PHANEROZOIC TAPHONOMY OF MARINE BENTHOS: EFFECTS OF SKELETAL DURABILITY ON MACROEVOLUTIONARY TRENDS

Secular patterns retrieved from the invertebrate fossil record may be subject to various taphonomic biases. In particular, if skeletal durability affects preservation potential, large-scale biases may distort the record of durable vs. fragile organisms. This pilot study - in conjunction with the Paleobiology Database [PBDB] (http://paleodb.org) - tests the taphonomic importance of durability for selected Phanerozoic benthic organisms. We focused on three major groups (bivalves, gastropods, and brachiopods) and restricted analyses to two time intervals, which currently are best represented in PBDB (Ordovician-Carboniferous and Jurassic-Paleogene). We used 300 genera with the highest number of occurrences (~63% of 63752 entries for the three groups; PBDB, 6/2002). Multiple species were scored for each genus in terms of body size, shell thickness, reinforcement structures (folds, ribs, spines), and shell mineralogy, using mature specimens from collections or literature sources. The average scores provide a taphonomic durability index for each genus. Preliminary results indicate that the frequency of occurrences of genera in PBDB is independent of durability-related characteristics. Thin-shelled and thick-shelled genera display occurrence-frequency distributions that are virtually identical in terms of shape and central tendency; results are similar for different shell mineralogies and body size classes. This pattern persists when data are analyzed separately for the Paleozoic and Meso-Cenozoic, despite striking increases over time in the proportion of aragonitic fauna, overall body size, and shell thickness. For example, even though Paleozoic aragonitic genera are scarce relative to calcitic ones, their average occurrence frequency is comparable. Although some factors are still poorly controlled and the database is preliminary in nature, these initial results constitute fundamentally good news for paleobiology. Taphonomic durability of shelly benthos apparently is NOT the primary determinant of fossil occurrence. Because a temperate-latitude bias in Meso-Cenozoic records should act against rather than for the observed secular changes in body size, shell thickness, and mineralogy, these trends are likely to reflect genuine biological patterns rather than taphonomic artifacts.