Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 440

presence of Gastrochaenolites and Trypanites not only in the rockground itself
but also in hardgrounds found within the Middle Jurassic (Bajocian-Bathonian)
Upper Inferior Oolite overlying the Carboniferous limestones. A more detailed
study of the rocky-shore bioerosion in the area was carried out by Cole and
Palmer (1999) . These authors characterized the assemblage as dominated by Try-
panites weisei (diameter 0.7-3.2 mm, maximum length 130 mm), which are gen-
erally very abundant, reaching densities of more than 100,000 borings per m 2 .
Larger worm borings ( Trypanites fosteryeomani ) and bivalve-produced cavities
( Gastrochaenolites ) are also common ( Fig. 6 C). Oysters ( Liostrea ) are found
encrusting the substrate, while the bivalve Hiatella and some gastropods
are found within the borings, probably indicating a nestling habit.
5.3 Miocene, Spain
Dom ` nech et al. (2001) published a detailed account on the bioerosion found
in rocky shores at several localities in the Camp de Tarragona Basin in NE
Spain. These rocky shores were formed during the Langhian-Serravalian
(Middle Miocene) transgression by the flooding of a previously existing
topography constituted by Mesozoic carbonates. In several localities, the
transgressive surface displays a planar morphology parallel to the overlying
Miocene beds, suggesting an interpretation as a wave-cut abrasion platform
( Fig. 7 A). Bioerosion is intense and dominated by Gastrochaenolites and
Entobia ( Fig. 7 B-D). Two ichnospecies of Gastrochaenolites were recog-
nized, G. lapidicus and G. torpedo , together with Phrixichnus isp. Sponge
borings ( Entobia ) occupy the area between Gastrochaenolites .Some Maean-
dropolydora decipiens ( Fig. 7 D) are occasionally found. Bivalve borings are
the most conspicuous feature on the surface, occurring often in densities
exceeding 1,500 individuals per m 2 ( Fig. 7 C).Theyareoftentruncated,most
likely due to the weakening of the substrate due to sponge activity, more
intense as transgression progressed. The rock substrate is generally covered
by heterometric breccias of a variable thickness, followed by coquines or calc-
arenites (Ardenya Unit). The clasts in the breccias are dolomitic and are also
intensely bored with the same ichnotaxa (plus Trypanites rectus in one local-
ity), but Entobia dominates here, while Gastrochaenolites is much more rare
except in big boulders. This is due to higher instability of such mobile
substrates that favor rapid growing borers such as sponges. No epilithobionts
have been found either on the rockground or on the clasts.
6. CONCLUSIONS
Rocky shores in the fossil record are found associated with regional unconfor-
mities. The right paleoenvironmental scenario for their formation and preserva-
tion occurs during major transgressive events capable of flooding rocky