The Ichnofacies Paradigm - Trace Fossils as Indicators of Sedimentary Environments - page 110

provide an excellent up-to-date synopsis of each ichnofacies, including the
ethological and trophic implications of the dominant tracemakers.
To illustrate the foregoing, neoichnological considerations of two of the most
common ichnofacies in shallow-marine settings ( Skolithos and Cruziana ) are com-
pared with one another. Ichnocoenoses attributable to the Skolithos and Cruziana
ichnofacies are readily observed in modern settings and bear out the idea that the
same ichnofacies, as expressed by the trace-fossil suites in the rock record, encom-
pass collective and recurring behavioral and tropic responses (see Tables 1 and 2 ).
Trace fossils that are commonly associated with the Skolithos Ichnofacies
include Skolithos , Ophiomorpha , and Diplocraterion , and to a lesser extent,
Rosselia and Conichnus . Modern analogs are known for all of these trace
fossils, and the ethologic purposes of the traces have been well documented
(e.g., Dashtgard and Gingras, 2012; Dashtgard et al., 2008; Gingras et al.,
1999, 2008a,b; H ¨ ntzschel, 1939; Reineck, 1965; Seilacher, 1964 ). For the
constituents of the Skolithos Ichnofacies, at least one of two important survival
strategies is consistently recognized (see Table 3 ): use of the biogenic structure
for (1) filter feeding; and/or (2) for coping with shifting substrates and/or epi-
sodic deposition (see Hiscott et al., 1984; Howard and Frey, 1984 ). The use of
some burrows for multiple purposes ensures that those that are suitable for
mobile substrates and energetic settings (typically characterized by the Sko-
lithos Ichnofacies) can also be employed in some lower-energy settings that
are characterized by the Cruziana Ichnofacies. For example, the domichnion
Ophiomorpha is produced in low-cohesive substrates and is used for filter-
feeding, farming (fermentation), and subsurface deposit-feeding; therefore,
Ophiomorpha is common to both high- and low-energy environments (e.g.,
Bromley, 1996; Heinberg and Birkelund, 1984 ).
By way of comparison, the Cruziana Ichnofacies is typified by trace fossils
such as Asterosoma , Scolicia , Planolites , Palaeophycus , Thalassinoides ,and
Teichichnus . All of these trace fossils have modern analogs that strongly support
the interpretation of these biogenic structures as deposit-feeding traces in resource-
rich sediments ( Dworschak and Ott, 1993; Gingras et al., 1999; Howard et al.,
1974;Wetzel, 2008 ). Importantly, each trace can also serve other purposes, includ-
ing acting as a domicile ( Teichichnus , Thalassinoides , Palaeophycus ,and Astero-
soma ), for motility ( Scolicia and Planolites ), or to reequilibrate to the sediment/
water interface (equilibrichnia, Lingulichnus ,some Siphonichnus and Teichich-
nus ). Some neoichnological studies suggest that Scolicia , in various occurrences,
maybeproducedbychemosymbionts(e.g., Bromley et al., 1995 ). Other trace fos-
sils, such as Arenicolites , Skolithos ,and Rosselia , commonly occur within suites of
the Cruziana Ichnofacies, but are not characteristic of deposit-feeding behaviors.
Nevertheless, vertical traces are used for a variety of feeding behaviors. For exam-
ple, Skolithos are used by some infauna for head-down deposit-feeding (e.g., Mal-
danus maldanus ; Gingras et al., 1999 ). Similarly, J-shaped Arenicolites is an
optimal configuration for mining sediment (e.g., Abarenicola pacifica ), whereas
conventional U-shapes are well suited for interface deposit-feeding (e.g.,