DATING THE UNDATEABLE: ASSESSING THE AGE OF EXTINCTION IN MIDWESTERN PROBOSCIDEANS

The timing of terminal Pleistocene extinctions in North America is key to establishing the likelihood of different--often highly debated--extinction scenarios. Adequate chronologies are important for assessing the contemporaneity of events, as well as determining whether the loss of megafaunal taxa was synchronous or time-transgressive. We use a new dataset of >100 direct-dated proboscideans from the Midcontinent to critically evaluate a series of extinction hypotheses using the freely available Bayesian calibration program, OxCal 4.2. Larger sample sizes improve both the accuracy and precision of extinction chronologies. The terminal age of mammoths in the study area is 11,300+/-100 ¹⁴C BP (13,390-13,020 cal BP). The youngest mastodon is roughly co-eval at 10,780+/-80 ¹⁴C BP (12,810-12,560 cal BP). However, since it is unlikely that the last surviving member of a species will be dated, terminal ages are a poor estimate of the age of extinction. Consideration of sample size and temporal frequency in ¹⁴C dates allows the calculation of a 95% probability boundary of the chronological dataset. This boundary is 13,460-12,160 cal BP for mammoths and 12,790-12,520 cal BP for mastodons. We are also able to quantitatively evaluate the likelihood of different extinction scenarios through sequence modeling of chronological events. Ultimately, causal factors in these extinctions remain obscure, despite increased rigor in the construction and analysis of extinction chronologies.