Ichnostratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 197

The biostratigraphic significance of Precambrian-Cambrian ichnofaunas has
been intensely explored since the 1970s (e.g., Alpert, 1977; Crimes, 1987, 1994;
Jensen, 2003; MacNaughton and Narbonne, 1999; Narbonne et al., 1987; Walter
et al., 1989 ). MacNaughton and Narbonne (1999) and Jensen (2003) presented a
biostratigraphic scheme that has been slightly modified more recently ( Fig. 4 ;
Buatois andM´ngano, 2011; Jensen et al., 2006 ). This scheme comprises twoEdi-
acaran zones and three EarlyCambrian zones. The lower Ediacaran zone includes
simple grazing trails (e.g., Helminthoidichnites , Helminthopsis , Archaeonassa )
as well as dickinsonid resting traces and Kimberella radular scratches
( Radulichnus ). The age of this interval is approximately 560-550 Ma ( Jensen
et al., 2006; Martin et al., 2000 ). The upper Ediacaran zone includes the oldest
branching burrow systems, represented by Treptichnus (non T. pedum ) and Strep-
tichnus , as well as trilobate trace fossils similar to Curvolithus . The age of this
zone is 550-542 Ma ( Grotzinger et al., 1995; Jensen et al., 2006 ). The lowermost
Early Cambrian zone is known as the T. pedum zone. It is of Fortunian age and its
base is marked by the first appearance of T. pedum ( Fig. 5 A). Other first appear-
ances in this zone are Gyrolithes polonicus and Bergaueria . The overlying zone is
referred to as the Rusophycus avalonensis zone and contains the oldest bilobate,
trilobite-type resting traces ( R. avalonensis ) together with the bilobate epichnial
trail “ Taphrelminthopsis” circularis ( Jensen et al., 2006 ). The age of this zone
ranges from the Fortunian to Cambrian Stage 2. Highest is the Cruziana tenella
(

problematica ) zone, which contains the oldest bilobate, trilobite-like furrow-
ing trails ( Cruziana problematica ), commonly associated with large backfilled
traces ( Psammichnites gigas ). The age of this zone is Cambrian Stage 2.
Even simple ichnotaxa with long stratigraphic ranges may be useful to
differentiate Precambrian and Phanerozoic strata. For example, the presence
of such long-ranging trace fossils as Planolites and Skolithos has proven to
be valuable in distinguishing between Cambrian and early Neoproterozoic sili-
ciclastic formations during bedrock mapping in northern Canada ( Aitken et al.,
1973; Fallas and MacNaughton, in press ).

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4.2 Stratigraphy

Cruziana stratigraphy actually is based on ichnospecies both of ribbon-like bilo-
bate traces, referred to as Cruziana sensu stricto ( Fig. 5 B), and short bilobate
traces, referred to as Rusophycus ( Seilacher, 1970, 1992, 1994 ). There is a
considerable functional and biostratigraphic information (e.g., Fortey and
Seilacher, 1997; Seilacher, 1985 ) to indicate that these structures were produced
primarily by trilobites, particularly in early Paleozoic marine deposits, but some
ichnotaxa could have been produced by other arthropods. Trilobites were well
suited for benthic life and inhabited a wide range of environmental settings
during the Early Paleozoic, potentially enhancing the utility of their traces
for biostratigraphy ( Crimes, 1975; Seilacher, 1970, 1992, 1994 ).
Cruziana and Rusophycus ichnospecies are defined based on fine morpho-
logical features. The so-called claw formula (i.e., the distinctive “fingerprint”

Cruziana