Distinguishing geology from biology in the Ediacaran Doushantuo biota relaxes constraints on the timing of the origin of bilaterians

Abstract

The Ediacaran Doushantuo biota has yielded fossils that include the oldest widely accepted record of the animal evolutionary lineage, as well as specimens with alleged bilaterian affinity. However, these systematic interpretations are contingent on the presence of key biological structures that have been reinterpreted by some workers as artefacts of diagenetic mineralization. On the basis of chemistry and crystallographic fabric, we characterize and discriminate phases of mineralization that reflect: (i) replication of original biological structure, and (ii) void-filling diagenetic mineralization. The results indicate that all fossils from the Doushantuo assemblage preserve a complex mélange of mineral phases, even where subcellular anatomy appears to be preserved. The findings allow these phases to be distinguished in more controversial fossils, facilitating a critical re-evaluation of the Doushantuo fossil assemblage and its implications as an archive of Ediacaran animal diversity. We find that putative subcellular structures exhibit fabrics consistent with preservation of original morphology. Cells in later developmental stages are not in original configuration and are therefore uninformative concerning gastrulation. Key structures used to identify Doushantuo bilaterians can be dismissed as late diagenetic artefacts. Therefore, when diagenetic mineralization is considered, there is no convincing evidence for bilaterians in the Doushantuo assemblage.

The Weng'an Biota (Doushantuo Formation): an Ediacaran window on soft-bodied and multicellular microorganisms

“…The softbodied organisms of the biota are three-dimensionally preserved in calcium phosphate and can be preserved at a subcellular level (Hagadorn et al 2006;Huldtgren et al 2011). However, even the best-preserved specimens are a complex amalgam of cements, making it challenging to determine which aspects represent preserved biology Bengtson 2003;Bengtson & Budd 2004;Cunningham et al 2012a;Schiffbauer et al 2012). The preservation of Weng'an fossils is discussed in Box 1.…”

“…An alternative approach is to characterize the mineral phases that preserve biological anatomy v. geological artefacts. This has been carried out in fossils where there is agreement regarding affinity and anatomy, and has allowed textural and chemical criteria to be established and then applied to more contentious Weng'an material (Cunningham et al 2012a(Cunningham et al , 2014. In particular, geological artefacts tend to have larger, euhedral crystals with a preferred orientation, higher X-ray attenuation and characteristic void-filling textures.…”

“…Chen et al 2009a). In each case, the specimens can be alternatively interpreted as examples of embryolike fossils that have undergone taphonomic and diagenetic processes (Huldtgren et al 2011;Cunningham et al 2012a). Specimens interpreted as possible bilaterian or cnidarian gastrulae and larvae (Chen et al 2000(Chen et al , 2002Chen & Chi 2005) are more probably deformed cysts filled with phosphatic cements .…”

“…The embryo-like fossils have been described under various genus and species names that are now considered to be different developmental stages or taphonomic variants of a single taxon (Huldtgren et al 2011;Cunningham et al 2012a;L. Chen et al 2014;Xiao et al 2014a,b).…”

“…The names Megasphaera (single-celled specimens), Parapandorina (multiple polyhedral cells) and Megaclonophycus (large numbers of usually spheroidal cells) are now widely considered to be synonyms (e.g. Huldtgren et al 2011;Cunningham et al 2012a;Xiao et al 2014a,b). However, because of different taxonomic interpretations, different researchers have referred to this taxon as either Megasphaera (e.g.…”