Nagler Christina, Hyžný Matúš, Haug Joachim T
Functional morphology group, Department of Biology II, Ludwig-Maximilians-University, Großhaderner Strasse 2, 82152, Planegg-Martinsried, Germany.
Department of Geology and Palaeontology, Faculty of Natural Sciences, Comenius University, Mlynská dolina, Ilkovičova 6, 84215, Bratislava, Slovakia.
BMC Evol Biol. 2017 Mar 9;17(1):76. doi: 10.1186/s12862-017-0915-1.
Isopods (woodlice, slaters and their relatives) are common crustaceans and abundant in numerous habitats. They employ a variety of lifestyles including free-living scavengers and predators but also obligate parasites. This modern-day variability of lifestyles is not reflected in isopod fossils so far, mostly as the life habits of many fossil isopods are still unclear. A rather common group of fossil isopods is Urda (190-100 million years). Although some of the specimens of different species of Urda are considered well preserved, crucial characters for the interpretation of their lifestyle (and also of their phylogenetic position), have so far not been accessible.
Using up-to-date imaging methods, we here present morphological details of the mouthparts and the thoracopods of 168 million years old specimens of Urda rostrata. Mouthparts are of a sucking-piercing-type morphology, similar to the mouthparts of representatives of ectoparasitic isopods in groups such as Aegidae or Cymothoidae. The thoracopods bear strong, curved dactyli most likely for attaching to a host. Therefore, mouthpart and thoracopod morphology indicate a parasitic lifestyle of Urda rostrata. Based on morphological details, Urda seems deeply nested within the parasitic isopods of the group Cymothoida.
Similarities to Aegidae and Cymothoidae are interpreted as ancestral characters; Urda is more closely related to Gnathiidae, which is therefore also interpreted as an ingroup of Cymothoida. With this position Urda provides crucial information for our understanding of the evolution of parasitism within isopods. Finally, the specimens reported herein represent the oldest parasitic isopods known to date.
等足目动物（潮虫、鼠妇及其近亲）是常见的甲壳类动物，在众多栖息地中数量丰富。它们采用多种生活方式，包括自由生活的食腐动物和捕食者，但也有专性寄生虫。到目前为止，等足目动物化石并未体现出现代这种多样的生活方式，主要是因为许多化石等足目动物的生活习性仍不清楚。Urda（1.9亿至1亿年前）是一类相当常见的化石等足目动物。尽管Urda不同物种的一些标本被认为保存完好，但迄今为止，对于解读其生活方式（以及系统发育位置）的关键特征仍无法获取。
利用最新的成像方法，我们展示了距今1.68亿年的Urda rostrata标本口器和胸足的形态细节。口器具有吸刺式形态，类似于艾氏等足科或扇肢亚目等外寄生等足目动物代表的口器。胸足带有强壮的弯曲指节，很可能用于附着在宿主身上。因此，口器和胸足的形态表明Urda rostrata具有寄生生活方式。基于形态细节，Urda似乎深深嵌套在扇肢亚目寄生等足目动物之中。
与艾氏等足科和扇肢亚目的相似性被解释为祖先特征；Urda与颚虱科关系更为密切，因此颚虱科也被解释为扇肢亚目的一个内群。基于这一位置，Urda为我们理解等足目动物寄生现象的进化提供了关键信息。最后，本文报道的标本代表了迄今为止已知最古老的寄生等足目动物。
