Harsh climate kept plant-eating dinosaurs away from the equator

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Published: June 16, 2015 10:07:20 PM

A harsh and unpredictable climate prevented big plant-eating dinosaurs from living around the equator some 200 million years ago, a new study has found".

A harsh and unpredictable climate prevented big plant-eating dinosaurs from living around the equator some 200 million years ago, a new study has found.

For more than 30 million years after dinosaurs first appeared, they remained inexplicably rare near the equator, where only a few small-bodied meat-eating dinosaurs eked out a living.

The age-long absence of big plant-eaters at low latitudes is one of the great, unanswered questions about the rise of the dinosaurs.

Now, an international team of scientists has provided a detailed picture of the climate and ecology more than 200 million years ago at Ghost Ranch in northern New Mexico, a site rich with fossils from the Late Triassic Period.

The new findings show that the tropical climate swung wildly with extremes of drought and intense heat. Wildfires swept the landscape during arid regimes and continually reshaped the vegetation available for plant-eating animals.

“It was a time of climate extremes that went back and forth unpredictably and large, warm-blooded dinosaurian herbivores weren’t able to exist nearer to the equator – there was not enough dependable plant food,” said study co-author Randall Irmis, curator of paleontology at the Natural History Museum of Utah.

The study, led by geochemist Jessica Whiteside from University of Southampton, UK, is the first to provide a detailed look at the climate and ecology during the emergence of the dinosaurs.

The results are also important for understanding human-caused climate change. Atmospheric carbon dioxide levels during the Late Triassic were four to six times current levels, researchers said.

“If we continue along our present course, similar conditions in a high carbon dioxide world may develop, and suppress low-latitude ecosystems,” Irmis said.

In the study, the researchers focused on Chinle Formation rocks, which were deposited by rivers and streams between 205 and 215 million years ago at Ghost Ranch.

The researchers reconstructed the past by analysing several kinds of data including fossils, charcoal left by ancient wildfires, and stable isotopes from organic matter and carbonate nodules that formed in ancient soils.

Fossilised bones, pollen grains and fern spores showed the types of animals and plants living at different times, marked by layers of sediment.

Dinosaurs remained rare among the fossils, accounting for less than 15 per cent of vertebrate animal remains. They were outnumbered in diversity, abundance and body size by the reptiles known as Pseudosuchian archosaurs, the lineage that gave rise to crocodiles and alligators.

The sparse dinosaurs consisted mostly of small, carnivorous theropods.

Big, long-necked dinosaurs, or sauropodomorphs – already the dominant plant-eaters at higher latitudes – did not exist at the study site or any other low-latitude site in Triassic Pangaea, as far as the fossil record shows, researchers said.

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