Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK.
School of Earth Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, Western Australia 6009, Australia.
Biol Lett. 2021 Jan;17(1):20200809. doi: 10.1098/rsbl.2020.0809. Epub 2021 Jan 20.
The somasteroids are Lower Palaeozoic star-shaped animals widely regarded as ancestors of Asterozoa, the group of echinoderms that includes brittle stars and starfish. However, the origin of asterozoans, the assembly of their distinctive body organization, and their relationships with other Cambrian and Ordovician echinoderms remain problematic owing to the difficulties of comparing the endoskeleton between disparate groups. Here, we describe the new somasteroid , a primitive asterozoan from the Early Ordovician Fezouata Lagerstätte in Morocco. shares with other somasteroids a unique endoskeletal arm organization and the presence of rod-like virgal ossicles that articulate with the ambulacrals, but differs from all other known asterozoans in the absence of adambulacral ossicles defining the arm margins, evoking parallels with non-asterozoan echinoderms. Developmentally informed Bayesian and parsimony phylogenetic analyses, which reflect the homology of the biserial ambulacral ossicles in Palaeozoic echinoderms according to the extraxial-axial theory, recover as the earliest divergent stem-group asterozoan. Our results illuminate the ancestral morphology of Asterozoa, and clarify the affinities of problematic Ordovician Asterozoa. Bayesian inference and parsimony demonstrate that somasteroids represent a paraphyletic grade within stem- and crown-group Asterozoa, whereas stenuroids are paraphyletic within stem-group Ophiuroidea. Our results also offer potential insights on the evolutionary relationships between asterozoans, crinoids and potential Cambrian stem-group representatives.
有栉水母类是下古生代广泛认为是星型动物海星和海胆的棘皮动物祖先的动物。然而，由于难以比较不同群体之间的内骨骼，棘皮动物的起源、它们独特身体组织的组装以及它们与其他寒武纪和奥陶纪棘皮动物的关系仍然存在问题。在这里，我们描述了来自摩洛哥早期奥陶世 Fezouata 层位的新型有栉水母类，这是一种原始的棘皮动物。与其他有栉水母类一样，它具有独特的内骨骼臂组织和杆状 virgal 小骨存在，这些小骨与步带区相连，但与所有其他已知的棘皮动物不同，它没有定义臂缘的 adambulacral 小骨，这让人联想到非棘皮动物的棘皮动物。基于发育的贝叶斯和简约系统发育分析，根据外轴 - 轴理论反映了古生代棘皮动物双列步带小骨的同源性，将有栉水母类作为最早分歧的棘皮动物冠群。我们的研究结果阐明了 Asterozoa 的祖先形态，并阐明了有问题的奥陶纪 Asterozoa 的亲缘关系。贝叶斯推断和简约性表明，有栉水母类代表棘皮动物冠群和干群内的并系等级，而 Stenuroids 在棘皮动物干群内是并系的。我们的结果还为棘皮动物、海百合和潜在的寒武纪干群代表之间的进化关系提供了潜在的见解。
