Storms routinely wash marine matter ashore, which land-living animals and other organisms exploit as food sources. This is known as marine subsidization (MS). Today, MS is particularly common in coastal ecosystems and is enhanced among animals when land ecosystems yield less food than usual, for example during droughts. A new Frontiers in Ecology and Evolution study has shown that dinos and their prey, too, may have indulged in 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 gets passed from food into the body of the consumer. When an animal eats, the δ13C value is incorporated into its tooth enamel with an upward shift in the δ13C value to a value higher than that of its food. This difference is between 11 and 13 parts per thousand in the tooth enamel of living animals.
In dino teeth, however, this difference has been found to be consistently higher, creating a mystery around carbon signatures of dino teeth. Their δ13C values were also higher than expected for animals that solely eat land-growing plants, which have lower δ13C values than most marine plants. If dinos or their prey snacked on marine matter, however, it could explain this 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," explained Forster.
To do so, the researchers used fossils from (once) coastal sites 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 land-locked 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 these fossils to analyze isotopic composition during both ages and determined the latitudes on which the organisms lived.
The analyses showed that the δ13C value in fossils from coastal sites was not only higher than that from the land-locked formation, but also was consistent between 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 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," 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."
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 found to be similar to that of animals which are alive today and consume land-growing plants. Therefore, higher δ13C values likely reflect dietary preferences rather than geological processes that changed chemical traces in tooth enamel after 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 in different latitudes and time periods such as the preceding Jurassic Period or the subsequent early Cenozoic Era.
"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."
