Ichnostratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 201

significance ( Seilacher, 2000 ). Of these, the most distinctive ichnotaxon is
Arthrophycus ( Fig. 5 C), the ichnospecies of which can be used for biostrati-
graphy in Ordovician-Silurian rocks. Five ichnospecies are thought to have bio-
stratigraphic significance: A. minimus (Upper Cambrian to Lower Ordovician),
A. brongniartii (Lower Ordovician to Lower Silurian), A. alleghaniensis
(Lower Silurian), A. lateralis (Lower Silurian), and A. parallelus (Carboni-
ferous) ( Buatois and M´ngano, 2011; Seilacher, 2000 ). Unfortunately, the
evidence supporting the scheme is poor. A few ichnospecies are widespread
(e.g., A. alleghaniensis ), but others are only known from one or two localities,
and accurate, independent biostratigraphic control is lacking in most cases. As
with Cruziana stratigraphy, Arthrophycid stratigraphy is a potential means to
date and correlate otherwise unfossiliferous early Paleozoic sandstone-
dominated strata, but much more work is needed to solidify these ichnostrati-
graphic schemes and test their validity.
5. DEEP-MARINE ENVIRONMENTS
Ichnostratigraphy in deep-marine strata has not been explored to the same degree
as in shallow-marine counterparts. Interestingly, the paucity or lack of body fos-
sils in many early Paleozoic deep-marine successions hampers high-resolution
relative dating of these deposits, and so trace-fossil data can be highly useful in
this regard. In these strata, even long-ranging ichnotaxa can provide useful age
control (e.g., Pickerill and Fyffe, 1999 ). The ichnogenus Oldhamia ( Fig. 6 A),
widespread in Early Cambrian deep-marine deposits, has long been considered
a characteristic fossil in the Cambrian of Ireland (e.g., Baily, 1865 ) andmay have
broader potential in biostratigraphy as pointed out by several authors (e.g.,
Lindholm and Casey, 1990; MacNaughton, 2007; Seilacher, 1974, 2007;
Seilacher et al., 2005 ). Oldhamia typically occurs in intensely tectonized and
slightly metamorphosed rocks that are devoid of body fossils, which has ham-
pered the calibration of this ichnotaxon against other biostratigraphic data. Old-
hamia curvata , O. radiata , and O. flabellata are known from Early Cambrian
rocks, and O. antiqua has been recorded in Early Cambrian to, more rarely, early
Middle Cambrian rocks ( Seilacher et al., 2005 ). An extensive review by
Herbosch and Verniers (2011) suggests that the best-constrained occurrences
of Oldhamia are within Cambrian Stages 3-5. Some authors have proposed that
evolutionary lineages can be recognized within this ichnogenus ( Lindholm and
Casey, 1990; Seilacher, 1974 ). Such lineages could lend themselves to biostra-
tigraphy, but convincing documentation remains elusive. In any case, Oldhamia
is a Cambrian indicator and has allowed distinction from Precambrian rocks
worldwide (e.g., Mirr´ and Ace˜olaza, 1972 ).
Another Paleozoic deep-marine ichnogenus with restricted stratigraphic dis-
tribution is Dictyodora ( Fig. 6 B) ( Benton and Trewin, 1980; Seilacher 1967;
Uchman, 2004 ). D. simplex is recorded from the Cambrian and Ordovician;
D. zimmermanni seems to be restricted to the Ordovician; D. scotica and
D. tenuis are Ordovician-Silurian, and D. liebeana is only known from the
Early Carboniferous.