Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 733

Small-chambered Entobia fossils ( Fig. 1E and F ) are frequently considered the
work of clionaid sponges (e.g., E. cretacea , E. ovula, E. retiformis, and E. laquea ).
Borings resembling Entobia have been ascribed to sponge activity several times
from Paleozoic rocks, from Cambrian archaeocyathid reefs ( Kobluk, 1981 )to
Ordovician trilobite carapaces ( Lindstr¨m, 1979 ). Despite their presence, small-
chambered Entobia are either underreported or rare until the Jurassic, when they
became a standard component of the bioeroding community ( Bromley, 2004 ).
Large-chambered Entobia devonica ( Fig. 1G ), with a structure analogous to
cavities formed by Aka , first appear in Devonian reef and peri-reef facies, but
disappear following the Frasnian ( Sch¨nberg and Tapanila, 2006b; Tapanila,
2006 ). The large-chambered entobians return by the Early Cretaceous and
are part of modern bioerosion assemblages (e.g., E. solaris and E. gigantea :
Bromley and D'Alessandro, 1984; Mikul ´ˇ , 1992 ).
2.1.4 Barnacles and Bryozoans
Twenty-four genera and 102 nominal species of barnacles have been recorded
as boring and embedding into live coral, of which 67 species are extant ( Ross
and Newman, 2002 ), and traditionally they have been placed within the Pyrgo-
matidae. A recent molecular study by Simon-Blecher et al. (2007) has suggested
that they are not a monophyletic group and at least one genus is far closer to
free-living barnacles, suggesting that the ability to bore and embed into coral
has evolved more than once in the group.
The ancient record of acrothoracican barnacles starts at least by Devonian
time, and may extend back to the Silurian ( Bromley, 2004 ). Rogerella , a small
pouch-shaped boring with a slit-like aperture, is the most commonly referenced
ichnogenus. In fossil reef substrates, barnacle borings are usually less abundant
and cause less damage than sponge, worm, and bivalve excavators, but impor-
tant exceptions may occur in reefs from deeper or cooler water settings ( Perry
and Bertling, 2000 ).
Borings by ctenostome bryozoans ( Fig. 1H ) are known from Late Ordovi-
cian to modern carbonate substrates and consist of elongate cavities connected
by narrow canals, which correspond to zooids and stolons (e.g., Ropalonaria :
Pohowsky, 1978 ). Cheilostome bryozoans produce shallow etchings and pits
on carbonate substrates, and are recognized from as early as the Early Creta-
ceous (e.g., Leptichnus : Taylor et al., 1999 ).
2.2 The Grazers
External erosion or grazing also occurs on both live and dead coral but by
different organisms. Scarids or parrotfish, now regarded as belonging to the
Labridae family ( Cowman et al., 2009 ), are important grazers on modern-
day reefs ( Fig. 2C ). Recent studies by Bellwood (2003) have shown that they
can be divided based on jaw morphology into excavators that remove pieces