Shallow-Marine Carbonates - Trace Fossils as Indicators of Sedimentary Environments - page 714

Kendall et al., 1994 ). These crusts have a patchy cement distribution with
numerous crevices, which provide shelter and food supply for a diverse fauna.
Semiconsolidated substrate is bioeroded by cyanobacteria, bivalves ( Botula ,
Lithophaga , and Gastrochaena ) and the shrimp Alpheus (cf. Asgaard et al.,
1997; Fischer and Meyer, 1985 ). Muddy and firm substrate along eroding chan-
nel margins is intensively burrowed by crustaceans, polychaetes, echiurans, and
other worms ( Fig. 12 D and E). Lower intertidal mud flats host burrows of
deposit-feeding ocypodid crabs (mainly Macrophthalmus depressus , see
Basson et al., 1977 ) producing Macanopsis -, Psilonichnus -, Skolithos -, and
Gyrolithes -like burrow morphologies ( Fig. 12 F), with the echiuran Ikeda tae-
nioides leaving deep subvertical burrows ( Hornby, 2005 ), and with bivalves.
A rich association of avifauna traces is present, including trackways of herons,
flamingo, plovers, sandpipers, gulls, and terns. On protected mud flats, the
Greater Flamingo ( Phoenicopterus roseus ) leaves extensive feeding traces when
sifting through the silt while turning its body slowly around ( Allen, 1956 ). The
circular and semicircular structures are approximately 1.2-1.6 m in diameter and
comprise a central pedestal surrounded by a feeding trough ( Fig. 12 G). The sur-
face of tidal pools and soupy channel mud banks is covered with minute trails
produced by foraminifers and abundant trails of cerithid snails with orientation
toward the water.
5.2.4 Subtidal Lagoon with Sand Bars
The lagoonal areas between barrier islands and the tidal flat are mainly sandy
and subordinately muddy. Subaqueous cementation is common and leads to the
synsedimentary development of extensive hardgrounds, which become
bioeroded by algae, grazing snails, endolithic sponges ( Entobia -like borings),
bivalves ( Gastrochaenolites -like borings), crabs, and stomatopods. Patch reefs
consisting of corals and algae grow where a suitable hard substrate is available
and are subject to bioerosion not only by the same kinds of organisms but also
by sea urchins, polychaetes, sipunculids, and fish. A complex system of tidal
currents within the lagoon gives rise to the accumulation of various sand bodies
such as bars, spits, and ebb-tidal deltas consisting of ooids. This sandy bottom is
inhabited by a diverse meiobenthic fauna in the interstices as well as by macro-
benthic animals with a burrowing behavior. Different snails and sea urchins
(sand dollars and heart urchins) leave their trails on the sediment surface or
dig slightly deeper. Bivalves, crustaceans, and worms all bioturbate the sand,
but their burrows are poorly known. Common features are sand cones up to
1 m in height, produced by callianassid shrimp, while excavating an extensive
burrow system—known from the fossil record as Ophiomorpha , Spongeliomor-
pha , and Thalassinoides (see Section 5.1 ). Burrows made by alpheid shrimp are
frequently inhabited by gobies, a small commensal fish. Larger fish (e.g., soles)
hide in the sand while waiting for prey, whereas rays form deep plug-shaped
feeding excavations due to hydraulic jetting. If fossilized, such traces are known