Generations of giant sloths died of an “apparent mass-death” after congregating in a shallow, oxygen-deprived marsh sometime during the last Ice Age.

During the late-Pleistocene, a multigenerational group of at least 22 giant ground sloths (Eremotherium laurillardi) somehow died and were subsequently preserved in perpetuity in a region along the southwest coast of Ecuador known as Tanque Loma. Now extinct, giant sloths were common in the New World Quaternary, the era spanning the last 2.6 million years or so, yet relatively little is known about individual species’ behavior and their social structure.

Published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology, the giant sloth graveyard was found in what is known as an asphaltic site. Ancient flora and fauna have been uncovered in about a dozen asphaltic sites, also known as “tar pits,” around the world. Naturally seeping asphalt works as an in-situ preservative, conserving bones and other material.

“Tar pits are incredibly intriguing, and Rancho La Brea is probably the most well-known fossil site in the world, but, despite more than a century of research, we’re learning that we have only just scratched the surface,” said study author Dr Emily Lindsey in a statement. Lindsey serves as the assistant curator and site director at La Brea Tar Pits in Los Angeles.

“While asphaltic sites are rare, they can preserve both a tremendous quantity and variety of fossils, making exploring these sites a scientific priority,” said Lindsey. “Tar pits are one of the only types of fossil sites where we can find remnants of an entire ecosystem preserved – leaves and bones, mega mammals and tiny insects, seeds and shells. As such, they are extremely valuable for paleontologists investigating important topics like evolution, paleoecology, and climate change, and are especially crucial in areas like the neotropics where the Pleistocene fossil record is sparse."

Scientists excavated Tanque Loma and conducted radiocarbon dating, cautiously estimating that most of the Eremotherium bones range from about 18,000 to 23,000 years ago. Sedimentological, geochemical, and taphonomic data were also collected, showing that the environment was anoxic, meaning that it lacked oxygen and was likely a marshy aquatic environment that underwent periodic droughts.

Such mass graveyards can arise either gradually or from one catastrophic event, putting forth three theories for the death. Firstly, the researchers suggest that Tanque Loma may have served as a watering hole that was frequented by the animals over an extended period of time and died off slowly from various causes.

Based on the accumulation of the remains, the varying ages of the individuals, and an abundant presence of poop consistent with plants that the animals ate, researchers believe that the massive animals died in one event. It's possible that the giant sloths died at once from selective predation – either from humans or other carnivorous predators – or a geological event, such as a volcanic eruption or flood. More likely, the sloths may have been suffering from a regional drought and sought the watering hole for reprieve and, already sickly and disease-ridden, continued to degrade in health before their ultimate demise.

The authors conclude that “this death event could have resulted from drought and/or disease stemming from the contamination of the wallow, paralleling situations observed among hippopotamus populations in watering holes on the present-day African savannah.”

However they met their end, the giant sloths provide insight into how these enigmatic creatures lived and the dramatic ways in which they died. Also discovered in the graveyard were five other large mammals, including a mylodont sloth, an elephant-like gomphothere, an ancient armadillo relative known as a pamphadere, as well as a horse and a deer.

“The discoveries we make at tar pits help us understand how past species and ecosystems responded to late-Pleistocene climate changes and human activities – processes that are once again impacting life on Earth today,” noted Lindsey.