Evolution of the hardground community

Hardground communities first appeared in the late Middle Cambrian but they werenot common before the Ordovician. Two factors had a major influence on the earlydevelopment of hardgrounds and resulted in abrupt and rapid increase in hard-ground area as well as in community density and diversity. The first factor was thechange from an aragonite to a calcite sea epoch; the second factor was positive feed-back between the expansion of hardgrounds and the increase in carbonate produc-tion by members of hardground communities. Stemmed echinoderms played a keyrole in the development of hardgrounds.