Paleontological evidence

The fossil evidence for escalatory coevolution in predators is less convincing than for prey, but it is worth looking at both.

Predators

• The earliest predatory 'molluscivores' in the fossil record were 'whole body ingestors' who simply swallowed molluscan prey whole; this is an unspecialized method of eating a mollusc and for that reason perhaps not a particularly menacing one.

• By the Devonian, specialized shell breakers have appeared - in the case of fish this can be inferred from their teeth - and by the late Devonian there were already 10 fish families of specialized shell breakers.

• Decapods, which crush shells in their claws, proliferated in the Triassic (an image of the decapod Mecochirus longimanatus along with its trail is shown opposite); gastropods that drill holes in other shelled species appear in the late Triassic; and starfishes, with their specialized suckers and pullers that enable them to prize bivalved shells apart, first appear in the Jurassic.

• By the late Mesozoic, many more shell breaking fish forms had appeared.

This narrative is consistent with an increase in the number, and specialization, of molluscivores through time; however, it is not persuasive evidence for escalation. To show escalation, we have to show the numbers of escalated predatory forms has proportionally increased. That requires looking at the evidence for the prey species.