Written in bones: palaeoclimate histotaphonomic history inferred from a complete Megatherium skeleton preserved in the Atacama Desert
Luisa Straulino Mainou, Jacqueline Correa-Lau, Rafael Labarca, Natalia A. Villavicencio, Vivien G. Standen, Susana Monsalve, Paula C. Ugalde, Sergey Sedov, Teresa Pi Puig, Alan Ulises Loredo-Jasso, Francisco J. Caro, Gabriela M. Jarpa, Patricia Hernández-Michaud, Claudio Latorre, Calogero M. Santoro
Summarized by William Pagan. William Pagan is an environmental earth systems major at Binghamton University. He plans to earn his degree and enter the workforce for a few years before attending graduate school. When he’s not busy with his studies, he is usually hiking or playing games with his friends.
What data were used? A nearly complete fossilized giant sloth skeleton found in the Atacama Desert in Chile was utilized, which was discovered prior to the study. The scientists collected carbonates from soils as well as rhyzoliths- trace fossils left by plant roots and cemented together by carbonates- that were found nearby the skeleton were also utilized.
What was the hypothesis being tested? The researchers aimed to understand how the climate of the Atacama Desert has changed since the deposition of the Megatherium skeleton since the end of Pleistocene Epoch (around 16,000-13,000 years ago) by analyzing the mineral composition and weathering patterns of the bones.
Methods: Researchers set up a 1×1 m grid system throughout the site in the Atacama Desert (Fig.1 C), covering 54 m2 in total. The site is mainly sand-sized dry sediment, with little to no plant life and almost no water present. The giant sloth bones were present in 25 grids, and nine of these grids were excavated in 5 cm deep intervals. Loose sediment was put through a mesh sieve to gather the smaller bone fragments, while the larger pieces were carefully excavated from the rock.Scientists separated the bone fragment specimens used in the study into three sample groups. The scientists either kept the samples whole, sliced them into thin sections to look at under microscopes, or ground them in a mortar and pestle. The bone fragments kept whole were mostly pieces of teeth of the Megatherium and were used to analyze physical characteristics -such as color and pores in the bone; some tooth samples were set aside for more in depth analysis. The thin sections were analyzed under microscopes in order to get an understanding of the mineral composition of the bones as well as the internal structure and impacts of weathering. The bone section that was ground up was passed through a machine that uses X-ray waves to identify the minerals that were present in the samples. The carbonate rocks and a sample of bone were used for radiocarbon dating, which uses the isotope C14 to get a more accurate and precise idea of the age of the sample.
Results: Researchers found that the bones had increased levels of porosity and cavities that bioerosion- or the breakdown of the bones by bacteria- occurred shortly after the Megatherium specimen died. The bioerosion was thought to have happened before and after burial. Bacteria that cause bioerosion rely on environments with both oxygen and water, leading to the conclusion that the Megatherium skeleton was located in a place that was submerged in water, but dried out over time. After this bioerosion took place, iron and magnesium were deposited into the spaces left in the bone. Iron and magnesium deposition like this is known to take place in anoxic (lacking oxygen) conditions. This shift from oxygenated to un-oxygenated conditions could have been caused by the water becoming stagnant opposed to running or the water becoming deeper, but overall represents an environmental shift. The bones then started to crack as conditions became drier, and minerals such as calcium carbonate, halite, and gypsum were encrusted onto the bones. These minerals all dissolve well in water, and the fact that they were deposited around the bones hints towards the evaporation of water and the shift to more arid conditions in the environment. The dates obtained by the carbonate rocks and bone fragments show that the Megatherium is a minimum of 16,250 years old and as young as 13,000 years old., which lines up with a known large rainfall event in the area, further confirming the wet depositional environment. The increased rainfall would have lasted for roughly 4,000 years. Overall, the Megatherium bones displayed excellent evidence of the changing conditions that line up well with the evidence found in the surrounding rock record.
Why is this study important? This study is important because it is a stellar example of how researchers can use bone samples- even smaller fragments with a lot of erosion and alterations- to get a better understanding of the paleoclimate of the Atacama Desert. This study also gives direct evidence into the past climate of the Andes Region, highlighting times of drought and increased rainfall, while explaining what features of the Megatherium bones helped the researchers reach these conclusions.
Broader Implications beyond this study: This study uses fossil data to better understand the broader pictures of how examining the fossilization of bones can give hints into past climate conditions. Scientists have used various methods in the past to try and reveal a broader picture of how regions and climates have changed, but the researchers in this study offer a unique angle of climate discovery by using the giant sloth bones to do it while combining many different disciplines and sciences to do it.
Citation: Straulino Mainou, L., Correa-Lau, J., Labarca, R., Villavicencio, N. A., Standen, V. G., Monsalve, S., Ugalde, P. C., Sedov, S., Puig, T. P., Loredo-Jasso, A. U., Caro, F. J., Jarpa, G. M., Hernández-Michaud, P., Latorre, C., & Santoro, C. M. (2025). Written in bones: Palaeoclimate histotaphonomic history inferred from a complete Megatherium skeleton preserved in the Atacama Desert. Palaeontology, 68(4), e70011. https://doi.org/10.1111/pala.70011