A new species of the Cambrian bivalved euarthropod Pectocaris with axially differentiated enditic armatures

Although first described in 1999, the early Cambrian Pectocaris remains a poorly known euarthropod genus. As of now, two species, Pectocaris eurypetala and Pectocaris spatiosa, are known from the Chengjiang biota. Here, we report a third species, Pectocaris inopinata sp. nov., which bears a pair of stalked eyes attached to an anterior sclerite, a pair of short but stout antennae, 29–35 cephalothoracic segments each corresponding to a pair of biramous appendages, 11–12 limbless segments carrying two rows of ventral spines and one row of dorsal spines, and a pair of broad telson flukes decorated with short marginal spines. Most strikingly, the endopods of anterior trunk appendages are each axially differentiated with two rows of setulose endites proximally and claw-like paired spines distally, in contrast to the endopods of posterior trunk appendages that carry two rows of setulose endites but lack claw-like paired spines. Phylogenetic analysis shows that P. inopinata, the other two Pecto...尽管射齿虾属（Pectocaris）早在1999年就被首次记述，但这种寒武纪早期的真节肢动物（euarthropod）属至今仍鲜有研究。截至目前，该属已知的物种有两种：宽瓣桨虾（Pectocaris eurypetala）与宽体桨虾（Pectocaris spatiosa），均产自澄江生物群（Chengjiang biota）。本文报道第三种桨虾属物种：意外桨虾（Pectocaris inopinata sp. nov.），该新种具有一对着生于前骨片（anterior sclerite）的柄眼（stalked eyes）、一对短粗的触角（antennae）；头胸甲体节（cephalothoracic segments）共29~35节，每节对应一对双分支附肢（biramous appendages）；无附肢体节（limbless segments）11~12节，具两排腹侧棘刺（ventral spines）与一排背侧棘刺（dorsal spines）；另有一对宽大的尾扇叶（telson flukes），边缘装饰有短小棘刺。最引人注目的是，其躯干前部附肢的内肢（endopods）沿轴向发生分化：近端具两排具刚毛的内叶（setulose endites），远端则为爪状成对棘刺（claw-like paired spines）；与之形成鲜明对比的是，躯干后部附肢的内肢仅具两排具刚毛的内叶，并无爪状成对棘刺。系统发育分析（phylogenetic analysis）显示，意外桨虾（P. inopinata）与另外两种桨虾属（Pecto...）