THE CRYOGENIAN EMERGENCE OF SPONGE-GRADE ANIMALS AND THEIR ENVIRONMENTAL CONSEQUENCES

While molecular clock studies suggest a Tonian-Cryogenian (~800–635 Ma) emergence of the Porifera, convincing fossil evidence of sponges is seen only as far back as ~530 Ma. The >100 Ma lacuna for sponges represents a critical missing piece of the Neoproterozoic puzzle. Assembling an evolutionary framework requires that Poriferan antiquity be understood in terms of sponge form and function, and the emergence of suspension-feeding amid profound environmental and climatic change. Here we report newly-discovered biomineralized fossils of articulated sponge-grade animals in the pre-Marinoan (ca. 650 Ma) Trezona Formation of South Australia. These structures alternate with pink laminated or peloidal microbial mats to form discrete columns, with detrital flake and bioclast breccias infilling depressions. The articulated remains are characterized by pleated syconid-to-leuconid fabrics composed of sparry calcite and barite surrounded by a dense ‘crown of thorns’ network of fibrous quartz crystals (interpreted as recrystallized silica spicules) cemented with glauconite and, more distally, by mottled calcite, with darker areas characterized by a dense constellation of 5–20 µm diameter pyrite framboids, while the lighter areas are defined by thin sparry carbonate rims surrounding micritic interiors lacking in framboids. The detrital components include red bioclasts characterized by a thin porous and laminated shell composed of calcite and quartz that is often punctuated with sub-spherical carbonate structures, possibly related to composite spicules, along with an interconnected and tiered network of internal channels. Interior to the laminated coating is a heavily pyritized zone, internal sparry calcite, and sediment-filled internal channels of various size and orientation. These bioclasts are remarkably similar to those discovered in Shuram Excursion strata from the Patom Uplift of central Siberia, which we similarly propose to represent sponge-grade animals. In both examples, the fossils are associated with strong negative carbon isotope anomalies recognized worldwide. We discuss the implications of the emergence of sponge-grade animals in the Cryogenian and Ediacaran periods, and the environmental and climatic consequences of their modification of Neoproterozoic ecosystems.