Reevaluating climate change responses in Rancho La Brea birds and mammals: new dates and new data

Abstract The Pleistocene/Holocene transition furnishes a classic example of apparent evolutionary stasis during a period of major environmental change, where observed environmental clines might predict evolutionary change. We have previously attempted to assess whether or not body size and shape were static in the extensive Quaternary vertebrate fauna of Rancho La Brea (RLB). However, the validity of time-series studies depends on dating, and there are indications that previous approaches based on pit mean radiocarbon ages may be misleading. Here we have compiled and recalibrated all available RLB radiocarbon ages, reanalyzed our morphometric data using a novel method for bootstrap resampling of calibrated C age distributions to estimate a time series of populations, and fit the time series with a range of simple evolutionary time-series models. Although the shortness of our time series tends to favor nondirectional models and is insufficient to allow reliable discrimination between punctuated and gradual change, the results can still be clearly interpreted. The population means for most anatomical elements in most species at RLB do genuinely appear to be static through the Pleistocene/Holocene transition, as previously published. Some species exhibit previously undetected changes in population mean size and shape, including Smilodon , Gymnogyps , and Equus. However, the timing of change is variable among the non-static species and generally does not correspond to changes in temperature, and thus resists a Bergmann’s rule interpretation. Considering the species by ecological category may reveal more about the effects of climate regime shifts.