Comparative embryology is integral to uncovering the pattern and process of metazoan phylogeny1, 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 far2,3,4,5 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 Markuelia3, whose life-history strategy contrasts both with the primitive condition inferred for metazoan phyla and with many proposed hypotheses of affinity3,6, 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.
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We thank W. Guo for field assistance, and S. Bengtson and S. Conway Morris for discussion. This work was supported by grants from the National Natural Science Foundation of China, Laboratory of Paleobiology and Stratigraphy of the Nanjing Institute of Geology & Paleontology, Chinese Academy of Sciences and the Ministry of Education of China (to X.-P.D.); the Royal Society and Natural Environment Research Council (to P.C.J.D.); and the Ministry of Science and Technology of China (to J.-B.L.).
The authors declare that they have no competing financial interests.
Supplementary Information: This details the cladistic analysis underlying the results presented in Dong et al. It includes a datamatrix, datasources, character descriptions, justification of coding strategies and experimental procedures employed, description of component and statistical analyses, and a comparison to existing morphology, molecular and total evidence analyses of ecdysozoan interrelationships. (RTF 2400 kb)
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