Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 430

some sponges (clionaids), polychaete worms or lithophagous bivalves, that bore
permanent dwellings into the substrate, and some regular echinoids, which
excavate depressions in it. Searching for protection in existing cavities is also
a valid mechanism employed by cryptic taxa that seek sporadic or permanent
shelter in caves and fissures (crabs, brachiopods, and bryozoans).
Animals living on rocky coasts display three main feeding strategies: graz-
ing, filter-feeding, and predation. Vagile epilithozoans (whelk gastropods,
crabs, and starfish) are mostly carnivorous, preying mainly on mollusks. Sessile
epilithozoans (barnacles, mussels, oysters, sabellid, and spirorbid polychaetes)
are filter-feeders, except for the anemones, which actively capture live small
organisms.

2.3 Preservation Potential of Rocky-Shore Biota
in the Fossil Record
As stated above, rocky-shore environments are basically erosional and associ-
ated with high-energy, unpredictable conditions. This scenario is not favorable
to the preservation of inhabiting biotas in the geological record. Among the
diversity of forms described in the previous section, it is easy to understand that
only those capable of modifying the substrate (bioeroders) or of strongly attach-
ing to it (cementers) would have a good chance to be preserved as in-situ fossils.
Other organisms with hard skeleton can be preserved only as transported ele-
ments in high-energy sedimentary deposits located in the vicinity of the rocky
shore.
Ancient rocky shores therefore typically preserve two types of fossils:
bioerosional trace fossils and shells of encrusting epilithobionts. Among the lat-
ter, red algae, cirripedes, and oysters are common, particularly in the Cenozoic
(e.g., de Gibert et al., 1998 ). In contrast to borings, they have a higher potential
of being destroyed during the biostratonomic phase of fossilization or even after
burial. Thus, bioerosion structures are the most common fossils in ancient rocky
shores and, in consequence, they are paramount for their identification and
interpretation. Even among bioerosion structures, the preservation of superficial
structures (e.g., grazing traces such as Gnathichnus or Radulichnus )is

rocky shore in Palam ´ s at a water depth of 30 m with the branching gorgonian coral (Cnidarian)
Paramuricea clavata . Approximate width of photograph: 80 cm. (C) Midlittoral ( ¼ littoral) zone
( 2 m) with the mussel Mytilus galloprovincialis and the fish Sarpa salpa at Port de la Selva.
Approximate width of photograph: 30 cm. (D) A variety of encrusting organisms in the sublittoral
zone ( 25 m) at Illes Medes, showing two demosponges, Phorbas tenacior (blue) and Antho incon-
stans (orange), the dendrophyllid coral Leptosammia pruvoti (yellow), and the branching bryozoan
Myriapora truncata (orange). Approximate width of photograph: 20 cm. (E) Sublittoral zone with
the bryozoan Sertella septentrionales and the demosponge Spirastrella cuncatrix (red). Approxi-
mate width of photograph: 20 cm. (F) Sublittoral zone ( 30 to 40 m) at Cap de Creus with the

demosponges Phorbas tenacior (bluish) and Antho inconstans (orange), the corals Corallium
rubrum (red), and Leptosammia pruvoti (yellow). Approximate width of photograph: 30 cm. Pho-
tographs (B-E), courtesy of Roger de Marf`; (F), courtesy of Creu Palac´n.