Data from: Fossil embryos from the Middle and Late Cambrian period of Hunan, south China

Comparative embryology is integral to uncovering the pattern and process of metazoan phylogeny, but it relies on the assumption that life histories of living taxa are representative of their antecedents. Fossil embryos provide a crucial test of this assumption and, potentially, insight into the evolution of development, but because discoveries so far lack phylogenetic constraint, their significance is moot. Here we describe a collection of embryos from the Middle and Late Cambrian period (500 million years ago) of Hunan, south China, that preserves stages of development from cleavage to the pre-hatching embryo of a direct-developing animal comparable to living Scalidophora (phyla Priapulida, Kinorhyncha, Loricifera). The latest-stage embryos show affinity to the Lower Cambrian embryo Markuelia, whose life-history strategy contrasts both with the primitive condition inferred for metazoan phyla and with many proposed hypotheses of affinity, all of which prescribe indirect development. Phylogenetic tests based on these embryological data suggest a stem Scalidophora affinity. These discoveries corroborate, rather than contradict, the predictions of comparative embryology, providing direct historical support for the view that the life-history strategies of living taxa are representative of their stem lineages.比较胚胎学（Comparative embryology）是揭示后生动物（metazoan）系统发育模式与演化过程的核心研究手段，但该领域的研究依赖一项前提假设：现生类群的生活史可代表其祖先类群的生活史。化石胚胎为验证这一假设提供了关键依据，亦有望为发育演化研究提供新视角，但目前已发现的化石胚胎均缺乏系统发育约束（phylogenetic constraint），其研究意义长期存疑。本文记述了产自中国南方湖南地区中寒武世至晚寒武世（约5亿年前）的一批胚胎化石集合，其保存了从卵裂期到孵化前胚胎的完整发育阶段，该类群为一类直接发育动物，与现生动吻总类群（Scalidophora，涵盖鳃曳动物门Priapulida、动吻动物门Kinorhyncha与铠甲动物门Loricifera）具有可比性。最晚发育阶段的胚胎化石显示出与下寒武统马克尔虫（Markuelia）的亲缘关系，而马克尔虫的生活史策略既不同于后生动物门的推断原始性状，也与诸多已提出的亲缘关系假说相悖——这些假说均认为该类群应具备间接发育（indirect development）特征。基于这些胚胎学数据开展的系统发育分析显示，该化石类群属于干群动吻总类群（stem Scalidophora）。本研究结果证实而非推翻了比较胚胎学的相关预测，为"现生类群的生活史策略能够代表其干谱系（stem lineages）"这一观点提供了直接的历史实证支持。