate portion, flanked by a fringe of lepidotrichia-like aligned scales. The discovery significantly widens the distribution of Silurian placoderm-grade gnathostomes in South China, and provides a range of morphological disparity for the outgroup comparison to the earliest evolution of jaws, teeth and pectoral fin in crown-group gnathostomes. We also demonstrate that the previously reported Silurian placoderms from central Vietnam are maxillate placoderms close to Qilinyu, Silurolepis, and Bianchengichthys, corroborating the paleogeographic proximity between the Indochina and South China blocks during the Middle Paleozoic.