Page 335 - Introduction to Paleobiology and The Fossil Record

322 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

ral zone to the edge of the continental shelf biting marked ecophenotypic variation (Box
at depths of about 200 m. Nevertheless a few 12.9).
intertidal forms are known, while some bryo- Bryozoans have successfully pursued several
zoans have been dredged from depths of over different life modes. Encrusting, erect, unat-
8 km in oceanic trenches; moreover numerous tached or rooted phenotypes all refl ect adap-
species have been recorded from the hulls of tive strategies in response to ambient
ships. Most species are sensitive to substrate environmental conditions. Shallow-water col-
types, turbulence, water depth and tempera- onies, particularly in the subtidal zone, are
ture together with salinity. The shape of colo- and were dominated by encrusting, erect,
nies can be very plastic, adapting to rooted and free-living forms. But deeper-water
environmental conditions, with erect, tree- environments, over 1 km deep, are character-
like colonies varying their branch thickness ized by mainly attached and rooted forms.
according to depth. In addition spines may be Nevertheless bryozoan colonies have occa-
induced by high current velocities or by the sionally formed reefs or bryoherms, par-
presence of predators (Taylor 2005). Bryozo- ticularly during the mid-Silurian and
ans are thus typical facies fossils exhi- Carboniferous.

Box 12.9 Bryozoans and environments

The majority of bryozoans grow as mounds, sheets or runners parallel to the substrate, many grow
erect colonies perpendicular to the seabed and some colonies are actually mobile. There have been

a number of growth–mode type classifications, some associated with particular genera, construc-
tional geometry or based on autecology. A more comprehensive way at looking at these complex
colonies is to combine attachment modes, construction orientation and the geometry of the individual
zooids (Hageman et al. 1997). Such a hierarchical growth–mode classification can be used to describe

regional biotas and predict paleoenvironments on limited datasets. However, as in many ecological
studies, the most common species or growth forms can swamp the overall ecological signal; some
form of scaling is needed. We can ask a couple of questions: How important is D at locality 1 rela-
tive to other occurrences of D and how important is D relative to all the other localities? Firstly a
simple data table is set up with growth forms along the y-axis and localities along the x-axis (see
below). One method of standardizing the data is to: (i) divide the number of growth type D at local-
ity 1 by the product of all the different growth types and the total at this one locality [10/(45 * 22)];
2
and (ii) this is then multipled by 100 to scale values to roughly between 0 and 100. This equals
101; this growth is clearly important at this locality. The relative importance of each growth form
at each locality can be plotted in a histogram.

Form A Form B Form C Form D Sum

Locality 1 20 10 5 10 45
Locality 2 20 40 10 5 75
Locality 3 20 40 40 5 105
Locality 4 20 120 60 2 202
Sum 80 210 115 22

This type of study has been expanded to an analysis of the distribution of growth forms across
the shelf-slope transition on the Lacepede Platform, southern Australia. A distinct pattern emerged
with free-living forms most important on the inner shelf and rigid cone-disk forms most important
on the deep slope (Fig. 12.20).