Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 396

in salinity (due to freshwater input), while low diversities are maintained in the
center of the lake due to continued chemical stratification ( Cohen, 2003 ). Areas
with localized freshwater input (e.g., spring-fed pools and marshes, delta plains)
favor the concentration of many species of insects, mammals, birds, and rep-
tiles. Local development of microbial mats, associated with hypersaline condi-
tions and/or hot springs, may favor burrowing by certain insects (e.g.,
staphylinid and heterocerid beetles). Matgrounds help to stabilize the substrate
or contribute to its early cementation, increasing the preservation potential of
both vertebrate and invertebrate biogenic structures ( Scott et al., 2007 ).
The Scoyenia Ichnofacies is widespread around lakes in underfilled basins.
Lake-margin assemblages mostly comprise root traces of saline-tolerant sedges
and grasses, arthropod trackways (e.g., Diplichnites , Umfolozia ), meniscate traces
(e.g., Scoyenia , Taenidium ), bilobate traces (e.g., Cruziana , Rusophycus ), chiro-
nomid dipteran, coleopteran, and annelid dwelling and feeding traces (e.g., Laby-
rintichnus ), and vertebrate trackways produced by sauropsids, amphibians,
dinosaurs, birds, andmammals.Due to the influence of salt efflorescence andother
destructive taphonomiceffects in subaeriallyexposed, hypersaline settings, poorly
preserved vertebrate footprints and infaunal burrows may be the only preserved
evidence of the Scoyenia Ichnofacies, especially in areas where capillary evapo-
ration of saline porewaters occurs (e.g., Cohen et al., 1991; Rodr´guez-Aranda and
Calvo, 1998; Scott et al., 2010 ). High-density, monospecific to low-diversity
assemblages dominated by sharp-walled vertical burrows ( Skolithos ) in subaerial
substrates of both high-energy and low-energy eulittoral areas without the influ-
ence of freshwater are attributable to the Skolithos Ichnofacies.
Progressively desiccated, fine-grained substrates typically preserve better
defined trackways emplaced in dewatered sediment, which commonly cross-
cut less-defined imprints that were formed in less-firm substrates ( Buatois
et al., 1997; Zhang et al., 1998 ). The density of trackways may be high, with
forming arthropod-tracked omission surfaces, or those representing a period
of non-deposition ( Fig. 10 E; e.g., Minter et al., 2007; Zhang et al., 1998 )or
high-density tetrapod tracksites (e.g., Farlow and Galton, 2003; Szajna and
Hartline, 2003 ). Some omission surfaces may represent sequence boundaries
expressed by coplanar surfaces, or those representing both lowstand and subse-
quent flooding. Lake-level fluctuations are conducive to complex cross-cutting
relationships and trace-fossil suite overprinting, particularly where the surfaces
involve more than one transgressive/regressive cycle ( Scott et al., 2009 ).
Relatively extreme variations in lake levels and water-table depths in under-
filled basins can lead to the presence of traces typically associated with terres-
trial environments within lacustrine and lake-margin deposits. Depending on the
conditions of the initial depositional environment, terrestrial trace-fossil assem-
blages (e.g., Celliforma , Termitichnus ) may overprint previously emplaced
lacustrine or lake-margin suites, or they may be emplaced within lacustrine
deposits that do not preserve other evidence of biogenic activity. In both cases,
the drop in lake level, and thus the water table depths within the zone of