CLOSING THE PHOSPHATIZATION WINDOW: TESTING FOR THE INFLUENCE OF TAPHONOMIC MEGABIAS ON THE PATTERN OF SMALL SHELLY FOSSIL DECLINE

Small shelly fossils (SSFs) are a group of mostly problematic, small skeletal elements preserved primarily through phosphatization. SSFs dominate Early Cambrian diversity, but appear to suffer a sharp decline at the end of the Botomian Stage. This observed decline coincides with a significant reduction in phosphogenesis, suggesting that it may be attributable to the closure of the phosphatization window. I tested for the influence of taphonomic bias on observed patterns of SSF extinction at the end of the Botomian using a preliminary dataset consisting of 911 Cambrian skeletal genus occurrences compiled from 138 references. Analyses indicate that SSF preservation is significantly enhanced by, and for most taxa, restricted to, a phosphatization window. Independent proxies indicate that phosphatizing conditions declined in abundance by almost 75% from Atdabanian/Botomian time to Toyonian/Middle Cambrian time, coincident with a severe reduction in observed SSF diversity. Subsampling methods that control for variations in phosphate abundance were used to construct a new SSF diversity curve. The corrected curve suggests that although the decline of SSFs was real, it has been significantly exaggerated by the closure of a phosphatization window. Further investigation of phosphatic facies from Middle Cambrian rocks may yield more SSF taxa, indicating that, rather than short-lived experiments in early animal evolution, SSFs may have been long-ranging, important components of Cambrian communities.