A Jurassic shell clump may preserve a predator’s rejected meal

A meal may leave a fossil even when the diner does not. In rock cores drilled offshore Norway, geologist Dirk Knaust found crowded clusters of Jurassic shells that he interprets as possible remains brought back up by a predator. The difficulty is that moving water and burrowing animals can gather shells, too. To make the case, he compares three arrangements preserved in the rock.

One figure in Knaust’s study shows what each explanation looks like:

• A layer: Mostly complete bivalve shells lie in a sandy bed, many curved side up. Their surfaces are only slightly worn. Knaust interprets the layer as material gathered by moving water.
• A tube: Shell and plant fragments fill the outline of a burrow. The passage gives the debris its shape; tight packing alone would not make it evidence of a meal.
• A rounded clump: Roughly 10 centimetres wide and 15 centimetres long, the illustrated cluster contains whole and broken bivalve shells, the hard tubes made by marine worms, plant fragments and a belemnite fossil. Some shells nest inside others. Their surfaces are irregular and etched, and packed remains partly mark out zones around the clump’s edge.

The third pattern is the puzzle. Its mixed contents are confined to a small patch rather than spread through a bed, yet they do not form an obvious burrow-shaped tube. Knaust argues that the clump’s outline, nested and broken pieces, and altered surfaces together fit hard material swallowed and later regurgitated by an animal. No single clue proves that account: shells can break or change after burial, and the burrow in the same figure shows how another process can pack fragments closely. Mineral cement also appears in these rocks; it helped make the illustrated burrow fill visible and affected the clump’s matrix, so cement by itself cannot identify a rejected meal.

Knaust examined about 20–30 such clumps in cores from several Jurassic formations. He cannot give an exact count because some have gradually disintegrated. The deposits were busy places where sediment accumulated and animals disturbed it. Burrowing, reworking and later changes could blur a clump’s original outline. A core offers a valuable view through the rock, but only part of a once larger deposit.

The setting raises a second question: how did the remains get there? Many clumps occur in deposits associated with a delta front, where incoming freshwater changed salinity. Yet they include remains of marine organisms that needed steadier saltwater conditions. The Norwegian Offshore Directorate independently describes the Sognefjord Formation, one of the formations Knaust studied, as coastal and shallow marine. Knaust proposes that a predator fed farther offshore and released indigestible remains nearer the delta. The clumps cannot establish where it fed or trace how the remains moved afterward.

Nor can they name the animal. Knaust considers long-necked plesiosaurs called cryptoclidids the likeliest producers, based on his assessment of feeding anatomy and which animals are known from the basin. He says hybodont sharks remain possible. No predator is preserved with a clump.

That leaves a useful, bounded discovery: the arrangement of fossils may record an act of feeding that an isolated shell could not reveal. The layered bed, filled burrow and rounded mixture show why reading that behavior from rock requires comparing several explanations—and why the identity and route of the suspected diner remain open.