Ortega-Hernández Javier, Janssen Ralf, Budd Graham E
Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK; Museum of Comparative Zoology and Department of Organismic and Evolutionary Biology, Harvard University, 26 Oxford Street, Cambridge, MA 02138, USA.
Department of Earth Sciences, Palaeobiology, Uppsala University, Villavägen 16, Uppsala se 752 36, Sweden.
Arthropod Struct Dev. 2019 Mar;49:155-158. doi: 10.1016/j.asd.2018.11.003. Epub 2018 Nov 29.
The Ecdysozoa is a major animal clade whose main uniting feature is a distinctive growth strategy that requires the periodical moulting of the external cuticle. The staggering diversity within Ecdysozoa has prompted substantial efforts to reconstruct their origin and early evolution. Based on palaentological and developmental data, we proposed a scenario for the early evolution of the ecdysozoan clade Panarthropoda (Onychophora, Tardigrada, Euarthropoda), and postulated that a terminal mouth is ancestral for this lineage. In light of the accompanying comment by Claus Nielsen, we take this opportunity to clarify the significance of our argumentation for Panarthropoda in the phylogenetic context of Ecdysozoa, and Bilateria more broadly. We conclude that the ancestral ecdysozoan most likely had an adult terminal mouth, and that the last common ancestors of all the phyla that constitute Ecdysozoa almost certainly also had an adult terminal mouth. The occurrence of a ventral-facing mouth in various adult ecdysozoans - particularly panarthropods - is the result of convergence. Despite the paucity of embryological data on fossil taxa, we contemplate the likelihood that a developmentally early ventral mouth opening could be ancestral for Ecdysozoa, and if so, then this would represent a symplesiomorphy of Bilateria as a whole.
蜕皮动物总门是一个主要的动物分支，其主要的共同特征是一种独特的生长策略，即需要定期蜕去外部角质层。蜕皮动物总门内惊人的多样性促使人们付出巨大努力来重建它们的起源和早期进化。基于古生物学和发育学数据，我们提出了泛节肢动物（有爪动物门、缓步动物门、真节肢动物）蜕皮动物分支早期进化的一种设想，并假设终端口是该谱系的祖先特征。鉴于克劳斯·尼尔森的相关评论，我们借此机会阐明我们关于泛节肢动物在蜕皮动物总门以及更广泛的两侧对称动物系统发育背景下的论证意义。我们得出结论，蜕皮动物的祖先很可能具有成体终端口，并且构成蜕皮动物总门的所有门的最后共同祖先几乎肯定也有成体终端口。各种成年蜕皮动物——尤其是泛节肢动物——中腹向口的出现是趋同进化的结果。尽管关于化石类群的胚胎学数据匮乏，但我们推测在发育早期出现腹侧口可能是蜕皮动物总门的祖先特征，如果是这样，那么这将代表整个两侧对称动物的一个共有衍征。
