Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 349

is pending, the authors described simple horizontal trails ( Cochlichnus ,
“ Hormosiroidea meandrica ”) and meniscate burrows ( Taenidium ), accompa-
nied by five tetrapod-track types (total ichnodiversity

8) from channel-bar
surfaces. The tetrapod tracks are assigned to bipedal tri-tetradactyl tracks with
thick digits ( Neotrisauropodus ), bipedal tridactyl tracks with stout digits
( Anomoepus , Moyenisauropus ), quadrupedal pentadactyl digitigrade tracks
( Episcopopus ), and chirotheriid tracks. In this case, an analysis of the best pre-
served trackways helped in the reconstruction of substrate consistency and of
the paleolandscape ( Smith et al., 2009 ). The example from the Miocene of
Argentina was recorded from the top of sand bars in an anastomosed dryland
river. The ichnofauna (ichnodiversity

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10) is dominated by eight types of avian
and mammalian tracks (many of them left in open nomenclature) and subordi-
nate simple trails ( Helminthopsis ) and horizontal burrows ( Palaeophycus ). Ver-
tebrate tracks include three bird-like footprint types (including Fuscinapeda )
and five mammal footprint types (including Macrauchenichnus and Venato-
ripes ). Krapovickas et al. (2009) emphasized the importance of seasonal flood-
ing events for preservation of tetrapod tracks on bars of anastomosed fluvial
systems in a semiarid climate.
A comparison between trace-fossil assemblages from floodplain and associ-
ated channel-belt deposits from the same lithostratigraphic units (28 case studies)
may be more helpful for highlighting the differences between both settings
( Fig. 8 B). These cases would reflect the contrasting behavior of biological com-
munities under similar physical constraints in a fluvial system. In most of these
case studies, a higher (up to six times) ichnodiversity is recorded in overbank
facies than in associated channel facies (e.g., D'Alessandro et al., 1987; Kim
et al., 2002; Melchor et al., 2006 ). In a few cases, the ichnodiversity is greater
in channel facies (e.g., Aramayo and Bocanegra, 2003; Fuglewicz et al.,
1990 ). MacNaughton and Pickerill (1995) explained the occurrence of a moder-
ately diverse ichnofauna in Triassic braided-fluvial channels from Canada, as a
consequence of higher moisture in coarse-grained channel fluvial deposits in a
semiarid climate. In this example, there is no record of overbank trace fossils.
The selected trace-fossil assemblages from floodplain and channel-belt deposits
fromthe same lithostratigraphic units ( Fig. 8 B) underscore a similar participation
in both facies of meniscate burrows, simple vertical burrows, and J-, Y-, or
U-shaped vertical burrows, which seems to contradict previous inferences about
the dominance of vertical burrows in channel facies and horizontal burrows in
overbank facies ( Maulik and Chaudhuri, 1983 ). However, for some trace fossils,
this relationship can apply. The trace fossil Capayanichnus vinchinensis from the
Neogene ofArgentina preferentially occurs in abandoned channel depositswith a
dominantly vertical to oblique orientation and in crevasse-splay facies with a pre-
vailing horizontal orientation ( Melchor et al., 2010 ). This particular distribution
and burrow orientation was interpreted as reflecting a marked dependence of the
producer of the local water table or moist substrates. Some other notorious dif-
ferences between channel and floodplain in these selected cases are the larger

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