Embryogenesis of the early Cambrian medusozoan (Cnidaria) Octapyrgites elongatus

Abstract

New post‐embryonic specimens and the first known embryos of Octapyrgites elongatus , a medusozoan cnidarian in the family Olivooidae, are described from Member 5 of the Yanjiahe Formation (Cambrian Stage 2) in western Hubei Province, South China. This discovery extends the embryological record of olivooids beyond the Kuanchuanpu biota and from the Fortunian Stage to Cambrian Stage 2. Micro‐CT scans of three embryos revealed internal anatomical structures, including relict soft tissue and stacked sets of four and eight embryonic oral lobes, with the four‐lobed set situated closest to the oral pole. Together, available specimens of O. elongatus record a mostly complete developmental cycle extending from the early organogenesis stage to hatchling specimens showing 12 annulations in the aboral portion of the theca. Co‐occurring, morphologically similar spheroidal specimens probably represent the earlier cleavage, blastula, or gastrula stages. Embryos of O. elongatus are relatively large, averaging 720 μm long, though many embryos have undergone diagenetic shrinkage and deformation. As in other olivooids, development was direct, with adoral migration of the embryonic oral lobes. The early ontogeny of O. elongatus is most similar to that of Quadrapyrgites quadratacris , further suggesting that Octapyrgites and Quadrapyrgites , both of which exhibit tetraradial symmetry, were more closely related to each other phylogenetically than either was to Olivooides or to the three other, pentaradially symmetrical olivooid genera.