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inhabitable water ways. For pioneering aquatic organisms that can withstand
harsh environmental conditions including perennial ice, low water tempera-
tures, high turbidity, and dissolved solids, only topographic barriers, in the
form of spillways, stand in the way.
Paleobiogeographical and paleoecological inferences using bedding-plane
trace fossils are tenuous and require identification of tracemakers—a challenge
given that the tracemakers are not deep infauna and do not produce trace fossils
characteristic of their particular trophic position or of their anatomy with regu-
larity. Because of the abundance of well-preserved bedding-plane trace fossils
within the Quaternary glacial rhythmites and the ability to utilize records of mod-
ern species occurrences as a starting point, identification in some cases has been
more than speculative. Comparisons can bemade between the assemblages of the
Late Paleozoic glacial strata and those of Cenozoic deposits. The replacement of
merostome and myriapod trace fossils of the Paleozoic trace-fossil assemblages
by subaquatic crustaceans in the Cenozoic glaciolacustrine deposits is probably a
consequence of evolution, as most merostome arthropods were extinct at the end
of the Paleozoic. However, ichnofacies relationships and ecological niche occu-
pation remain similar between the old and the young deposits, which are an indi-
cation of the constancy of biotic reaction to glacial events through time.
ACKNOWLEDGMENTS
The research summarized in this chapter was supported by the Brazilian National Council
for Scientific and Technological Development (CNPq grants 479457/2007-7, 503487/
2007-4, 305208/2010-1, and 401826/2010-4 to R. G. N.), Tufts University Faculty (Research
Award to J. S. B. and J. C. R.), National Science Foundation (Award 0639830 to J. C. R.),
Natural Sciences and Engineering Research Council (NSERC) (Discovery Grants 311727-
05/08 to M. G. M. and 311726-05/08 to L. A. B.), Jagillonian University (DS funds to A.
U.). The critical reviews made by J. Isbell, H. Walter, and P. Suhr were capital to improve
this chapter.
REFERENCES
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