Early Neoproterozoic origin of the metazoan clade recorded in carbonate rock texture

Abstract

Early Neoproterozoic reefs (older than 779, younger than 1083 Ma) contain a carbonate rock texture, already familiar from sponge-rich Phanerozoic limestones, characterized by authigenic Ca carbonate and irregular, secondary voids containing internal sediment (poly-mud fabric). In Holocene sediment, this texture develops by calcification of degrading extra-cellular collagenous matrix (ECM) of siliceous sponges. ECM calcification of siliceous sponges is evident throughout the Phanerozoic, and the related polymud fabric is a diagnostic petrographic feature of Phanerozoic sponge-rich carbonate mudmounds. The authigenic Ca carbonate of polymuds is interpreted to result from connective tissue calcification just beneath the seafloor, that, in a kind of taphonomic race, takes place at the same time as tissue oxidation and associated void formation. It is intriguing that ECM is a fundamental character of the metazoan clade, and so the presence in Early Neoproterozoic rocks of a texture directly associated with it implies the existence of metazoan-grade organisms at that time. This observation pushes back the earliest geological evidence for animals by about 200 m.y. The timing corroborates results of an integrated phylochronology and supports the concept of a biosphere that persisted through the snowball Earth interval.