Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 736

3. REEF ICHNOLOGY THROUGH TIME
3.1 Fossil Reefs
Most fossil reef studies are qualitative, but some have attempted quantitative
methods to describe and contrast bioerosion with the modern record. This
section attempts to summarize key trends in fossil reef ichnology through the
Phanerozoic eras.

3.1.1 Paleozoic
Archaeocyathid calcareous sponges are the first metazoan animals to produce
reefs, and they preserve the earliest evidence of macrobioerosion in the fossil
record ( James et al., 1977 ). Early Cambrian archaeocyathan reefs from Labrador
(eastern Canada) contain the simple cylindrical boring Trypanites , which pene-
trates the skeletal fabric of the framebuilders as well as the early cemented car-
bonate muds that surround and infill the central osculum of the sponges ( Fig. 1B ).
With few exceptions, Trypanites borings are the most common Paleozoic reef
(and non-reef) hard-substrate trace fossil. After their first appearance in the Early
Cambrian, and an absence during the Middle Cambrian, Trypanites are next
recorded in Late Cambrian and Early Ordovician hardgrounds (e.g., Chow and
James, 1992 ; Rozhnov, 1994 ). By the Middle to Late Ordovician, the endolithic
habit expands in parallel with the Great Ordovician Biodiversity Event ( Webby
et al., 2004 ). Termed the Ordovician Bioerosion Revolution ( Wilson and Palmer,
2006 ), dwelling cavities made by macroborers in reef substrates increase in abun-
dance and intensity during the Middle Ordovician (e.g., Kapp, 1975 ), and by the
end of the Ordovician, the diversity of ichnotaxa includes seven distinct forms.
Despite the increased diversity in ichnogenera among various carbonate sub-
strates, reef macrobioerosion remains monotonously dominated by Trypanites .
Studies on the Late Ordovician to Early Silurian carbonate ramp preserved
on Anticosti Island (eastern Canada) exemplify the early Paleozoic pattern of
Trypanites -dominated bioerosion. Tapanila et al. (2004) examined more than
2500 coral and stromatoporoid sponge substrates from patch reef and off-reef
facies, examining boring frequency and intensity. Trypanites accounted for
>

99.9% of all macroborings, with rare off-reef occurrences of Petroxestes
pera . Although an average of 30% of reef constructors had been bored, corals
and stromatoporoids living in off-reef settings were more frequently bored
(40-50%) and with greater intensity (up to 35 Trypanites per 4 cm 2 of substrate
surface) ( Tapanila et al., 2004 ). Increased postmortem exposure time above the
sediment/water interface was determined to be the most important environmen-
tal control to promote bioerosion in a substrate. The relatively lower frequency
of borings in reef constructors, as compared to off-reef corals and stromatopor-
oids, may be due to increased competition for space in the reef from encrusters,
coupled with the flatter growth forms that dominate Anticosti's reef substrates.
Tapanila et al. (2004) found that, regardless of facies, tabular (flatter) growth