Fossil record misconstrued: Sponge-like fabrics reflect incipient carbonate metamorphism

The earliest named stromatolite Cryptozoon Hall, 1884 (Late Cambrian, ca. 490 Ma, eastern New York State), was recently re-interpreted as interlayered microbial mat and non-spiculate (keratosan) sponge deposit. This “classic stromatolite” has come to be central to a fundamental debate concerning the significance or even existence of non-spiculate sponges in carbonate rocks of the Neoproterozoic (Tonian) onward. We determine herein that Cryptozoon has three types of primary carbonate layers: clotted-pelletoidal micrite with microbial filaments, clotted-pelletoidal micrite with vesicular structure, and dense microcrystalline laminae. Using contextual fabric analysis, elemental mapping, cathodoluminescence microscopy, fluid inclusions, electron backscatter diffraction, U–Pb carbonate dating, and regional burial history, the sponge interpretation is rebutted. We conclude that suspect fabric elements are secondary in nature and best explained as products of deep burial alteration. Incipient carbonate metamorphism is early Carboniferous in age (Mississippian, terminal Acadian orogeny). Key petrographic observations include heterogenous recrystallization (aggraded Ostwald ripening) associated with interfingering reaction fronts typical for partially miscible fluids, a granoblastic calcite texture exhibiting preferred crystallographic orientation, and subsequent authigenic white mica (deepest burial; late Carboniferous and Permian Appalachian orogeny). Topotype Cryptozoon is a sub-greenschist metacarbonate stromatolite. The published Tonian to Phanerozoic record of non-spiculate sponges needs to be reassessed.