While mosasaurs were established as the preeminent marine predators during the age of dinosaurs, recent investigations have brought to light that terrestrial realms, specifically rivers, were not beyond their predatory reach.
A collaborative effort involving researchers from Sweden, the United States, and the Netherlands involved the meticulous examination of isotopic signatures present in numerous mosasaur teeth recovered from various locales within North Dakota. This analysis has substantiated the hypothesis that these formidable ancient marine reptiles were capable of inhabiting environments characterized by freshwater.
Based on the morphological characteristics observed in a tooth discovered within an inland flood plain, it is inferred that this specimen originated from a lineage of mosasaurs that could attain impressive dimensions, potentially reaching approximately 11 meters (36 feet) in length.
This revelation introduces a profoundly unsettling dimension of peril to the act of seeking hydration at natural water sources. Dinosaurs, in addition to being vigilant against land-based threats, were compelled to remain alert for the sudden emergence of colossal, bus-sized predators from the very waters they approached.
“The sheer magnitude of this creature suggests it would have been comparable to the largest killer whales, presenting an extraordinary predatory encounter in fluvial environments that, prior to this finding, were not typically associated with such gargantuan marine inhabitants,” stated Per Ahlberg, a vertebrate paleontologist affiliated with Uppsala University in Sweden.
Mosasaurs were predatory aquatic reptiles that thrived during the latter part of the Cretaceous period. Although certain species were relatively diminutive, the majority were of immense size, a factor contributing to their dominance of the ancient oceans for extended geological epochs.
Consequently, the discovery in 2022 of a mosasaur tooth within an inland flood plain, juxtaposed with a Tyrannosaurus rex tooth and a crocodilian mandible, presented a significant enigma. The pertinent question was whether its owner habitually resided in this freshwater fluvial setting, or if the specimen had been transported there from marine origins.
To resolve this uncertainty, the research team embarked on an isotopic analysis of the tooth’s enamel, subsequently comparing the gathered signatures with those derived from comparable fossilized remnants, including shark teeth and ammonites.
Elements can exist in various forms, termed isotopes, distinguished by differences in the neutron count within their atomic structure. Investigations into the relative abundance of these isotopes within a sample can yield insights into an organism’s diet and habitat.
In this particular inquiry, the researchers focused on the ratios of oxygen, strontium, and carbon isotopes.
Oxygen isotopes, for instance, prove exceptionally valuable in distinguishing between marine and freshwater environments. The lighter isotope, 16O, exhibits a greater propensity for evaporation from oceanic bodies and subsequent precipitation, resulting in freshwater ecosystems possessing a higher concentration of 16O and a comparatively lower presence of the heavier 18O isotope when contrasted with seawater.
Indeed, the isotopic signature of oxygen and strontium within the mosasaur tooth unequivocally indicated that the animal was entirely adapted to and comfortable within a freshwater milieu.
“Upon examining two supplementary mosasaur teeth unearthed at adjacent, slightly older sites in North Dakota, we observed analogous freshwater isotopic signatures,” reported Melanie During, a fellow vertebrate paleontologist at Uppsala.
“These examinations demonstrate that mosasaurs inhabited riverine environments during the final million years preceding their extinction.”
The carbon isotope ratios further corroborated these findings, adding a disquieting detail: this formidable riverine inhabitant did not shy away from preying on dinosaurs.
“Carbon isotope compositions within teeth generally serve as a proxy for an organism’s dietary intake,” explained During.
The research team posits that the transition from marine to freshwater ecosystems may have represented a late adaptive strategy employed by mosasaurs within the final millennium or so preceding the mass extinction event that led to their demise, along with that of the dinosaurs.
This groundbreaking research has been formally published in the esteemed journal BMC Zoology.
