Fluvial Environments - Trace Fossils as Indicators of Sedimentary Environments - page 337

reflecting colonization of active channels, and the vertical burrows were ten-
tatively considered as domiciles of suspension feeders ( Buatois and M´ngano,
2004, 2007 ). Alternatively, similar assemblages may be interpreted as subaer-
ial in origin, implying channel abandonment or exploitation of sandy crevasse
deposits ( Netto, 2007; Schlirf et al., 2001 ). The latter interpretation is sup-
ported by the observation of similar dense vertical to oblique burrows pro-
duced by staphylinid beetles in modern abandoned anabranches and braided
bars during low-water stages of

the Platte and Loup rivers, Nebraska

( Smith and Heim, 1971 ).

3.7 Ichnofacies

The recently proposed Grallator Ichnofacies ( Hunt and Lucas, 2007 ) can be
distinguished in fluvial facies (e.g., Gierli ´ ski et al., 2004; Johnson, 1986;
Krapovickas et al., 2009 ), although the typical occurrence is from lakeshore
facies ( Fig. 1 G). This ichnofacies is typified by the abundance of bipedal tridac-
tyl or tetradactyl tracks of avian and non-avian theropods (bird tracks and
bipedal tri-tetradactyl trackways with thick digits; see Fig. 6 A and B). Other
undiagnostic components are bipedal trackways with thick short digit imprints,
simple horizontal burrows, simple trails, arthropod trackways, burrow systems,
meniscate burrows, and swim traces. The Grallator Ichnofacies was proposed
only for tetrapod tracks, but here we include the associated invertebrate trace
fossils in order to provide a more complete picture of the animal community.
In consequence, the examples of the Grallator Ichnofacies can be envisaged
as sharing components with the Scoyenia Ichnofacies (meniscate burrows,
arthropod trackways). The ichnofaunas are of moderate to high diversity and
almost invariably contain invertebrate trace fossils (which are less diverse than
vertebrate trace fossils) in addition to tetrapod tracks. The observed ichnodiver-
sity ranges from 3 to 13 and averages 8 trace-fossil morphologies.

Grallator

3.8 Ichnofacies

The Batrachichnus Ichnofacies was defined by Hunt and Lucas (2007) as
ichnofaunas in which the majority of tracks are of quadrupedal carnivores with
moderate to high diversity (four to eight ichnogenera). The authors did not pro-
pose morphological attributes that can be used for distinguishing carnivore
tracks. In consequence, the recognition of this ichnofacies relies essentially
on the interpretation of the possible producer of the tracks and/or on the pres-
ence of some of the typical ichnogenera (e.g., Batrachichnus , Amphisauropus ,
Ichniotherium , chirotheriid tracks; see Fig. 6 C-E). Considering the examples
mentioned by Hunt and Lucas (2006, 2007) , at least eight trace-fossil assem-
blages from fluvial deposits belong to this ichnofacies ( Fuglewicz et al.,
1990; Lucas et al., 2004; Mack et al., 2003; Melchor and Sarjeant, 2004 ). Most
of these assemblages also contain invertebrate trace fossils that are typical of the
Scoyenia Ichnofacies (mainly meniscate burrows and various arthropod

Batrachichnus