A new ichthyodectiform (Pisces, Teleostei) from the Lower Cretaceous of South Korea and its paleobiogeographic implication

Abstract

Here we report the first ichthyodectiform in the Jinju Biota from the Lower Cretaceous Jinju Formation, Shindong Group, at Hyojagyo, Jinju City, South Korea. It shows the following ichthyodectiform characters: 1) jaw teeth in single series; 2) coracoid expanded ventrally; 3) dorsal fin situated posteriorly, with its origin opposite to that of long anal fin. Besides, its caudal skeleton also exhibits certain features often seen in basal ichthyodectiforms and some other primitive teleosts. The fish is noticeably similar to the species referred to Chuhsiungichthys from the upper Lower Cretaceous Dobaru and Kumagai formations of the Wakino Subgroup of Kanmon Group in Kyushu, Japan, and Chuhsiungichthys tsanglingensis from the Jiangdihe Formation of similar age from Chuxiong, Yunnan Province, China. Chuhsiungichthys was, in turn, comparable with Mesoclupea showchangensis from the upper Lower Cretaceous Shouchang Formation in Shouchang and Linhai, and Guantou Formation in Zhuji, Zhejiang Province, China. The latter two genera also show the ichthyodectiform characters mentioned above. Nevertheless, the fish from Korea, those referred to Chuhsiungichthys from Japan and China, and Mesoclupea can easily be distinguished from other ichthyodectiforms in their higher body depth and shorter length; proportionally longer head; longer dorsal and anal fins; lower number of vertebrae, with abdominal less than caudal; vertebrae deeper than long. Among them the Jinju fish is distinct from the other two genera in even higher and shorter body, and presence of urodermal. In addition, the Jinju fish is different from Mesoclupea showchangensis in having a much longer dorsal fin and lower number of vertebrae, but different from Chuhsiungichthys tsanglingensis in having a much higher number of vertebrae and more ridges and grooves on the lateral face of vertebrae. Thus, the Jinju fish cannot be referred to either Chuhsiungichthys or Mesoclupea, and establishing a new genus and species for it is warranted. The Asian ichthyodectiforms, Mesoclupea, Chuhsiungichthys, and Jinjuichthys, may most probably form a monophyletic group – Chuhsiungichthyidae, known so far restricted to the upper Lower Cretaceous fresh and/or brackish waters of East Asia. The new ichthyodectiform materials from Korea are better preserved than those from Japan, thereby allowing a relatively complete description of the fish, providing more information for discussion of its phylogenetic position, enriching the diversity of the local fish fauna, and helping us better understand the paleobiogeographical distribution of the group and its geological background.