T

Early Late E.M. Ediacaran Cambrian

Late Ordovician — Geological time

Recent Neogene

Figure 4.21 Continued

During the last 600 million years the Earth has oscillated at least five times between icehouse and greenhouse conditions, spending most of the time in greenhouse climates (Fig. 4.23). For much of the Precambrian the Earth probably endured relatively cool climates. The Earth is generally divided into five climate zones: humid tropical (no winters and average temperatures above 18°C), dry subtropical (evaporation exceeds precipitation), warm temperate (mild winters), cool temperate (severe winters) and polar (no summers and temperatures below 10°C). But can these zones be recognized through deep time and be

Box 4.6 Shell concentrations

Shell concentrations of various types can tell us a huge amount about environments of deposition but also can act as a proxy for biological productivity through time (Kidwell & Brenchley 1994). Moreover, it is possible that evolutionary changes in the diversity and ecology of organisms that produce and destroy calcareous skeletons suggest that the nature of these concentrations may have changed through the Phanerozoic. Data from marine siliciclastic rocks, silicate-based clastic sediments, of Ordovician-Silurian, Jurassic and Neogene ages show a significant increase in the thickness of densely packed bioclastic concentrations, from thin-bedded brachiopod-dominated concentrations in the Ordovician-Silurian to a mollusk-dominated record with more numerous and thicker shell beds in the Neogene (Fig. 4.22). Jurassic shell beds vary in thickness depending on whether they have Paleozoic or modern affinities as the main components. This suggests that the Phanerozoic increase in shell-bed thickness was not controlled by diagenesis or by a shift in taphonomic conditions on the seafloor, but rather by the evolution of biogenic clast producers, themselves - i.e. groups with, firstly, more durable low-organic skeletons, secondly, greater ecological success in high-energy environments, and thirdly higher rates of carbonate production. These results indicate that (i) reproductive and metabolic output has increased in benthic communities over time; and (ii) the scale of time averaging in benthic assemblages has increased owing to greater hard-part durability of modern groups. New data, however, suggest that brachiopods were probably just as durable as mollusks, but their communities simply did not produce so many shells. The frequency and thickness of shell beds through time may simply be down to the relative biological productivity of different groups of organisms.

Modern fauna I Paleozoic fauna

0 0

Post a comment