Geology Reference
In-Depth Information
Micro- and macroborers produce trace fossils
The traces of micro- and macroborers are known
throughout the Phanerozoic.
Microborers
at a scale < 1
m to about 100
m tun-
nel their way into the substrate using weak acids. They
include bacteria, cyanobacteria, fungi as well as fora-
minifera and bryozoans.
Macroborers
are visible with the naked eye, and have
diameters of 1 mm and greater. They use physical and
physicochemical techniques for boring.
Endolithic microborers attack hard substrates (skel-
etons, hardgrounds, carbonate grains, rocks) and pro-
duce borings that are filled with cement or sediment,
or remain open. The products of boring activities are
tunnels and caves whose shape may correspond to the
morphology of the borers, thus allowing taxonomic as-
signments of the trace fossils. Most borings excavated
by bacteria, algae or fungi are too small to be success-
fully studied in thin sections. Studies dealing with mod-
ern and ancient microborers are mainly based on SEM
investigations of artificial casts produced by filling the
boreholes with resin (Golubic et al. 1970, 1983; Pl. 52/
1-4). Some of the macroborings can be morphologi-
cally differentiated in thin sections and attributed to
systematic groups (Box 9.4).
Fig. 9.11.
Sponge borings
dominate in modern and Neogene
environments, but are already known in the Cambrian.
Sponges are capable of destroying large amounts of reef
frameworks. The
Modiola
mussel is intensively bored by a
sponge (
Cliona
) characterized by a network structure. The
borings consist of a series of galleries branching and anasto-
mosing within the substrate and communicating with the sur-
face via numerous small round pores that house the inhalant
and exhalant papillae of the sponge. The sponge tissue has
specialized etching edges responsible for boring, which re-
sults in freeing of characteristically shaped chips. These chips
with a size of 40-60
m are expelled by the oscula und car-
ried away by currents. Clionid borings preserved as trace fos-
sils are called
Entobia
. X-ray photograph. Scale is 2 cm
.
pact on the destruction of modern reef surfaces. The
first unequivocal fossil representative of the Scaridae
is known from the Middle Miocene (Bellwood and
Schultz 1991); scarids may represent 'the reefs latest
trophic innovation' (Bruggemann 1994).
Grazers, scrapers and swallowers
Bioeroding organisms include not only borers, but
also grazers, scrapers and swallowers, which have a
major impact on the bioerosion of modern shallow-
marine carbonates (Hutchins 1986).
Grazers
(e.g. echinoids, gastropods, chitons, and
fishes) are organisms feeding mainly on plants, e.g. al-
gal and microbial mats. Most grazers search for algae
in living or dead substrates. Grazing echinoids are ma-
jor eroders of modern coral reefs. Chitons, already
known from the Late Cambrian, are common grazers
of endo- and epilithic algae along carbonate coastlines
(Rasmussen and Frankenberg 1990). Carbonate re-
moval by grazers may be responsible for a high per-
centage of the total bioerosion (Chazottes et al. 1995).
Intensive microbial mat grazing started in the Late Pre-
cambrian. Grazing by conodontophorids and fishes may
have become more intensive in the Middle and Late
Cambrian.
Grazing fishes are the dominant herbivores in most
tropical environments. Herbivorous grazing fish spe-
cies are important bioeroders on tropical shallow reefs
(up to 7 kg/m
2
/year). Substrates already infested by bor-
ing algae and of low skeletal density are eroded fastest
(Bruggemann et al. 1995). Scarid parrotfish and acan-
thurid surgeonfish, feeding by scraping off algae that
grow on and in dead coral skeletons, have a major im-
Scrapers
(e.g. regular echinoids, limpids, some par-
rot fishes) use beaks or claws to scratch skeletons in
order to remove microbial nutrients or algal/microbial
cover. Regular echinoids are known since the Carbon-
iferous.
Swallowers
are detritus feeders that swallow sedi-
ment grains that are comminuted through their diges-
tive tracks, usually producing pellets (e.g. holothuri-
ans, jawed fishes). Holothurians are known since the
Middle Cambrian, jawed fishes since the Wenlockian.
9.3.1 Recent and Fossil Microborers
Microboring is not only a product of light-depen-
dent photosynthetic algae but also of heterotroph bac-
teria and fungi. Therefore, microborings have a wide
bathymetric distribution ranging from supratidal and
intertidal environments to subtidal and deeper-water
environments.
Many microborers are concentrated within shallow
subtidal and intertidal depths and exhibit distinct zona-
tion patterns. Endolithic biodegradation is common and
highly diverse in near-coastal environments and warm
