ASSESSING UNCERTAINTY IN EARLY CAMBRIAN BIOSTRATIGRAPHIC FIRST APPEARANCE DATA FROM DYNAMIC PROGRAMMING-DETERMINED δ13CCARB CORRELATIONS

Aligning Cambrian Series 1–2 δ¹³C_carb records from fossiliferous but temporally unresolved stratigraphic sections (e.g., Mongolia, Siberia, China) to the U-Pb radiometrically calibrated Moroccan δ¹³C_carb record allows for the extrapolation of the Moroccan age-model to the former locales and reveals the absolute timing of animal first appearance data (FADs; Maloof et al., 2010; Smith et al., 2016). The inferred diversification trend, however, hinges on the underlying age model which assumes steady sediment accumulation between five ash beds. We constructed two alternative age-models for the Moroccan section. The first assumes exponential decay, as might be expected for a subsiding passive margin; the second arises from the Bayesian Bchron age-depth modeling algorithm (Haslett and Parnell, 2008). The three classes of age model differ from ~541– 525 Ma and show similarities between 525–520 Ma. We employed a dynamic programming (DP) technique for least-squares optimization (Hay et al., 2019) to catalogue plausible alignments of the Mongolia, Siberia, and China δ¹³C_carb records to the Moroccan δ¹³C_carb record, thereby producing ~one dozen sub-variants of the three age models. We quantify the DP precision of a local FAD as the range in ages arising from multiple plausible alignments to a single Moroccan δ¹³C_carb age-model. We assess class precision as the degree of similarity between the three age-models for the suite of plausible alignments. We find that none of the local FADs for Cambrian Series 1–2 taxa of Mongolia, Siberia, or China are of both high DP precision (≤ 1 Myr difference between alignments) and high class precision (≥ 50% similarity of the assigned age distribution of a given local FAD between all three age-models). For example, the FAD of Anabarites in Siberia is of high DP precision (0 Myr) but low class precision (<10%), while the FAD of Anabarites in Mongolia is of high class precision (>90%) but low DP precision (>10 Myr). The FAD uncertainty reflected in age class and DP precision challenges previous inferences of early Cambrian diversification; we conclude that, without additional geochronological constraints, only low-resolution diversification is discernable from the early Cambrian biostratigraphic record.