Hardground communities first appeared in the late Middle Cambrian but they were not common before the Ordovician. Two factors had a major influence on the early development 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 the change from an aragonite to a calcite sea epoch; the second factor was positive feedback between the expansion of hardgrounds and the increase in carbonate production by members of hardground communities. Stemmed echinoderms played a key role in the development of hardgrounds.
HARDGROUNDS, areas of synsedimentarily lithified carbonate sea floor, occurred for the first time in the late Middle Cambrian and were widely distributed in the Ordovician. The time of their occurrence and wide distribution coincided with the Ordovician radiation of marine biota, which resulted in the replacement of the Cambrian Evolutionary Fauna by the Paleozoic Evolutionary Fauna (Sepkoski 1979,1981,1984) that was to dominate the remainder of the Paleozoic. A significant increase in biodiversity was connected with this radiation.
The lack of appearance of new taxa of rank higher than class and subphylum, apart from the Bryozoa, was characteristic of the Ordovician radiation. In comparison, the previous major radiation, during the Precambrian-Cambrian interval, led to the formation of new phyla and subphyla. After the Permian extinction, no new taxa above subclass, and generally not higher than ordinal rank, arose. New taxa of marine biota at the Cretaceous-Tertiary boundary did not exceed superfamilial and subordinal rank (Valentine 1992) (figure 11.1).
The Cambrian-Ordovician transition is the most interesting interval for the study of the evolution of higher taxa of marine biota. One of the major radiations at high taxonomic level in the history of the marine fauna took place at this time. Because ancestors of many Ordovician organisms already had skeletons in the Cambrian, it is possible to study the Ordovician radiation. We can thus compare these two consec-
PHYLA AND CLASSES AND SUBCLASSES ORDERS AND
CLASSES SUBCLASSES AND ORDERS SUBORDERS
PRECAMBRIAN/ CAMBRIAN/ PERMIAN/ CRETACEOUS / TERTIARY CAMBRIAN ORDOVICIAN TRIASSIC
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