Plesiosaurs had a dining problem. The long-necked marine reptiles that patrolled Jurassic seas appear to have collected shellfish from the seafloor, but their teeth were not built to crush shells, and their bodies apparently could not pass the hard leftovers through. New research published by Cambridge University Press proposes an unexpectedly unglamorous solution: the animals swam from open water into river deltas so they could throw up.
The study appears in Geological Magazine and was led by paleontologist Dr. Dirk Knaust. He identified fossilized plesiosaur vomit pellets in sediments that once formed the shallow waters of river deltas, roughly 50 kilometers (31 miles) off the coast of what is now Norway. Reported on September 22, 2026, the finding adds a behavioral dimension to a creature more often studied for its anatomy and mode of swimming.
An unglamorous but effective survival strategy
Earlier work on plesiosaur anatomy and movement suggests the iconic long-necked reptile angled its head downward to gather shellfish from the seafloor. Because the animal is thought to have lacked the dental equipment to break shells or other skeletal material, its stomach is believed to have compacted the indigestible fragments into pellets that were then expelled — a process familiar today in owls and other pellet-producing animals.
That much was already part of the picture. What Knaust's analysis adds is evidence about where the vomiting happened, and what that geography implies about the animal's behavior. If plesiosaurs were merely coughing up shells wherever they fed, the debris should turn up in open marine sediments. Instead, it was recovered from delta deposits.
The salt problem at the heart of the discovery
The shellfish Knaust identified inside the vomit needed stable, ocean-like salt levels to survive. That is a crucial detail. The deposits where the vomit was found formed in a river delta, where inflowing freshwater would have continually shifted salinity up and down. For the sensitive shellfish species preserved in the pellets, such fluctuating conditions would have been impossible to endure.
In other words, the shells could not have grown where they were ultimately dumped. They must have been eaten elsewhere — in deeper, more saline water — and carried inside a plesiosaur's gut to the delta, where the animal finally rid itself of them. That chain of reasoning is what pushed Knaust toward the conclusion that plesiosaurs deliberately swam shoreward, into estuarine, freshwater-influenced settings, in order to vomit.
Twenty lumps of 160-million-year-old vomit
The evidence rests on more than 20 regurgitalites — the technical term for clumps of fossilized vomit. The specimens range in size from about a walnut to a mango. One drill core image shows 160-million-year-old delta sand containing a clump of shells alongside plant remains, interpreted as plesiosaur vomit. The material therefore dates to the Jurassic, placing the behavior deep in the age of these marine reptiles.
Regurgitalites are not the most celebrated fossils, but they are unusually informative. A single bone can tell a paleontologist about an animal's skeleton; a preserved stomach ejection can capture a moment of behavior — what was eaten, what was rejected, and where the rejection occurred. Knaust's samples preserve both the rejected shellfish and the geological setting of the act.
Modern animals do something similar
The proposed behavior has living parallels. Fur seals and marine crocodiles are known to regurgitate indigestible material, and owls famously produce pellets of fur, bone, and other hard remnants their digestive systems cannot break down. In each case, the strategy allows an animal to exploit prey it cannot fully process, extracting nutrition and discarding the rest.
For a plesiosaur, that strategy would have opened up a rich food source on the seafloor that its teeth alone could not have handled. The cost was a commute: feeding in deeper, saltier water, then traveling into coastal deltas to disgorge the remnants.
Why this is a first
Knaust framed the significance of the work in terms of what fossil vomit research has historically covered. "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," he said. "This is the first time that plesiosaur vomit, and the shellfish in it, have been identified, and these creatures tell us a great deal."
That emphasis matters. Most regurgitalite studies reconstruct predator-prey relationships among backboned animals, tracing who ate whom. This case instead documents a marine reptile feeding on invertebrates and then exporting the waste to a completely different environment, linking two habitats through a single act of digestion.
What the finding implies
- Plesiosaurs likely fed on shellfish in deeper, higher-salinity water where those shellfish could live.
- They then moved into river deltas, where freshwater shifted salt levels, and vomited the hard remains.
- The vomit was preserved as regurgitalites in 160-million-year-old delta sand, about 50 kilometers (31 miles) off the Norwegian coast.
- The behavior mirrors that of modern fur seals and marine crocodiles, and of pellet-producing birds such as owls.
- It is the first identification of plesiosaur vomit containing shellfish, according to the study author.
The research, published by Cambridge University Press and reviewed under a fact-checked editorial process, also included an artist's impression of plesiosaurs feeding on the seafloor, credited to Lente Artemieff and produced with AI assistance.
The bigger picture
Plesiosaurs were among the most successful marine reptiles of the Mesozoic, and their long necks and four flipper-like limbs have made them a staple of paleontological study. Much of that work has focused on how they moved and how they fed. Knaust's contribution shifts attention to a messier question: what they did with the parts of their meals they could not use.
The answer, written in delta sand off Norway, appears to be that they swam to shore and got rid of them. It is not a dignified image, but it is a behavioral one — and it suggests these reptiles were navigating between open-ocean feeding grounds and coastal, brackish environments in ways that fossilized bones alone would not reveal.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
