Hughes Ian V, Evans Scott D, Droser Mary L
Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138, USA.
Earth, Ocean, and Atmospheric Sciences, Florida State University, Tallahassee, FL 32304, USA.
Curr Biol. 2024 Dec 16;34(24):5782-5788.e1. doi: 10.1016/j.cub.2024.10.030. Epub 2024 Nov 18.
Molecular clocks and Cambrian-derived metazoans strongly suggest a Neoproterozoic origin of many animal clades. However, fossil bilaterians are rare in the Ediacaran, and no definitive ecdysozoan body fossils are known from the Precambrian. Notably, the base of the Cambrian is characterized by an abundance of trace fossils attributed to priapulid worms, suggesting that major divisions among ecdysozoan groups occurred prior to this time. This is supported by ichnofossils from the latest Ediacaran or early Cambrian left by a plausible nematoid, although definitively attributing this inferred behavior to crown-Nematoida remains contentious in the absence of body fossils. Given the high probability of the evolution of Ecdysozoa in the Proterozoic, the otherwise prolific fossil record of the Ecdysozoa, and the identification of more than 100 distinct Ediacaran genera, it is striking that no Ediacaran body fossils have been confidently assigned to this group. Here, we describe Uncus dzaugisi gen. et. sp. nov. from the Ediacara Member (South Australia), a smooth, vermiform organism with distinct curvature and anterior-posterior differentiation. The depth of relief of Uncus is unique among Ediacara fossils and consistent with a rigid outer cuticle. Ecological relationships and associated trace fossils demonstrate that Uncus was motile. Body morphology and the inferred style of movement are consistent with Nematoida, providing strong evidence for at least an ecdysozoan affinity. This validates the Precambrian origin of Ecdysozoa, reconciling a major gap between predicted patterns of animal evolution and the fossil record..
分子钟和寒武纪衍生的后生动物强烈表明许多动物类群起源于新元古代。然而，埃迪卡拉纪的化石两侧对称动物很少见，且前寒武纪尚无确凿的蜕皮动物实体化石。值得注意的是，寒武纪底部的特征是有大量归因于鳃曳虫类蠕虫的遗迹化石，这表明蜕皮动物类群之间的主要分化发生在此之前。这一观点得到了来自最新埃迪卡拉纪或寒武纪早期一种似线虫动物留下的遗迹化石的支持，尽管在缺乏实体化石的情况下，将这种推断行为明确归因于冠轮动物门线虫纲仍存在争议。鉴于蜕皮动物在元古代演化的可能性很高，蜕皮动物丰富的化石记录，以及已识别出100多个不同的埃迪卡拉纪属，令人惊讶的是，尚无埃迪卡拉纪实体化石被确凿地归为这一类群。在此，我们描述了来自埃迪卡拉生物群（南澳大利亚）的佐氏钩形虫，这是一种体表光滑、蠕虫状的生物，具有明显的弯曲和前后分化。钩形虫的浮雕深度在埃迪卡拉纪化石中是独一无二的，且与坚硬的外部角质层一致。生态关系和相关的遗迹化石表明钩形虫是可移动的。身体形态和推断的运动方式与线虫纲一致，为至少具有蜕皮动物亲缘关系提供了有力证据。这证实了蜕皮动物的前寒武纪起源，弥合了动物进化预测模式与化石记录之间的一个主要差距。
