Taphonomic approaches to interpretation of the fossil record of chitons (Mollusca, Polyplacophora).

摘要

Biases in the fossil record of chitons (Mollusca, Polyplacophora) were evaluated using data compiled on Phanerozoic occurrences and the effects of taphonomic processes on modern valves. Most chiton fossils comprise relatively rare, worn, and disarticulated valves. The generally poor preservation may accurately represent the shallow coastal settings that most chitons have inhabited since the Late Cambrian, but the fossil record of chitons had not been assessed from a taphonomic perspective.;The occurrence data indicate that the fossil record of chitons is incomplete, sporadic, and geographically limited partly due to collector sampling biases. Disarticulated skeletal elements of Mississippian multiplacophorans, an extinct chiton order with seventeen valves rather than the usual eight, recovered at four Midwestern localities provided further evidence of sampling biases. Study of the disarticulated material combined with examination of two nearly complete and partly articulated Mississippian multiplacophoran specimens provided architectural details used in a new reconstruction of the entire skeleton. The studies allowed Order Multiplacophora to be placed in Subclass Neoloricata of Class Polyplacophora, but the order is distinguished by unique shared characters that have not been observed in more typical chitons, fossil or modern.;The taphonomic data suggest that the chiton fossil record is incomplete partially due to taphonomic biases. The condition of Katharina tunicata and Mopalia muscosa valves collected on rocky shores of San Juan Island, Washington demonstrates that preservation potential of chitons varies depending on locality, species, and type of valve. Katharina tunicata intermediates have the highest preservation potential compared to terminal valves or Mopalia muscosa intermediates. The field observations were consistent with laboratory tests of valve strength and resistance to dissolution and abrasion. The experimental results combined with a microstructural study and finite element analyses of simple three-dimensional models of intermediates indicate the differential preservation is due mainly to intrinsic differences in valve morphology, thickness, and microstructure. These characters similarly affect the durability of disarticulated valves in the natural environment. Taxon specific compounds and other differences in decomposition rates seen in a novel method investigating post-burial decay processes suggest there also may be specific differences in preservation potential of soft tissues.