Transgression-regression cycles drive correlations in Ediacaran-Cambrian rock and fossil records

Strata of the Ediacaran Period (635-538.8 Ma) yield the oldest known fossils of complex, macroscopic organisms in the geologic record. These “Ediacaran-type” macrofossils (known as the Ediacaran biota) first appear in mid-Ediacaran strata, experience an apparent decline through the terminal Ediacaran, and directly precede the Cambrian (538.8-485.4 Ma) radiation of animals. Existing hypotheses for the origin and demise of the Ediacaran biota include: changing oceanic redox states, biotic replacement by succeeding Cambrian-type fauna, and mass extinction driven by environmental change. Few studies frame trends in Ediacaran and Cambrian macroevolution from the perspective of the sedimentary rock record, despite well-documented Phanerozoic covariation of macroevolutionary patterns and sedimentary rock quantity. Here we present a quantitative analysis of North American Ediacaran–Cambrian rock and fossil records from Macrostrat and the Paleobiology Database. Marine sedimentary rock quantity increases nearly monotonically and by over a factor of five from the latest Ediacaran to the late Cambrian. Ediacaran–Cambrian fossil quantities exhibit a comparable trajectory and have strong (rs > 0.8) positive correlations with marine sedimentary area and volume flux at multiple temporal resolutions. Even so, Ediacaran fossil quantities are dramatically reduced in comparison to the Cambrian when normalized by the quantity of preserved marine rock. Although aspects of these results are consistent with the expectations of a simple fossil-preservation induced sampling bias, together they suggest that transgression-regression and a large expansion of marine shelf environments coincided with the diversification of animals during a dramatic transition that is starkly evident in both the sedimentary rock and fossil records.

Methods
Two existing datasets, Macrostrat's database of rocks/stratigraphy and a subset of Paleobiology Database (PBDB) fossil occurrence data, were merged on the basis of their shared rock unit name field for Ediacaran-Cambrian age (635-485.4 Ma) rocks/fossils of North America. Once PBDB fossil occurrences were matched to Macrostrat rock units in time and space (and checked), the fossil occurrence age ranges were modified based on the Macrostrat provided age model of a given fossil occurrences' host rock. Time series of fossil occurrences were generated from this updated data. Correlation coefficients were calculated from the generated time series of fossil occurrences and rock quantities through the Ediacaran-Cambrian geologic time Periods.