Cambrian to Cretaceous changes in hardground communities

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Abstract
The changing nature of the communities of boring and encrusting taxa found on upward-facing hardgrounds was studied from the standpoints of diversity, faunal composition, and nature of the niches occupied. After a rapid initial increase in the early Paleozoic, diversity remained at much the same level from the Middle Ordovician until the Late Cretaceous. There is a considerable turnover in the identity of the individual taxa between successive sample intervals. The incoming and outgoing of the major groups parallel their fortunes in the marine realm as a whole. Niche analysis suggests that the same feeding levels are occupied for most of the history of hardground communities, but Mesozoic faunas contain a much higher proportion of species with true exoskeletons, or which lived infaunally. The evolution of these forms was probably influenzed by the Mesozoic radiation of marine predators and duriphages, but it also resulted in Mesozoic hardground faunas being more resistant than their Paleozoic counterparts to episodic corrosion. Resulting higher population densities in the Mesozoic were probably one reason why cavity faunas beneath some of these hardground surfaces are more diverse than those beneath Paleozoic examples.