The full understanding of fossils must include consideration of their burial (taphonomy) and subsequent alteration (diagenesis). Analysis of time and mode of burial enhances interpretation of depositional environments. Furthermore, insight into diagenetic changes in fossiliferous rocks may be acquired from studies of physical changes in fossils. Yet these important aspects of the history of fossils commonly are neglected in both paleontologic and petrologic studies.
Field and laboratory studies of “silicified” Permian bivalves in Wyoming have provided a complex case history of one fossil assemblage. These surficial and shallow-burrowing pelecypods commonly are broken and randomly scattered through about 6 in. of calcarenite. Circumstantial evidence suggests that the bottom disturbance was caused by rooting predatory fish. Subsequently, valves were dissolved selectively from hardening sediment not far below the sea bottom.
Induration of sediment and removal of buried shells apparently took place during an interruption in sedimentation. At that time burrowers penetrated the bottom and introduced younger quartz sand and fine shell debris into the substrate and into some of the shallowest molds of valves. The infilling produced detrital casts. Unfilled molds then were lined with precipitated fine-grained quartz, chalcedonic laminae, and euhedral quartz crystals, as in geodes. Such “silicified” fossils are essentially silica casts rather than the result of replacement in the usual sense.
Finally, the fossiliferous stratum was buried under a new increment of pebbly, calcareous quartz sand similar to that of the detrital casts below the now-obscure stratigraphic discontinuity.