For decades, paleontologists have quibbled over the absence of herbivorous dinosaurs in equatorial regions during the Late Triassic. Although these giant plant eaters first emerged in the Middle Triassic, some 230 million years ago, their absence in low latitudes persisted for almost 30 million years, even after they had become abundant to the north and south. Now, scientists think they know why.
To solve this mystery, a team of paleontologists scoured the famed New Mexico fossil site, Ghost Ranch, which during the time period in question was only about 12 degrees north of the equator, on the supercontinent, Pangea. They hoped the evidence they gathered would enable them to reconstruct the ancient environments of the Triassic.
"For several decades, researchers noticed [that] large, plant-eating dinosaurs seemed to be much more common at high latitudes during the Triassic," lead researcher of the new study, Jessica Whiteside says. "However, in the past 10 years we've realized that they're completely missing from the tropics, where only a few small carnivorous dinosaurs dwelled."
By analyzing carbon and oxygen isotopes within the rocks recovered at Ghost Ranch, the scientists were able to reconstruct the ancient environment, and what they found were carbon dioxide levels four to six times higher than today's. Their evidence also pointed to persistent wildfires that ravaged the leafy vegetation on which large plant eaters relied.
"Rapid climate swings, extremes of drought, and intense heat continually reshaped the vegetation available for warm blooded plant-eating dinosaurs, suppressing their ability to live in the low-latitudes for millions of years," said Whiteside, a paleoclimatologist at the University of Southampton. "These data suggest there are profound consequences for our modern world if they enter the high CO2 conditions that are predicted to occur in the next 100 years."
Their research, which appears this week in the Proceedings of the National Academy of Sciences, is the first multiproxy study to examine the interplay between climate change and ecosystem evolution at low latitudes.
"This is the first really detailed look tying together climate and paleoecology in the Late Triassic of western North America," says Randal Irmis, a vertebrate paleontologist at the Natural History Museum of Utah. "There have been previous related studies from eastern North America where we find early dinosaur footprints, but this is really the first study of its kind from a Triassic site known for its abundant fossil bones of early dinosaurs and other reptiles."
Irmis hopes their research will inspire further interdisciplinary studies linking climate, habitat, and species range in order to further elucidate the complex evolutionary processes of the dinosaurs.
"I think it really emphasizes the interplay between climate and dinosaur evolution," Irmis says. "Our results go to show that the rise of dinosaurs was a multi-faceted event that occurred in several stages, and occurred at different rates in different areas of the supercontinent Pangaea."