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. Phy...比较胚胎学（Comparative embryology）是揭示后生动物系统发育（metazoan phylogeny）模式与过程的核心研究手段，但该领域的研究依赖于一项前提假设：现存类群的生活史可代表其祖先类群的生活史。化石胚胎（Fossil embryos）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），因此其研究意义尚存争议。本文报道了产自中国南方湖南中晚寒武世（距今5亿年）的一批胚胎化石，其保存了从卵裂（cleavage）到直接发育动物孵化前胚胎的各个发育阶段，该类群与现生动吻总门（Scalidophora，包含鳃曳动物门Priapulida、动吻动物门Kinorhyncha、铠甲动物门Loricifera）类群具有可比性。最新阶段的胚胎化石显示其与下寒武统化石马克尔虫（Markuelia）具有亲缘关系，而马克尔虫的生活史策略既不同于后生动物各门的推断原始状态，也与诸多已提出的亲缘关系假说相悖——这些假说均预设其发育模式为间接发育（indirect development）。Phy...