The Ichnofacies Paradigm - Trace Fossils as Indicators of Sedimentary Environments - page 107

that differentiating shallow-tier associations frommid- and deep-tier associations
as discrete ichnofacies is not really very different from the distinction between the
Zoophycos Ichnofacies (dominated by deep-tier associations) and the Nereites
Ichnofacies (largely defined by shallow-tier suites). We believe, however, that
this distinction can be captured within the details of the hardground suites them-
selves and do not require a discrete ichnofacies. In defense of the Zoophycos and
Nereites ichnofacies distinction, it is the presence of graphoglyptids (organized
farming and grazing/foraging structures) that is the diagnostic indicator of the
Nereites Ichnofacies (e.g., Frey and Seilacher, 1980; Seilacher, 1967 ), not merely
shallow-tier associations. The graphoglyptids tend to be cast on the soles of low-
energy and/or low-density turbidites that were emplaced into the deep basin. In no
instances known to the authors have thin, low-energy tempestites been found to
cast shallow-tier associations containing graphoglyptids within the Zoophycos
Ichnofacies. That is, settings residing above storm wave base on the shelf do
not appear topossess graphoglyptidassociations, indicating that the twoarchetypal
ichnofacies are geographically discrete. Suites attributable to continental slope
associations of the Zoophycos Ichnofacies do have the potential, however, for
some overlap with the Nereites Ichnofacies (cf. Hubbard et al., 2012).
The foregoing demonstrates that medium-scale subdivisions of the
Seilacherian ichnofacies ultimately may be warranted. However, we believe
that, (1) it is necessary to demonstrate their recurrence in time and space in a
manner similar to the original Seilacherian ichnofacies, and (2) ichnosubfacies
must demonstrate their application to the characterization of subenvironments,
before being formally designated and applied. Until so shown, such ichnosub-
facies cannot be regarded as formal ichnofacies and are better regarded as dis-
crete ichnofabrics or trace-fossil suites. Indeed, MacEachern et al. (2007c) and
MacEachern and Bann (2008) have characterized trace-fossil suites in the con-
text of “proximal”, “archetypal”, and “distal” expressions of some Seilacherian
ichnofacies, in order to define some subenvironments.
Finally, “ichnofacies” have also been employed to characterize specific
recurring assemblages in particular rock units (e.g., D'Alessandro et al.,
1986; Hayward, 1976; Hunt et al., 1994; Lockley, 2007; Lockley et al.,
1994 ). This approach has become particularly common for workers focused
on the widespread and extensive tetrapod trackways of the continental realm
(e.g., Hunt and Lucas, 2007; Hunt et al., 1994; Lockley et al., 1994 ). Several
workers established that specific associations of track types and coprolites rep-
resent particular depositional environments and subenvironments (e.g., Hunt
et al., 1994, 1995, 1998; Lockley et al., 1994 ). Between the establishment of
the first tetrapod ichnofacies ( Lockley et al., 1994 ) and 2007, approximately
15 tetrapod and vertebrate coprolite ichnofacies were named.
Although commonly characteristic of specific environments, most named
vertebrate ichnofacies are limited in temporal distribution ( Hunt and Lucas,
2007; Santi and Nicosia, 2008 ). The Chelichnus and Brasilichnium ichnofacies,
characteristic of eolian successions, are limited to the Permian and Late
Triassic/Early Jurassic, respectively ( Hunt and Lucas, 2006, 2007; Lockley