Rocky Shorelines - Trace Fossils as Indicators of Sedimentary Environments - page 434

some other taxa are known to produce Entobia -like borings ( Bromley, 2004 ).
Sponges are chemical borers, and their borings are restricted to carbonate
substrates. Entobia is present in almost all described Cenozoic carbonate
rocky shores, but their presence in Mesozoic rocky shores is limited to the
Cretaceous ( Fig. 5 ).
3.3 Worm Borings
A variety of worm borings are known in fossil rocky shores, the most common
being Trypanites , Maeandropolydora, and Caulostrepsis . Trypanites ( Fig. 4 C)
is a simple, cylindrical boring similar to those produced today by some sipun-
culans and polychaetes ( Bromley, 1978 ). It has a long fossil record from the
Cambrian ( James et al., 1977 ) to theHolocene. It is often the only bioerosion struc-
ture in Paleozoic rocky shores, but their presence in Mesozoic and Cenozoic
assemblages is, in most cases, a secondary component ( Fig. 5 ). Caulostrepsis
( Fig. 4 D) and Maeandropolydora ( Bromley and D'Alessandro, 1983 ) are two ich-
nogenera generally attributed to the work of spionid polychaetes ( Dom ` nech
et al., 2008; Voigt, 1965 ). Caulostrepsis is a U-shaped boring, while Maeandro-
polydora consists of winding cylindrical tubes sometimes connected to pouch-like
structures. Both ichnogenera are well known from the Mesozoic and Cenozoic,
but some occurrences have been reported in the Paleozoic ( Bromley, 2004 ). They
are present inmany Neogene rocky shores ( Fig. 5 ), where they occur as secondary
components together with Gastrochaenolites and Entobia . They may be found in
the rockground or, more often, affecting the shells of epilithic mollusks.
3.4 Echinoid Borings
Some regular echinoids are important rock borers in very shallow waters today
( Fig. 3 A). They typically inhabit shallow, bowl-shaped depressions, which they
bore mechanically by using the jaws and spines ( Martinell, 1981; Warme,
1975 ). This allows for bioerosion of lithologically diverse substrates. Echinoid
borings are known since the Jurassic ( Bromley, 2004 ), and Mikul ´ˇ (1992)
erected the ichnogenus Circolites ( Fig. 4 F) to name this type of trace fossil.
Circolites has been identified in Cretaceous and Neogene rocky shores ( Fig. 5 ).
3.5 Cirripede Borings
Acrothoracican cirripedes bore small, narrow, pouch-shaped cavities with nar-
row apertures ( Seilacher, 1969 ). They are assigned to the ichnogenus Rogerella
( Bromley and D'Alessandro, 1987 )( Fig. 4 E), which ranges from the Devonian
to the Holocene ( Bromley, 2004 ). Although not very abundant, Rogerella has
been described from Paleozoic (Carboniferous), Mesozoic (Cretaceous), and
Neogene rocky shores ( Fig. 5 ).