Dinosaur snacked on seaweed washed up on North American coasts, fossil teeth suggest

Storms routinely wash marine material ashore, providing food for land animals and other organisms. This is known as marine subsidization (MS). Today, MS is particularly common in coastal ecosystems and is especially important to animals when land ecosystems produce less food than usual, such as during droughts. A new Frontiers in Ecology and Evolution study has shown that dinos and their prey, too, may have eaten seafood washed onto beaches.

“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.”

Mystery teeth

Plants contain “light” (C-12) and “heavy” (C-13) carbon isotopes. The ratio of these isotopes, expressed as δ¹³C, is a chemical “fingerprint” that passes from food into a consumer’s body. When an animal eats, the δ¹³C value is incorporated into its tooth enamel, where it is higher than in its food. In living animals, this difference is 11–13 parts per thousand.

In dino teeth, however, this difference has consistently been higher, creating a mystery around their carbon signatures. Their δ¹³C values were also higher than expected for animals that eat only land-growing plants, which have lower δ¹³C values than most marine plants. If dinos or their prey snacked on marine matter, however, that could explain the discrepancy.

“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 do so, the researchers used fossils from sites that were once coastal along the Western Interior Seaway—a vast inland sea that split North America into two landmasses around 100 million years ago—and the ancient Gulf of Mexico coastline. They also included fossils from landlocked sites. These sites formed during the early Albian (around 113–107 million years ago) or early Cenomanian (approximately 100–96 million years ago). The team used powder from the fossils to analyze isotopic composition from both ages and determined the latitudes at which the organisms lived.

The analyses showed that the δ¹³C value in fossils from coastal sites was higher than that in fossils from the landlocked formation. It was also consistent among coastal sites, regardless of latitude or age. The study is the first to identify MS in prehistoric ecosystems.

Suspect: Seaweed

“Coastal-dwelling organisms must have eaten some kind of organic matter from the ocean, or prey that had done so. 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,” Forster explained. The source also must have been an organism that lived 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,” Forster said. “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.”

But not all dinos exploited marine resources. Tenontosaurus tilletti, a large herbivorous dinosaur found in many locations throughout the Cretaceous, for example, did not use marine sources for food. Its δ¹³C value was similar to that of living animals that eat land-growing plants. Therefore, higher δ¹³C values likely reflect dietary preferences rather than geological processes that changed chemical traces in tooth enamel after the animals died.

While the dataset clearly demonstrates MS in the coastal deposits the researchers examined, it lacks data from polar and equatorial latitudes during the early Albian and early Cenomanian. Future research needs to determine whether MS was common at different latitudes and during other periods, such as the preceding Jurassic Period or the subsequent early Cenozoic Era.

“Our study emphasizes the connections between terrestrial and marine ecosystems,” Forster concluded. “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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Gaby Clark

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Robert Egan

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