Continental trace fossils and museum exhibits - Geological Curators ...

2000, 2004). These categories are based on thedistribution of extant organisms and theirphysiological requirements for water (e.g., Kevan1962, Wallwork 1970, Hasiotis and Mitchell 1993,Hasiotis 2000), as well as the distribution ofichnofossils, sedimentary structures and pedogenicfeatures exhibited in outcrop and core (e.g., Stanleyand Fagerstrom, 1974, Bown and Kraus, 1983,Hasiotis and Mitchell 1993, Hasiotis and Dubiel1994, Hasiotis and Honey 2000). Organisms livingabove the water table in the well-drained, uppermostparts of the vadose zone construct terraphilic traces.These organisms have low tolerance for areas ofprolonged high moisture levels, tolerate short periodsof 100 % soil moisture and live in areas with relativelylittle available water. Surface-dwelling, trackwaymaking,shallow-nesting organisms construct surfacetracks, trails and traces, and are termed epiterraphilic.Epiterraphilic tracemakers and their traces can cooccurwith other behavioural categories at groundlevel during periods of elevated moisture levels.Organisms living within the intermediate and lowerparts of the vadose zone construct hygrophilic traces.This category includes organisms that live aboveground, but burrow to higher moisture levels in thesubstratum for reproduction. They obtain their oxygenfrom the soil atmosphere and above-groundatmosphere via the main shaft or tunnel of the burrowor nest. Hydrophilic traces are constructed byorganisms that live below the water table within a soiland within the substratum in open bodies of waterwhere the water table intersects the land surface orwater is perched above the surface by an impermeablelayer, such as clay, to form rivers, swamps and lakes.These organisms obtain oxygen from the water, butcan also use high levels of soil moisture to keep theirgills wet for short periods of time (e.g., Horwitz andRichardson 1986, Hasiotis and Mitchell 1993). Thiscategory includes organisms that burrow to a positionbelow the water table and maintain the whole burrow,including the entrance at the surface.The depth and cross-cutting relationships ofcontinental traces, also known as tiering, can be usedto approximate the position and fluctuation of theunsaturated and saturated zones of the palaeogroundwaterprofile (Hasiotis and Dubiel 1994).These interpretations are verified independently byexamining the association of primary and secondarysedimentary structures, and the development of suchpedogenic features as mottling, ped structures,colouration, micromorphology, texture andgeochemistry (e.g., Retallack 1990, 1997). In severalcases in the Willwood Formation in the BighornBasin, Wyoming, red palaeosols interpreted asrepresenting a well-drained environment containrelatively deep, penetrating rhizoliths consisting ofgrey, iron-depletion zones with red rims, indicatinghaematite accumulation. Powdery calcium carbonateis present locally within the grey depletion zones.These features indicate surface water gley (i.e.,standing water on the surface) processes that causediron and manganese to move from the root channeloutward to the soil matrix and carbonate precipitationin the channel as the soil dried. Burrows are diverse,abundant and distributed deeply in these palaeosols.Purple palaeosols interpreted as representing morepoorly-drained environments have rhizolithsconsisting of iron depletion zones surrounded byyellow-brown rims composed of goethite, indicatingsurface water gley processes. Burrows are lessdiverse, but abundant in these palaeosols to shallowerdepths. Palaeosols that are even more poorly drainedcontain rhizoliths preserved in jarosite, which is anoxidation product of pyrite, and are associated withvery few and shallow penetrating trace fossils. Verypoorly drained, low chroma palaeosols contain sparserhizoliths that do not penetrate deeply and burrowsare very rare to absent. In all of these palaeosols, theposition of the water table is marked by the placewhere the primary sedimentary structures and nearlyoriginal sedimentary layering is still preserved (Krausand Hasiotis 2006, Hasiotis and Kraus unpublisheddata).Trace fossils record in situ evidence of food-web andother ecological relations between fossorial, terrestrialand aquatic communities. For example, borings ondinosaur bones suggests scavenging by dermestidbeetles of the postmortem dinosaur carcass (Hasiotiset al. 1999, Hasiotis 2004). Modern forensic studieshave demonstrated that there is a succession ofnecrophilous (dead-flesh eating) and saprophagous(feeding on dead or decaying material) insectsthroughout the stages of decay on carcasses (Smith1986). An ecological succession of insects resultsfrom changes in the attractive nature of a carcassleading to the complete decomposition of the animal(e.g., Reed 1958, Payne 1965). The dermestids areone of the last arthropods to arrive to a carcass duringthe dry stage where they feed on skin, fur and horns,and sometimes bore into bone to pupate. The tracefossils of dermestid feeding and pupation are the onlyevidence of this type of detritivore recycling duringthe Late Jurassic (Hasiotis et al. 1999, Hasiotis 2004).In other examples, trace fossils interpreted as thoseof dung beetles suggest strongly the presence ofherbivores and edible plants, and their interactions(e.g., Chin and Gill 1996, Hasiotis 2002, 2004, Radieset al. 2005).-216-