Page 106 - Introduction to Paleobiology and The Fossil Record
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PALEOECOLOGY AND PALEOCLIMATES 93
contains more dissolved carbon dioxide although other factors such as the substrate
(CO 2) providing a means to corrode carbon- were also infl uential.
ate. At given depths in the world’s oceans, Paleontologists were slow to adopt these
carbonate material begins to dissolve at insights from modern marine biology. There
so-called compensations depths. Below the were a few pioneer studies on Carboniferous
carbonate compensation depth (CCD) the assemblages in the 1930s, but it was the classic
dissolution of calcium carbonate exceeds work by Alfred Ziegler in the 1960s that
supply and at about 4–5 km calcite is not really brought these methods to the attention
preserved. The depth is shallower for arago- of paleontologists. He identifi ed fi ve depth-
nite, with the aragonite compensation depth related, brachiopod-dominated communities
(ACD) placed at 1–2 km. Both the CCD and in the Lower Silurian rocks of Wales and the
ACD vary with latitude, being shallower at Welsh borderlands (see Chapter 12). These
higher latitudes, and both parameters have communities stretched from the intertidal
varied throughout geological time. Neverthe- zone in the east to the deep shelf and conti-
less, depth alone probably has little effect on nental slope towards the west, matching per-
biotic distribution, rather the many depth- fectly the ancient paleogeography. This whole
related factors can be used to reconstruct system has been revamped and is now more
the water depths of ancient marine widely known as the benthic assemblage zones
communities. (Fig. 4.15). These zones are defined on a wide
Finally, the state of the substrate, rates of range of faunal and sedimentological criteria
sedimentation and turbidity dramatically and may be subdivided, internally, on the
affect the distributions of benthic organisms basis of, for example, substrate type and the
(Brenchley & Pickerill 1993). Organisms have degree of turbulence (Brett et al. 1993).
complex ecological requirements, some pre-
ferring a particular grain size, a certain type Describing fossil communities
of organic material or they even respond to
chemical signals (chemotaxic). There are also Sometimes the simplest jobs are the hardest
complex taphonomic feedback processes, to do properly. For over a hundred years,
where biogenic substrates such as shell paleontologists have provided lists of species
pavements can form attachment sites for new from particular localities, but these are not
communities. In general terms, within near- helpful for ecological work unless the relative
shore environments, there is a broad correla- abundances of the different species are docu-
tion between community distribution and mented as well. We need to know which
grain size. Diversity tends to be highest in species dominate (sometimes one species
muddy sands, moderate in sandy muds, makes up more than 50% of the sample) and
low in pure sands and virtually zero in soft which are rare (i.e. less than 5% of the col-
muds. Moreover whether the sediments lection). Now it is more common to docu-
form soupy muds, loose sands, fi rmgrounds ment the absolute and relative abundance of
or hardgrounds will infl uence faunal each organism, illustrated graphically with
distributions. frequency histograms, and based on data
derived from line transects, quadrats or more
commonly now from bed-by-bed collecting or
Paleocommunities
bulk samples.
Paleocommunities are recurrent groups of Counting conventions remain a problem.
organisms related to some specifi c set of envi- With many organisms it is relatively simple to
ronmental conditions or limiting factors. calculate how many individuals were actually
Many of the concepts and techniques applied represented in a given assemblage: univalved
to marine fossil communities are based on the species (e.g. gastropods) count as one, whereas
work of biologists such as the Danish scientist twin-valved species (e.g. bivalves and brachio-
Carl Petersen, researching in the late 1800s pods) may be assessed by adding the most
and early 1900s. Petersen recognized a series common valve (right or left, dorsal or ventral)
of level-bottom benthic communities around to the number of articulated or conjoined
the Scandinavian coasts; the major control on shells. Animals that molt, such as ostracods
community distribution was water depth, and trilobites, colonial organisms and those
