“The 92% extinction estimate we get when we consider fossil leaf species across the K-Pg boundary should be taken as a maximum” Stiles said. “We were surprised to find such high extinction levels compared with the 60% extinction rate seen in North America. Nonetheless, we observed a sharp drop in plant species diversity and a high species-level extinction.”
Ecosystem recovery likely took millions of years, added Stiles, which is a small fraction of Earth’s nearly 4.5-billion-year history.
Stiles also led a novel morphospace analysis to identify changes in leaf shape from the Cretaceous to the Paleogene, as such changes could provide clues to the kinds of environmental and climatic occurrences that took place across the boundary interval. She studied each leaf fossil for nearly 50 features, including shape, size and venation patterns.
The analysis showed a higher diversity of leaf forms in the Paleogene, which surprised the researchers given the high species-level extinction and drop in number of species at the end of the Cretaceous. They also found an increase in the proportion of leaf shapes typically found in cooler environments, which suggests that climatic cooling occurred after the end-Cretaceous extinction event.
The researchers’ findings, combined with those of previous studies, suggest that despite the high species-level extinction at the end of the Cretaceous, South American plant families largely survived and grew more diverse during the Paleogene. Among the survivors were the laurel family, which today includes plants such as bay leaves and avocados, and the rose family, which includes fruit like raspberries and strawberries.
“Plants are often overlooked in these big events in geologic history,” Stiles said. “But really, because plants are the primary producers on terrestrial landscapes and sustain all other life forms on Earth, we should be paying closer attention to the plant fossil record. It can tell us how the landscape changed and how those changes affected different groups of organisms.”
Other contributors included Maria Gandolfo, Cornell University; and Ruben Cuneo, MEF.
The Geological Society of America, Mid-American Paleontological Society, National Science Foundation and Penn State, through a Charles E. Knopf, Sr. Memorial Scholarship and the Paul D. Krynine Memorial Fund, supported this research.