Reefs and Mounds - Trace Fossils as Indicators of Sedimentary Environments - page 732

2002; Wilson and Palmer, 1988 ; Fig. 1C ). Whitfield's (1893) discovery of Co-
rallidomus scobina mytilid bivalves preserved inside Petroxestes borings pro-
vides unusual, but compelling, evidence for the tracemaker identity ( Pojeta and
Palmer, 1976 ). By contrast, the affinity of the earliest Gastrochaenolites is not
as straightforward.
Gastrochaenolites is an elongate boring that enlarges toward the base, and has
been described variously as pouch shaped, flask shaped, or clavate ( Kelly and
Bromley, 1984 ; Fig. 1D ). The distinct morphology of the cavity corresponds
to a tracemaker that grows in width as it deepens its boring into the substrate.
The ichnogenus is most frequently cited as a product of bivalve bioerosion, a
claim validated by modern activities of gastrochaenid and pholadid bivalves.
Fossil borings often preserve the tracemaking bivalve inside the cavity (although
“nestling” fauna could be mistaken for a tracemaker: e.g., Kelly, 1980 ) and in the
instance of pholadid mechanical borings, bioglyphic ornament from the rasp-like
shell adorn the internal surface of the boring ( Evans, 1970 ). The earliest record of
the Gastrochaenolites morphology comes from Early Ordovician hardgrounds,
although these are unlikely to be the product of bivalves ( Benner et al., 2004;
Ekdale and Bromley, 2001 ). Gastrochaenolites preserved in reef substrates are
absent in the fossil record until the Jurassic.
2.1.3 Sponges
Members of several orders of sponges bore into modern-day coral substrates,
including Hadromerida (Clionaidae, perhaps Spirastrellidae, and Alectonidae),
Poecilosclerida (Acarnidae, viz. , the genus Zyzzya ), Halichondrida (Halichondrii-
dae, viz. , the genus Amorphinopsis ), and Haplosclerida (Phloeodictyidae, viz. ,the
genus Aka ) and there are a few “maybe eroders” in other Orders ( Hooper and Van
Soest, 2002 ; Christine Sch¨nberg, Australian Institute of Marine Science,
personal communication, 2011). Sponges bore by the chemical etching of the
substrate and the mechanical removal of the resultant sponge chips (for a review
on sponge boring, see Sch¨nberg, 2008 ). Rates of boring vary according to spe-
cies of sponge, substrate density ( Edinger and Risk, 1997; Highsmith et al., 1983 ),
location, and depth ( L ´ pez-Victoria and Zea, 2005 ), with most material being
removed frommassive, less porous coral species ( Buddemeier et al., 1974 ). Rates
vary over the lifetime of the sponge colony, with highest rates of growth and ero-
sion exhibited by newly settling larvae or sponge fragments and the lowest rates
by established colonies ( Neumann, 1966; R¨tzler, 1975 ).
Entobia consists of one or more chambers interconnected by narrow tunnels
and leading tomultiple apertures at the surface of the substrate. Much of the Ento-
bia excavation occurs below the substrate surface, which often obscures the mag-
nitude of substrate removal. Several ichnospecies are named under Entobia (see
reviews by Bromley andD'Alessandro, 1984, 1989 ), and can be categorized—for
the sake of simplicity—within two end-member morphologies: those colonies
comprised of many small chambers and those with few large chambers.