Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 432

uncommon, and rocky shore trace-fossil assemblages are generally dominated
by borings produced by endolithobionts.

3. MAIN BIOEROSION ICHNOTAXA
Both micro- and macrobioerosion occur in rocky shores. Nevertheless, little
work has been done on the first, compared to the attention paid to macroborings,
which are obviously more conspicuous and easily identifiable. Thus, this review
focuses on macroscopic bioerosion structures, which are usually without great
difficulty assignable to a variety of groups of invertebrates. They are described
here according to their abundance in ancient rocky shores.

3.1 Bivalve Borings
Several families of bivalves, such as Pholadidae, Gastrochaenidae, and Mytili-
dae, alongwith some genera in other families, are active borers inmodernmarine
environments ( Kleemann, 1973; Warme, 1975 ). They produce flask-shaped bor-
ings with a narrow aperture and a larger ovoid chamber known from the fossil
record as Gastrochaenolites ( Kelly and Bromley, 1984 )( Figs. 3 B and 4 B). This
trace fossil became common in the Jurassic ( Bromley, 2004 ) and thus, its fossil
record parallels that of boring bivalves. Paleozoic Gastrochaenolites are known
from the Ordovician ( Wilson and Palmer, 1988; Benner et al., 2004; Ekdale and
Bromley, 2001 ) and Carboniferous ( Wilson and Palmer, 1998 ), but the identity
of their tracemakers is still unknown. Gastrochaenolites is a ubiquitous element
in Mesozoic and Cenozoic rocky-shore trace-fossil assemblages ( Fig. 5 ), gener-
ally being the most conspicuous and abundant trace fossil. As bivalves include
both chemical andmechanical boring representatives, their bioerosion structures
may be present in substrates of a variety of lithologies.

3.2 Sponge Borings
Entobia is the most widespread and characteristic ichnogenus attributed to
sponges ( Fig. 4 A). This complex boring consists of an anastomosing network
of canals and chambers with numerous apertures connecting the system to
the substrate surface ( Bromley and D'Alessandro, 1984 ). Species belonging
to the family Clionaidae are the dominant endolithic sponges today, although

FIGURE 3 Modern bioerosion in littoral settings. (A) Midlittoral platform with abundant Para-
centrotus lividus within their borings, and some abandoned cavities occupied by sea anemones,
Bakio, Basque Country, N Spain. Scale bar 10 cm. Photograph courtesy of Oriol Ferrer. (B) Cryp-

toendolithic bivalves occupying previously bored Gastrochaenolites , Monte Le´n, Puerto Santa
Cruz, Patagonia, Argentina. Scale bar

¼

2 cm. (C) Bioerosion produced by the periwinkle gastropod
Littorina neritoides in the supralittoral zone, Salou, NE Spain. Scale bar ¼ 2 cm. (D) Radulichnus -
like traces produced by the gastropod Patella sp., Monte Le´n, Puerto Santa Cruz, Patagonia, Argen-
tina. Scale bar ¼ 2 cm. (E) Bioerosion produced by Patella caerulea and the polyplacophoran Chiton
olivaceus , Tarragona, NE Spain. Scale bar ¼ 2 cm.

¼