When lost giants were the architects of Neotropical forests

New fossil evidence from Chile reveals that the South American mastodon Notiomastodon platensis was a regular fruit-eater and an essential long-distance seed disperser. This discovery confirms a long-standing idea: many large-fruited plants evolved in partnership with extinct megafauna whose movements shaped the structure and resilience of South American forests.The analysis of fossil teeth from paleontological sites across 1,500 km of Chile, using a combination of methods similar to forensic work: the chemistry of tooth enamel, microscopic wear marks, and plant remains trapped in ancient dental plaque. Together, these techniques revealed clear traces of fruits, such as starch grains and plant tissues from palms and other fleshy-fruited trees, proving that mastodons were not just leaf-eaters but important fruit consumers and seed dispersers.The study also shows that these animals lived in a variety of environments, from dry woodlands to temperate rainforests, and probably moved long distances while feeding. As modern elephants do today, they would have transported large seeds across the landscape, helping forests regenerate and expand.To understand the long-term consequences of their extinction, a machine-learning model was used to evaluate the conservation status of tree species that once depended on large animals to spread their seeds. The results reveal a marked pattern: in central Chile—where no medium-sized mammals remain to replace the mastodons—40% of these tree species are now threatened with extinction. This is four times higher than in tropical regions where tapirs, monkeys or other animals still act as seed dispersers.This work provides the long-awaited proof that ancient megafauna shaped the forests we see today. It shows that the disappearance of mastodons broke a vital ecological partnership, leaving many plant species vulnerable for thousands of years. This study highlights how understanding the past is essential for conserving the ecosystems of the future.