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Flowering plants are the most successful group of plants on Earth. This type of pervasive vegetation dominates forests and makes up 90 percent of all plant life on land.
The first fossils of flowering plants, or angiosperms, resembled the brush that grows along fast-flowing streams and rivers. How did these 140-million-year-old shrubs eventually become vast forests?
Researchers already knew that angiosperms had diversified and spread before the dino-killing meteorite smashed into Earth and reset life on the planet 65 million years ago. But the fossil evidence is ambiguous as to whether the mass extinction was a tipping point, or if angiosperms were already on their way to world domination before the impact.
In the new study, scientists sought clues by comparing modern forests to fossil plants. The team measured leaves from top to bottom in two tropical forests in Panama and one temperate forest in Maryland.
In modern tropical forests, sun-loving trees grab the most energy with tightly packed leaf veins, while trees relegated to the shade have leaves with veins spaced wider apart. This leaf "vein density" is a hallmark of photosynthesis, or how fast a leaf can transport water and take in carbon dioxide.
A canopy crane in the Parque Natural San Lorenzo (tropical rainforest), Smithsonian Tropical Research Institute, Panama.
(Image credit: Smithsonian Tropical Research Institute)
The scientists also looked at leaf litter, the detritus that falls to the forest floor.
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"This was particularly important because a litter assemblage is the closest analog to a fossil flora — the leaves that have fallen from the trees, accumulated on the ground and eventually fossilized," said lead study author Camilla Crifò, a biologist at the Smithsonian Tropical Research Institute in Panama during the study.
The modern leaf grouping was most similar to fossils from forests that grew 58 million years ago, Crifò reports in the September 2014 issue of the journal Geology. The modern leaves were compared to fossil angiosperms from as far back as 132 million years ago. The results suggest angiosperm forests resembling today's tropical forests dominated after the meteorite impact, not before.
Vein density is a "promising and exciting technique" for gathering information on plant metabolism from fossils, Crifò said in an email interview. "We hope to be able to use vein density to reconstruct the photosynthetic capabilities of plants that lived 140 million years ago."
Becky Oskin covers Earth science, climate change and space, as well as general science topics. Becky was a science reporter at Live Science and The Pasadena Star-News; she has freelanced for New Scientist and the American Institute of Physics. She earned a master's degree in geology from Caltech, a bachelor's degree from Washington State University, and a graduate certificate in science writing from the University of California, Santa Cruz.
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