Data from: 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约霍国家公园斯蒂芬山柯林斯采石场的中寒武世（第3统，第5阶）布尔吉斯页岩组（Burgess Shale Formation）的两个新双瓣节肢动物（bivalved arthropods）：带棘尾铠虾（Loricicaris spinocaudatus gen. et sp. nov.）与布里格斯泳虾（Nereocaris briggsi sp. nov.）的形态学特征。该批标本最初被归入鳃虾属（Branchiocaris），但其独特的性状组合足以将其划归至其他类群。带棘尾铠虾具有与同期透虾属（Perspicaris）和加拿大盾虫（Canadaspis）相似的细长带棘腹部，同时拥有与鳃虾属（Branchiocaris）类似的螯状头部第二附肢以及亚三角形外肢。布里格斯泳虾具有侧扁的壳瓣、细长纤弱的附肢，以及着生于菱形眼柄上、位于一对侧眼之间的中眼。尽管其无疑与产自布尔吉斯页岩组层位稍年轻的瘦泳虾（Nereocaris exilis）同属，但布里格斯泳虾在整体比例与体节数量上均存在差异，因此应被定为新种。研究人员将本次记述的新类群编码至一项涵盖755个性状、312种已灭绝与现生泛节肢动物（panarthropods）的大型分支系统学分析（cladistic analysis）中，类群范围涵盖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基部的并系类群（paraphyletic assemblage）。研究发现，节肢动物演化史上的多项重要革新，包括头部中脑附肢特化、从浮游底栖沉积食性向底栖腐食/捕食食性的转变，以及真节肢动物（Euarthropoda，冠群节肢动物）的祖征食性状态，均发生在基干双瓣节肢动物类群当中。