A prehistoric graveyard containing the remains of more than 20 mammoths is shedding light on ancient conditions that killed the giants.

Preserved alongside horse and bison skeletons, researchers have long thought the dozens of animals buried at the Waco Mammoth National Monument were killed by a flood or landslide event. New research from PhD candidate Donald Esker illustrates a different scenario: a massive drought that brought animals from many herds to one remaining watering hole.

“The most significant big-picture implication of my findings is what it says about Ice Age paleoclimate,” researcher Donald Esker told IFLScience. “The (potential!) presence of multiple herds at a single diminishing watering hole points to a regional megadrought around 67,000 years ago.”

Esker compares this drought event to the Dust Bowl, only much more drawn out.

“It’s a good reminder that the massive climate swings during the Pleistocene didn't just involve changes in temperature, but changes in precipitation too,” he said. “The role of droughts in the lead-up to the eventual extinction of the megafauna 11,500 years ago deserves closer examination.”

Previous studies of the Waco Mammoth National Monument fossils focused on the demographics of the mammoths and their spatial distribution, leading researchers to believe the herd was killed by a flood or landslide. That’s where Esker’s research differs.

“I'm looking at the site from a more geochemical perspective, using serial sampling of mammoth teeth to produce a detailed record of where the mammoths traveled in the lead up to their death,” he told IFLScience. “The results point towards a drought that drew multiple herds of mammoths to the last remaining watering hole.”

To see where the mammoths had traveled from, Esker collected and sampled vegetation preserved in rocks from across Texas. They then took a series of enamel samples from Waco mammoths’ teeth using a technique that “minimizes damage” but still leaves small grooves in the enamel. These samples were then sent to a special lab to analyze strontium isotope ratios in grass and tooth enamel.

“By comparing the ratios found in the teeth to those found in the vegetation, I was able to determine roughly where the mammoths had been,” said Esker, who continues that these results differed considerably from samples collected during a pilot study in 2017. “My tentative conclusion is that the mammoth I reported on last year was not a member of the same herd in this year's report.”

It’s important to note that different teeth were used between the pilot study and this year’s research, which could potentially introduce contamination. Esker says he plans to re-sample the pilot study tooth using this year’s method to account for this.

"Carbon isotopes will tell us about what the mammoths were eating, and oxygen isotopes can tell us about environmental conditions during the last few years of [the mammoths’] lives,” continued Esker. “The oxygen analysis, in particular, could support or refute the new drought hypothesis.”

The findings were presented at the annual meeting of the Society of Vertebrate Paleontology and will need to pass a doctoral committee to be published in a peer-reviewed journal.

[H/T: Live Science]