Reconstructing ancient climate provides clues to climate change

Knowledge of the natural world millions of years ago is fragmented, but a 15-year study of a site in Bolivia by an international team led by Case Western Reserve University provides a comprehensive view of an ancient ecosystem when the Earth was much warmer than today.

A synthesis of the team's in-depth research was published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology.

The site, known as the Quebrada Honda Basin (QHB) in the Andes mountains in southern Bolivia, encompasses a time period 13 million years ago during the Miocene Epoch. During the Miocene, the Earth's climate rebounded from the cooling of the prior epoch, with global temperatures and mammal biodiversity markedly increasing.

Globally, temperatures were 3-4 degrees Celsius warmer than today. Understanding ecosystems of the past helps predict what might happen in the future due to human-related climate change.

"Sites like this one in Bolivia are essential for helping us calibrate climate models," said Darin Croft, professor of anatomy at Case Western Reserve's School of Medicine, who led the QHB team. "Our understanding of climate change is based on models, and those models are based on information from the past. We are getting into uncharted territory in terms of climate, and you have to go deeper in time to get conditions that are similar."

The site is 11,500 feet (3,500 meters) above sea level. When its fossils accumulated, it was lower, but exactly how much has been a matter of debate. Previous studies using geochemistry concluded that the Miocene QHB was relatively high, close to 10,000 feet (3,000 meters).

But the current publication favors warmer temperature and lower elevation, likely less than 3,000 feet (1,000 meters), meaning the Andes uplift happened more recently in geologic time than previously thought.

The team found fossils of many different types: bones and teeth of mammals and other vertebrates, microscopic plant remains, ancient soils and tracks and traces of insects and other invertebrates. Cold-blooded animals found at the site -- a giant tortoise, a side-necked turtle and a very large snake -- suggest the site's elevation when these animals lived was less than 1,000 meters, based on modern-day distributions of closely related species.

The team concluded that the QHB was a dry forest or wooded savanna with palms and bamboos -- which grow at lower elevations -- with no similarity to any modern ecosystem. First author Caroline Strömberg, biology professor at the University of Washington, studied fossilized phytoliths, microscopic pieces of silica found in the cell walls of plants, characteristic of the types of vegetation they come from. She compared the fossilized phytoliths with those found in contemporary vegetation to identify the mix of plants at the site.

Layers of volcanic ash and magnetic signatures in rocks allowed the fossils to be accurately dated. The diversity of preserved material allowed Croft's team to make detailed reconstructions of the plants and animals and their living conditions. The team named 13 new species of fossil mammals based on remains from the site, including marsupials, hoofed mammals, rodents and armadillos. Most of the species have not been found anywhere else in South America and have no modern descendants.

"Nature has a wide variety of body plans, often much greater than the limited variety we see today," said Russell Engelman, a Case Western Reserve biology graduate student who worked on the mammal fossils.

Other coauthors include: Beverly Saylor, professor of earth, environmental and planetary sciences at Case Western Reserve; Angeline Catena, geology professor at Diablo Valley Community College in Pleasant Hill, Calif.; Daniel Hembree, professor of earth and planetary sciences at the University of Tennessee; and Federico Anaya, geology professor at Universidad Autonóma Tomás Frías in Potosí, Bolivia.

Between 2007 and 2017, Croft and Anaya led six international teams to the QHB, funded primarily by the National Science Foundation (NSF). Six years after Croft's second NSF grant ended, the findings are still yielding data and publications.

"Field paleontology is a really good investment for the NSF, because the dividends far outweigh the costs," said Croft, who's seeking funding to study another Bolivian Miocene site of a similar age but over a longer time-period.