Ichnostratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 200

left by claws or setae present in the distal part of the walking appendages) is
particularly important, but other features reflecting the producers' gross mor-
phology or burrowing behavior (e.g., presence of cephalic impressions, coxal
marks, exopodal brushings, pleural or genal spine impressions) are also signif-
icant traits to be taken into account. The underlying assumption is that, if leg
morphology displays high rates of evolutionary change, coupled with signifi-
cant changes in burrowing behavior, it is possible to establish relatively narrow
stratigraphic ranges for the different ichnospecies of Cruziana and Rusophycus
( Buatois and M ´ ngano, 2011; Seilacher, 1970, 1992 ).
Ichnospecific classification of Cruziana and Rusophycus is challenging and
may not always be achievable. The claw formula is not always easy to
determine as there are common taphonomic constraints to overcome. Even if
the claw formula can be determined from a single, well-preserved specimen,
this may not be sufficient for reliable ichnospecific assignment. Accurate recon-
struction of the burrowing technique also is essential and typically requires a
representative sample of several specimens. The possibility of convergency also
needs to be carefully considered (compare, e.g., Magwood and Pemberton,
1990 , with Seilacher, 1994 ). Nonetheless, if both morphological and behavioral
traits are taken into consideration, a relatively solid biostratigraphic scheme
emerges (e.g., M´ngano and Buatois, 2003; Seilacher, 1990, 1992 ).
Cruziana stratigraphy has been developed and applied almost entirely with
reference to Gondwana, where more than 30 ichnospecies of Cruziana and
Rusophycus with biostratigraphic significance have been identified ( Seilacher,
1970, 1992 ). Subsequent efforts have expanded the model into Laurentia
( Seilacher, 1994 ) and Baltica ( Jensen et al., 2011; Knaust, 2004 ). The strati-
graphic ranges of these ichnospecies comprise between one and four series,
but the majority of these ichnotaxa are restricted to only one or two series.
However, the available volume of information is uneven. The most extensive
dataset is for the Late Cambrian to Middle Ordovician series (e.g., Baldwin,
1977; Crimes, 1970, 1975; Fillion and Pickerill, 1990; Knaust, 2004;
M´ngano and Buatois, 2003; M´ngano et al., 1996; Pickerill and Fillion,
1983 ), but some significant proposals have been made more recently for the
Early Silurian (Llandovery) ( Seilacher, 1996 ). In the stratigraphic scheme pro-
posed, Cruziana and Rusophycus ichnospecies are in turn clustered into groups
(e.g., dispar group, semiplicata group, rugosa group) ( Seilacher, 1970, 1992 ).
The problems involved in Cruziana stratigraphy have been recently explored
by Buatois andM ´ ngano (2011) , who underscored the need for critical analysis
basedonadequatesamplingandthelackof independent biostratigraphic evi-
dence in the case of many ichnospecies (see also MacNaughton, 2007 ).
4.3 Arthrophycid Stratigraphy
Still more controversial is the so-called Arthrophycid stratigraphy, based on
various ichnotaxa ( Arthrophycus , Daedalus , Phycodes ) included in the ichno-
family Arthrophycidae, which were proposed to have biostratigraphic