Reassessment of Archaeooides based on new material from the Fortunian (early Cambrian) of China infers algal affinity

Abstract

Archaeooides originally refers to a group of early Cambrian globular microfossils with regularly arranged nodular surface ornaments, and of a diameter ranging from less than 400 μm to more than 2 mm. The traditional taxonomy assigns all Archaeooides-like microfossils to a single morphotaxon, Archaeooides granulatus, irrespective of their differences in size and surface ornamentation. Archaeooides has been interpreted as a possible diapause embryo because of its inferred palintomical cell division and thick wall with vesicular structures. However, this interpretation needs further testing based on an updated taxonomy and differentiation of taphonomic artifacts from biological structures. Here, we report new, three-dimensionally phosphatized, Archaeooides-like microfossils from the Fortunian of China. In addition to A. granulatus with nodular ornaments, we recognized three new taxa, i.e., Shaanisphaera spinosa gen. et sp. nov. with spine-like ornaments, Qinlingisphaera conica gen. et sp. nov. with cone-like ornaments, and Dahisphaera plana gen. et sp. nov. with oblate ornaments. Each taxon has a wide size interval, ranging continuously from several hundred micrometers to one or two millimeters, which is different from animal eggs that generally have a narrow and conservative size interval. Micro-CT analyses and thin sections reveal that the vesicular structures may be taphonomic in nature, and that the wall thickness is specimen-dependent. Collectively, the evidence at hand does not support an embryo interpretation for Archaeooides. Instead, the wide size interval probably represents ontogenetic variations that are common in many fossil and modern algae, and thus further suggests an algal affinity for Archaeooides as well as the other Archaeooides-like microfossils.