REVISED 14C CHRONOLOGIES CONFIRM EVOLUTIONARY STASIS DURING RAPID CLIMATE SHIFTS AT RANCHO LA BREA

The tar pits at Rancho La Brea (RLB) preserve an unparalleled variety and abundance of late Pleistocene vertebrate skeletal remains; they sample a diverse single ecological community from 37ka to 9ka. Regional temperature is also reasonably well constrained by pollen and oxygen isotope data. The RLB collection thus presents an opportunity to investigate mean body size and shape in a whole array of populations as the climate cooled into the last glacial maximum and then warmed again into the Holocene. Since 2010, we have collected morphometric data sets for the 28 most common vertebrate species at RLB, mostly measurements on long bones, and conducted pairwise, ANOVA, and time-series analyses on each species in a series of papers addressing the hypothesis that Pleistocene glacial-interglacial cycles applied evolutionary pressure to body size. The ANOVA and pairwise comparisons consistently found minimal changes in population body size mean over the late Pleistocene across all species,even though many of these species are subject to Bergmann's rule in modern populations. The glacial-interglacial cycle appears to be too fast to produce an evolutionary response in population mean size in any species, even small nonmigratory birds with limited ranges.

However, the treatment of age in these studies was simplistic and unsatisfactory, based only on estimated mean age for each pit. This model does not account for the unique taphonomy at RLB, where each pit opens, absorbs remains for some time, and then closes, producing completely time-averaged fossil deposits. As part of an ongoing revision of chronologies for Late Quaternary North American vertebrate sites, a revised chronology has been computed for each of the pits, producing age boundary estimates based on all known high-quality radiocarbon ages for each pit. In this study, we redid the time-series analyses on the basis of these new dates, using a uniform probability model for the age of specimens within each pit, and additionally compared our measurements directly with several temperature variables. This allows us to more accurately reassess the sequential shifts in body size and shape in each population over time and their association with climate. Our new statistical methods validate our previous conclusions and provide a more sound basis for our inferences about the effect of short-term climate fluctuations on body size.