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Paper No. 103-2
Presentation Time: 8:00 AM-5:30 PM
THE INVERSE RELATIONSHIP BETWEEN THE STRATIGRAPHIC DISTRIBUTION OF RUSOPHYCUS AND BIOTURBATION THROUGH TIME: IMPLICATIONS FOR SECULAR CHANGE IN SEDIMENT RHEOLOGY
The key transition between Proterozoic matgrounds to Phanerozoic mixed grounds (i.e., the Agronomic Revolution) resulted from an increase in the diversity and complexity of trace making behavior from the Ediacaran to the late Cambrian. Current understanding of the Agronomic Revolution outlines that the development of the mixed layer took place over ~120 million years, reaching depths similar to the modern mixed layer in the late Silurian. In the absence of widespread mats before the development of a deep mixed layer, fine grained siliciclastic marine sediments were poorly mixed with low water content, leading to the widespread formation of hydroplastic, cohesive, and erosion resistant firmgrounds. Firmgrounds are characterized by their faithful preservation of shallow tier trace fossils that are otherwise lost in well-mixed sediments, representing a unique taphonomic window in the Paleozoic that also influenced the nature of animal-substrate interactions. Therefore, the timing of the decline in firmgrounds has broad implications for our understanding of the trace fossil record and animal evolution. We utilize global trends in the occurrence of the ichnogenus Rusophycus, which requires firmground conditions to be preserved, to assess trends in firmground occurrence throughout the Paleozoic. Results normalized to geologic period length and marine siliciclastic rock area demonstrate that Rusophycus is most common in the Cambrian, after which there is a steep and continuous decline towards the end of the Paleozoic. This trend is not consistent with patterns of trilobite evolution. We interpret the decline of Rusophycus occurrences towards the end of the Paleozoic as a marker for the decline in firmgrounds and the closing of the taphonomic window for surficial, shallow tier trace fossils. This highlights how this secular change in seafloor sediment rheology shaped the trace fossil record.