Early ecdysozoan body shape evolution revealed by new fossil embryos from the basal Cambrian of China

Eggs and embryos are rare in fossil record; however, when preserved, they can offer crucial insights into the evolutionary developmental biology of extinct animals. Here we report new exceptionally preserved fossil embryos from the early Fortunian (early Cambrian) of China, which are assigned to two new taxa, Saccus xixiangensis gen. et sp. nov. and Saccus necopinus gen. et sp. nov. They have a bag-shaped body, a non-ciliated epidermis, radially arranged sclerites apically, and bilaterally arranged sclerites abapically, but lack a mouth and anus at this embryonic stage. The arrangement of the abapical sclerites imparts bilaterality, and the non-ciliated epidermis further suggests an ecdysozoan affinity. The bag-shaped body lacking an introvert and featuring paired limbs suggests a total- or stem-group ecdysozoan affinity, possibly related to Saccorhytus. This study expands the morphological disparity and diversity of early Fortunian fossil embryos and ecdysozoans, and indicates that a bag-shaped body was primitive to ecdysozoans.