To regurgitate shells and other indigestible components of their seafood meals, plesiosaurs swam into river deltas.
Plesiosaurs fed on shellfish from the seabed in deeper water before swimming to the coast to regurgitate the hard, indigestible parts of their marine banquet. That is the conclusion of palaeontologist Dirk Knaust in Geological Magazine (22 September), based on detailed analyses of more than twenty lumps of fossilised vomit. The prehistoric sea reptiles were probably unable to grind shells and other skeletal material with their teeth; instead, these remains compacted in the stomach and the plesiosaur expelled them as a pellet.
Knaust identified the fossilised remains in what were once the shallow waters of a river delta, some fifty kilometres off the Norwegian coast. Based on the irregular shape and characteristic mineralogy of the lumps, the palaeontologist deduced that they represented vomit rather than faeces. Remarkably, he found within the fragments the remains of, among other things, bivalves and crinoids – animals that depend on a stable saltwater environment. In the river delta, where freshwater input caused salinity levels to fluctuate considerably and continuously, these sensitive prey animals could not possibly have survived. The animals responsible for the regurgitated remains must therefore have gathered their food elsewhere.
It was not immediately clear which animal species the regurgitators belonged to; an extensive comparative analysis followed. The predator must have been large enough to expel the lumps, which range in size from a walnut to a mango. In addition, none of the vomit fragments contained fish remains, which allowed Knaust to rule out sharks and crocodiles – which were also abundant in the area at the time but had fish as their primary dietary staple. The palaeontologist ultimately arrived at the Cryptoclididae: a family of plesiosaurs that lived from the Middle Jurassic to the Early Cretaceous, although Knaust cannot entirely exclude hybodont sharks. Their size and feeding habits make these animals the most likely candidates.
Although a great deal of research into fossilised vomit has been conducted over the past decade, this is the first time that such pellets have been attributed to plesiosaurs. Knaust suggests that the Cryptoclididae may have come close to the coast to rest. They probably regurgitated their stomach contents at that point, in a manner comparable to the behaviour seen today in fur seals and sea crocodiles.
An earlier version of this article incorrectly referred to plesiosaurs in the headline as ‘sea dinos’ and spoke of ‘dinosaur vomit’. Plesiosaurs were not dinosaurs but sea reptiles that lived during the same period. The text has been amended accordingly. Apologies for this inaccuracy.