Microbioerosion - Trace Fossils as Indicators of Sedimentary Environments - page 225

While such euendoliths indicate cool- or cold-water conditions by their absence,
only few ichnotaxa have been identified to date that indicate low paleotempera-
tures by their presence. These are the fungal trace Saccomorpha cf. terminalis
( Radtke, 1991; Wisshak, 2006 ; new ichnotaxon currently under description),
the possible thraustochytrid trace F. baiulus ( Wisshak and Porter, 2006 ), and
the presumed foraminiferal boring S. pulchra as discussed by Bromley et al.
(2007) .
There are three possible pitfalls linked to the evaluation of microborings
with respect to (paleo)temperature. First, some traces might have been produced
during a relatively short time interval during a warm summer spell by euendo-
liths that would not survive winter temperatures in the given area or vice versa .
Second, it is paramount to acknowledge the fact that paleotemperature does not
necessarily allow deductions on the (paleo)latitude despite a certain degree of
correlation, since “cold-water species” may well occur in tropical latitudes in
cold bottom waters or shallow upwelling areas. The proposed cold-water indi-
cator F. baiulus , for instance, was found as far south as the warm-temperate to
tropical Florida Escarpment but only in cold bottom waters in 3000 m depth
( Wisshak and Porter, 2006 ) and S. pulchra was reported from cold upwelling
waters off Mauretania ( Glaub, 2004 ). Third, temperature and latitude bear a cer-
tain degree of correlation with the trophic regime that shows an overall trend of
eutrophication toward higher latitudes. Hence, an apparent temperature depen-
dency might actually reflect a certain nutrient demand instead. However, there
are a number of taxa ( Table 3 ) that can yield us valuable information on the
paleotemperature of a depositional system, provided that the whole ichnocoe-
nosis is considered and other biogeographic indicators are taken into account.
Our present knowledge on the biogeographic distribution, particularly of the
named cold-water indicators, is still limited, calling for more research in this
direction.
4.3 Paleosalinity
Microendoliths occur over a wide range of salinities from hypersaline lagoons
to pure freshwater environments, implying the feasibility of applying microbor-
ing trace fossils as paleosalinity indicators, even though they have to date not
been widely appreciated as such. Whereas many euendoliths are obligate open
marine and thus strictly stenohaline, a number of euryhaline representatives
have been identified ( Table 4 ). The endolithic bio- and ichnodiversity strongly
decreases when salinity differs from open marine conditions—a trend that is
only partially an artifact of the highly understudied freshwater and hypersaline
settings.
Radtke (1991, 1992) describes a number of ichnotaxa from brackish-water
paleofacies of parts of the Paleogene Paris Basin, France, and concludes that
boring assemblages from brackish environments consist mainly of heterotro-
phic endoliths. From a drill-core in an Oligocene limnic-brackish lake setting