Geology Reference
In-Depth Information
abundant in Mesozoic and Cenozoic limestones. The
oldest representatives occur in Early Cambrian reefs
(James and Debrenne 1980; Rowland and Gangloff
1988). Many fossil records are attributed to marine
annelid worms, specifically to polychaete worms, which
include several rock- and wood-boring families.
In thin sections borings attributed to polychaete an-
nelids exhibit circular or thumb-shaped cross sections.
The boring extends as a long cylindrical tube from a
single entrance.
9.3.3 Micro- and Macroboring through Time
Major boring organisms have very long ranges (Fig.
9.14), but the contribution of the groups varied strongly
during the Phanerozoic. Older bioerosion processes that
can be qualitatively and quantitatively compared with
modern processes are relatively young and have been
effective since the Late Tertiary. It has been suggested
that bioerosion, predation and grazing features were
only fully present in modern Mesozoic-Cenozoic reefs
(Wood 1998, 1999).
The biological destruction of recent coral reefs is an
impressive example of the overwhelming role of or-
ganisms in degradation and recycling of carbonate sedi-
ments (Bromley 1978; Hutchings 1986). Although micro-
borers have been known since the Precambrian and
macroborers since the Cambrian, processes comparable
in quality and quantity with the biological destruction
in modern reefs seem to have developed relatively late
in the Phanerozoic and did not reach their full intensity
in reefs before the Late Tertiary (Fig. 9.15). Examples,
indicating strong bioerosion are known however, from
Early Paleozoic stromatoporoid reefs (Nield 1984),
from Triassic coral reefs (Stanton and Flügel 1989) and
from Jurassic sponge and coral reefs (Pisera 1987;
Reitner and Keupp 1991; Flügel 1993).
A common trace fossil often (but not unequivocally)
interpreted as borings of polychaete worms is
Trypanites
,
first described from the German Triassic, and now
known from the Early Cambrian to the recent. These
borings may be observed in thin sections of reef-build-
ing organisms (e.g. stromatoporoids, rugose corals,
bryozoans; Pemberton et al. 1988) and reef sediments
as well as in hardgrounds (Palmer 1982) and at discon-
tinuity surfaces (Pemberton et al. 1980).
Trypanites
is
characterized by simple unbranched vertical to sinu-
ous borings with a single opening to the surface, and
with or without a flared entrance. The diameter (< 1 to
about 2 mm) is more or less the same throughout the
entire length of the boring which varies between a few
millimeters to more than 20 millimeters. Geopetal sedi-
ment fillings within the boring may bear testimony to a
reorientation of the bored fossils.
9.3.3.1 Qualitative Changes in Micro and
Macroborer Groups
Cirripedians:
Some ascothoracican Cirripedia living
in shallow seas create cavities in skeletons (e.g. corals,
bivalves, echinoids) and various carbonate substrates
including hardgrounds. Records of Paleozoic cirripe-
dian borings (Webb 1987) are rare as compared with
the Mesozoic (Fig. 9.13; Fürsich and Wendt 1977) and
the Cenozoic. The millimeter-sized borings are shoe-
or sack-like with a narrow opening at a neck to the ex-
terior. A slit-like extension at the anterior end gives the
aperture the shape of a comma.
Microborers were important in Paleozoic and post-
Paleozoic times, but the impact and the composition of
macroborer communities seem to have increased dur-
ing the Mesozoic and the Cenozoic, as exemplified by
Fig. 9.15. The oldest macroborers in the Early Cam-
brian and Ordovician occurred in reef environments.
In contrast, the most intensive boring in the Silurian
took place not in reefs but in deeper-water muddy ramp
settings. Copper (2002) discussed the possibility that
boring developed in deeper shelf settings before invad-
ing shallow reef platforms. Macroboring occurred in
Early and Middle Devonian reefs, but is only poorly
documented in the Famennian and was not a dominant
process in Carboniferous and Early Permian reefs
(Webb 2002; Wahlman 2002). The rate of the destruc-
tion of Late Permian reefs by borers and grazers was
low, because free substrates were rapidly occupied by
encrusting organisms. Macroborers (predominantly
sponges) were more important than microborers
(Weidlich 2002). Interestingly, the composition of
microborers did not change from the Permian to the
Triassic, despite significant changes in the reefal host
Phoronids and sipunculids:
Less common macro-
borer trace fossils are interpreted as being produced by
phoronids and sipunculids.
Phoronid
borings
have been
described from the German Triassic Muschelkalk and
from the Late Cretaceous chalk. The borings correspond
to small irregularly winding tubes with diameters less
than 0.5 mm. Modern
sipunculid
borings
are common
in coastal rocks and reef limestones. They appear as
clubs in thin sections (Rice and Macintyre 1972). If all
or some
Trypanites
borings were attributed to spirun-
culids instead of polychaete worms, the stratigraphic
record would range from the Cambrian to the Quater-
nary.
