Dinosaur tooth chemistry provides new understanding of ancient ecosystems

Illustration by Luke Dickey for Western University

About 75 million years ago, southern Alberta was a lush and warm coastal floodplain rich in plant and animal life, similar to Louisiana’s environment today. New isotopic evidence suggests large herbivores in this system co-existed in the same habitats, contrary to earlier hypotheses that some stuck to coasts and others to inland forests.

An international team of scientists, including Fred Longstaffe from Western University, has 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, and it has been a mystery just how so many big animals co-existed in such a small area.

It has been proposed 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. This idea remained somewhat untested, however, without being able to directly observe dinosaur behaviour and ecosystems.

To solve this conundrum, a research team — led by researchers from the Field Museum and Royal Ontario Museum — has now 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) that do not change into other elements over time. When animals consume food and water, the stable isotopes of the elements that make up those resources are passed to the animal’s tissues, including tooth enamel.

Illustration by Luke Dickey for Western UniversityAnalyses were performed to measure the stable carbon and oxygen isotopes preserved in the fossils from this site, including the teeth of large herbivorous dinosaurs like this Styracosaurus.

The stable carbon and oxygen isotope compositions of these herbivorous dinosaurs were measured using various methods. The primary approach was laser gas chromatography isotope ratio mass spectrometry conducted at Western’s Laboratory for Stable Isotope Science (LSIS) by Longstaffe, Western Research Scientist Li Huang and project lead Thomas Cullen from the Field Museum.

“This approach allowed us to analyze very small samples, and because of that, to extend the science of isotope ecology back into the time of the dinosaurs,” says Longstaffe, Canada Research Chair in Stable Isotope Science. “Normally my isotope ecology work is focused on Ice-Age animals and the reasons for their disappearance or survival. To attempt to reach back much deeper to the time when the dinosaurs lived was both challenging and exciting.”

The study, published in the scientific journal Geology, compared results for numerous individuals of each species to those of other animals in this ancient ecosystem. While multiple ecological patterns are evident in the results, and differences found in some species, the stable carbon and oxygen isotope ranges for large herbivorous dinosaurs were found to strongly overlap, providing direct evidence against the habitat use hypothesis.

“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 who has been extinct for millions of years,” explains Cullen, currently a Research Associate at the Field Museum and a Postdoctoral Research Scholar at NC State University.

“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, carbon dioxide in the atmosphere was higher than it is today, and there was little to no ice at the poles. 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,” says Cullen.

The new study is one of the largest ever conducted on a dinosaur ancient ecosystem, involving more than 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 David Evans, Temerty 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.

Evans, the senior author on the paper and Cullen’s PhD supervisor when he conducted this research during his studies at the University of Toronto, has been researching dinosaur communities for more than 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,” says Evans. “The environmental conditions were probably quite similar, and the number of the animals there probably had similar lifestyles to those found in dinosaur ecosystems. That gives a great deal of control when exploring our data.”

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