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invertebrate representatives, with trackways, burrows, trails and particularly
nests becoming dominant landward, delineating the transition into terrestrial
ichnofacies, such as
Scoyenia
and
Coprinisphaera
(
Buatois and M´ngano,
1995; Genise et al., 2000
). Among the vertebrates, mammals, reptiles, and birds
produce a wide variety of structures, including burrows and trackways (
Baucon,
2008; Frey and Pemberton, 1986
). Ichnological observations in supratidal
deposits are essentially restricted to modern environments, there being a
remarkable dearth of studies in ancient deposits. In addition, given the low pre-
servation potential of many of the supratidal biogenic structures (root traces and
crustacean burrows being the exception), it is still premature to provide an
ichnological classification of associated ancient analogs.
2.2 Mud Flats
Mud flats constitute the upper zone of tidal flats, depositional processes being
dominated by the fallout of suspended sediment comprising sortable silts, flocs,
and aggregates (
Chang et al., 2007; Flemming, 2012
), as well as biodeposition
due to the production of fecal pellets and pseudofeces (
Fig. 2
;
Augustinus, 2002;
De Boer, 1998; Potter et al., 2005
). Mud-flat deposits are dominated by parallel-
laminated or massive mudstone, lenticular-bedded siltstone and very fine-
grained sandstone being less common (
Fig. 4
A). Salinity shifts, together with
changes in exposure and temperature, are typically dramatic in the upper inter-
tidal zone (
Newell, 1979; Reise, 1985
).
Modern mud flats are intensely bioturbated, polychaetes, gastropods, and
bivalves being common producers (
Reineck and Singh, 1980; Sch¨fer,
1972
). However, limited lithological contrast commonly precludes visualiza-
tion and preservation of biogenic structures (
M´ngano et al., 2002a
). Accord-
ingly, discrete trace fossils are relatively rare and an indistinct mottled
texture is commonly observed instead, it being locally accompanied by more
distinct polychaete and crustacean burrows (
Baucon, 2008
). In the fossil record,
elements of the
Cruziana
Ichnofacies may be preserved where lithological
interfaces are available (e.g.,
M´ngano et al., 2002a
). Typical components
are
Cruziana
,
Rusophycus
,
Palaeophycus
,
Psammichnites
,
Lockeia
, and
Proto-
virgularia
. The presence of these ichnogenera may either reflect a wide envi-
ronmental adaptation of these bioturbators, or the short-term incursion into
this zone by faunal elements of the middle and lower intertidal zones
(
M
´
ngano et al., 2002a
). Migrations may be regulated by tidal cyclicity in con-
nection with the search for food. Also, invertebrate trackways and simple graz-
ing trails, such as
Helminthopsis
and
Helminthoidichnites
, are common
(
Figs. 3
B, C and
4
B); their preservation potential is low, but it may increase
in the presence of associated microbial mats (e.g.,
Desjardins et al., 2010a
).
The mud flat also hosts vertebrate trackways that illustrate some of the ichno-
coenoses of the
Brontopodus
and
Batrachichnus
ichnofacies of
Hunt and Lucas
(2007)
.
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