3D RECONSTRUCTIONS OF AN EDIACARAN SPONGE-GRADE ANIMAL FROM THE PATOM UPLIFT OF SIBERIA

As a basal metazoan group, sponges are thought to have been among the earliest multicellular animals to develop on Earth. However, the oldest widely accepted examples of sponges in the fossil record date back only to the Cambrian Period, despite estimates from molecular clocks that place their emergence deep into the Neoproterozoic Era. Although reports of Precambrian sponges have been numerous, definitive identification has been complicated by a lack of morphological details or diagnostic features. Here, we provide an update on new work describing recently discovered fossils interpreted as early biomineralizing sponge-grade animals. The fossils are from the Patom Uplift (~565 Ma) in central Siberia, stratigraphically a few hundred meters above the nadir of the Shuram carbon isotope excursion. The centimeter-scale fossil fragments make up a detrital hash, and are preserved three dimensionally in a limestone matrix through mineral replacement of the original tissue by carbonate and pyrite. Earlier descriptions of the fossils highlighted calcified structures interpreted as original biomineralization of a carbonate shell, as well as complex internal canals. We now describe new findings from 3D reconstructions generated via serial sectioning as well as acid maceration. These new analyses have provided an extremely detailed view of the morphology of the fossils. We have expanded upon observations made previously through petrography and microCT observations, and built a more complete model of the fossil organism as displaying mineralized surficial features, an internal lattice-like network of consistently shaped nodes and branches, and abundant exterior pores. These observations are most consistent with a sponge-grade animal assignment. This fossil discovery provides strong evidence for the presence of sponge-grade animals prior to the Cambrian Period, as well as a link between biological evolution and profound changes in the global carbon cycle as revealed by chemostratigraphic events. These observations have significant implications for our understanding of Ediacaran ecology and the evolution of animals.