Page 119 - Introduction to Paleobiology and The Fossil Record

106 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

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 sedi-

ments, 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 condi-
tions on the seafl oor, 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) repro-
ductive 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.

8
Modern fauna
Paleozoic fauna
4
oysters oysters other epifaunal bivalves infaunal bivalves cheilostomes
Thickness (m) 2 other mollusks

brachiopods archaeogastropods stenolaemate bryozoans brachiopods cephalopods

1

0
0 100% 0 100% 0 100%
Ord-Sil Jurassic Neogene
(n = 85) (n = 139) (n = 218)
% of shellbeds
Figure 4.22 Thicknesses of shell concentrations during the Ordovician-Silurian, Jurassic and
Neogene. Thick shell beds are a phenomenon of the Modern fauna, mainly generated by
bivalves. (From Kidwell & Brenchley 1994.)