Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 728

2. TRACES AND TRACEMAKERS OF THE REEF
2.1 Macroborings and Bioclaustrations
The ichnoguild concept of Bromley (1990, 1996) (see also the guilds for
biotaxa: Bambach, 1983 ) provides an ecological basis for categorizing hard-
substrate dwelling structures by combining the traits of ethology, vagility,
and trophic level to unite the activities of disparate phyla ( Tapanila, 2008b ).
In this framework, all hard-substrate dwelling cavities made in reef settings
are constructed by two superguilds: the heterotrophic endolithic guild, which
includes all macroborings, bioclaustrations, and a few heterotrophic microbor-
ings; and the autotrophic endolithic guild, which includes all algal and cyano-
bacterial microborings. The latter comprise a significant aspect of reef
ichnology, and they are treated thoroughly by Wisshak (2012) .
Heterotrophic endoliths either bore or embed their way into a substrate,
although a combination of processes may be used in live skeletal substrates.
A wide variety of organisms have been documented as macroborers, including
sponges, bivalve molluscs, sipunculans, polychaetes, and phoronids ( Bromley,
1978; Hutchings, 1986 ).
Themanifold ecological similarities among animals of the heterotrophic endo-
lithic guild permit some generalizations. The great majority of macroborings and
all bioclaustrations are produced for the primary function of dwelling, and are con-
structed by sessile, soft bodied, suspension feeders (predatory borings, such as
Oichnus , are important exceptions). Adoption of a lifestyle confined within a hard
substrate cavity precludes locomotion, and has minimized the need for a protec-
tive exoskeleton, thus, with the exception of the rasping shell of pholadid bivalves,
most heterotrophic endoliths have reduced, or lack, external hard parts. As
suspension feeders, environmental conditions often dictate where heterotrophic
endoliths will flourish. It is generally accepted that increased nutrients
(e.g., oligo- and eutrophic waters) benefit endolithic activity, whereas increased
siliciclastic input may inhibit some endoliths by reducing the efficiency of feeding
currents or by enhancing the burial of their substrate ( Edinger and Risk, 1994 ).
By contrast to their shallow-marine coralline substrates, heterotrophic endo-
liths are eurytopic with respect to water temperature, turbidity, and light intensity
( Cortes and Risk, 1985 ). Macroborers of dead substrates are not usually limited
by substrate type, although factors that increase exposure time of the substrate to
the water column, such as substrate height above the sea floor, often encourage
greater intensities of bioerosion ( Tapanila et al., 2004 ). Endoliths of live coralline
substrates often show some degree of host specificity, especially among embed-
ding symbionts that make bioclaustrations ( Tapanila, 2005 ).
The most recent literature reviews on the history of hard substrate tracemakers
and their trace fossils are the following: Taylor and Wilson (2003) and Bromley
(2004) for macroborings, Glaub and Vogel (2004) and Wisshak (2012) for
microborings, and Tapanila (2005, 2008a) for bioclaustrations.