New research published by Cambridge University Press suggests that plesiosaurs, long-necked marine reptiles that lived during the Mesozoic Era, swam into river deltas to vomit up shells and other indigestible parts of their meals. This behavior is similar to that of modern-day fur seals and marine crocodiles. Previous studies on plesiosaur anatomy suggested that these creatures fed on shellfish from the seafloor, but they lacked the ability to crush hard materials with their teeth. As a result, it is believed that their stomachs compacted these materials into pellets, which they later vomited up, much like owls do today. In a study published in Geological Magazine, paleontologist Dr. Dirk Knaust uncovered new insights into the behavior of these ancient marine reptiles. He identified fossilized remains of plesiosaur vomit—known as regurgitalites—in what was once the shallow water of a river delta, about 50 kilometers off the Norwegian coast. These vomit pellets contained shellfish that required stable salt levels to survive, which would not have been possible in a river delta where freshwater constantly altered salinity. This led Knaust to conclude that plesiosaurs must have eaten these shellfish in deeper, saltier waters before swimming to the shore to vomit them up. Knaust's detailed analysis of more than 20 regurgitalites—ranging in size from a walnut to a mango—revealed that these plesiosaurs likely hunted in the open ocean and then returned to the delta to expel indigestible material. He emphasized that while fossilized vomit has been studied in recent years, this is the first time plesiosaur vomit has been identified with such clarity. The discovery provides valuable insight into their feeding habits and behavior. Dr. Knaust has spent over three decades analyzing drill core samples from North Sea wells operated by Equinor ASA, the Norwegian energy company where he works. The vomit fossils he studied were found near the Troll gas and oil field, where a subsea storage site for CO2 was being developed. He speculated that plesiosaurs may have come close to shore to rest or sleep after hunting, and that's when they vomited. While modern animals like fur seals go onto land, plesiosaurs, due to their size and shape, likely remained submerged. Near the fossilized vomit, Knaust also discovered a bone and coprolites (fossilized feces), which led to an intriguing mix of species. Initially, he thought the bone might be from a dinosaur, but a CT scan revealed it was from an ichthyosaur—a marine reptile unrelated to dolphins. However, the composition of the vomit and feces suggested they might have come from different animals. Knaust identified various shells and exoskeletal remains in the vomit, which pointed to a predator that specialized in eating hard-shelled prey from the seafloor. After considering various marine animals, Knaust concluded that plesiosaurs were the most likely culprits. The size and composition of the vomit matched that of plesiosaurs, which were among the most successful marine reptiles in history. The study adds to our understanding of plesiosaur behavior and highlights the value of examining fossilized remains of ancient animals' meals. As Knaust notes, this is just the beginning of what can be learned from the remnants of extinct creatures' dinners.