Origin of modern rainforests traced to end-Cretaceous asteroid impact
In an analysis of thousands of fossil pollen and leaves spanning the Cretaceous-Paleogene (K/Pg) boundary, researchers found that the cataclysmic asteroid impact that resulted in the destruction of nearly 75% of all terrestrial life on Earth drastically restructured tropical forests, setting the stage for the evolution of what has become one of the planet’s most diverse ecosystems – the neotropical rainforest. While the end-Cretaceous impact nearly 66 million years ago was catastrophic for terrestrial ecosystems worldwide, its long-term effects on tropical forests have remained a mystery. This is largely due to the lack of palaeobotanical exploration in the region, which has only just begun to provide the data needed to evaluate these questions. Mónica Carvalho and colleagues used fossil pollen and leaves recovered from Colombia to characterize how the impact changed South American tropical forests, finding large-scale changes in species composition and forest structure. According to the findings, late Cretaceous rainforests were characterized by an open canopy environment. However, plant diversity declined by roughly 45% at the K/Pg boundary and extinctions were widespread, particularly among seed-bearing plants. While the forests recovered over the subsequent six million years, angiosperms, or flowering plants, came to dominate the forests. This transition led to the closed canopy structure and the layered, vertical distribution of plant biodiversity that defines modern tropical rainforests. In an accompanying Perspective, Bonne Jacobs and Ellen Currano discuss how the findings from this study and others demonstrate that the recovery and lasting effects of the K/Pg impact and extinction event were variable and depended largely on proximity to the crater and local conditions such as climate. “Today, the world is experiencing a sixth mass extinction event, but this time, there is no place on Earth far from the ultimate cause – humans,” write Jacobs and Currano. “It seems that proximate perturbations are and will be substantial everywhere, even if they vary.”
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