Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 731

Most ancient bioclaustrations are thought to be made by polychaete worms, as
are many modern equivalents, for example, Serpulidae and Sabellariidae ( Nishi
and Nishihira, 1999 ). Much of the fossil literature on symbiotic bioclaustrations
is confined to coral- and sponge-hosted associations. The earliest known coral-
hosted bioclaustration, Chaetosalpinx , is also the most pervasive ichnogenus of
the Paleozoic. Chaetosalpinx is a straight cavity formed by the interaction of a
symbiont residing for up to 5 years in the skeleton of Columnopora tabulate coral
hosts from Late Ordovician patch reefs in eastern Canada ( Tapanila, 2002 ). Heli-
cosalpinx is a spiraled tube in tabulate corals and stromatoporoids frombiostromes
of the Late Ordovician to Late Devonian ( Tapanila, 2004 ). Hicetes is a complex
spiraled U-shaped bioclaustration associated with Devonian Pleurodictyum tabu-
late corals frommuddy-bottom facies ( Brett and Cottrell, 1982 ). Bioclaustrations
show strong host specificity—from the family to genus level—and their strati-
graphic record mirrors that of their preferred host clade ( Tapanila, 2004, 2008a ).
2.1.2 Bivalves
Within the bivalve molluscs, boring species are found in several families on mod-
ern reefs. The families Petricolidae, Pholadidae, and Clavagellidae are represented
by only a few species and genera, but by far most species boring into dead coral
belong to the Lithophaginae and the Gastrochaenidae. Species restricted to boring
into live coral belong to theMytilidae and some genera of the Lithophaginae. Some
speciesof Lithophaga can live in several coral species,whereas others are restricted
to a single species. Host specificity among bivalve borers may indicate how they
evolvedwith respect to their host-coral substrates. For example, the composition of
corals found in the Atlantic and Indo-Pacific differ with only 7% generic overlap
and none at the specific level. The less specialized, dead coral-boring bivalves that
occur in several species are restricted to the coral genera that have wide geograph-
ical distributions, whereas the boring species that are restricted to live coral are
found only in coral species that have limited distributions. This suggests that boring
bivalves evolved as the corals themselves diversified ( Morton, 1990; Rosen, 1984 ).
In addition to bivalvemolluscs, there are some species ofmodern gastropods (all in
the family Coralliophilidae) that can bore into living corals ( Soliman, 1969 ),
although fossil gastropod borings have yet to be found.
Petroxestes pera is a slot-shaped boring with a wide aperture found in Ordo-
vician and Silurian stromatoporoids and hardgrounds ( Tapanila and Copper,
off-reef facies, Early Silurian, Go´land Member, Jupiter Formation, Anticosti Island, Qu´bec,
Canada. Width of image is 8 cm. (D) Gastrochaenolites oriented parallel to scleractinian corallites,
Oligocene, Lares Quarry, Puerto Rico. Width of image is 9 cm. (E) Multichambered Entobia
retiformis in reef rubble; Miocene, Rio Guajataca locality, Puerto Rico. Width of image is
1.7 cm. (F) Entobia in rudist bivalve, Cretaceous, Huqf area, Oman. Width of image is 7 cm.
(G) Large chambers of Entobia devonica in a stromatoporoid, Devonian, Iowa. Width of image
is 8 cm. (H) Bryozoan borings in scleractinian coral, Miocene, Rio Guajataca locality, Puerto Rico.
Width of image 5 cm. (Photos: L. Tapanila, except E, which was provided by D. Knaust.)