The King of the Riverbanks of North America — Mosasaurs

Mosasaurs are among the most famous marine reptiles. Because of their enormous size and formidable predatory habits, they are widely recognized as some of the most representative marine predators of the Late Cretaceous. In 2022, however, a mosasaur tooth fossil—catalogued as NDGS 12217—was discovered in the Hell Creek Formation of North Dakota. What makes this specimen particularly intriguing is the nature of the surrounding deposits. Instead of marine fossils, the sedimentary environment contains remains of freshwater and even terrestrial organisms. This unusual context raises the possibility that mosasaurs were not strictly marine animals. The central question becomes whether the owner of this tooth actually lived in freshwater environments or merely entered rivers occasionally. The study discussed here seeks to explore that question in detail.

Mosasaurs were in fact a highly diverse group. Some species reached lengths of more than ten meters and ranked among the largest predators of their time, while others were only two to three meters long. Their teeth also varied greatly: some species possessed blade-like teeth well suited for slicing flesh, whereas others developed rounded crushing teeth adapted for breaking shells. These different dental forms reflect distinct ecological strategies. Although most mosasaurs are known from shallow coastal seas, discoveries of mosasaur fossils in estuarine or freshwater deposits increasingly challenge the long-standing view that they were exclusively marine reptiles.

Photo of NDGS 12217; the red box in F marks the location where NDGS 12217 was discovered, beside a Tyrannosaurus tooth fossil (NDGS 15125)（Image source：During, M. A. D et al. (2025 ). ， CC BY 4.0 ）

An examination of the burial conditions of NDGS 12217 shows no evidence that the tooth had been transported from elsewhere. This strongly suggests that the animal itself inhabited the freshwater environment represented by the Hell Creek deposits rather than having been washed in after death. The tooth's morphology provides additional clues. Its enamel bears fine veined textures and lacks serrations along the cutting edges, characteristics consistent with members of the tribe Prognathodontini. Mosasaur material discovered at nearby localities may belong to the same taxonomic group, suggesting that individuals of this lineage could have reached lengths of up to about eleven meters.

The fossil was preserved in a carbon-rich mudstone layer interpreted as part of a floodplain environment near a stream. Animal carcasses accumulated in this area and were subsequently scavenged and dismembered by other animals before burial. As a result, most fossils from the site occur as fragmented and scattered remains. Bones belonging to many different species are commonly mixed together. Near the mosasaur tooth NDGS 12217, for example, researchers also recovered a tooth of Tyrannosaurus (NDGS 15125), a maxilla belonging to the crocodilian Brachychampsa (NDGS 18199), and remains of the hadrosaur Edmontosaurus.

Previous studies had already hinted that some mosasaurs might have tolerated or even inhabited freshwater environments. In particular, smaller species with limb structures suggesting adaptation to shallow or restricted habitats have been investigated through isotopic analysis. Examples include an indeterminate specimen of Plioplatecarpus from Canada and the Hungarian species Pannoniasaurus inexpectatus. Strontium and oxygen isotope studies of these fossils indicated that they may have lived in freshwater settings. However, relying on a single isotopic proxy cannot fully exclude the possibility that fossils were displaced after death or that chemical alteration occurred during fossilization.

To address this limitation, the present study applied multiple isotopic techniques to the enamel of NDGS 12217. Researchers analyzed oxygen and carbon isotopes from the carbonate component of the enamel, oxygen isotopes from phosphate, and strontium isotope ratios. By combining these independent datasets, the study sought to eliminate the possibility that the results were caused by diagenetic alteration during burial rather than reflecting the animal's true living environment.

The isotopic results indicate that NDGS 12217 formed in a freshwater setting and that the animal occupied a relatively high trophic level within its ecosystem. Interestingly, ammonite fragments and shark teeth from roughly the same geological interval were analyzed for comparison, and their isotopic signatures point clearly to marine conditions. This contrast led researchers to propose that the local water column contained a strong halocline—a stratified system in which salinity changes sharply with depth. In such a situation, freshwater could have formed a surface layer while denser saltwater remained below. The mosasaur would have inhabited the upper freshwater layer, whereas sharks and ammonites lived in the underlying marine water.

Map of the Western Interior Seaway, Laramidia, and Appalachia during the Cretaceous period（Image source：Scott D. Sampson, Mark A. Loewen, Andrew A. Farke, Eric M. Roberts, Catherine A. Forster, Joshua A. Smith, Alan L. Titus ， CC BY 4.0 ）

During the Late Cretaceous, a vast inland sea known as the Western Interior Seaway stretched across North America, dividing the continent into two landmasses: Appalachia in the east and Laramidia in the west. This seaway supported a rich marine fauna, including mosasaurs. Toward the end of the Cretaceous, however, the seaway gradually retreated and its salinity decreased. During this period of environmental change, some mosasaurs—including members of the Prognathodontini and possibly species of Mosasaurus—may have begun adapting to freshwater environments. The large river systems that developed as the seaway withdrew could have supported sizable aquatic predators, producing a distinctive ecosystem in which mosasaurs inhabited freshwater channels and floodplains.

This capacity to tolerate changing environments may help explain why mosasaurs occupied such a wide range of ecological niches during the Late Cretaceous. Their adaptability likely allowed them to exploit habitats beyond the open ocean. Future research, particularly additional isotopic studies of other mosasaur specimens, will be important for clarifying how widespread this ecological flexibility was and how these reptiles responded to the dramatic environmental shifts that preceded their extinction.

Author: Bai Leng

Reference:

During, M. A. D., Van Vranken, N. E., Boyd, C. A., Ahlberg, P. E., Warmerdam-Verdegaal, S. J. A., Van der Lubbe, J. H. J. L. (2025). "King of the Riverside", a multi-proxy approach offers a new perspective on mosasaurs before their extinction. BMC Zoology.

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