New insights into dinosaur ecosystems along North America’s inland sea

TORONTO, May 04, 2020 — An international team of scientists have revealed new insights into the inner workings of ancient dinosaur communities. Seventy-five million years ago, North America was divided into western and eastern landmasses by a shallow inland sea. The western part of this land was home to an extremely rich diversity of dinosaurs. How so many big animals co-existed in such a small area has been a mystery which has baffled scientists. The popular theory proposes that this incredible diversity was maintained by dividing up the landscape and food sources to accommodate so many species. For example, horned dinosaurs (ceratopsians) may have stuck to coastal areas, while duck-billed dinosaurs (hadrosaurs) preferred more inland habitats. However, without being able to directly observe dinosaur behaviour and ecosystems this theory remained untested.

To solve this conundrum, researchers from the Royal Ontario Museum (ROM) and Field Museum compared the compositions of stable isotopes in fossil teeth from these dinosaurs. Stable isotopes are naturally occurring varieties of chemical elements (like carbon or oxygen, for example) that don't change into other elements over time. When animals consume food and water, the stable isotopes of the elements that make up those resources (for example, carbon, oxygen, etc.) are passed on to the animal’s tissues, including tooth enamel.

"Differences in the sources of water and types of food being consumed, as well as the physiology of the animal itself and the habitats they live in, will all result in small differences in the relative amounts of the stable isotopes of a given element, for example, carbon-13 versus carbon-12, present in their body tissues," says Dr. Thomas Cullen, lead researcher on the project, which formed a component of his recently completed PhD thesis at the University of Toronto and the ROM. "Measuring the ratios of the different isotopes of elements such as carbon or oxygen in tissues like tooth enamel gives us a unique window into the diet and habitat of an animal which has been extinct for millions of years."

The new study is one of the largest ever conducted on a dinosaur ancient ecosystem, involving over 350 isotopic measurements from 17 different species whose fossils had all accumulated in a single ancient wetland deposit. Even more uniquely, the authors combined this information with measurements from 16 living species that the team previously sampled from a modern coastal wetland in Louisiana.

"Most of the time when these types of studies are done, the size of the dataset is much smaller and doesn't take into consideration how dinosaur ecosystems compared to modern ones," says Dr. David Evans, James and Louise Temerty Endowed Chair and Senior Curator of Vertebrate Paleontology at the Royal Ontario Museum and Associate Professor of Ecology and Evolutionary Biology at the University of Toronto. Dr. Evans, the senior author on the paper and Cullen’s PhD supervisor, has been studying dinosaur communities for over 15 years, and explained that this was one of the most exhaustive studies on a single dinosaur ecosystem ever conducted. "Louisiana was the perfect place to use as a comparison with the dinosaur communities we studied. The environmental conditions were probably quite similar, and a number of the animals there probably had similar lifestyles to those found in dinosaur ecosystems. That gives us a great deal of control when exploring our data."

The team’s research, published in the scientific journal Geology, compares stable carbon and isotope compositions from each species in this ancient ecosystem. While multiple ecological patterns are evident in the results, the stable carbon and oxygen isotope ranges for large herbivorous dinosaurs were found to strongly overlap, providing direct evidence against the habitat use hypothesis.

“We were surprised that the large herbivores didn’t appear to be separating across different habitats,” says Cullen, now a Research Associate at the Field Museum and Postdoctoral Research Scholar at NC State University. “This suggests that they may be doing something different to avoid competition with one another, such as shifting their feeding across the landscape seasonally or feeding on different parts of the same plants. Due to an abundance of plants, competition may have even been somewhat relaxed compared to modern ecosystems, reducing the need to partition their resource use as much.”

The fossils from this site also allowed for surprisingly accurate estimation of environmental conditions. By using an approach that combined average oxygen isotope compositions from the sampled species, new estimates of mean annual temperatures for the area could be calculated. They found that 75-million years ago, this area of southern Alberta to northern Montana had a mean annual temperature of about 16-20 °C, a stark contrast to the current range of about 5-7 °C in that region.

“Dinosaurs lived in a weird world: broad-leafed and flowering plants were much less common, it was warm enough in high latitudes to support crocodilians, CO2 in the atmosphere was higher than it is today, and there was little to no ice at the poles,” says Cullen. “It’s not like anything we as humans have any direct experience with, but it may be the direction we are headed, so it’s critical that we understand how ecosystems and environments function under those sorts of conditions so we can better prepare ourselves for the future.”

Illustration by Danielle Dufault. © Royal Ontario Museum

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ABOUT THE ROM 

Opened in 1914, the Royal Ontario Museum showcases art, culture and nature from around the world and across the ages. Among the top 10 cultural institutions in North America, Canada’s largest and most comprehensive museum is home to a world-class collection of 13 million art objects and natural history specimens, featured in 40 gallery and exhibition spaces. As the country’s preeminent field research institute and an international leader in new and original findings, the ROM plays a vital role in advancing our understanding of the artistic, cultural and natural world. Combining its original heritage architecture with the contemporary Daniel Libeskind-designed Michael Lee-Chin Crystal, the ROM serves as a national landmark, and a dynamic cultural destination in the heart of Toronto for all to enjoy. 

ABOUT THE FIELD MUSEUM

Located on Chicago’s iconic Lake Michigan shore, the Field Museum opened its current building to the public in 1921—but our story began years earlier. Our collection grew out of items on display in the 1893 World’s Columbian Exposition in the “White City.” The exposition delighted visitors with 65,000 exhibits filled with natural wonders and cultural artifacts, many of which later found a permanent home in Chicago at the newly created Field Columbian Museum. Our museum name still honors Marshall Field, who donated $1 million to make the collective dream of a permanent museum a reality. Since opening the Museum in 1894, our collection has grown to nearly 40 million artifacts and specimens. The breadth of our mission has expanded, too. We continue to research the objects in our collections, as well as document previously unknown species, conserve ecosystems in our backyard and across the globe, educate budding scientists, invite cross-cultural conversation, and more—all to ensure that our planet thrives for generations to come.