Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 437

(1994) and Wilson and Palmer (1998) from boulders associated to a Carboni-
ferous rocky shore. Thus, Paleozoic rockground bioerosion is essentially
typified by low-diversity (monotypic) assemblages of worm borings, maybe pro-
duced by sipunculids (e.g., Pemberton et al., 1980 ). The Carboniferous example
may suggest the incorporation of new boring taxa to Paleozoic rocky-shore
communities, although more examples would be needed to confirm this tendency.
No Permian or Triassic bioerosion in rocky shores is known to the authors to
have been reported, which does not allow the assessment of the possible impact of
the end-Permian extinction on coastal borers. On the contrary, several Jurassic
examples are known ( Fig. 5 ), which are characterized by bivalve borings
( Gastrochaenolites ), whichmay be associatedwith Trypanites . The assemblages
known from the Cretaceous take on a more modern aspect. The sponge boring
Entobia is present in most cases, while at least one example ( Mikul ´ˇ , 1992 )
has diversity comparable to Neogene and modern assemblages, including not
only bivalve and sponge borings but also echinoid and cirripede trace fossils.
There is a considerable gap of information concerning Paleogene rocky
shores ( Johnson, 1988a ). Nevertheless, Neogene and Pleistocene records of
bioerosion are very abundant ( Fig. 5 includes only some examples). Trace-fossil
assemblages are the same as those seen in modern coasts and can be typified by
the presence of bivalve and sponge borings ( Gastrochaenolites and Entobia ,
respectively), often accompanied by worm borings ( Maeandropolydora and
Caulostrepsis ), echinoid bioerosional bowls ( Circolites ), and cavities produced
by acrothoracican cirripedes ( Rogerella ). This general increase in diversity is
even more remarkable in some examples, such as in the Pliocene of Greece
( Bromley and Asgaard, 1993a,b ) or the Plio-Pleistocene of Italy ( Bromley
and D'Alessandro, 1987 ), where other trace fossils such as Gnathichnus , Radul-
ichnus , and ' Conchotrema '( =Talpina ) also occur.
This trend in the diversification of rocky-shore bioerosion during the Phane-
rozoic was already acknowledged by de Gibert et al. (1998) , and it shows a clear
parallelism with the pattern exhibited by boring assemblages in marine hard-
grounds in general ( Palmer, 1982; Taylor and Wilson, 2003; Wilson and
Palmer, 1990, 1992 ) and also to the general trend to increasing diversity of
rocky-shore biotas shown by Johnson and Baarli (1999) .
4.3 Rocky-Shore Ichnofacies
Bioerosional trace-fossil assemblages were not incorporated into the Seilacher-
ian or archetypal ichnofacies model until Frey and Seilacher (1981) defined the
Trypanites Ichnofacies. These authors characterized it as constituted by borings,
raspings, and gnawings formed in a variety of hard substrates, including not
only rocky coasts, beachrocks, and sublittoral hardgrounds but also organic
substrates such as shell, bone, and wood. Later, Bromley et al. (1984) erected
the Teredolites Ichnofacies for marine woodgrounds. Thus, the Trypanites