Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 377

Many biogenic structures produced in lake basins have not yet been formally
defined. Differences between invertebrate ichnotaxa typical of marine environ-
ments and similar trace fossils in continental settings (e.g., Taenidium , Sponge-
liomorpha ), and their implied behavioral and ecological significances, may lead
to conceptual ambiguities where ichnological analyses are not accompanied by
detailed accounts of associated lithofacies. Also, the supposed presence in
continental settings of typical marine ichnogenera (e.g., Paleodictyon , Nereites ,
Scolicia , Chondrites ) has been a matter of confusion (e.g., Archer and Maples,
1984; Bohacs et al., 2007b; Hasiotis, 2002, 2004; Hu et al., 1998; Kim et al.,
2005; Pickerill, 1990; Smith et al., 1982 ). Careful reanalysis indicates that the
forms reported in continental rocks do not display the diagnostic features of cor-
responding marine ichnogenera, being relatively simple in morphology ( Buatois
andM ´ ngano, 2007 ). Asimilar problemresults fromadopting an environmentally
based ichnotaxonomy, such as the use of Isopodichnus for continental examples of
Cruziana and Rusophycus ( Trewin, 1976 )or Naktodemasis for continental exam-
ples of Taenidium ( Smith et al., 2008 ). If the depositional environment is used as a
basis for ichnotaxonomy, the use of trace fossils as an aid in sedimentary facies
analysis would become pervasively affected by circular reasoning.
Vertebrate footprints tend to be named according to the interpreted
tracemaker, which can cause difficulties in recognizing recurrent paleoeco-
logical patterns through geological time ( Hunt and Lucas, 2007 ). In defense
of this practice, it should be noted that most vertebrate tracemakers do produce
morphologically distinct footprints from other vertebrates at the level of genus
to family. In some cases, the morphological variants present in vertebrate tracks
coincide with a higher level of taxonomic resolution than is possible with inver-
tebrate traces ( Voigt et al., 2007 ). Similarly, some of the complex structures
produced by Cenozoic social insects may be attributable to tracemakers at
the level of subfamily or even genus (e.g., Macrotermes ).
Some trace-fossil assemblages in lake basins are recurrent throughout geo-
logical time. In part due to the effects of hydrochemical conditions and other
physical parameters that affect sedimentation as well as the distribution and
behavior of organisms, these recurrent assemblages may be closely associated
with lithofacies that typify depositional systems in different lake-basin zones
( Table 1 ; e.g., Melchor et al., 2006 ). Whenever there is a hiatus between initial
deposition and the production of traces within a substrate, a change in environ-
mental conditions between the two events becomes more likely as the length of
time increases, particularly if the change is related to subaerial exposure of a
substrate deposited subaqueously. Thus, the close association of trace-fossil
suites with particular lithofacies and sedimentological features (e.g., desicca-
tion cracks) is more probable for non-pedogenically altered deposits, which
do not typically undergo large gaps in time between the initial deposition
and the recorded biogenic activity.
The recognition of hiatuses may be one of the most applicable aspects of
continental ichnology, by facilitating the recognition of stratigraphic surfaces