Evidence for escalation?

Evidence for evolutionary escalation has particularly been collected by Vermeij. He has found two kinds of evidence, from biogeography and paleontology. The evidence comes mainly from molluscs (pictured opposite) in shallow-water marine environments: the molluscs are abundant as fossils; and the nature of a shell itself can reveal how strongly adapted a species was, as a predator or a prey.

Characters of strongly defended shells:

• general thickening, or thickening concentrated around their apertures;

• burrowing or cementing themselves down.

The degree of escalation in a habitat can be estimated by the proportions of more and less powerfully defended prey, and equivalent measures of the power of predators.