New ecdysozoan fossil embryos from the basal Cambrian of China

Abstract

Invertebrate embryos have relatively low fossilization potential; 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. All specimens are three-dimensionally phosphatized, akin to the Orsten-type preservation. The embryos are enclosed within a thin and smooth envelope ranging in diameter from 730 μm to 1 mm. The embryos have a bag-shaped body, a non-ciliated integument, radially arranged sclerites apically, and bilaterally arranged sclerites abapically, but lack any orifice, thus representing a developmental stage prior to the formation of a mouth or anus. The integument shows soft deformation, while the sclerites remain largely undeformed, suggesting that the sclerites are stiffer and likely cuticularized. The arrangement of the abapical sclerites imparts bilaterality, classifying these new embryos as bilaterians. Furthermore, the absence of cilia or cilium insertion sites and the presence of cuticularized sclerites suggest an ecdysozoan affinity. The relatively large embryo size indicates yolk-rich eggs, and the presence of cuticle implies that the embryos are in later embryonic stages, possibly close to hatching. In the absence of hatched specimens, it is inferred that these embryos undergo indirect development, hatching as lecithotrophic (yolk-feeding) larvae. In this case, the morphology of the juvenile and adult forms is uncertain due to metamorphosis during later growth. Alternatively, the embryos could undergo direct development, hatching as lecithotrophic juveniles, with both juveniles and adults potentially having a similar bag-shaped body without an introvert or paired limbs, closely reminiscent of Saccorhytus. This study expands our understanding of the morphological disparity and diversity of early Fortunian fossil embryos and ecdysozoans.