First putative occurrence in the fossil record of choanoflagellates, the sister group of Metazoa

Abstract

Choanoflagellates are microeukaryotes that inhabit freshwater and marine environments and have long been regarded as the closest living relatives of Metazoa. Knowledge on the evolution of choanoflagellates is key for the understanding of the ancestry of animals, and although molecular clock evidence suggests the appearance of choanoflagellates by late Neoproterozoic, no specimens of choanoflagellates are known to occur in the fossil record. Here the first putative occurrence of choanoflagellates in sediments from the Cretaceous (Cenomanian–Turonian) is described by means of several cutting-edge petrographic techniques, and a discussion of its paleoenvironmental significance is performed. Furthermore, their placement in the organic matter classification systems is argued, with a placement in the Zoomorph Subgroup (Palynomorph Group) of the dispersed organic matter classification system being proposed. Regarding the ICCP System 1994, incorporation of choanoflagellates is, at a first glance, straightforward within the liptinite group, but the definition of a new maceral may be necessary to accommodate the genetic origin of these organisms. While modern choanoflagellates may bring light to the cellular foundations of animal origins, this discovery may provide an older term of comparison to their extant specimens and provide guidelines for possible identification of these organic components in other locations and ages throughout the geological record.