Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 738

influence from macroborers, and are dominated by borings from fringe ele-
ments, the barnacles and phoronids—the latter are unknown in modern reefs,
but common off-reef. The rapid diversification of scleractinians in the Middle
Triassic establishes their future dominance as reef constructors ( Fl¨gel, 2002 ),
while the first fully endolithic bivalves (i.e., lithophagids and gastrochaenids)
adapt to a reef destroyer lifestyle ( Carter and Stanley, 2004; Fl¨gel and
Senowbari-Daryan, 2001 ). From the Late Triassic onward, bivalves become
a significant component of the macroboring community, producing trace fossils
belonging to the ichnogenus Gastrochaenolites . The few studies on Late Trias-
sic reef bioerosion show boring frequencies on a par with Ordovician-Silurian
reef values. For example, 30% of the corals and spongeliomorphids in latest Tri-
assic (Rhaetian) Austrian reefs show evidence of worm and bivalve macrobor-
ings ( Perry and Bertling, 2000; Stanton and Fl¨gel, 1989 ). A general reef trend
through the Mesozoic is a transfer of dominance from “worm” borers to bivalve
borers, although local environmental conditions may upset this trend ( Perry and
Bertling, 2000 ). Debate remains over the earliest clionaid sponge, Entobia , rec-
ognized by small, multichambered borings (Late Permian: Weidlich, 1996 ; Late
Triassic: Perry and Bertling, 2000 ; Early Jurassic: Fraser et al., 2004 ), but it is
evident that they do not become significant bioeroders until the Late Jurassic.
Jurassic reef studies more frequently comment on bioerosion as an impor-
tant and common process, with bivalve and worm borings being most abundant,
followed by sponge excavations ( Perry and Bertling, 2000 ). Warme (1977)
illustrates the great intensity of Middle Jurassic excavation of scleractinian skel-
etons by deeply penetrating (lower tier) Gastrochaenolites and shallow (upper
tier) Entobia . Grazers, including echnioids, first appear in reefs and associated
microbial crusts during the Late Jurassic ( Leinfelder, 1986 ).
Bivalves and worms continue to be the most abundant contributors to coral
reef erosion during the Early Cretaceous, although few studies are available to
draw upon ( Perry and Bertling, 2000 ). The Late Cretaceous rudist buildups, by
contrast, show increased sponge bioerosion, but the relative dominance among
bivalves, “worms”, and sponges appears to correlate with environmental factors
(shelf position and siliciclastic input) rather than an intrinsic quality of the
bioeroding community ( Perry and Bertling, 2000 ).
Following the end-Cretaceous mass extinction, trace fossils from reef envi-
ronments show the same morphologies, demonstrating the resilience and con-
servatism of the marine bioeroders. Bivalve borings continue to dominate over
those made by worms and sponges during the Paleocene-Eocene interval. But,
by the Late Oligocene and Miocene, reef bioerosion assemblages become dom-
inated by sponges over bivalves and worms. The zonation of bioerosion across
reef facies shows a greater intensity and diversity of bioeroders in back-reef and
lagoon facies and lower intensity and diversity at the reef crest and fore-reef
sites ( Edinger and Risk, 1994; Frost et al., 1983; Perry, 1996; Pleydell and
Jones, 1988 ). The composition and zonation of the reef bioeroding community
that is most prevalent in Quaternary oceans was thereby established already