Rebecca Terry, Mark Novak*, Patrick DeLeenheer
Knowing how time is distributed in the fossil record is fundamental to paleobiological inference because the myriad phenomena that lead to time-averaging can have both positive and negative consequences for the biological information it retains. Increasing effort is thus being devoted to characterizing fossil age-frequency distributions to infer rates of decay and the consequent temporal resolution of the fossil record. From studies of surficial marine assemblages, a common pattern and analytical approach has emerged: age-frequency distributions are right-skewed (dominated by specimens of younger age). This has prompted the fitting of exponential decay models to estimate rates of specimen loss over time. However, two deviations from exponential decay are common: distributions tend to show fewer-than-expected specimens of the youngest ages, and are characterized by heavier-than-expected tails of older age specimens. Here we present a simple model of fossil record formation that provides a mechanistic understanding of how time becomes distributed across a stratigraphic sequence. By adding to the dimension of time the second dimension of depth our model explains the persistent deviations from exponential distributions and offers the following testable predictions: (i) age-frequency distributions will exhibit a secular trend from right- to left skew with increasing stratigraphic depth, and (ii) will exhibit a higher temporal acuity (effective temporal resolution) than evidenced by the total extent of their time-averaging. We confirm these predictions using empirical age-frequency distributions of 80 kangaroo rat (Dipodomys sp.) femora, dated by AMS 14C, from the modern to early Holocene record of Homestead Cave, Utah. Our validated model offers several new insights into the relationship between stratum depth and time-averaging with broad-reaching implications for the interpretation of modern, subfossil, and fossil assemblages. In particular, we show: (i) how the windows of temporal acuity and total time-averaging converge with increasing stratigraphic depth, (ii) how models fit to surficial strata will always overestimate specimen decay rates because age-frequency distribution shape is dependent on both decay and vertical mixing, and (iii) how asymmetric mixing imposes a expectation for interpreting the dynamics of biodiversity over time.