Paleontologists studying Cretaceous fossils have repeatedly found that the teeth of many ancient animals contain more of a heavier carbon isotope, carbon-13, than their presumed plant-based diets would predict. Proposed explanations have ranged from dinosaur physiology to forest canopy effects to higher carbon dioxide in the atmosphere. A team of scientists from the University of Arkansas and Auburn University offers a different one: seaweed and other marine plants carry a distinctive carbon signature, and animals that eat them, or eat creatures that do, should show it in their tooth enamel.
This chart shows the average chemical signatures in the tooth enamel of different animal groups from the Cretaceous period. The vertical axis tracks carbon: the higher an animal’s carbon-13 reading, the more its diet likely came from the sea. The horizontal axis tracks oxygen, which reflects the local water the animals drank, such as a river, lake or seaway. Image credit: Forster et al., doi: 10.3389/fevo.2026.1895247.
“We show that coastal terrestrial organisms in the greenhouse climates of the Cretaceous relied on marine resources to supplement their diets in a similar way that modern organisms do,” said first author Dr. Clayton Forster, a geologist at the University of Arkansas.
“We can identify that marine resources are passed along the food chain and incorporated in the minerals of bones and teeth of dinosaurs, crocodiles, turtles, and fish.”
In their research, Dr. Forster and colleagues analyzed fossils of fish, crocodile relatives, turtles and dinosaurs from six North American formations dating to roughly 100 to 113 million years ago.
Five were coastal, in Canada, Idaho, Utah, Oklahoma and Texas. One, the Cloverly Formation of Wyoming and Montana, was an inland deposit.
“By determining the carbon isotope composition of dinosaur, fish, and crocodile tooth enamel, we can determine what their primary dietary source was and if they were different between regions,” Dr. Forster said.
Enamel from the coastal sites was consistently richer in carbon-13 than enamel from the inland site.
“Coastal-dwelling organisms must have eaten some kind of organic matter from the ocean, or prey that had done so,” Dr. Forster said.
“This pattern is shared from fish to megaherbivores and indicates that the extra carbon source must have been low in the food-chain to affect both aquatic and terrestrial animals.”
“The source also must have been an organism living in coastal, but not inland, habitats and remained available over many millions of years.”
“Few organisms meet these criteria besides marine macroalgae or macrophytes — seaweeds.”
“Given the almost ubiquitous behavior of large coastal herbivores today to supplement their diet with seaweeds, it’s likely that most of the sampled herbivorous dinosaurs were no different.”
“Our study emphasizes the connections between terrestrial and marine ecosystems.”
“They are deeply intertwined and have been for hundreds of millions of years.”
“It highlights the importance of environmental linkages across time and space and protecting them where they exist today.”
The findings were published September 30 in the journal Frontiers in Ecology and Evolution.
_____
Clayton W. Forster et al. 2026. Marine subsidization of dinosaurian ecosystems. Front. Ecol. Evol 14; doi: 10.3389/fevo.2026.1895247
