Dinosaurs may have been eating seaweed 100 million years ago

Dinosaurs living along ancient coastlines may have enjoyed an unexpected addition to their diets: seaweed and other food from the ocean. New research suggests that marine resources washed onto prehistoric beaches helped feed dinosaurs and their prey millions of years ago.

Today, powerful storms frequently deposit seaweed and other ocean material along shorelines, creating opportunities for animals that live on land to find additional food. Scientists call this process marine subsidization (MS). It is especially common in coastal environments and can become more important when food on land is limited, such as during droughts.

A new study published in Frontiers in Ecology and Evolution suggests that this relationship between land and sea existed during the age of dinosaurs. By examining chemical signatures preserved in fossils, researchers found evidence that ocean resources contributed to prehistoric food chains.

"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."

Mysterious Chemical Clues in Dinosaur Teeth

The discovery helps explain a longstanding mystery involving the chemical composition of dinosaur teeth.

Plants contain two forms of carbon known as isotopes: 'light' (C-12) and 'heavy' (C-13). Scientists can measure the relative amounts of these isotopes, expressed as δ13C, to identify a chemical fingerprint that passes from food into the animals that consume it.

As animals eat, this carbon signature becomes incorporated into their tooth enamel. The δ13C value in enamel is typically higher than that of the food consumed, with living animals generally showing an increase of between 11 and 13 parts per thousand.

However, previous measurements of dinosaur teeth have consistently revealed unusually high values. These readings also exceed what scientists would expect from animals that ate only plants growing on land, which generally have lower δ13C values than most marine plants.

One possible explanation is that dinosaurs, or the animals they consumed, were obtaining some of their nutrients from the ocean.

"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," explained Forster.

Fossils Reveal a Prehistoric Ocean Food Source

To investigate, researchers examined fossils from (once) coastal locations bordering the Western Interior Seaway and the ancient coastline of the Gulf of Mexico. Around 100 million years ago, the Western Interior Seaway was an enormous inland sea that divided North America into two separate landmasses.

The team also analyzed fossils from locations farther inland, allowing them to compare animals that lived near the ocean with those that occupied landlocked environments.

The fossil deposits formed during two intervals of the Cretaceous Period: the early Albian (around 113-107 million years ago) and the early Cenomanian (approximately 100-96 million years ago).

Researchers used powdered fossil material to measure carbon isotope signatures from both periods. They also determined the latitudes where the animals had lived to investigate whether geography influenced the results.

A clear pattern emerged. Fossils from coastal environments consistently contained higher δ13C values than those from the inland formation. Remarkably, the coastal signatures remained similar across different locations, regardless of latitude or geological age.

These findings provide the first evidence of marine subsidization in prehistoric ecosystems, indicating that nutrients originating in the ocean were entering food chains on land millions of years ago.

Seaweed May Have Been on the Dinosaur Menu

The researchers next considered what ocean food source could explain the widespread chemical signatures.

"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.

For the same chemical pattern to appear in such a wide variety of animals, the marine food source would likely have entered the food chain near its base. It also needed to be something that existed along coastlines but was absent farther inland, while remaining available for millions of years.

Seaweed emerged as the most likely explanation.

"Few organisms meet these criteria besides marine macroalgae or macrophytes - seaweeds," said Forster. "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."

Modern plant-eating animals living near coastlines often consume seaweed to supplement their diets. The fossil evidence suggests that many herbivorous dinosaurs may have behaved similarly, with marine nutrients also passing to other animals through the food chain.

Not Every Dinosaur Had a Taste for the Sea

Despite the widespread evidence of marine nutrients in coastal food webs, the researchers found that not every dinosaur took advantage of these resources.

One notable exception was Tenontosaurus tilletti, a large plant-eating dinosaur whose fossils have been discovered at numerous locations dating to the Cretaceous Period.

Its tooth enamel contained δ13C values comparable to those of modern animals that feed on plants growing on land. This suggests that Tenontosaurus tilletti did not rely on marine food sources, even though other dinosaurs apparently did.

The difference also strengthens the researchers' interpretation of the fossil evidence. If geological processes occurring after death had altered the carbon signatures in dinosaur teeth, the same unusual values would be expected more broadly. Instead, the variation between species suggests that the elevated readings reflect genuine differences in diet.

An Ancient Connection Between Land and Ocean

Although the findings provide strong evidence that marine resources supported the coastal ecosystems examined, important questions remain.

The available fossils do not include specimens from polar or equatorial regions during the early Albian and early Cenomanian. As a result, scientists cannot yet determine how widespread this feeding behavior was across different latitudes.

Additional studies will also be needed to establish whether similar relationships existed during other geological periods, including the earlier Jurassic Period and the later early Cenozoic Era.

Beyond revealing a surprising possibility about what dinosaurs ate, the discovery highlights how closely life on land has been connected to the ocean throughout Earth's history.

"Our study emphasizes the connections between terrestrial and marine ecosystems," concluded Forster. "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."