蜕皮动物是蜕皮的原虫,包括节肢动物,缓步,和线虫。分子和化石记录都表明Ecdysozoa是一个起源于元古代的古老群体,和特殊的化石生物在显生代初期显示出它们的优势和多样性。然而,由于现存的蜕皮虫的极端形态多样性,很难确定蜕皮虫共同祖先的性质,以及古代生物化石中缺乏早期分歧的类群。
在这里,我们重新描述了来自云南早寒武纪澄江生物群的Acosmiamaotiania,中国并将其分配给茎组Ecdysozoa。Acosmia有一个由两部分组成的身体,前鼻带有末端嘴和肌肉咽部,和一个带有直肠的后环状躯干。使用简约对原生质体进行形态学系统发育分析,最大似然和贝叶斯推理,编码是由已发表的实验衰变研究提供的,每个人都将Acosmia作为姐妹分类单元放置在Cycloneuralia节肢动物中,即茎组Ecdyosoa。计算了关键蜕皮动物节点的祖先状态概率,为了测试从化石中推断为Ecdyosoa祖先的字符。结果支持冠状蜕皮动物的祖先,它们共享一个环状的Vermiform体,末端像Acosmia,而且还具有寒武纪旋回神经和双足动物的咽部电枢和周围结构。
Acosmia是第一个放置在蜕皮动物茎组中的分类单元,并为检验蜕皮动物早期进化的假设提供了约束。我们的研究表明获取咽部电枢,因此改变了喂养策略(例如捕食),可能表征了Acosmia样祖先的冠状蜕皮动物的起源和辐射。
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.