For years, palaeontologists have had different theories about why they could find no evidence of large, long-necked, herbivore dinosaurs (sauropodomorphs) 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).

Published today in Proceedings of the National Academy of Sciences, the new research suggests a highly unpredictable hot and dry climate, linked with elevated levels of CO2, prevented larger herbivore dinosaurs from inhabiting the area.

The climate, 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° 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,” says lead-author Dr Jessica Whiteside of the University of Southampton. “The fluctuating and harsh climate with widespread wild fires meant that only small two-legged carnivorous dinosaurs, such as Coelophysis, could survive.”

Credit: Victor Leshyk

Scientists took 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.

Co-author Dr Sofie Lindström, of the Geological Survey of Denmark and Greenland, says: “When these rocks were deposited during the Late Triassic, northern New Mexico was very close to the equator at about 12° north in latitude - around the same latitude as the southernmost tip of India sits today.”

After analysis of the rocks, by crushing the sample and separating isotopes of the elements carbon and oxygen using an electromagnet and counting them with high-precision detectors, the team were able to ascertain changes in ecosystem productivity and estimates of atmospheric CO2 levels. They also determined estimates of wildfire temperatures (from fossil charcoal), the types of plant living in the region (from fossil pollen and spores) and the types of vertebrate animals living in the region.

Co-author Dr Randall Irmis, a palaeontologist at the Natural History Museum of Utah and assistant professor at the University of Utah, says: “Throughout this period, levels of CO2 were four to six times higher than the levels we observe today, but the findings do indicate that if we continue our present course of human-caused climate change, similar conditions could develop and suppress equatorial ecosystems.”

Dr Whiteside comments: “This is the first multi-proxy study of climate and associated faunal change for this ecosystem, containing an extensive vertebrate fossil record. For the first time, we can examine the interplay between climate change and ecosystem evolution at low latitudes, shedding light on what had been a major unresolved aspect of the rise of dinosaurs.”

The study Extreme ecosystem instability suppressed tropical dinosaur dominance for 30 million years was funded by the US National Science Foundation and various other bodies.