Ancient Coastlines Fed the Dinosaurs: Marine Nutrients Shaped Cretaceous Ecosystems

On the coastlines of the Cretaceous world, more than 66 million years ago, dinosaurs and other terrestrial vertebrates may have been quietly dining on resources that originated in the sea. A new study published in Frontiers in Ecology and Evolution presents stable carbon isotope evidence that marine-derived organic matter flowed into terrestrial food webs along ancient North American shorelines, a phenomenon the researchers call marine subsidization. The finding suggests that coastal dinosaur ecosystems were more nutritionally connected to the ocean than previously appreciated, and it raises questions about how scientists reconstruct the diets of extinct herbivores from fossilized tooth enamel.

Marine subsidization is a well-documented process on modern coastlines. Marine algae, seagrasses, and other coastal plants produce organic matter that washes ashore or is carried inland by tides, storms, and scavenging animals, enriching terrestrial soils and feeding land-based consumers. Ecologists have long recognized this cross-boundary nutrient transfer in contemporary settings, from seabird colonies fertilizing islands to kelp forests sustaining coastal forests. Yet, according to the authors, this phenomenon has rarely been considered in reconstructions of Cretaceous terrestrial coastal ecosystems, despite the fact that many of the most famous dinosaur-bearing rock formations in North America were deposited along the margins of the Western Interior Seaway, a vast inland sea that split the continent.

The research team focused on a geochemical signature that had puzzled paleontologists for years. Studies of Cretaceous coastal ecosystems frequently reported anomalously high carbon isotope values, expressed as δ13C, in the bioapatite of dinosaurs and other vertebrates, values much higher than expected when compared to the presumed dietary carbon sources of these animals. In terrestrial C3 plant communities, which dominated Cretaceous floras, δ13C values are relatively low. Marine plants and macroalgae, however, use a modified C3 photosynthetic pathway that results in comparatively enriched δ13C values. Any animal consuming marine-derived plant material or prey that had themselves fed on such resources should therefore carry a distinctive isotopic fingerprint in its tissues.

To test whether Cretaceous coastal vertebrates were actually tapping into marine food sources, the researchers analyzed stable carbon isotopes preserved in tooth enamel from a range of vertebrate taxa, including fish, neosuchian crocodilians, and dinosaurs. Tooth enamel is remarkably resistant to chemical alteration over geological time, making it one of the most reliable archives of an animal’s dietary carbon signature. The team compared enamel from coastal assemblages against enamel from the Cloverly Formation of Wyoming and Montana, an inland intercontinental deposit where marine influence was expected to be minimal based on established stratigraphic interpretations.

The coastal formations examined span a broad geographic and temporal range across western North America. They include the Dunvegan Formation of British Columbia and Alberta in Canada, the Wayan Formation and Draney Limestone of Idaho, the Antlers Formation of Oklahoma and Texas, the Woodbine Group and Lewisville Formation of Texas, and the Mussentuchit Member of the Cedar Mountain Formation in Utah. Together, these units represent diverse coastal environments that existed at various points during the Cretaceous period, from estuarine and deltaic settings to nearshore floodplains.

The results were striking. Coastal vertebrate enamel exhibited δ13C values up to more than 5.5 per mil above what would be expected from animals feeding exclusively on terrestrial C3 vegetation. This enrichment is consistent with the incorporation of marine-derived carbon into the diets of coastal organisms. In contrast, the upland assemblages from the Cloverly Formation showed no comparable enrichment, supporting the interpretation that the elevated isotope values in coastal settings reflect genuine marine subsidization rather than some artifact of preservation, diagenesis, or analytical methodology.

The implications of this finding extend beyond simply confirming that some dinosaurs ate things that came from the sea. Back-calculations of dietary plant δ13C values from dinosaur herbivore enamel have traditionally relied on assumptions about the isotopic composition of terrestrial C3 plants. If coastal herbivores were consuming marine algae, seagrasses, or animals that had fed on such resources, then previous estimates of dietary plant δ13C values systematically underestimated the true range of carbon sources available to these animals. Future estimations of Cretaceous vertebrate isotopic offsets must therefore account for the influx of 13C-enriched, marine-derived organic matter into terrestrial food webs.

Beyond the technical implications for isotope ecology, the study suggests that marine subsidization may have functioned as a major ecosystem and trophic driver in Cretaceous coastal environments. If substantial quantities of marine nutrients and energy were regularly flowing onto land, they could have supported higher densities of vertebrate consumers than terrestrial primary production alone would allow. This could help explain the remarkable abundance and diversity of large-bodied dinosaurs and other vertebrates in coastal depositional settings, which have sometimes been attributed solely to favorable terrestrial conditions or preservational biases.

The research also highlights the value of comparative approaches that pair coastal and inland deposits. By establishing that the Cloverly Formation, an inland deposit, lacks the isotopic enrichment seen in coastal assemblages, the study provides a baseline against which marine influence can be detected. This framework can now be applied to other Cretaceous formations around the world, potentially revealing the geographic extent and temporal variability of marine subsidization during the age of dinosaurs.

As paleontologists continue to reconstruct ancient ecosystems with ever-greater precision, the recognition that ocean and land were deeply intertwined during the Cretaceous adds a new dimension to our understanding of dinosaur ecology. The giants that roamed the shorelines of the Western Interior Seaway were not simply inhabitants of a terrestrial world; they were participants in a dynamic system where marine and terrestrial energy flows converged, shaping the structure and productivity of ecosystems in ways that scientists are only now beginning to appreciate.

Subject of Research: Marine-derived nutrient subsidies in Cretaceous coastal dinosaur ecosystems revealed by stable carbon isotope analysis of tooth enamel

Article Title: Marine subsidization of dinosaurian ecosystems

Article References: Marine subsidization of dinosaurian ecosystems. (n.d.). https://doi.org/10.3389/fevo.2026.1895247

Image Credits: AI Generated

DOI: 10.3389/fevo.2026.1895247

Keywords: marine subsidization, Cretaceous, dinosaurs, stable carbon isotopes, tooth enamel, Western Interior Seaway, paleoecology, coastal ecosystems, δ13C, food webs, Frontiers in Ecology and Evolution, vertebrate paleontology