Highly unpredictable hot and dry climate, linked with elevated levels of carbon dioxide (CO2), prevented larger herbivore dinosaurs from inhabiting the tropics for more than 30 million years, says a new study.
For years, paleontologists have had different theories about why they could find no evidence of large herbivore dinosaurs living at low latitudes, until at least 30 million years after they first appeared on Earth, and 10 to 15 million years after they became abundant at higher latitudes (both north and south of the equator).
The new study, published in the journal Proceedings of the National Academy of Sciences, suggests that the climate in the tropics, characterised by wet seasons in some years and extreme droughts in others, was punctuated by raging wildfires every few dozen years that reached temperatures of up to 600 degrees Celsius.
The conditions would have made it difficult for abundant vegetation to grow and survive; vegetation that the Triassic predecessors of more well-known Jurassic sauropods (like Brachiosaurus, Diplodocus and Brontosaurus) would have fed on.
"The conditions would have been something similar to the arid western United States today, although there would have been trees and smaller plants near streams and rivers and forests during humid times," said lead-author Jessica Whiteside from University of Southampton in Britain.
"The fluctuating and harsh climate with widespread wild fires meant that only small two-legged carnivorous dinosaurs, such as Coelophysis, could survive," Whiteside said.
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For the study, scientists analysed rock samples from a location called Ghost Ranch, in New Mexico, where a number of Triassic dinosaur fossils have been discovered.
The rocks were deposited by rivers and streams between 205 and 215 million years ago, during the Late Triassic Period.
"When these rocks were deposited during the Late Triassic, northern New Mexico was very close to the equator at about 12 degree north in latitude -- around the same latitude as the southernmost tip of India sits today," co-author Sofie Lindstrom from Geological Survey of Denmark and Greenland, said.