Page 107 - Introduction to Paleobiology and The Fossil Record

94 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

Benthic Wave
assemblage base Storm evidence Photic zone evidence
BA–1
wave ripples amalgamated beds stromatolites

BA–2 thrombolites

gutter casts winnowed coquinites hummocky cross stratification cyclocrinitids (large) green algal borings blue-green algal borings trilobites with well developed eyes
BA–3 tabulate-stromatolite reefs

BA–4 mud tempestites small bioherms (small) receptaculitids

BA–5
?

BA–6

Figure 4.15 Silurian marine benthic assemblage zones and identifying criteria. (From Brenchley &
Harper 1998.)

that easily fragment (e.g. bryozoans) and ecospace, and they may be dominated by one
those with multi-element skeletons (e.g. cri- or two very abundant opportunistic species,
noids and vertebrates) require more special- in contrast to long-established and rather
ized counting techniques. These basic data are stable equilibrium communities where rela-
then transformed into a more realistic picture tively high diversities of more or less equally
of ancient communities populating past land- abundant animals are present. The ecological
scapes and seascapes, through histograms and relationships between organisms is also an
pie charts. Raw numerical data are extremely important aspect of community development
useful, but these can also be converted to (Box 4.3).
diversity, dominance and evenness parame-
ters, and parameters for taxonomic distinc- Paleocommunity development through time
tiveness (Box 4.2). These together can give
us a rich overview of the composition and Communities undoubtedly change with time.
structure of the paleocommunity and allow Factors such as environmental fl uctuation,
numerical comparisons with other similar immigration and emigration of animals and
assemblages. Such approaches have become plants, evolution and extinction of species
routine in studies of invertebrate paleoecology and coevolutionary changes will alter the

but it is much more difficult to apply these composition and structure of a community.
methods to vertebrate assemblages where But are the components of communities tightly
sample sizes are generally much smaller. linked and thus evolve together or is it a rather
Detailed analysis of paleocommunity struc- haphazard random process? Living communi-

tures has permitted recognition of a number ties, when first established, show initial high

of specific community types. Pioneer commu- rates of replacement and instability, whereas
nities are those that have just entered new later stages are more stable with little change,