The final nail in the coffin of Patagonian megafaunal extinctions

Are our ancestors responsible for Late Pleistocene megafaunal extinctions? Were the Ice Age giants doomed to extinction because they couldn’t adapt or is it human fault that there is no woolly rhino, giant deer and cave bear today?

Cueva del Milodón (Mylodon Cave) in Patagonia. Source: WikimediaCommons

Cueva del Milodón (Mylodon Cave) in Patagonia. Source: WikimediaCommons

A new contribution to the neverending debate came from an international collaboration led by Alan Cooper. Results of the study were published in Scientific Advances with Jessica Metcalf as the first author.
Following the success of the previous study on Holarctic megafaunal extinctions, which was published in Science last year, Cooper continued with the approach of generating time series’ of radiocarbon dates to find patterns shared by multiple species, but this time focusing on Southern Hemisphere.

It’s about the synergy

Already in 2010, Anthony D. Barnosky and Emily Lindsey used a small set of radiocarbon dates to show that “a synergy of human impacts and rapid climate change” was probably the final nail in the coffin for the Neotropical Pleistocene megafauna.
However, analyzing 93 dates spread across 15 genera and different locations can’t possibly give enough resolution to resolve South American megafaunal extinctions. Therefore, Barnosky & Lindsey (2010) finished the abstract introducing the studies to come: “These results highlight the need for future intensive dating efforts on South American megafauna and archaeological remains.”

The Last Hope of South American paleogenetics

Despite the call for more radiocarbon dates, Barnosky and his team came back in 2015 with another compilation of previously published dates. To get a finer-scale time series, they zoomed in on a specific region within the continent – the holy grail of South American Late Pleistocene paleontology – called Última Esperanza. If there is hope for well-preserved samples in South America, it’s coming from there.
To estimate putative extinction dates of Patagonian megafauna, Villavicencio et al. (2015) used Gaussian-resampled, inverse-weighted method (GRIWM; Bradshaw et al. 2012).

“This approach deals with non-random fossilization by progressively up-weighting the gap sizes the closer they are to the time of disappearance from the fossil record. It also takes into account the uncertainties associated with the radiometric dates, providing a 95% confidence band around the estimated time of extinction.”

When compared to estimated human arrival, climatic and vegetation records, the extinction dates supported the synergistic human-climate extinction pressure. Villavicencio and colleagues conclude that humans are probably responsible for the mega-carnivore extinction, but herbivores seem to do fine until climate change brings spreading Nothofagus forests.

Fig. 4. Villavicencia et al. (2015)

Fig. 4. Villavicencia et al. (2015)

Intensive dating

It’s probably about time to get back to the new study by Metcalf et al. (2016), though I think that this introduction was necessary. Metcalf and colleagues replied to the six-year-old call of Barnosky & Lindsey and more than doubled the radiocarbon data.
More than anything, the value of this study is in giving time stamps to 71 megafaunal samples from southern Patagonia. Considering the price for accelerator mass spectrometry (AMS), budget or connections must have been generous.
Cooper’s crew estimated extinction ages with the Phase model option in OxCal 4.2 with General Outlier analysis detection. Since that approach assumes a uniform Poisson distribution of radiocarbon dates, individual age calibration was used for taxa with less than 11 dates.

“Using Bayes’ theorem, the algorithms employed sample possible solutions with a posterior probability that is the product of the prior and likelihood probabilities; the outlier option was used to detect ages that fall outside the calibration model for each group and, if necessary, down-weight their contribution to the final age estimates. Taking into account the deposition model and the actual age measurements, the posterior probability densities quantify the likeliest age distributions.”

Unlike Villavicencio et al. (2015), Metcalf and colleagues didn’t find carnivore extinctions to precede those of herbivores. Surprisingly, extinctions of several taxa seem to overlap in time at about 12,280 ± 110 years ago, which is more than a thousand years after humans came to the region.

Fig. 1. Metcalf et al. (2016)

Fig. 1. Metcalf et al. (2016)

The extinction window coincides with the warming period following the Antarctic Cold Reversal (ACR). Arid and windy grassland environment became replaced by Nothofagus forests, and together with increasing numbers of humans in the area, it was too much for the Ice Age beasts.

“The initial presence of humans in the area during the ACR stadial conditions was apparently insufficient to drive megafaunal extinctions, in direct contrast to the Blitzkrieg model, which suggests that the naïveté of megafauna to human hunting led to rapid extinction. However, human presence, in combination with the rapid advance of forests and environmental changes associated with the ensuing warming phase, appears to have led to the collapse of the megafaunal ecosystem within a few hundred years.”

L. Metcalf, C. Turney, R. Barnett, F. Martin, S. C. Bray, J. T. Vilstrup, L. Orlando, R. Salas-Gismondi, D. Loponte, M. Medina, M. De Nigris, T. Civalero, P. M. Fernandez, A. Gasco, V. Duran, K. L. Seymour, C. Otaola, A. Gil, R. Paunero, F. J. Prevosti, C. J. A. Bradshaw, J. C. Wheeler, L. Borrero, J. J. Austin, A. Cooper. Synergistic roles of climate warming and human occupation in Patagonian megafaunal extinctions during the Last Deglaciation. Sci. Adv. 2 (6): e1501682 (2016).
D. Barnosky, E. L. Lindsey. Timing of Quaternary megafaunal extinction in South America in relation to human arrival and climate change. Quat. Int. 217, 10–29 (2010).
J. A. Bradshaw et al. Robust estimates of extinction time in the geological record. Quat. Sci. Rev. 33, 14–19 (2012).
A. Villavicencio, E. L. Lindsey, F. M. Martin, L. A. Borrero, P. I. Moreno, C. R. Marshall, A. D.Barnosky. Combination of humans, climate, and vegetation change triggered Late Quaternary megafauna extinction in the Última Esperanza region, southern Patagonia, Chile. Ecography 38, 125–140 (2015).

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