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paleoreliefs devoid of sedimentary cover. Marine evidence of submersion
(bioerosion, body fossils, tidal, or wave erosion) commonly overprints other
features formed during the period of subaerial exposure (e.g., karstification
in carbonate rocks) preceding the transgression. Although it is possible to iden-
tify sedimentary deposits linked to rocky shores, these are essentially erosional
environments, in which bioerosion is often the best aid for their identification in
the fossil record. Bioeroded rocky-shore surfaces can be extensive and, given
the adequate outcrop conditions, can be recognized and correlated through dif-
ferent localities in a basin, thus contributing to its paleogeographic reconstruc-
tion. Such large surfaces are often time-averaged as a consequence of the
overprinting of succeeding communities as the transgression proceeded.
Bioerosion can be also a very precise indicator of maximum sea level in those
areas where borings disappear above a certain altitude, a feature that can be
related with other morphological characteristics indicative of sea level, such
as erosional notches. Identifying these paleopositions of sea level may be
important to interpret eustatic sea-level changes or relative changes due to tec-
tonic activity.
Rocky-shore bioerosion assemblages are typified by the
Trypanites
Ichno-
facies characterized by the presence of deep-tier dwelling borings produced by a
variety of invertebrates (sponges, worms, bivalves, cirripedes, and echinoids)
seeking refuge from hostile conditions and predators. Two clearly distinct
recurrent assemblages can be recognized when analyzing trace-fossil associa-
tions in rocky shores throughout the Phanerozoic. They do not correlate with
significant paleoenvironmental differences but rather reflect variations in the
complexity of the ecological structure of the benthic community related to evo-
lutionary aspects or substrate limitations. Thus, trace-fossil assemblages in
Paleozoic rocky shores previous to the Jurassic diversification of borers, but
also some younger occurrences on non-carbonate substrates, exhibit a low
diversity of vertical, deep-penetrating borings (
Trypanites
and
Gastrochaeno-
lites
). Meanwhile, more diverse assemblages consisting of shallow- and
deep-tier structures, most typically
Entobia
and
Gastrochaenolites
, are found
in Cretaceous and Cenozoic rocky shores on carbonate substrates.
ACKNOWLEDGMENTS
This research was funded by the Spanish Ministry of Research and Innovation as a contribu-
tion to the projects CGL 2007-60507/BTE and CGL 2010-15047/BTE.
REFERENCES
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