Ancient plesiosaurs swam to shore to regurgitate indigestible prey, fossil study reveals

A paleontologist working through drill cores extracted from the North Sea has identified something that changes how we understand what plesiosaurs actually did when they weren't hunting. Dr. Dirk Knaust, a specialist in fossil traces left by invertebrates, found more than twenty fossilized vomit pellets in sediment that would have formed in shallow river deltas off the Norwegian coast roughly 50 kilometers from open water. The pellets ranged in size from a walnut to a mango, and they contained the hard remains of creatures that should never have been there.

The puzzle began with the shellfish themselves. Bivalves, serpulid tubes built by filter-feeding worms, crinoids anchored to the seafloor, and the hard parts of squid-like cephalopods—all of these require the steady salt levels of open ocean to survive. Yet Knaust found them in a river delta, a place where freshwater constantly flowed in and disrupted the salinity. These creatures could not have lived there. So how did their remains end up fossilized in that location? The answer emerged from careful analysis: a large marine animal had eaten them from the seafloor in deeper water, then swam into the shallows to vomit them up.

Knaust spent three decades analyzing drill core samples from North Sea wells operated by the Norwegian energy company Equinor, work that gave him access to thousands of meters of subsurface rock. The vomit fossils came largely from cores extracted during the development of a subsea storage site for carbon dioxide near the Troll gas and oil field. What he was looking at were regurgitalites—compacted clumps of fossilized vomit that had survived because they were tightly packed and coated in mucus. Most such pellets would have been torn apart by waves and tides, but some were quickly buried by sediment flowing from rivers, and others settled into burrows left by crustaceans on the seafloor, where they fossilized intact.

Identifying the culprit required elimination. Knaust first considered invertebrates like cephalopods, but the size and composition didn't match. Fish vomit was too small. Sharks and crocodiles were abundant in the area and large enough, but they hunted fish, not hard-shelled prey from the seafloor—and Knaust found no fish skeletons in the vomit. The profile pointed to plesiosaurs: long-necked marine reptiles that could grow from two to thirteen meters in length and were among the most successful marine animals ever to exist. The cryptoclidid plesiosaurs, which Knaust believes produced the vomit, measured four to eight meters and lived between roughly 174 and 100.5 million years ago. Their teeth could not crush shells or skeletal material, so their stomachs compacted these hard pieces into pellets to be regurgitated, much as owls do today.

The behavior itself resembles what modern fur seals and marine crocodiles do, though with a crucial difference. Those animals haul out onto land to rest and digest. Plesiosaurs, constrained by their weight and body shape, likely remained in shallow water when they came close to shore. Knaust suggests they may have approached these river deltas to relax or sleep after hunting, and that is when they vomited. The discovery adds texture to what we know about these animals—not just their anatomy and movement, but their actual daily rhythms and the places they sought out.

In one core, just centimeters from the vomit, Knaust found a bone and fossilized feces. A CT scan revealed the bone belonged to an ichthyosaur, a marine reptile unrelated to plesiosaurs. The feces, he determined, likely came from a crocodile. But the vomit itself—distinguished by its higher proportion of undigested food, its scattered irregular shape, and its distinctive mineral composition—belonged to the plesiosaur. Knaust could make this distinction because he compared his findings to vomit remains studied over the past century across Europe and North America.

Plesiosaur skeletons have been found worldwide, but the United Kingdom is particularly rich in them. Knaust, who searches for fossils on England's Jurassic Coast, notes that in Jurassic times, the well site in Norway and the coastal cliffs of southern England were part of the same narrow sea, similar to the North Sea today. This suggests the behavior he has documented in Norwegian sediments may have occurred along many coastlines. The research, published in Geological Magazine by Cambridge University Press, opens a new window into how these animals lived—not just what they ate, but where they went to process their meals and what that tells us about their patterns of movement and rest.