Data from: Occurrence of the isopod Archaeoniscus coreaensis new species from the Lower Cretaceous Jinju Formation, Korea

The fossil isopod crustacean genus Archaeoniscus has been known to occur in England, France and Germany during the Upper Jurassic, and in Mexico and Egypt during the Lower Cretaceous. The morphology of this genus is unique in having dorsoventrally compressed body, the cephalon set deeply into the first pereionite, pleon as wide as pereion, and a broad semicircular pleotelson. These features have resulted in placing the classification of the genus in the monotypic family Archaeoniscidae. However, due to the lack of detailed morphological data, suprafamilial classification of this genus has remained unclear, as well as its ecology and lifestyle. Here we report Archaeoniscus coreaensis n. sp. from the Jinju Formation, Gyeongsang Basin, Korea. The occurrence of Archaeoniscus in the East Asia implies that the genus may have had a worldwide distribution. The Gyeongsang Basin was a Cretaceous backarc basin, which consists of exclusively non-marine sedimentary sequences. The occurrence of this genus, therefore, indicates that Archaeoniscus successfully adapted to a freshwater ecosystem as well. Detailed anatomy including antennulae, antennae, pereiopods, and uropods was observed from well-preserved multiple specimens, which allows better understanding of the morphology of Archaeoniscus. The axial structure in the posterior part of the body, which was previously interpreted as a unique brood pouch characterizing the family, turned out to be a remnant of the hindgut. Females of all isopods and most of the members of the superorder Peracarida have a thoracic ventral brood pouch, modified from the thoracic coxal endites. Based on the morphology of the largely unmodified ambulatory pereiopods of A. coreaensis, the possibility of Archaeoniscus being ectoparasitic is discounted. Instead, the flattened body and the form of limbs of A. coreaensis would have been suitable for a benthic lifestyle.等足目甲壳动物化石属古异足虫属（Archaeoniscus）此前已被报道分布于英格兰、法国与德国的上侏罗统地层，以及墨西哥与埃及的下白垩统地层。该属的形态特征独具一格：身体背腹扁平，头部深陷于第一胸节（pereionite）内，腹部与胸部等宽，且具有宽大的半圆形尾节复合体（pleotelson）。这些特征使得该属被归入单型科古异足虫科（Archaeoniscidae）。然而，由于缺乏详细的形态学数据，该属的超科级分类地位及其生态与生活方式始终未能明确。 本文报道了产自韩国庆尚盆地晋州组的一新种朝鲜古异足虫（Archaeoniscus coreaensis n. sp.）。古异足虫属在东亚的发现暗示该属可能曾具有全球分布范围。庆尚盆地为白垩纪弧后盆地，仅发育非海相沉积序列，因此该属的发现表明古异足虫属已成功适应淡水生态系统。 本次研究通过多件保存完好的标本，对其小触角（antennulae）、大触角（antennae）、步行胸肢（pereiopods）以及尾肢（uropods）等细节解剖结构进行了观察，这有助于我们更深入地理解古异足虫属的形态特征。此前被认为是该科标志性独特育幼囊的身体后部轴状结构，后续被证实为后肠（hindgut）残留物。所有等足类以及囊虾总目（Peracarida）的绝大多数类群，其胸部腹面的育幼囊均由胸肢基节内叶特化而来。 基于朝鲜古异足虫基本未特化的步行胸肢形态，古异足虫属为外寄生物的可能性被排除。相反，朝鲜古异足虫扁平的身体与肢型结构，更适合底栖生活方式。