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Abstract
Fossil bone preservation – both the condition of the bone tissue itself and the materials that infill or permineralize voids within the bone – can vary over short stratigraphic distances, suggesting that late-diagenetic or regional factors are not important drivers of preservation. Instead, preservation is hypothesized to be driven by the experience of the bone in the initial post-mortem and early diagenetic window. Brief exposure before permanent burial below the surface mixed layer should result in well-preserved bone, while prolonged taphonomic exposure should result in more damaged bone and more complex infill. To test this hypothesis in the fossil record, bones were sampled from marine stratigraphic sequences where burial rate could be inferred from sequence stratigraphic position, sampling from within aggradational systems tracts (rapid burial) and along stratigraphic surfaces of prolonged exposure and/or erosion. In the third-order siliciclastic sequence in Wadi Al-Hitan (Birket Qarun and Qasr el-Sagha Formations, Eocene, Egypt), where bones are extensively permineralized by calcite, distinct taphonomic and authigenic features still distinguished bones according to their depositional context. Importantly bones from the incised valley filled showed evidence of collagen shrinking, terrestrial-form microbial tunneling, and oxidized minerals, all indicating post-mortem subaerial exposure. In the more complex, second-order sequence at the Calvert Cliffs (Calvert, Choptank, and St. Marys Formations, Miocene, Maryland), which is highly condensed and unlithified, microtaphonomic and authigenic features of bones preserved evidence of higher-order cyclicity that was otherwise cryptic. Finally, bones sampled from the interval of maximum transgression (marine inundation) within the third-order Aguja Formation (Cretaceous, Big Bend National Park, Texas) also display evidence of subaerial exposure rather than exclusively submarine conditions, again providing evidence of higher-order cyclicity. The hypothesis that early postmortem conditions determine bone preservation is thus supported across all three sedimentary basins. Moreover, variability in bone preservation is predictable when the sequence stratigraphy is understood, and taxonomically-insignificant bone fragments can provide unique insights into the otherwise cryptic sedimentary dynamics of stratigraphic surfaces and very thin records.