MICROFOSSIL SIZE TRENDS ACROSS THE LATE ARCHEAN AND PROTEROZOIC: A MAXIMUM LIKELIHOOD APPROACH

Previous studies have shown that the size of Proterozoic acritarchs seems to exhibit fluctuating and non-directional trends. However, whether the trends truly exhibit random variation, stasis or directional change over the proterozoic has not been explicitly tested. Furthermore, whether other proterozoic microfossil groups share similar size trends has not been examined, and whether patterns of size evolution vary among different paleoenvironments or taphonomic modes has not been investigated. To fill these gaps, we investigate whether size change in Archean-Proterozoic spheroidal and filamentous microfossils is best characterized by models of directional change, random variation or stasis, and whether size trends appear different when data are limited to samples collected from particular lithologies. Size measurements of ~500 Archean-Proterozoic spheroidal and filamentous microfossils were compiled from the literature. These measurements were used to calculate the average size of all taxa present in a particular time bin. Time bins were all approximately ~150 Myr. We used the package ‘paleoTS’ in R to fit maximum likelihood models corresponding to directional change (the biased random walk), random variation (the unbiased random walk), and stasis to our size data. The relative fit of models was assessed by comparing their Aikaike’s information criterion (AIC) scores. Preliminary results indicate that both directional change (the biased random walk) and random non-directional change (the unbiased random walk) can be rejected in favor of stasis for spheroidal microfossils extracted from shales. For spheroidal microfossils preserved in cherts both stasis and the unbiased random walk provided good fits to the data. Size trends for filamentous microfossils are also well fit by both the unbiased random walk and stasis. Spheroidal microfossils from shales had significantly larger average sizes than their counterparts from shallow subtidal and peritidal carbonates.