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The calcareous skeletons of modern bryozoans are major contributors to sediments only outside the tropics, and many temperate limestones are dominated by bryozoans, often in conjunction with mollusks in the so-called 'bryomol' grain association. Can the presence of rock-forming abundances of bryozoans be used to recognize nontropical sediments in the geologic past? We have addressed this question by assembling a database of 176 Ordovician-Pleistocene bryozoan-rich deposits and plotting them on paleogeographical maps to obtain their latitudes of formation. The results reveal a striking difference between post-Paleozoic and Paleozoic latitudinal distributions. Whereas the great majority (94%) of post-Paleozoic bryozoan-rich deposits formed outside the tropics, Paleozoic bryozoan-rich deposits have a more variable latitudinal distribution, most (68%) having formed within the tropics. The presence of rock-forming abundances of bryozoans in Paleozoic sediments should not, therefore, be used to infer nontropical carbonate deposition. The change in latitudinal distribution between the Paleozoic and post-Paleozoic corresponds with a major taxonomic turnover in bryozoan faunas. Biotic interactions, especially with predators, may have led to the exclusion of sediment-producing bryozoans from the post-Paleozoic tropics. These evolutionary changes have had a more profound effect on carbonate platforms than has been previously appreciated.

The Middle Cambrian Burgess Shale was deposited under variable levels of oxygenation. During periods of oxygenation, low-diversity shell beds were formed and the muds were colonized by infauna. Under these circumstances pyrite was restricted to anoxic microenvironments and formed small (1 mm) discrete aggregated masses. Soft-bodied organisms were preserved under low-oxygen conditions typified by evenly dispersed pyrite, and an absence of trace fossils and shell beds.

Preservation of soft-bodied fossil biotas (Konservat-Lagerstatten) that preserve traces of volatile nonmineralized tissues (readily degraded by bacteria) are not evenly spaced through geologic time. When compared to outcrop area, exceptional faunas appear to be over-represented in the Cambrian and Jurassic. These concentrations n time correspond to particular environments, indicating that controls on the distribution of exceptional faunas may have operated on a global scale. The reduction in the number of exceptional faunas after the Cambrian may reflect the evolution and diversification of deep bio-turbators. Specific conditions favoring stagnation and episodic burial were required to ensure preservation in younger rocks.

Species diversity varies with latitude. Almost all marine sedimentary rock in the Paleozoic of Europe and North America was deposited in the tropics, whereas only 24% was deposited in these latitudes during Mesozoic and Tertiary time. A new plot of species diversity through the Phanerozoic takes into account the paleolatitudinal distribution of marine sedimentary rock. This plot indicates that the Permian witnessed the acme of species diversity prior to the Cretaceous, and that the Triassic recovery following the end-Permian extinction was much less pronounced than suggested by previous studies.

Geologic evidence based on the preservation of soft-bodied fossil biotas suggests that deep bioturbators radiated from onshore to offshore habitats through the early Palezoic. Its also indicates that deep burrowers only became significant during the Carboniferous and that during the Cambrian there was a general lack of bioturbators in deep-water habitats. However, insufficient sampling of early Paleozoic deep-water sequences has led to a possible miscomprehension of the abundance and diversity of ichnotaxa and extent of sediment reworking.