Carbon locked in Cretaceous teeth points to coastal food webs quietly subsidized by the sea some 100 million years ago.

Every storm that rakes a coastline leaves something behind. Kelp, sea grass, stranded fish and tangled mats of algae pile up along the tide line, and the land animals living nearby rarely let the windfall go to waste. Ecologists call this flow of food from ocean to land marine subsidization. Today it is especially common in coastal ecosystems, and it tends to intensify when food on land runs short, such as during droughts.

A new study suggests the habit is far older than anyone had documented. Writing in Frontiers in Ecology and Evolution, a team led by geologist Clayton Forster of the University of Arkansas reports chemical evidence that dinosaurs, crocodilians, turtles and fish living along Cretaceous coastlines were drawing on marine resources roughly 100 million years ago, most likely seaweed and the animals that ate it.

“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,” Forster said. The signal, he added, travels up the food chain and into the skeleton itself: “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.”

A puzzle written in carbon

The work hinges on carbon isotopes. Plants take up both lighter carbon-12 and heavier carbon-13, and the balance between the two, expressed as δ13C, acts as a chemical fingerprint that passes from food into the body of whatever eats it. In tooth enamel, that fingerprint shifts upward by a predictable amount. In living animals, enamel values run between 11 and 13 parts per thousand higher than the diet.

Dinosaur teeth have long refused to follow the rule. Their δ13C values have consistently come out higher than expected, including for animals assumed to have eaten only land plants, which carry lower values than most marine plants. Something in the Mesozoic menu was pushing the numbers up. A regular helping from the sea would do it.

“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,” Forster explained.

To test the idea, the researchers gathered fossils from sites that once sat along the shores of the Western Interior Seaway, the vast inland sea that split North America into two landmasses around 100 million years ago, as well as along the ancient Gulf of Mexico coastline. For comparison, they added fossils from formations that had been landlocked. The assemblages dated to two windows: the early Albian, roughly 113 to 107 million years ago, and the early Cenomanian, roughly 100 to 96 million years ago. Among the specimens were crocodilian teeth and teeth from the predatory dinosaur Deinonychus antirrhopus and the armored plant-eater Sauropelta edwardsi, drawn from the Cloverly Formation.

The team analyzed powder drilled from the fossils to measure their isotopic composition and reconstructed the latitudes at which each community had lived.

A coastal signature

The pattern was clear. Fossils from coastal sites carried higher δ13C values than those from the landlocked formation, and the coastal values were consistent with one another regardless of latitude or geological age. According to the authors, it is the first time marine subsidization has been identified in a prehistoric ecosystem.

“Coastal-dwelling organisms must have eaten some kind of organic matter from the ocean, or prey that had done so,” 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.”

That narrowed the list of suspects considerably. The extra carbon had to come from something near the base of the food web, something that grew in coastal but not inland habitats, and something that stayed available across many millions of years.

“Few organisms meet these criteria besides marine macroalgae or macrophytes – seaweeds,” Forster said. Modern behavior makes the case more plausible still. “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.”

The image is a vivid one: big plant-eaters working the wrack line after a storm, and the predators that hunted them carrying the same marine carbon in their own teeth.

Not everyone came to the beach

One dinosaur stood apart. Tenontosaurus tilletti, a large herbivore found at many Cretaceous localities, showed no sign of marine food. Its δ13C values resembled those of living animals that eat land plants. That exception matters to the study’s logic. If chemical changes to the enamel after burial had inflated the readings, Tenontosaurus should have been affected too. Its ordinary values suggest instead that the elevated numbers in other animals reflect what they actually ate.

Gaps in the map

The authors acknowledge the dataset’s limits. It lacks samples from polar and equatorial latitudes for the two time windows studied, so it cannot yet say whether coastal subsidies were universal. Future work will need to test whether the pattern holds at other latitudes and in other periods, such as the Jurassic that came before or the early Cenozoic that followed.

For Forster, the broader lesson concerns the seam between land and sea. “Our study emphasizes the connections between terrestrial and marine ecosystems,” he said. “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.”

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Notes

1. Forster, C., et al. “Marine Subsidization of Dinosaurian Ecosystems.” Frontiers in Ecology and Evolution, 30 September 2026. DOI: 10.3389/fevo.2026.1895247.
2. Frontiers. “Dinos snacked on seafood washed up on ancient beaches, fossil study shows.” EurekAlert!, 30 September 2026. https://www.eurekalert.org/news-releases/1144410.

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