TREE seedling experiments are helping to resolve an ancient question: why did so many giant dinosaurs prosper in the mid-Cretaceous period?
Fossil evidence shows that at this time, some 100 million years ago, dinosaurs reached a peak in both body size and population. Western North America supported 10,000-strong herds of herbivorous dinosaurs, each weighing over a tonne. Much larger carnivores and herbivores weighing more than 10 tonnes were lumbering about elsewhere, including the biggest known land animal, the 35-metre-long Argentinosaurus from South America, which weighed 100 tonnes.
Sara Decherd of North Carolina State University wondered if plentiful food was the key. Were plants growing unusually fast because the atmosphere was rich in greenhouse gases? Geological evidence suggests carbon dioxide levels were 5 times what they are now, and oxygen levels were 1.5 times today鈥檚. To test this, Decherd grew ginkgo plants, which were common 100 million years ago, in a simulated mid-Cretaceous atmosphere. She found that photosynthesis rates in the seedlings were 3 times what they would be in today鈥檚 conditions.
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She says this suggests plants grew faster during the mid-Cretaceous than they do now. The plentiful vegetation could have supported larger and more numerous herbivores. They in turn would have been prey for the carnivores, allowing these dinosaurs to grow larger and more numerous as well.
