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）对于揭示后生动物（Metazoa）系统发育的模式与过程至关重要，但该学科依赖于一项核心假设：现生类群的生活史能够代表其祖先类群的生活史。化石胚胎可为该假设提供关键检验依据，同时有望为发育演化研究提供新的见解，但由于迄今发现的化石胚胎均缺乏系统发育约束，其学术价值始终存在争议。本文报道了采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，其保存了从卵裂期到孵化前胚胎的完整发育阶段，所属类群为直接发育型动物，与现生的动吻动物总门（Scalidophora，包含曳鳃动物门Priapulida、动吻动物门Kinorhyncha、铠甲动物门Loricifera）类群具有可比性。该批化石中最晚阶段的胚胎与早寒武世马克尔虫属（Markuelia）的化石胚胎具有亲缘关系；马克尔虫属的生活史策略既不同于后生动物门类的推断祖征状态，也与诸多已提出的亲缘关系假说相悖——这些假说均主张生物为间接发育型。基于该胚胎学数据开展的系统发育检验结果表明，该化石类群隶属于动吻动物总门的基干干群。本研究发现不仅未否定比较胚胎学的相关预测，反而为其提供了佐证，同时为"现生类群的生活史策略能够代表其干群支系的生活史"这一观点提供了直接的历史实证支持。