Coevolution - What is an evolutionary 'arms race' ?
Types of shell
The escalation of molluscan prey defences is also suggested by a trend in the proportion of different types of gastropod shells through time. Shells that lie loose, we have seen, are less well defended than burrowers and attached shells, and if we look at the proportions of the different types through time, we see that the loosely attached forms have decreased. There is also limited evidence that the better defended burrowers increased from being about 5 - 10% of genera in the Late Carboniferous - Late Triassic to about 37% in the Late Cretaceous and 62 - 75% in modern formations.
This evidence would not be persuasive enough to convince a skeptic. The evidence is not abundant; it is noisy; the patterns are not all consistent; and it is difficult to prove that the fossil record is unbiased. For example, could there be some unidentified reason why shell forms with narrower apertures became relatively more likely to be preserved as time passed? However, it does then suggest that some escalation of prey defences and predatory weapons has taking place in evolution.