title: First macrobiota biomineralization was environmentally triggered.
abstract: Why large and diverse skeletons first appearedca550 Ma is not well understood. Many Ediacaran skeletal biota show evidence of flexibility, and bear notably thin skeletal walls with simple, non-hierarchical microstructures of either aragonite or high-Mg calcite. We present evidence that the earliest skeletal macrobiota, found only in carbonate rocks, had close soft-bodied counterparts hosted in contemporary clastic rocks. This includes the calcareous discoidal fossilSuvorovella,similar to holdfasts of Ediacaran biota taxa previously known only as casts and moulds, as well as tubular and vase-shaped fossils. In sum, these probably represent taxa of diverse affinity including unicellular eukaryotes, total group cnidarians and problematica. Our findings support the assertion that the calcification was an independent and derived feature that appeared in diverse groups where an organic scaffold was the primitive character, which provided the framework for interactions between the extracellular matrix and mineral ions. We conclude that such skeletons may have been acquired with relative ease in the highly saturated, high alkalinity carbonate settings of the Ediacaran, where carbonate polymorph was further controlled by seawater chemistry. The trigger for Ediacaran biomineralization may have been either changing seawater Mg/Ca and/or increasing oxygen levels. By the Early Cambrian, however, biomineralization styles and the range of biominerals had significantly diversified, perhaps as an escalating defensive response to increasing predation pressure. Indeed skeletal hardparts had appeared in clastic settings by Cambrian Stage 1, suggesting independence from ambient seawater chemistry where genetic and molecular mechanisms controlled biomineralization and mineralogy had become evolutionarily constrained.
authors.affiliation: School of GeoSciences, University of Edinburgh, James Hutton Road, Edinburgh EH9 3FE, UK rachel.wood@ed.ac.uk.
authors.forename: Rachel
authors.identifier: http://orcid.org/0000-0002-0165-5987
authors.initials: R
authors.lastname: Wood
authors.affiliation: Borissiak Paleontological Institute, Russian Academy of Sciences, ul. Profsoyuznaya 123, Moscow 117997, Russia.
authors.forename: Andrey Yu
authors.initials: AY
authors.lastname: Ivantsov
authors.affiliation: Department of Biological Evolution, Faculty of Biology, Lomonosov Moscow State University, Leninskie gory 1(12), Moscow 119234, Russia.|Geological Institute, Russian Academy of Sciences, Pyzhevskiy per. 7, Moscow 119017, Russia.
authors.forename: Andrey Yu
authors.identifier: http://orcid.org/0000-0003-1611-6916
authors.initials: AY
authors.lastname: Zhuravlev
mesh_terms: D000818:Animals; D002113:Calcification, Physiologic; D002119:Calcium Carbonate; D005580:Fossils; D008903:Minerals; D012623:Seawater
chemical_list: D008903:Minerals; D002119:Calcium Carbonate
keywords: Ediacaran; biomineralization; carbonate supersaturation; macrobiota; metazoans