Aboard the Isotope Time Machine.

Abstract

Several years ago, paleontologist Shaena Montanari was doing field work in Mongolia’s Gobi Desert when she noticed fragments of dinosaur eggshells in the sand. She knew them by distinctive lines and bumps on their surfaces. Montanari, then a graduate student at the American Museum of Natural History, and now a Royal Society fellow at the University of Edinburgh, realized she could use the fragments to understand the climate that these dinosaurs lived in about 80 million years ago. That’s because an isotopic signature in the calcium carbonate shells—the ratio of oxygen-18 to oxygen-16—can tell researchers about the same signature in the water the dinosaurs drank. Such water isotopes, geochemists have shown, depend on how wet or dry a certain climate is. Montanari eventually found that the oviraptorid theropod dinosaurs that made those eggshells lived in arid conditions, much like the Gobi has today. Stable isotopes found in fossilized dinosaur eggshells (left), bones and teeth like mammoth tusks (top right), and insects like chironomids (bottom right, 1 mm across) can provide clues to past climates and the diets of extinct animals. Credits: Shaena Montanari, Optimarc/Shutterstock, Matthew Wooller. Montanari wasn’t the only one to use these fossilized shells as a window to the past. Chemist Robert Eagle, then of California Institute of Technology, realized that the eggshells might also shed light on an ongoing debate in paleontology: whether dinosaurs were warm-blooded like birds and mammals or cold-blooded like reptiles. Eagle, now at the University of California, Los Angeles, was inspired by a relatively recent advance called clumped isotope analysis, which can accurately predict the temperature at which a carbonate mineral was formed. Pioneered by John M. Eiler and colleagues at Caltech, it has been used to more accurately reconstruct temperature deep into the past by analyzing carbonate in ancient corals and shells in ocean sediments. Though scientists have long been analyzing the stable isotopes of fossils to study the past, recent advances like clumped isotope analysis are helping push the boundaries of what paleontologists, archeologists, and paleoanthropologists can learn about life thousands to millions of years ago. Through signatures preserved in eggshells, bones, teeth, and even the molecular remains of primeval plants, researchers are piecing together the physiology and diet of ancient animals and humans, and figuring out how climate and environmental changes might have influenced evolution and extinction events. Eagle thought clumped isotope analysis on the oviraptorid theropod eggshells could tell him about the body temperature of the creature that formed them. In clumped isotope analysis, scientists use a mass spectrometer to simultaneously measure the stable isotopes of carbon and oxygen in carbon dioxide derived from a carbonate sample. They then determine how often the different isotopes are paired together in the carbonate material. The temperature at which carbonate is formed influences how often certain carbon and oxygen isotopes join up in the crystal, a phenomenon called isotopic “clumping”. Eagle first validated his hunch by analyzing eggs laid by modern birds. He then visited Montanari at the museum and worked with her to study the Gobi eggshells, as well as ones from titanosaurid sauropods, plant-eaters that resemble Brontosaurus, found in Argentina. Paleontologist Shaena Montanari (shown holding fossilized bones) collected dinosaur eggshells in the Gobi Desert and analyzed them to understand ancient climates. Credit: Courtesy of Shaena Montanari. Their results show that the titanosaurids had a body temperature of about 38 °C, similar to modern birds, which are warm-blooded. In contrast, the oviraptorid dinosaurs had body temperatures of about 32 °C—not quite as warm, but warmer than their environment by about 6 °C, meaning they weren’t cold-blooded either. Thus, they may have had an intermediate metabolism called mesothermy, like modern great white sharks and echidnas, Montanari says.