1] Department of Geology and Geophysics, Yale University, PO Box 208109, New Haven, Connecticut 06520, USA [2] Research Unit Palaeontology, Department of Geology and Soil Science, Ghent University, Krijgslaan 281/S8, B-9000 Ghent, Belgium.
1] Department of Zoology, University of Oxford, The Tinbergen Building, South Parks Road, Oxford OX1 3PS, UK [2] Oxford University Museum of Natural History, Parks Road, Oxford OX1 3PW, UK.
Nature. 2015 Jun 4;522(7554):77-80. doi: 10.1038/nature14256. Epub 2015 Mar 11.
Exceptionally preserved fossils from the Palaeozoic era provide crucial insights into arthropod evolution, with recent discoveries bringing phylogeny and character homology into sharp focus. Integral to such studies are anomalocaridids, a clade of stem arthropods whose remarkable morphology illuminates early arthropod relationships and Cambrian ecology. Although recent work has focused on the anomalocaridid head, the nature of their trunk has been debated widely. Here we describe new anomalocaridid specimens from the Early Ordovician Fezouata Biota of Morocco, which not only show well-preserved head appendages providing key ecological data, but also elucidate the nature of anomalocaridid trunk flaps, resolving their homology with arthropod trunk limbs. The new material shows that each trunk segment bears a separate dorsal and ventral pair of flaps, with a series of setal blades attached at the base of the dorsal flaps. Comparisons with other stem lineage arthropods indicate that anomalocaridid ventral flaps are homologous with lobopodous walking limbs and the endopod of the euarthropod biramous limb, whereas the dorsal flaps and associated setal blades are homologous with the flaps of gilled lobopodians (for example, Kerygmachela kierkegaardi, Pambdelurion whittingtoni) and exites of the 'Cambrian biramous limb'. This evidence shows that anomalocaridids represent a stage before the fusion of exite and endopod into the 'Cambrian biramous limb', confirming their basal placement in the euarthropod stem, rather than in the arthropod crown or with cycloneuralian worms. Unlike other anomalocaridids, the Fezouata taxon combines head appendages convergently adapted for filter-feeding with an unprecedented body length exceeding 2 m, indicating a new direction in the feeding ecology of the clade. The evolution of giant filter-feeding anomalocaridids may reflect the establishment of highly developed planktic ecosystems during the Great Ordovician Biodiversification Event.
古生代保存异常完好的化石为节肢动物的进化提供了重要的见解，最近的发现使系统发育和特征同源性成为焦点。在这些研究中，异常虾类是一个重要的分支，它们的形态特征阐明了早期节肢动物的关系和寒武纪的生态。尽管最近的工作集中在异常虾类的头部，但它们的躯干性质一直存在广泛争议。在这里，我们描述了来自摩洛哥早奥陶世 Fezouata 生物群的新的异常虾类标本，这些标本不仅显示出保存完好的头部附肢，提供了关键的生态数据，而且还阐明了异常虾类躯干瓣的性质，解决了它们与节肢动物躯干肢的同源性问题。新的材料表明，每个躯干节都有一对单独的背侧和腹侧瓣，在背侧瓣的基部有一系列刚毛叶片。与其他谱系的节肢动物的比较表明，异常虾类的腹侧瓣与足肢的游走肢和真节肢动物的双肢分支肢的后肢同源，而背侧瓣和相关的刚毛叶片与具鳃的足肢动物的瓣（例如，Kerygmachela kierkegaardi，Pambdelurion whittingtoni）和“寒武纪双肢分支肢”的外肢同源。这一证据表明，异常虾类代表了外肢和后肢融合成“寒武纪双肢分支肢”之前的一个阶段，证实了它们在真节肢动物主干中的基础地位，而不是在节肢动物冠群或与轮虫类蠕虫中。与其他异常虾类不同，Fezouata 分类群将头部附肢结合在一起，这些附肢适应于过滤式进食，而且身体长度超过 2 米，这表明该分支的进食生态学出现了一个新的方向。巨型滤食性异常虾类的进化可能反映了在大奥陶纪生物多样性事件期间高度发达的浮游生态系统的建立。
