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ICHNOLOCJY OF LACUSTRINE SYSTEMS 299

Brncken and Pieard, I 984; Squiros and Advocate, 1984; P'Alossandro ot al., 1987; Debriette and Gand, 1990; Sarkar and Chaudhiiri, 1992; Smith, 1993; Aceflola/a and Buatois, 1993; Bualois ot al., 1996a; Kim and Paik, 1997: Gand ot al., 1997; Eberth et al., 2000; Savrda et al., 2000; Aramayo and Bocanegra, 2003; Buatois et al., in press). Desiccated overbank ichnofaunas almost invariably belong to the Scoyenia ichnofacies (Buatois and Mangano, 2002, 2004). Analysis of tracę fossil morphology commonly roveals features indica-tive of firm substrates, such as striated walls in Scoyenia and    Spongell iomorpha,    sharp scratch

marks in Tainbia, Cruzinnn, and Rusophycus, and well-defined appendage imprints in arthropod track-ways. In some cases, two distinct suites can be recognized: one 'pre-desiccation suitę' characterized by meniscate, backfilled structures without ornamentation (e.g., Tnmidiiim, Beaconites) developed in a soft substrate, and the second or 'desiccation suitę' typified by striated traces (e.g., Scoyenia, Spongelliomorpha), cross-cutting the former and devel-oped in a firm substrate (Buatois et al., 1996a; Savrda et al., 2000; Buatois and Mangano, 2002, 2004). The resulting palimpsest surfaces record tapho-nomic pathways due to progressive desiccation of floodplain sediments. Desiccated overbank ichnofaunas tend to domina te in distal overbank settings and/ or arid to semiarid settings (Buatois and Mangano, 2004).

Ichnofaunas from overfilled overbank deposits consist of simple grazing trails (e.g., Helminthopsis, Helminthoidichnites), locomotion trails    (e.g.,

Cochlichnus), horizontal dwelling burrows (e.g., Ctenopholeus, Palaeophycus), and resting traces (e.g., Lockeia). Tetrapod tracę fossils, arthropod track-ways, backfilled tracę fossils, and bilobated traces with scratch marks either are subordinate elements or are directly absent. Most of the tracę fossils are oriented parallel to the bedding piane and reflect very shallow tier emplacement. Ichnodiversity is Iow torarely moderate. Examples of these ichnofaunas are aiso well doctimented in the literaturę (Turner, 1978; Fordyce, 1980; Miller, 1986; Pollard and Hardy, 1991; Ciuszek, 1995; Buatois et al., 1997a; Buatois and Mangano, 2002; Keighley and Pickerill, 2003; Ochman et al., 2004). Tracę fossils typically occur in parafie! slrałi/ied, climbing ripple cross-laminated or oirrtmt ripple cross-laminated very fine- to medium* i'JairterI ^amNtone and mudsłone. Physical structures indkatiye of subaeTial exp(»^{uire are absent, reflecting "verb*nk crtif al aci retton rather łhan desiccation uf ^ body Morphologic details of the tracę fossils *^t¥ ^ery pondy preserwd, suguesting that fheV were ^f,rfnetf in a water"Mturated substrate •eg, Buatois et al., 1997a). Overall features of these ichnofaunas reflect the subaqueous naturę of the associated environment. Poorly preserved traces may be crosscut by better defined softground tracę fossils reflecting improving taphonomic conditions due to increasing firmness of the substrate. Although formed in flood-plains, these ichnofaunas lack most of the diagnostic features of the Scoyenia ichnofacies and are regarded as examples of a depauperate Mermia ichnofacies (Buatois and Mangano, 2002, 2004). The lower ichnodiversity of the Mermia ichnofacies in these floodplain deposits in comparison with their equiva-lents from lacustrine basins is probably an expression of the less stable conditions and temporary naturę of overbank environments. Overfilled overbank ichnofaunas tend to dominate in proximal overbank settings of meandering systems and/or temperate and humid settings (Buatois and Mangano, 2004).

ICHNOLOGY OF LACUSTRINE SYSTEMS

Information on the ichnology of lacustrine deposits is summarized in Table 17.3 based on 70 data entries. Biogenic structures formed in lacustrine sediments probably have the highest preservation potential of all Continental tracę fossils. Preservation of biogenic structures is particularly favored in low-energy areas of lacustrine systems. For example, altemation of very fine sand and mud deposited from underflow or turbidity currents is conducive to preservation of tiny surface to very shallow traces (Buatois and Mangano, 1995a, 1998). In low-energy Coastal areas of lakes, preservation of biogenic structures is commonly linked to rapid influx of sand via non-erosive sheet floods (e.g., Zhang et al., 1998). Although monospe-cific tracę fossil assemblages are present, moderately diverse ichnofaunas are common in lacustrine deposits (Table 17.3). Lacustrine systems can be divided into hydrologically open (i.e., with an outlet) and hydrologically closed (i.e., without an outlet) (Gore, 1989).

Closed lakes are very stresstul i\\vs\‘stv'n\s, char-acterized by high salinity and rapidly tluctuating shorelines (Gore, 19g9), Due to harsh i.otuiitions, fauna 1 diversity is verv Iow and bvoge\\\c structures emplaced under permanent sul\tv|uiH'us conditions are scarce or absent (e.g., Prieo and Nh^.'ann, 1990; Uchman and Alvany ,'000). 1 lowwer, nuH.1erab'lv diverse ichnofaunas mav occur at the lakę margins. recnrding tlu* activitv ot terrestri«d rather than av|uatic faunas (e.g, /hang et al., 1998; Scott et al, in press). (lyerall, closed lakę ichntUaunas consist ot plant