Jurassic plesiosaurs likely swam into river deltas to vomit indigestible shells, study suggests

Plesiosaurs swam into river deltas to vomit up shells and other indigestible elements of their marine meals, suggests new research published by Cambridge University Press. The behavior resembles that of today's fur seals and marine crocodiles.

Previous research into plesiosaur anatomy and movement suggests the iconic long-necked marine reptile angled its head down to collect shellfish from the seafloor. The animal couldn't crush shells or other skeletal material with its teeth, it is thought, so its stomach compacted it to be vomited as pellets, as animals including owls do today.

In a study published in Geological Magazine, paleontologist Dr. Dirk Knaust offers new insights into the behavior of these successful sea monsters. Knaust identified the fossilized remains of plesiosaur vomit pellets in what would have been the shallow waters of river deltas, 50 kilometers (31 miles) off the Norwegian coast.

The shellfish, which Knaust identified in the vomit, needed the kind of steady salt levels found in the open ocean to survive. But Knaust discovered that the vomit was deposited in a river delta in which incoming freshwater would have continually changed the salt levels, making it impossible for these sensitive creatures to live there. So how did their remains get into the delta?

Knaust's detailed analysis of more than 20 regurgitalites—clumps of fossilized vomit ranging in size from a walnut to a mango—led him to the conclusion that plesiosaurs must have eaten the shellfish from the seafloor in deeper water before swimming to shore to vomit up their hard, indigestible parts.

Knaust said, "There has been a lot of research into fossilized vomit over the last decade, but most of it has focused on vertebrates eating other vertebrates. This is the first time that plesiosaur vomit, and the shellfish in it, have been identified, and these creatures tell us a great deal."

Over the last three decades, Knaust—a specialist in the fossil traces left by invertebrates—has analyzed several thousand meters of drill core samples taken from North Sea wells operated by his employer, the Norwegian energy company Equinor ASA. The vomit fossils he studied mainly come from the development of a subsea storage site for CO2 in the vicinity of the Troll gas and oil field.

"Plesiosaurs may have come close to shore to relax or sleep after hunting, and that's likely when they vomited," Knaust said. "We see fur seals and marine crocodiles doing something similar today, but while they go onto land, plesiosaurs probably remained submerged due to their weight and shape."

While dangers lurked in the deep, dinosaurs—two-legged predators and long-necked herbivores—roamed the delta lands nearby. Knaust and his colleagues have identified parts of their footprints in some of the extracted cores.

Knaust said, "This vomit survived partly because it was so compacted and covered in mucus. Even so, most pellets would have been completely broken up by the waves and tides. Some clusters were, however, quickly covered by sediment arriving from the rivers, and some ended up in burrows left by crustaceans on the seafloor, where they were partially preserved and fossilized."

Just centimeters away from the fossilized vomit in one core, Knaust was excited to find a bone and coprolites (fossilized feces).

"This was quite spectacular," Knaust said. "To begin with, I thought it was a dinosaur bone, but when we looked at the CT scan, we realized, no, it must be from an ichthyosaur—a group of marine reptiles similar to but unrelated to dolphins. So then I thought the vomit and feces might have belonged to an ichthyosaur, but it wasn't that simple. There were many other marine animals to consider, and the composition of their vomit differs."

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Knaust could distinguish vomit from feces due to the higher amount of undigested food remains in the vomit specimens, their scattered and irregular shape, and their distinctive mineralogy. Throughout his research, he compared the fossils to previously studied vomit remains discovered over the last century in many parts of Europe and North America.

Knaust identified bivalve shells, serpulid shells (hard, tube-dwellings built by filter-feeding worms), crinoids (invertebrates that attached themselves to the seafloor by a stalk), and belemnites (the hard parts of squid-like cephalopods). This mix pointed to an animal specialized to eat hard-shelled and exoskeletal prey on the seafloor. The culprit also had to be large enough to allow it to vomit clumps up to 10 centimeters (4 inches) or more in size.

Knaust started by considering invertebrates including cephalopods (a group including squid, octopus and cuttlefish), but the size and composition of the vomit didn't match. Moving onto vertebrates, he found that fish vomit was too small. Higher up the food chain, he assessed more likely suspects, including sharks and crocodiles, which were abundant in the area.

Knaust thinks one of the coprolites (feces) found near the vomit and ichthyosaur bone came from a crocodile. However, Jurassic sharks and crocodiles hunted fish rather than eating shellfish from the seafloor, and Knaust didn't find any evidence of fish skeletons in the fossilized vomit.

This led Knaust to his prime suspect. "The plesiosaur most closely matches the size and feeding profile of the vomit producer," he said.

Plesiosaurs could measure anything from 2 to 13 meters (6.6 to 43 feet) in length and were some of the most successful marine reptiles to have ever lived. They survived for more than 140 million years through multiple extinction events. The cryptoclidid plesiosaurs, which Knaust holds responsible for the vomit, grew 4 to 8 meters (13 to 26 feet) long and lived from the Middle Jurassic to the Early Cretaceous (approximately 174–100.5 million years ago).

Plesiosaur skeletons have been found all over the world, but the United Kingdom is a particular hotspot. Knaust, who loves searching for fossils on England's Jurassic Coast, thinks that plesiosaurs could have displayed this behavior around Britain, as well as many other coastlines. In Jurassic time, the location of the well where the fossils were collected and the outcrops of rock along coastal cliffs in southern England belonged to the same narrow sea, similar to the North Sea today.

"The plesiosaur is such a fascinating animal," Knaust said. "These findings add to other kinds of evidence that help us to understand their abilities and behavior. And this is still just the beginning. There's so much more we can learn from picking through what extinct animals threw up after dinner."

The fate of supposed plesiosaur vomit from Jurassic delta deposits offshore Norway, Geological Magazine (2026). DOI: 10.1017/S0016756826100806

Provided by Cambridge University Press

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