Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 438

Ichnofacies was left to typify marine-bioerosion assemblages in rocky, sedi-
mentary, and skeletal substrates.
In their study of bioerosion in the Late Pliocene of Rhodes, Bromley and
Asgaard (1993a) proposed the existence of two different ichnofacies: (1) Entobia
Ichnofacies, dominated by deep-tier dwelling borings found in rocky-shore set-
tings; (2) Gnathichnus Ichnofacies, constituted mainly by epigenic and shallow-
tier trace fossils seen in shells and lithoclasts found in deeper water sediments.
They suggested that these two ichnofacies could be either subdivisions or even
substitutes of the Trypanites Ichnofacies. Later, de Gibert et al. (1998, 2007)
demonstrated the recurrence of these two assemblages since the Jurassic and val-
idated them as archetypal ichnofacies with paleoenvironmental significance.
Thus, there are two archetypal ichnofacies available for rocky-shore settings:
Trypanites and Entobia (de Gibert et al., 1998, 2007) . Both are characterized by
deeply penetrating dwelling borings (domichnia). Based on that, MacEachern
et al. (2007) considered both ichnofacies to be equivalent. Nevertheless, as
pointed out by de Gibert et al. (2007) , there are important particularities in the
Entobia Ichnofacies that reflect compositional and structural evolutionary
changes of endolithic rocky-shore communities. Our proposal here is to keep
the Trypanites Ichnofacies to encompass all bioerosional trace-fossil assem-
blages constituted by dwelling borings resulting from long-term colonization
in permanent or semipermanent hard substrates, such as those of rocky shores,
some hardgrounds, gravel pavements, and reefs, in contrast to the Gnathichnus
Ichnofacies (see de Gibert et al., 2007 ), which consists mostly of epigenic and
shallow-tier traces in ephemeral substrates such as shellgrounds, and some hard-
grounds and gravels. Nevertheless, this Trypanites Ichnofacies in Phanerozoic
rocky shores includes two clear and distinct recurrent assemblages ( Fig. 5 ).
The first corresponds to low-diversity bioerosion associations constituted by ver-
tical to subvertical borings and typifies most Paleozoic and Jurassic rocky-shore
trace-fossil assemblages and also some younger occurrences in non-carbonate
substrates ( Fig. 5 ). Meanwhile, the other one (the Entobia Ichnofacies of
Bromley and Asgaard, 1993a ) is characteristic of Cretaceous to modern carbon-
ate rocky shores, and it is characterized by a higher diversity with both shallow-
and deep-tier borings. The differences between both are linked to the nature of the
substrate and, most importantly, to the increasing complexity of marine benthic
hardground communities after the Mesozoic Marine Revolution.
5. CASE STUDIES
5.1 Early Devonian, Ontario
Among published descriptions of Paleozoic rocky shores, Kobluk et al. (1977)
and Pemberton et al. (1980) provide interesting details on bioerosional trace
fossils. This case refers to the Silurian-Devonian disconformity in southern
Ontario (Canada), where the Early Devonian (Emsian) Oriskany and Springvale