EBB AND FLOW AT THE DAWN OF ANIMAL LIFE: A COMMON CAUSE MECHANISM FOR CORRELATIONS IN ROCK AND FOSSIL QUANTITIES

Sedimentary rocks of the Ediacaran Period (635-541 Ma) yield the oldest known fossils of complex, macroscopic organisms in the geologic record. These fossils, referred to as ‘Ediacaran-type’ macrofossils, appear in mid-Ediacaran strata and represent communities of complex macroscopic organisms that directly precede the Cambrian (541-485.4 Ma) radiation of animals. There are many hypotheses involving the appearance and decline of Ediacaran-type macrofossils, including changing oceanic redox states, biotic replacement by succeeding Cambrian-type fauna, and mass extinction driven by environmental change. However, few studies frame Ediacaran and Cambrian macroevolution from the perspective of the sedimentary rock record, despite covariation of macroevolutionary patterns and sedimentary rock volume being well-documented in the Phanerozoic. Here we present a quantitative analysis of North American Ediacaran-Cambrian rock and fossil records from the Macrostrat data platform and the Paleobiology Database (PDBD). Trends in the number of Ediacaran-Cambrian fossil occurrences and genera are broadly correlated with trends in the quantity of preserved marine sedimentary rock volume (r² = 0.59 and 0.7, respectively). A stronger correlation between sedimentary volume flux and genera suggests that changes in Ediacaran-Cambrian rock and fossil quantities share a common cause mechanism; most likely that of continental transgression and regression. Ediacaran fossil occurrences, genus richness, and the proportion of the sedimentary record occupied by fossils are, however, dramatically reduced in comparison to the Cambrian, even when the number of fossil occurrences and genera are normalized by the quantity of preserved sedimentary rock. Our results do not preclude existing hypotheses of mechanisms driving macroevolutionary change across the Ediacaran-Cambrian boundary, but they do indicate that transgression and regression influenced both the temporal abundance and distribution of sedimentary rocks in addition to macroevolutionary patterns during the dawn of animal life.