New Middle Cambrian bivalved arthropod from the Burgess Shale (British Columbia, Canada)

The morphology of two new bivalved arthropods, Loricicaris spinocaudatus gen. et sp. nov. and Nereocaris briggsi sp. nov. from the middle Cambrian (Series 3, Stage 5) Burgess Shale Formation (Collins Quarry locality on Mount Stephen, Yoho National Park, British Columbia, Canada), is described. The material was originally assigned to the genus Branchiocaris, but exhibits distinctive character combinations meriting its assignment to other taxa. Loricicaris spinocaudatus possesses an elongate and spinose abdomen comparable to the contemporaneous Perspicaris and Canadaspis, as well as chelate second head appendages and subtriangular exopods, comparable to Branchiocaris. Nereocaris briggsi possesses a laterally compressed carapace, elongate and delicate appendages and a medial eye located between a pair of lateral eyes on a rhomboidal eye stalk. Although undoubtedly congeneric with Nereocaris exilis from a slightly younger horizon of the Burgess Shale Formation, N. briggsi differs in overall proportions and segment number, warranting assignment to a new species. The newly described taxa were coded into an extensive cladistic analysis of 755 characters, and 312 extinct and extant panarthropods, including a variety of Cambrian bivalved arthropods from both the Burgess Shale and the Chengjiang Lagerstätten. Cambrian bivalved arthropods consistently resolved as a paraphyletic assemblage at the base of Arthropoda. Important innovations in arthropod history such as the specialization of the deutocerebral head appendages and a shift from a nekton-benthic deposit feeding habit to a benthic scavenging/predatory habit, the symplesiomorphic feeding condition of Euarthropoda (crown-group arthropods), were found to have occurred among basal bivalved arthropods.

本文记述了产自加拿大不列颠哥伦比亚省幽鹤国家公园斯蒂芬山柯林斯采石场的中寒武统（第3统、第5阶）布尔吉斯页岩组（Burgess Shale Formation）的两个双瓣节肢动物（bivalved arthropods）新类群：刺尾铠虾（Loricicaris spinocaudatus，新属新种）与布里格斯泳虾（Nereocaris briggsi，新种）。该类群标本最初被归入鳃虾属（Branchiocaris），但其独特的性状组合表明其应归入其他类群。刺尾铠虾具有细长且具棘刺的腹部，这一特征与同时期的透虾属（Perspicaris）和加拿大虾属（Canadaspis）相似；同时其第二头部附肢为螯状，外肢呈亚三角形，这又与鳃虾属类群一致。布里格斯泳虾具有侧扁的背甲、细长纤弱的附肢，以及位于菱形眼柄上一对侧眼之间的中眼。尽管其与布尔吉斯页岩组稍年轻层位产出的逸泳虾（Nereocaris exilis）无疑同属，但布里格斯泳虾在整体比例与体节数目上存在差异，因此建立为新种。研究人员将本次新记述的类群纳入涵盖755个性状、312种已灭绝与现生泛节肢动物（panarthropods）的大规模支序分析中，其中包括布尔吉斯页岩与澄江生物群（Chengjiang Lagerstätten）中的多种寒武纪双瓣节肢动物。系统发育分析结果显示，寒武纪双瓣节肢动物始终被复原为节肢动物门（Arthropoda）基部的并系类群。节肢动物演化史中的若干重要创新事件，例如中脑头部附肢特化、从浮游底栖沉积食性向底栖食腐/捕食性的转变，以及真节肢动物（Euarthropoda，冠群节肢动物）的祖征性取食状态，均被发现发生在基部双瓣节肢动物类群之中。