Caron Jean-Bernard, Aria Cédric
Department of Natural History (Palaeobiology Section), Royal Ontario Museum, Toronto, Ontario, Canada.
Department of Ecology and Evolutionary Biology, University of Toronto, Toronto, Ontario, Canada.
BMC Evol Biol. 2017 Jan 31;17(1):29. doi: 10.1186/s12862-016-0858-y.
Arthropoda, Tardigrada and Onychophora evolved from lobopodians, a paraphyletic group of disparate Palaeozoic vermiform animals with soft legs. Although the morphological diversity that this group encompasses likely illustrates the importance of niche diversification in the early radiation of panarthropods, the ecology of lobopodians remains poorly characterized.
Here we describe a new luolishaniid taxon from the middle Cambrian Burgess Shale (Walcott Quarry) in British Columbia, Canada, whose specialized morphology epitomizes the suspension-feeding ecology of this clade, and is convergent with some modern marine animals, such as caprellid crustaceans. This species possesses two long pairs and four shorter pairs of elongate spinose lobopods at the front, each bearing two slender claws, and three pairs of stout lobopods bearing single, strong, hook-like anterior-facing claws at the back. The trunk is remarkably bare, widening rearwards, and, at the front, extends beyond the first pair of lobopods into a small "head" bearing a pair of visual organs and a short proboscis with numerous teeth. Based on a critical reappraisal of character coding in lobopodians and using Bayesian and parsimony-based tree searches, two alternative scenarios for early panarthropod evolution are retrieved. In both cases, hallucigeniids and luolishaniids are found to be extinct radiative stem group panarthropods, in contrast to previous analyses supporting a position of hallucigeniids as part of total-group Onychophora. Our Bayesian topology finds luolishaniids and hallucigeniids to form two successive clades at the base of Panarthropoda. Disparity analyses suggest that luolishaniids, hallucigeniids and total-group Onychophora each occupy a distinct region of morphospace.
Hallucigeniids and luolishaniids were comparably diverse and successful, representing two major lobopodian clades in the early Palaeozoic, and both evolved body plans adapted to different forms of suspension feeding. A Bayesian approach to cladistics supports the view that a semi-sessile, suspension-feeding lifestyle characterized the origin and rise of Panarthropoda from cycloneuralian body plans.
节肢动物、缓步动物和有爪动物由叶足动物进化而来，叶足动物是一个并系群，包含多种形态各异的古生代蠕虫状动物，腿柔软。尽管该类群所涵盖的形态多样性可能表明生态位多样化在泛节肢动物早期辐射演化中的重要性，但叶足动物的生态学特征仍鲜为人知。
在此，我们描述了一种来自加拿大不列颠哥伦比亚省中寒武世布尔吉斯页岩（沃尔科特采石场）的新的罗哩山虫类分类单元，其特殊形态体现了该类群的滤食生态，并且与一些现代海洋动物趋同，如麦秆虫类甲壳动物。该物种前部有两对长的和四对较短的细长多刺叶足，每只叶足上有两个细长爪，后部有三对粗壮叶足，每只叶足前端有一个强壮的钩状爪。躯干明显光秃，向后变宽，前端延伸到第一对叶足之外，形成一个小“头部”，有一对视觉器官和一个有许多牙齿的短吻。基于对叶足动物特征编码的批判性重新评估，并使用基于贝叶斯和简约法的系统发育搜索，得出了早期泛节肢动物演化的两种替代假说。在这两种情况下，怪诞虫类和罗哩山虫类被发现是已灭绝的辐射干群泛节肢动物，这与之前支持怪诞虫类作为总群有爪动物一部分的分析结果相反。我们的贝叶斯系统发育树拓扑结构发现罗哩山虫类和怪诞虫类在泛节肢动物基部形成两个连续的分支。差异分析表明，罗哩山虫类、怪诞虫类和总群有爪动物各自占据形态空间的一个独特区域。
怪诞虫类和罗哩山虫类具有相当的多样性和成功度，代表了古生代早期两个主要的叶足动物分支，并且都演化出了适应不同滤食形式的身体结构。一种基于贝叶斯方法的系统发育分析支持这样一种观点，即半固着的滤食性生活方式是泛节肢动物从环神经动物身体结构起源和兴起的特征。
