PALEOECOLOGY AND PALEOCLIMATES 93
contains more dissolved carbon dioxide
(CO
2
) providing a means to corrode carbon-
ate. At given depths in the world’s oceans,
carbonate material begins to dissolve at
so-called compensations depths. Below the
carbonate compensation depth (CCD) the
dissolution of calcium carbonate exceeds
supply and at about 4–5 km calcite is not
preserved. The depth is shallower for arago-
nite, with the aragonite compensation depth
(ACD) placed at 1–2 km. Both the CCD and
ACD vary with latitude, being shallower at
higher latitudes, and both parameters have
varied throughout geological time. Neverthe-
less, depth alone probably has little effect on
biotic distribution, rather the many depth-
related factors can be used to reconstruct
the water depths of ancient marine
communities.
Finally, the state of the substrate, rates of
sedimentation and turbidity dramatically
affect the distributions of benthic organisms
(Brenchley & Pickerill 1993). Organisms have
complex ecological requirements, some pre-
ferring a particular grain size, a certain type
of organic material or they even respond to
chemical signals (chemotaxic). There are also
complex taphonomic feedback processes,
where biogenic substrates such as shell
pavements can form attachment sites for new
communities. In general terms, within near-
shore environments, there is a broad correla-
tion between community distribution and
grain size. Diversity tends to be highest in
muddy sands, moderate in sandy muds,
low in pure sands and virtually zero in soft
muds. Moreover whether the sediments
form soupy muds, loose sands, fi rmgrounds
or hardgrounds will infl uence faunal
distributions.
Paleocommunities
Paleocommunities are recurrent groups of
organisms related to some specifi c set of envi-
ronmental conditions or limiting factors.
Many of the concepts and techniques applied
to marine fossil communities are based on the
work of biologists such as the Danish scientist
Carl Petersen, researching in the late 1800s
and early 1900s. Petersen recognized a series
of level-bottom benthic communities around
the Scandinavian coasts; the major control on
community distribution was water depth,
although other factors such as the substrate
were also infl uential.
Paleontologists were slow to adopt these
insights from modern marine biology. There
were a few pioneer studies on Carboniferous
assemblages in the 1930s, but it was the classic
work by Alfred Ziegler in the 1960s that
really brought these methods to the attention
of paleontologists. He identifi ed fi ve depth-
related, brachiopod-dominated communities
in the Lower Silurian rocks of Wales and the
Welsh borderlands (see Chapter 12). These
communities stretched from the intertidal
zone in the east to the deep shelf and conti-
nental slope towards the west, matching per-
fectly the ancient paleogeography. This whole
system has been revamped and is now more
widely known as the benthic assemblage zones
(Fig. 4.15). These zones are defi ned on a wide
range of faunal and sedimentological criteria
and may be subdivided, internally, on the
basis of, for example, substrate type and the
degree of turbulence (Brett et al. 1993).
Describing fossil communities
Sometimes the simplest jobs are the hardest
to do properly. For over a hundred years,
paleontologists have provided lists of species
from particular localities, but these are not
helpful for ecological work unless the relative
abundances of the different species are docu-
mented as well. We need to know which
species dominate (sometimes one species
makes up more than 50% of the sample) and
which are rare (i.e. less than 5% of the col-
lection). Now it is more common to docu-
ment the absolute and relative abundance of
each organism, illustrated graphically with
frequency histograms, and based on data
derived from line transects, quadrats or more
commonly now from bed-by-bed collecting or
bulk samples.
Counting conventions remain a problem.
With many organisms it is relatively simple to
calculate how many individuals were actually
represented in a given assemblage: univalved
species (e.g. gastropods) count as one, whereas
twin-valved species (e.g. bivalves and brachio-
pods) may be assessed by adding the most
common valve (right or left, dorsal or ventral)
to the number of articulated or conjoined
shells. Animals that molt, such as ostracods
and trilobites, colonial organisms and those