Mass extinction gave rise to modern tropical rainforests

Plant fossils reveal the devastating effect of the mass extinction at the end of the Cretaceous period, 66 million years ago, on tropical rainforests.

An international team of scientists, including Professor Daniele Silvestro from the University of Fribourg, have used an extensive collection of leaf fossils and pollen found in Colombia to better understand the fate of tropical rainforests after the mass extinction that brought about the disappearance of the dinosaurs. The study, published Friday 2 April in the journal Science, reveals that during the last stage of the age of dinosaurs, 72 to 66 million years ago, tropical rainforests differed markedly from their modern counterparts. Before the end of the Cretaceous period, tropical rainforests were dominated equally by ferns and flowering plants. Conifers, mainly relatives of the Araucaria and kauri trees, were also an important component. “This type of ecosystem has no living analogue today and undoubtedly functioned differently than modern-day tropical rainforests”, says Dr Mónica Carvalho, main author of the study and postdoctoral researcher at the Smithsonian Tropical Research Institute and at Del Rosario University in Colombia. According to Dr Carvalho, the tops of the trees did not form the dense and almost continuous cover (canopy) which characterises the tropical rainforests of today. Nitrogen-fixing legumes, abundant in today’s tropics, were not common then either.

A cataclysm which changed everything
Around 45% of plant species disappeared from the tropics at the end of the Cretaceous period following the impact of a meteorite. This event triggered an ecological catastrophe that enabled flowering plants to dominate the tropical ecosystems. The Araucaria were almost completely replaced and the ferns became only a minor component. The authors compared pre- and post-extinction floras with the modern rainforests of Amacayacu (Colombia) and Barro Colorado Island (Panama). They found that post-extinction floras were more similar to their modern counterparts than to those of the pre-extinction tropical rainforests. After the extinction, the legumes become dominant and diversified, as did the other tree families that characterise tropical rainforests today. The leaf fossils showed that the post-extinction tropical rainforests had no gaps between the tops of the trees and that there were numerous forest layers.

Impacts on insects
The study also showed that other ecological characteristics changed after the end of the Cretaceous period, such as the feeding patterns of herbivorous insects. This can be observed thanks to marks of chewing found on fossilized leaves. Before the extinction, one can see that each species of insect had a preferred plant to feed on. Afterwards, however, feeding patterns seem to diversify with abundant traces of leaf-chewing appearing in an indiscriminate manner on most species of plant. The dramatic changes that resulted from the cataclysmic event at the end of the Cretaceous period and to which the Colombian fossils bear witness, indicate that this event ultimately led to one of the most diverse ecosystems on the planet today. “It’s a nice example of the way in which a historical accident has changed the course of life on Earth forever”, says Prof. Silvestro enthusiastically, “not only in the animal kingdom but also for the tropical rainforests.”

A short video illustrating this study’s results is available on YouTube and the original article appeared in Science Mag.