Ichnostratigraphy - Trace Fossils as Indicators of Sedimentary Environments - page 191

2. CONTINENTAL ENVIRONMENTS
Tetrapod trackways have been extensively used for biostratigraphy in conti-
nental environments (e.g., Haubold and Katzung, 1978 )( Fig. 1 A-D). In a
recent review, Lucas (2007) recognized several global time intervals based
on the footprint record ( Table 1 ). Lucas (2007) noted that global biochronology
based on tetrapod trackways resolves geological time approximately 20-50%
as well as the body-fossil record. Notably, this resolution is better than that of
invertebrate trace fossils currently used in biostratigraphy (see Cruziana and
Arthrophycidae stratigraphy). Also, given the uneven resolution of continental
body fossils through time, the temporal resolution of tetrapod footprints may be
as good as that of body fossils for certain time intervals having a poor skeletal
record. Unfortunately, the reliability of vertebrate footprints in stratigraphy is
limited by the uneven quality of footprint ichnotaxonomy ( Lockley, 2000;
Rainforth, 2003 ). As noted by Lucas (2007) , biostratigraphic zonations using
ichnotaxa defined on the basis of extramorphological features (i.e., morpholog-
ical features resulting from foot/substrate interactions) give the false appear-
ance of stratigraphic resolution, but this is an artifact resulting from
a splitter approach to ichnotaxonomy. Zonations based on better defined
ichnotaxa (e.g., Haubold, 1996, 2000 ) are sound, albeit with limited resolution
( Lucas, 2007 ).
In addition to the extensive use of footprints, vertebrate coprolites may have
some utility for stratigraphic correlation. For example, Hunt et al. (1993, 1998)
found that vertebrate coprolites were useful for correlation in the Upper Triassic
of the western USA.
Invertebrate trace fossils in continental deposits offer relatively little scope
for ichnostratigraphy. The most common invertebrate ichnotaxa in continental
settings tend to have long stratigraphic ranges, and as a result, similarities in
ichnocoenoses between coeval successions are more a reflection of similar
environmental limiting factors than of a common age (e.g., MacNaughton
and Pickerill, 2010 ). One group of continental trace fossils with at least some-
what limited stratigraphic distribution is insect trace fossils in paleosols (see
review by Genise, 2004 ). Most of these ichnotaxa are not known in strata
older than Paleogene. For example, Eatonichnus is only known from the
Paleocene-Eocene and Teisseirei from the Eocene-Oligocene. Although the tem-
poral resolution of these ichnotaxa is quite limited, their presence may be used to
differentiate Cenozoic paleosols from older continental deposits ( Genise et al.,
2000 ). As the ichnotaxonomy of continental trace fossils improves, it is possible
that additional ichnotaxa will prove to have at least broad age implications. For
example, the burrow network Spongeliomorpha carlsbergi , which ranges from
the Late Triassic to the Miocene ( Melchor et al., 2010 ), may be useful for distin-
guishing between Mesozoic and upper Paleozoic continental deposits.