Eolian Environments - Trace Fossils as Indicators of Sedimentary Environments - page 408

for inhabitants of the dune field. Rarely is the stoss preserved in ancient
eolianites; it is the cross-stratified slip face deposits that make up the bulk of
ancient eolian sandstone beds.
Tracemaking animals that inhabit the dynamic environment of modern and
ancient dune fields consist almost exclusively of arthropods (chiefly insects and
arachnids) and vertebrates (chiefly reptiles and mammals). Thus, studies in
eolian ichnocoenoses necessarily include attention to both invertebrate and
vertebrate traces.
Although the obligatory arid conditions that characterize active dune envi-
ronments typically preclude high population densities, dune communities may
exhibit a surprising diversity of desert organisms and their traces ( Fig. 1 ). It may
seem counterintuitive that in modern dunes, the most distinct and most easily
identifiable traces occur in sand in the erosional stoss microenvironment, which
is preserved only very rarely in the geologic record. Instead, the traces produced
by animals in sand in the depositional slip face microenvironment have a much
greater preservation potential, but they are typically deformed by collapse and
localized slumping.
Plants are generally restricted to the stoss side of a dune, so plant trace fossils
(rhizoliths) in eolianites are to be expected only in situations where the stoss
facies of stabilized dunes are preserved. Typically, rhizoliths are observed more
commonly in coastal dune deposits rather than in inland erg deposits ( Curran,
2007; Curran and White, 2001 ). Animal traces in the stoss sands may include
both domichnia and repichnia (and rarely pascichnia), whereas in the slip face
sands, the traces are primarily repichnia.
2.2 Ichnofacies in Eolian Paleoenvironments
At present, there are three named ichnofacies that pertain specifically to trace
fossil associations in ancient eolianites—the Entradichnus Ichnofacies ( Ekdale
et al., 2007 ), Octopodichnus Ichnofacies ( Hunt and Lucas, 2007 ), both of which
are characterized by traces of invertebrate animals, and the Chelichnus Ichno-
facies ( Hunt and Lucas, 2007 ), which is characterized by vertebrate tracks. To
our knowledge, there is no named ichnofacies pertaining specifically to eolia-
nites infested by pervasive rhizomorphs, which often create strikingly promi-
nent ichnofabrics, nor do we suggest that a new ichnofacies should be named
for such plant-dominated situations. In fact, for the sake of broader utility
and simplicity in ichnology, it seems reasonable that the three aforementioned
ichnofacies, as well as rhizomorph-dominated ichnofabrics in eolianites, all
should be contained within a single ichnofacies designation, namely the
Entradichnus Ichnofacies.
The Chelichnus Ichnofacies was proposed “for ichnofaunas that have a low
diversity (less than five ichnogenera) of tetrapod tracks in which manual and
pedal tracks are equant in shape with short digit impressions and in shape often
have subequal manual and digit impressions” ( Hunt and Lucas, 2007 : 66).