Fossil vomit pellets in a Norwegian delta hold shells that needed open-ocean salt

Two things can carry a full-marine shell into brackish delta mud: water, or an animal. Storm currents and waves move shells routinely, so the packaging is what Knaust leans on. The shells arrive in clumps that were squeezed together and coated before burial.

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." Some of the same cores contain parts of dinosaur footprints, from two-legged predators and long-necked herbivores, so land was close by. phys.org's report describes the fossils as Jurassic.

Assigning a pellet to a producer is harder than finding one. Centimetres from the vomit in one core, Knaust found a bone and coprolites, and a CT scan indicated the bone came from an ichthyosaur. "So then I thought the vomit and feces might have belonged to an ichthyosaur, but it wasn't that simple," he said. "There were many other marine animals to consider, and the composition of their vomit differs." He separated vomit from feces on three grounds: more undigested food remains, a scattered and irregular shape, and a distinctive mineralogy, checked against European and North American material described over the past century. The contents, four invertebrate groups with hard skeletons, point to an animal specialized on hard-shelled prey. The plesiosaur attribution then rests on earlier anatomical work, which has the animal angling its head down to take shellfish off the seafloor, unable to crush shell with its teeth, compacting the debris in its stomach the way an owl does.

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 three decades Knaust has worked through several thousand metres of core from North Sea wells operated by Equinor, where he is employed, and this study reports more than 20 pellets. Read "several thousand" at its lowest, 2,000 metres, and the recovery is roughly one pellet per 100 metres of core. The holes were drilled where a subsea CO2 store and a gas field are.

The pellets do not settle whether the inshore trip was the point. Knaust said, "Plesiosaurs may have come close to shore to relax or sleep after hunting, and that's likely when they vomited." On the modern comparison he was explicit: "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."