Quadrapyrgites from the lower Cambrian of South China: growth pattern, post-embryonic development, and affinity

Abstract

Quadrapyrgites is a microfossil reported with embryonic and post-embryonic stages from the lower Cambrian small shelly fauna of South China, and is closely related to Olivooides. Their development mode and systematic positions are hot topics in palaeoembryology and evolutionary biology in recent years, but are still under debate. Here, we present a description on the growth pattern and post-embryonic development of Quadrapyrgitesquadratacris recovered from the lower Cambrian of South China. The growth zone of Q. quadratacris is situated at the oral end, and new terminal lobes are generated within the oral aperture. The terminal lobes are first in control of the opening and closure of the tubes, and later transferred to be crests, thus elongating the tubes. The post-embryonic development is characterized by a one-by-one addition of crests on the post-embryonic tissue, and an ontogenetic sequence with crest number from 1 to 17 is reconstructed. During the ontogeny, the embryonic tissue remains stable in size and morphology. The oral aperture is the only opening of the tubes, while a possible anus on the apical end does not occur. Quadrapyrgites was proposed to be the sister group of Olivooides, and now this is strengthened by the identical growth pattern and post-embryonic development. The comprehensive data from anatomy, growth pattern and post-embryonic development reject the stem-lineage cycloneuralian affinity for Quadrapyrgites and Olivooides, and instead support a coronate scyphozoan hypothesis.