Extreme heat and a lack of plants may be why dinosaurs didn’t start living in the tropics until about 30 million years after they arrived on Earth—and about 10 to 15 million years after they settled in at higher altitudes north and south of the Equator.
Researchers examined sedimentary rocks and fossil record preserved in the Chinle Formation in northern New Mexico to investigate the environment in tropical latitudes during the Late Triassic Period 235 to 201 million years ago.
Their findings suggest temperature fluctuations, associated with high atmospheric carbon dioxide, may have prevented the establishment of a diverse community of fast-growing, warm-blooded, large dinosaurs such as sauropods and their close relatives.
These dinosaurs likely required a productive and stable environment to thrive, conditions not present in equatorial North America until the beginning of the Jurassic Period around 200 million years ago.
Ghost Ranch fossils
For the study in the Proceedings of the National Academy of Sciences researchers examined rocks and fossils from a location called Ghost Ranch, an area noteworthy for a number of Triassic dinosaur discoveries including the early meat-eating dinosaurs Coelophysis and Tawa.
The rocks at Ghost Ranch were deposited by rivers and streams between 205 and 215 million years ago when the area was much closer to the equator. During the Late Triassic, northern New Mexico was at a latitude comparable to the southernmost tip of India today.
Researchers have worked in the area for nine years to document the ecological and evolutionary changes taking place during the period.
“By focusing so much effort at a single region like Ghost Ranch, we’ve made one of the largest and most detailed collections of Late Triassic fossil vertebrates in North America,” says Alan H. Turner, associate professor of anatomical sciences at Stony Brook University.
“Additionally, our team was able to provide the first detailed paleoenvironmental analysis for rocks that produce early dinosaur fossils.”
Arid western US
The analysis, which used stable carbon isotopes from fossilized organic matter, carbonate nodules in fossilized soil, and fossil charcoal, revealed data about climate and wildfire temperatures.
Wild climate swings correspond to changes in plant community structure, from a seed fern-dominated system to a conifer-dominated system. Individual plant groups repeatedly alternated from rare to common through time.
“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 Jessica Whiteside, a geochemist from 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.”
Researchers from Brown University, University of Utah, Geological Survey of Denmark and Greenland, Colby College, Rensselaer Polytechnic Institute, Virginia Polytechnic Institute, and Howard University contributed to the work.
Partial funding came from the National Science Foundation. The Richard Salomon Foundation, National Geographic Society Committee for Research & Exploration, the University of California Museum of Paleontology, the University of Utah, the Grainger Foundation, Dyson Foundation (MFS), and Field Museum Women’s Board – Geocenter Denmark also provided funding.
Fieldwork took place with the permission and support of the Ghost Ranch Conference.