60 million years of ecological shifts in large herbivore communities revealed by Network Analysis

The fossil record provides direct evidence for the behavior of biological systems over millions of years. In doing so, paleontological information becomes a key source to study the evolution of ecosystems and how they responded to major environmental shifts. Using network analysis over a dataset of worldwide large herbivores spanning the past 60 Myr, we found that large herbivore assemblages experienced long periods of ecological stability interrupted by irreversible reorganizations linked with abiotic events (tipping points). Initially, communities were characterized by mid-size browsers with low-crowned teeth and experienced increasing functional diversity until the formation of a land connection between Eurasia and Africa around 21 Ma. This was followed by a new functional system, the first tipping point, characterized by big-size browsers with mid to high crowned teeth. This new functional system experienced increasing functional diversity, with a peak ~10 Ma. It collapsed shortly thereafter, contemporaneous with global cooling and increased aridification that lead to the spread of C4-dominated grasslands, the second tipping point. It was replaced by a system characterized by a combination of grazers and browsers with high crowned teeth. Functional diversity has been decreasing since ~10 Ma, with an accelerated decline since ~2.5 Ma. Despite the diversity decline, the functional structure of terrestrial ungulate faunas has remained unaltered for the last part of the Cenozoic. The evolutionary history of large mammal assemblages is marked by two major functional transitions at critical points coincident with large-scale tectonic and climatic processes.