The early Cambrian fossil embryo Pseudooides is a direct-developing cnidarian, not an early ecdysozoan.

Baichuan Duan,Xi-Ping Dong,Luis Porras,John A Cunningham,Philip C J Donoghue,Kelly Vargas

doi:10.1098/rspb.2017.2188

Abstract

Early Cambrian Pseudooides prima has been described from embryonic and post-embryonic stages of development, exhibiting long germ-band development. There has been some debate about the pattern of segmentation, but this interpretation, as among the earliest records of ecdysozoans, has been generally accepted. Here, we show that the ‘germ band’ of P. prima embryos separates along its mid axis during development, with the transverse furrows between the ‘somites’ unfolding into the polar aperture of the ten-sided theca of Hexaconularia sichuanensis, conventionally interpreted as a scyphozoan cnidarian; co-occurring post-embryonic remains of ecdysozoans are unrelated. We recognize H. sichuanensis as a junior synonym of P. prima as a consequence of identifying these two form-taxa as distinct developmental stages of the same organism. Direct development in P. prima parallels the co-occuring olivooids Olivooides, and Quadrapyrgites and Bayesian phylogenetic analysis of a novel phenotype dataset indicates that, despite differences in their tetra-, penta- and pseudo-hexa-radial symmetry, these hexangulaconulariids comprise a clade of scyphozoan medusozoans, with Arthrochites and conulariids, that all exhibit direct development from embryo to thecate polyp. The affinity of hexangulaconulariids and olivooids to extant scyphozoan medusozoans indicates that the prevalence of tetraradial symmetry and indirect development are a vestige of a broader spectrum of body-plan symmetries and developmental modes that was manifest in their early Phanerozoic counterparts.

Highlights

Embryos are among the earliest unequivocal animal fossil remains in the geological record and, as such, they have the potential to provide fundamental insights into the embryology of animals during the evolutionary emergence of the fundamental metazoan body plans
Pseudooides fossils were subsequently interpreted as ecdysozoan embryos based on evidence of irregular spiral/radial cleavage, pinched germ bands comprising six paired, bilaterally arranged domains, interpreted to exhibit long germ-band development, and co-association with the preserved remains of post-embryonic segmented larvae that exhibit bilaterally arranged pairs of anatomical domains [4]
Our results reveal that the ‘germ band’ of Pseudooides develops into an aperture that unfolds along the midline furrow and ‘metamere’ boundaries, resulting in a 10-sided, bilaterally symmetrical theca with a concave base

Summary

Introduction

Embryos are among the earliest unequivocal animal fossil remains in the geological record and, as such, they have the potential to provide fundamental insights into the embryology of animals during the evolutionary emergence of the fundamental metazoan body plans. The phylogenetic affinity and, the evolutionary significance of most remain unresolved [1]. This is especially true of Pseudooides prima, originally described from spherical fossils of unknown affinity from the earliest Cambrian Kuanchuanpu Formation and equivalents from southern Shaanxi Province and northern Sichuan Province, South China [2,3]. Pseudooides fossils were subsequently interpreted as ecdysozoan embryos based on evidence of irregular spiral/radial cleavage, pinched germ bands comprising six paired, bilaterally arranged domains, interpreted to exhibit long germ-band development, and co-association with the preserved remains of post-embryonic segmented larvae that exhibit bilaterally arranged pairs of anatomical domains [4].

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