MEC International Joint Laboratory for Palaeobiology and Palaeoenvironment, Yunnan University, Chenggong Campus, Kunming, 650500, China.
Centre for Ecology and Conservation, University of Exeter, Penryn Campus, Cornwall, TR10 9TA, UK.
BMC Evol Biol. 2020 Nov 23;20(1):156. doi: 10.1186/s12862-020-01720-6.
Ecdysozoa are the moulting protostomes, including arthropods, tardigrades, and nematodes. Both the molecular and fossil records indicate that Ecdysozoa is an ancient group originating in the terminal Proterozoic, and exceptional fossil biotas show their dominance and diversity at the beginning of the Phanerozoic. However, the nature of the ecdysozoan common ancestor has been difficult to ascertain due to the extreme morphological diversity of extant Ecdysozoa, and the lack of early diverging taxa in ancient fossil biotas.
Here we re-describe Acosmia maotiania from the early Cambrian Chengjiang Biota of Yunnan Province, China and assign it to stem group Ecdysozoa. Acosmia features a two-part body, with an anterior proboscis bearing a terminal mouth and muscular pharynx, and a posterior annulated trunk with a through gut. Morphological phylogenetic analyses of the protostomes using parsimony, maximum likelihood and Bayesian inference, with coding informed by published experimental decay studies, each placed Acosmia as sister taxon to Cycloneuralia + Panarthropoda-i.e. stem group Ecdysozoa. Ancestral state probabilities were calculated for key ecdysozoan nodes, in order to test characters inferred from fossils to be ancestral for Ecdysozoa. Results support an ancestor of crown group ecdysozoans sharing an annulated vermiform body with a terminal mouth like Acosmia, but also possessing the pharyngeal armature and circumoral structures characteristic of Cambrian cycloneuralians and lobopodians.
Acosmia is the first taxon placed in the ecdysozoan stem group and provides a constraint to test hypotheses on the early evolution of Ecdysozoa. Our study suggests acquisition of pharyngeal armature, and therefore a change in feeding strategy (e.g. predation), may have characterised the origin and radiation of crown group ecdysozoans from Acosmia-like ancestors.
节肢动物是蜕皮动物，包括节肢动物、缓步动物和线虫。分子和化石记录都表明，蜕皮动物是一个起源于晚元古代的古老群体，特殊的化石生物群表明它们在显生宙早期的优势和多样性。然而，由于现存的节肢动物在形态上具有极端的多样性，以及古代化石生物群中缺乏早期分化的分类群，因此很难确定节肢动物的共同祖先的性质。
本文重新描述了来自中国云南澄江生物群的早寒武世的 Acosmia maotiania，并将其归为节肢动物的原始分支。Acosmia 具有两部分身体，前部有一个末端有口的喙和肌肉发达的咽，后部有一个有环的躯干，有一个贯穿的肠道。使用简约法、最大似然法和贝叶斯推断法对原口动物进行形态系统发育分析，并根据已发表的实验衰减研究对编码进行编码，每种方法都将 Acosmia 作为环神经动物+泛节肢动物的姐妹分类群，即节肢动物的原始分支。为了测试从化石中推断出的特征是否为节肢动物的祖先，对关键节肢动物节点进行了祖先状态概率计算。结果支持冠群节肢动物的祖先具有与 Acosmia 相似的环状蠕虫状身体，末端有口，但也具有环神经动物和环节动物特有的咽部装备和口周结构。
Acosmia 是第一个被归入节肢动物原始分支的分类群，为测试关于节肢动物早期进化的假说提供了一个约束条件。我们的研究表明，咽部装备的获得，以及因此而改变的摄食策略（例如捕食），可能是冠群节肢动物从 Acosmia 样祖先起源和辐射的特征。
