Large prehistoric animals living in modern-day California were likely the victims of major ecological changes driven by climate shifts and human impacts at the end of the last Ice Age.
As the 2.56 million-year-long Pleistocene epoch ended about 11,000 years ago, very large mammals were driven to extinction as climactic changes took hold and human populations began to exact their influence on landscapes and their fellow animals.
Now, researchers from the Natural History Museums of Los Angeles County have undertaken new radiocarbon dating of megafauna from the La Brea tarpits to understand the causes of their die-off.
Modern radiocarbon dating scrutinises differences in abundance of carbon isotopes within organic matter – particularly carbon-14 – using accelerator mass spectrometry. This technique requires researchers to purify samples of organic matter – in this case, isolating the collagen of preserved animal remains from the surrounding tar – then targeting the carbon-14 within them. As carbon-14 atoms decay to nitrogen-14 at a consistent rate (with a half-life of 5,730 years) these are directly measured relative to stable Carbon-12 and 13 atoms to determine the age of a sample.
Samples from 172 individuals across eight different species were obtained for the study – including about 30 each of sabre-toothed cat, ancient bison, dire wolf, coyote and the prehistoric wild horse Equus occidentalis. The analysis found that most of the species disappeared between 12,900 – 15,600 years ago.
These dates were then compared to sediment records to understand the reasons for their disappearance. The researchers found that the La Brea animals began to vanish from the region due to ‘state shifts’ in their environment.
Changes in vegetation and a drying of the landscape, potentially due to human impacts, were likely the cause of their disappearances, with large fire events likely major drivers of their disappearances.
This, their modelling suggests, was potentially due to a “positive feedback loop in which rising human populations caused enhanced fire activity”.
This feedback is put down to the loss of herbivore numbers through human activity, leading to more vegetation to burn, as well as direct human use of fire, and has been recorded over thousands of years across other countries including Australia and New Zealand.
“Our results also highlight the importance of considering extinction dynamics on ecologically relevant spatial, temporal, and taxonomic scales,” the authors write.