Dinosaur Teeth Hint at Seaweed Snacking 100 Million Years Ago

After a big storm, a modern beach can be littered with kelp and other ocean debris, and animals from the land side of the shoreline come down to eat it. Ecologists call this marine subsidization. It is most common along coasts, and it matters most when food inland runs short, as in a drought. A new study in Frontiers in Ecology and Evolution argues that the same arrangement was already operating more than 100 million years ago, and that dinosaurs were part of it.

The evidence sits in tooth enamel. Plants carry two forms of carbon, a lighter one (C-12) and a heavier one (C-13). The ratio between them, written as δ13C, passes from food into the animals that eat it and gets locked into their enamel. In living animals, enamel typically reads 11 to 13 parts per thousand higher than the diet that produced it. Dinosaur teeth, though, have repeatedly come back with unusually high values, higher than would be expected if the animals ate only land plants, which generally have lower δ13C than most marine plants. One possible explanation was that dinosaurs, or the animals they ate, drew some nutrition from the sea.

To test that, the team examined fossils from sites that were once coastal, along the Western Interior Seaway and the ancient Gulf of Mexico shoreline. Around 100 million years ago, the Seaway was an enormous inland sea splitting North America into two landmasses. The researchers also sampled fossils from farther inland to compare. The deposits date to two stretches of the Cretaceous: the early Albian, roughly 113 to 107 million years ago, and the early Cenomanian, roughly 100 to 96 million years ago. They measured carbon isotopes in powdered fossil material and worked out the latitudes where the animals lived, to check whether geography shaped the results.

The pattern was clear. Coastal fossils consistently had higher δ13C values than the inland ones, and the coastal signatures looked alike across sites, whatever the latitude or age. The authors say this is the first evidence of marine subsidization in a prehistoric ecosystem. First author Dr. Clayton Forster, a geologist at the University of Arkansas, said coastal organisms of the Cretaceous greenhouse relied on marine resources much as modern ones do, and that those resources show up in the bones and teeth of dinosaurs, crocodiles, turtles and fish.

What was the food? Because the signal appears in fish and in giant plant-eaters alike, Forster reasoned that the extra carbon must have entered low in the food chain. It also had to exist along coasts but not inland, and persist for millions of years. Few things fit, he said, besides marine macroalgae, meaning seaweeds. He noted that large coastal herbivores today routinely supplement their diets with seaweed, and that most of the sampled herbivorous dinosaurs were probably no different.

Not all of them were. Tenontosaurus tilletti, a large plant-eater found at many Cretaceous sites, had enamel values comparable to modern animals eating land plants, suggesting it did not lean on marine food. The researchers see that as support for their reading. If changes after death had altered the carbon in the teeth, the odd values would be expected more broadly; the differences between species point instead to real differences in diet.

Limits remain. The available fossils include no specimens from polar or equatorial regions for the early Albian and early Cenomanian, so how widespread the behavior was across latitudes is unknown. Further work will be needed to see whether the same relationships held in other periods, including the earlier Jurassic and the early Cenozoic. Forster closed by saying land and sea ecosystems are deeply intertwined and have been for hundreds of millions of years, which he said argues for protecting those links where they survive today.