Lacustrine Environments - Trace Fossils as Indicators of Sedimentary Environments - page 375

variety of benthic organisms capable of producing traces, such as annelids
(e.g., oligochaetes), aquatic to semiaquatic insects (e.g., dipterans, coleop-
terans), crustaceans (e.g., decapods, amphipods), and molluscs (e.g., bivalves,
gastropods) ( Cohen, 2003; White and Miller, 2008 ). Vertebrates that inhabit
lacustrine settings, including fish, amphibians, and aquatic reptiles, may also
interact with the substrate. In subaerially exposed lake-margin settings, trace-
producing organisms include semiaquatic to terrestrial insects (e.g., crickets,
earwigs, beetles, ants, termites), arachnids (e.g., spiders, mites), crustaceans
(e.g., crayfish), and molluscs (e.g., land snails). Vertebrate diversity may be
high in lake margins, even around saline lakes, and include trace producers such
as tetrapods (e.g., mammals, reptiles, amphibians) and birds (e.g., shorebirds,
flamingos). Plants that produce biogenic structures occur in the shallow,
oxygenated littoral zone of lakes (e.g., sedges), as well as in lake margins with
shallow water tables (e.g., grasses, sedges) and terrestrial and fluvial settings
near the lake (e.g., shrubs and trees, ferns, herbaceous plants). The diversity
and distribution of plants and their traces are also controlled by similar environ-
mental parameters as invertebrates and vertebrates (e.g., salinity, oxygen,
sediment texture and drainage; Cohen, 1982; Kraus and Hasiotis, 2006 ).
In modern freshwater lakes, the density and diversity of the aquatic benthic
fauna usually reach a maximum in the oxygenated sublittoral zone, referred to
as the concentration zone by Mackie (2001) . In most lakes, this zone occurs
between depths of 2 and 4 m, but may be as deep as 18 m in large well-mixed
lakes ( Mackie, 2001; Martin et al., 2005; White and Miller, 2008 ). Lakes with
high productivity of planktonic microorganisms (e.g., warm tropical lakes,
nutrient-rich lakes), and/or high salinities, are generally not well oxygenated,
restricting the biodiversity to the littoral to sublittoral zones ( Hammer,
1986 ). In lake-marginal settings with harsh conditions (e.g., high temperature,
high salinity), tracemakers may concentrate in localities where conditions
(e.g., available freshwater, food resources) are more suitable (e.g., Gierlowski-
Kordesch, 1991; Scott et al., 2007, 2009 ). Hydrochemical conditions, including
the availability of free oxygen, the pH, ionic concentrations, temperature, and
drainage, may be the primary factors affecting the presence and behavior of infau-
nal and epifaunal organisms in both aquatic lacustrine and subaerial lake-margin
environments with shallowwater tables. Energy levels, sedimentation rate, turbid-
ity, and substrate conditions may be secondarily important for the diversity and
distribution of tracemaking organisms, but these factors are certainly important
for the production and preservation of trace fossils.
The types of traces common in lakes and lake margins that are produced by
invertebrates in either subaqueous or subaerial substrates include simple surface
trails, open surface tunnels, trackways, burrows backfilled with waste material
and/or excavated sediment, branching 3D burrow systems, and vertical burrows
( Fig. 1 ). Vertebrates produce structures such as swim traces, footprints and track-
ways, trampled surfaces, nest-mounds, and burrows. Vertebrate traces may be
present in the profundal and sublittoral zones of a lake, or shallow lacustrine