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

Highlights

• We described a new ichthyodectiform genus and species, Jinjuichthys cheongi.

• The new fish is from the late Early Cretaceous, South Korea.

• We suggested an Asian monophyletic group within the Ichthyodectiformes.

• This group is distributed along the coastal margin of Asia with other cosmopolitan fishes.

• The presence of cosmopolitan fishes in the area indicates rising of West Pacific in the late Early Cretaceous.

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.

Introduction

A large-scale paleontological excavation was carried out by Haang-Mook Kim and his team from the Pusan University in the period of 2005–2007, in the area close to Jinju city, during the construction work for broadening the highway between Jibhyeon and Saengbiryang (Kim, 2009; Figs. 1 and 2A). In the excavation, numerous fossils, including vertebrates, invertebrates, and plants, were collected from the Jinju Formation (see Fig. 2B for the geological section of this part of the formation), and the name “Jinju Biota” was first used for these fossils. The Jinju Biota was then correlated with the Early Cretaceous Jehol Biota from northwestern China, particularly with the fossils from the Yixian Formation. In this paper we describe two teleost specimens collected from the Jinju Formation, Shindong Group, Gyeongsang Supergroup at Hyojagyo, Micheon-myeon, Jinju City, South Gyeongsang Province, South Korea. They show a close resemblance to the specimens described by Yabumoto (1994) from the Lower Cretaceous Dobaru and Kumagai formations of the Wakino Subgroup of Kanmon Group in Kyushu, Japan (Yabumoto et al., 2006). Yabumoto (1994) established two new species, namely, Chuhsiungichthys yanagidai (from Dobaru Formation) and C. japonicus (from Kumagai Formation), referred to the genus Chuhsiungichthys described by Lew (1974) from the Jiangdihe Formation of similar age from Chuxiong (the Pinyin spelling adopted after 1979 in mainland China that has replaced the old spelling of Chuhsiung in Wade-Giles romanization), Yunnan Province, China. Yabumoto (1994) was the first to refer the genus to the early teleost order Ichthyodectiformes (bulldog fish), and he established a new family Chuhsiungichthyidae to include Chuhsiungichthys and another genus Mesoclupea, described by Ping and Yen (1933) from the Lower Cretaceous Shouchang Formation, now designated as the basal part of Aptian (Jingeng Sha, pers. comm. 2013), from Shouchang, Zhejiang Province, China. Mesoclupea was later discovered also from a wider area and time range, i.e., the Shouchang Formation of Jiande, Zhuji, Chun'an, Pujiang, Linhai, and the Guantou Formation, now designated as early Albian (Jingeng Sha, pers. comm. 2013), of Zhuji, Zhejiang Province (Chang, 1963, Chang and Chou, 1977, Chang and Chou, 1986, Chang and Miao, 2004). Mesoclupea and Chuhsiungichthys were formerly placed in the Chirocentridae (living wolf herring) by Chang (1963) and Lew (1974) respectively, and Mesoclupea was also referred to the Chirocentridae by Bardack (1965). Bardack and Sprinkle (1969) established a new extinct group of teleosts, the Ichthyodectiformes, based on their detailed study on fishes of the two extinct families Ichthyodectidae and Saurocephalidae. Patterson and Rosen (1977) conducted a comprehensive anatomical survey of the Ichthyodectiformes, and considered the order as a basal teleostean group, sister to Tharsis dubius plus all extant teleosts. A recent work by Cavin et al. (2012) has shed much new light on this group. Although Cavin et al. (2012) excluded Ascalabothrissops (Arratia, 2000) and Pachythrissops (Woodward, 1919) from the Ichthyodectiformes in their comprehensive phylogenetic analysis we still cite some characters of these two taxa for the purpose of comparison, wherever information is available. Fish belonging to the Chuhsiungichthyidae (incorrectly spelt as “chuingichthiid”) (Order Ichthyodectiformes) was mentioned by Lee et al. (2001, p. 363) from “the Jinju Formation exposed in Donggogri (Habin-myeon, Dalseong-gun, North Gyeongsang Province)” among other fossil fishes, and listed in Lee et al. (2001), table 1 (p. 358). The age of the Shindong Group, the Jinju Formation being its youngest unit, has been designated as the Hauterivian–Barremian by palynologists and ostracod workers, and as Aptian to Albian by mollusc workers (Yang, 1982, Yi et al., 1994; and Choi, 1985 in Lee et al., 2001, Sha et al., 2012). Recently a U–Pb dating based on detrital zircons of the group was given by Lee et al. (2010) as ranging from the Aptian to Albian. In this paper, we describe the two well-preserved fossil fish specimens from Hyojagyo, Micheon-myeon and compare them with other known ichthyodectiforms, especially Chuhsiungichthys and Mesoclupea. For comparison, a specimen of well-preserved impression of Mesoclupea showchangensis without head region (GMC V 1007-1) is also described in details. We conclude that the specimens from the Jinju Formation described here cannot be assigned to any of the known ichthyodectiforms. Therefore, we must treat them as a new genus and species of the order. However, we feel that a meaningful phylogenetic analysis has to wait for sufficient morphological information of the three Asian forms to emerge from future discoveries. The Jinju fish fauna (Ichthyodectiformes, Osteoglossiformes etc.) is similar in composition to the fish fauna from Kyushu, Japan and southeast China, and forms one and the same ichthyofauna with those from the latter two areas. This ichthyofauna has a slightly younger age and different geological background than the fish fauna from the Jehol Biota, and contained several cosmopolitan fish forms.