Data from: Nectocaridid ecology, diversity, and affinity: early origin of a cephalopod-like body plan

Nectocaridids are soft-bodied early to middle Cambrian organisms known from Burgess Shale-type deposits in Canada, China, and Australia. Originally described as unrelated species, they have recently been interpreted as a clade; their flexible tentacles, camera-type eyes, lateral fins, internal gills, axial cavity, and funnel point to a relationship with the cephalopods. However, aspects of this reinterpretation, including the relevance of the group to cephalopod evolution, have been called into question. Here, I examine new and existing nectocaridid material, including a large new form that may represent a sexual dimorph of Nectocaris pteryx. Differences between existing taxa largely represent taphonomic variation between sites and specimens—which provides further constraint on the organisms' anatomy. I revise the morphology of the tentacles and fins, and describe mouthparts and phosphatized gills for the first time. A mathematical analysis supports the presence of the earliest known camera-type eyes, and fluid mechanical considerations suggest that the funnel is optimized for efficient jet propulsion in a low Reynolds number flow regime. Nectocaridids closely resemble coleoid cephalopods, but a position deeper within Cephalopoda raises fewer stratigraphic challenges. Whether its coleoid-like construction reflects common ancestry or profound convergence, the Nectocaris body plan adds substantially to Cambrian disparity, demonstrating the rapid colonization of nektobenthic niches after the Cambrian explosion.

泳虾类（Nectocaridids）是一类身体柔软的寒武纪早至中期生物，目前仅在加拿大、中国与澳大利亚的布尔吉斯页岩型沉积（Burgess Shale-type deposits）地层中被发现。该类群最初曾被归类为多个互不相关的物种，近期则被认定为一个单系群（clade）；其拥有的柔性触手、照相机型眼、侧鳍、内鳃、轴腔与漏斗结构，显示其与头足类（cephalopods）存在演化关联。不过，该重新诠释的若干方面——包括该类群对头足类演化研究的意义——仍受到学界质疑。本研究对新发现及已有的泳虾类标本进行了检视，其中包括一种大型新形态，其可能为翼泳虾（Nectocaris pteryx）的性别二态型（sexual dimorph）个体。已报道类群间的形态差异，本质上多为不同沉积位点与标本间的埋藏学（taphonomic）变异，这为泳虾类的解剖结构提供了更严格的限定条件。本研究修正了泳虾类的触手与鳍的形态描述，并首次报道了其口器与磷酸化（phosphatized）鳃结构。数学分析证实了该类群拥有目前已知最早的照相机型眼，而流体力学分析则表明，其漏斗结构在低雷诺数流场（low Reynolds number flow regime）中已适配高效喷射推进（jet propulsion）运动。泳虾类与鞘亚纲头足类（coleoid cephalopods）形态高度相似，但若将其置于头足类更基部的演化位置，则可减少地层学层面的演化时序矛盾。无论其鞘亚纲类的躯体构型是源自共同祖先还是趋同演化，泳虾的身体构型都极大拓展了寒武纪生物形态差异度，证明了寒武纪大爆发后生物快速占据游底栖生态位（nektobenthic niches）的过程。