Some dinosaurs that lived alongside ancient coastal zones may have included marine algae in their diet, or consumed animals that fed on them. The conclusion results from the analysis of carbon isotopes preserved in fossilized teeth of dinosaurs and other animals that lived about 100 million years ago.
The study, published in Frontiers in Ecology and Evolution, found consistently higher values of carbon-13 (C-13) in fossils from ancient coastal environments than in fossils from inland terrestrial regions. According to the researchers, this difference is compatible with the input of marine resources into terrestrial food webs.
The investigation focused on fossils found in ancient coastal zones of North America, including areas that, during the Cretaceous, formed the margins of the Western Interior Seaway, an enormous inland sea that divided the continent into two landmasses, and the ancient Gulf of Mexico coastline.
The researchers analyzed the carbon isotopic composition present in the enamel of teeth of dinosaurs, crocodiles, turtles and fish. The carbon present in foods leaves a kind of chemical signature in the tissues of the animals that consume them, allowing partial reconstruction of the origins of the resources that formed part of their diet.
Land plants generally show lower values of C-13 than many marine plants. The values found in coastal fossils, however, were higher than would be expected for animals whose diet relied exclusively on terrestrial plants.
For the researchers, a possible explanation is the so-called marine subsidization: organic matter transported from the ocean to land, for example during storms, that becomes available to terrestrial animals. This phenomenon currently occurs in several coastal ecosystems, especially when terrestrial food resources decline.
“We demonstrated that coastal terrestrial organisms in the Cretaceous greenhouse climates depended on marine resources to supplement their feeding, in a way similar to what happens today,” explains Clayton Forster, a geologist at the University of Arkansas and the study’s first author.
The next question was to identify which marine resource could be the source of this chemical signature. The researchers consider that large marine macroalgae, or seaweeds, are one of the most plausible explanations, because they are coastal organisms, are at the base of food chains, and have existed for millions of years.
“Few organisms meet these criteria besides macroalgae or marine macroalgae—the seaweeds,” says Forster.
The authors also point out that large coastal herbivores today often rely on algae to supplement their diet, which raises the possibility that some herbivorous dinosaurs may have adopted a similar behavior. However, the investigation cannot conclude that all coastal dinosaurs consumed algae directly.
One example that points to differences in dietary preferences is Tenontosaurus tilletti. Although it was a large herbivorous dinosaur found in several regions of the Cretaceous, its C-13 values were similar to those observed today in animals that feed on terrestrial plants. For the researchers, this suggests that the higher values found in other dinosaurs reflect differences in diet and not post-fossilization chemical changes.
The study is presented by the authors as the first identification of marine subsidization signals in prehistoric ecosystems. Still, the sample does not include data from regions near the poles or the equator for the periods analyzed, so it will be necessary to study other latitudes and times to understand to what extent this phenomenon was common.
The research highlights, according to the authors, the connection between terrestrial and marine ecosystems, showing that the transfer of resources between both may have played a role in food webs for hundreds of millions of years.
