New holozoans with cellular resolution from the early Ediacaran Weng'an Biota, Southwest China

Abstract

The embryo-like fossils from the early Ediacaran Weng'an Biota (Southwest China, 609 million years ago), widely interpreted as members of holozoans, potentially provide insights to understanding the early evolution of development of metazoans and the rise of the animal kingdom. However, the biodiversity of the embryo-like fossil assemblage is largely underestimated, and its more precise phylogenetic affinities within the holozoan tree are still under debate. Here we describe a new species of embryo-like fossil Ostiosphaera rara n. gen. n. sp. from the Ediacaran Weng'an Biota. These three-dimensional, phosphatized specimens exhibit a spherical morphology, an ornamented thick envelope with a circular opening and a membrane-bounded, multicellular inner body. In terms of biological characteristics, Ostiosphaera rara show similarities to a number of extant and fossil analogues including testate amoebae, unicellular green algae, cellular slime mold Fonticuida and co-occurring Weng'an embryo-like fossils. Although the phylogenetic affinity of Ostiosphaera rara is difficult to constrain very precisely based on the available evidence, it is reasonable to follow the holozoan interpretation for them, since that they share the same grade complexity with the co-occurring embryo-like fossils such as Megasphaera and Helicoforamina in terms of the combination of biological features. It is worth mentioning that the new holozoans resemble asexual reproductive gemmules of fossil and living demosponges in size, morphology, circular opening, and cellular anatomy. If the similarity between them reflects biological affinity rather than convergent evolution, this discovery would force us to rethink the evolutionary history of Precambrian sponges.Thematic collection: This article is part of the Advances in the Cambrian Explosion collection available at: https://www.lyellcollection.org/cc/advances-cambrian-explosion