Muckraking and mudslinging: The joys of deposit-feeding

Trace fossils, as everyone knows by now, provide us with direct information about fossil behavior, and they offer us a wide variety of mysteries to solve in the area of paleoethology (literally, "the study of ancient behavior"). Chief among these mysteries are "what?", Hwho?", "why?" and "where?". What we especially want to know is what a given trace fossil looks like in three dimensions, what type of organism(s) created it, for what reason(s) they made it, and under what type of environmental conditions the trace was made. Some of the most commonly encountered trace fossils by geologists are infaunal feeding burrows (fodinichnia, agrichnia, and pascichnia). The main reasons for this are that (a) the burrows usually are filled with sediment (often fecal) that differs signficantly in composition and texture from the host sediment, so they easily catch the eye, (b) they are often quite extensive and abundant within a highly localized area, and in some cases they comprise the most obvious sedimentologic or paleontologic feature in the rock unit, and (c) many of these feeding burrows are emplaced far below the water-sediment interface and therefore occupy the deepest tiers of a stratified ichnocoenosis (trace fossil community). The significance of this third point is that deep-tier burrows are the last burrows to be produced in a sequence of bioturbation events, so they overprint all previous burrows that were emplaced at shallower depths in the sediment. Moreover, they were emplaced in a stratum that already has undergone some degree of compaction and is fairly stiff, so the burrow margins are sharp and clear, unlike those of the higher-tier burrows, which often are smashed and smeared, possess .diffuse burrow margins, and exhibit faint color contrasts with the surrounding sediment (Bromley and Ekdale, 1986). In fact, Bromley (1990) described many of these infaunal feeding structures as "elite" structures that dominate the sediment fabric, because they are highlighted by excellent preservation and sometimes are enhanced diagenetically (e.g., by secondary ferruginization or silicification). Fodinichnia (e.g., Chondrites, Ophiomorpha, Thalassinoides, and Zoophycos) are the organized mining traces of deep-dwelling, deposit-feeding animals that efficiently excavate sediment for whatever organic matter that they can find to eat (Seilacher, 1967b, 1986). Pascichnia (e.g., Helminthoida, Nereites, Scolicia, and Spirorhaphe) are the grazing traces of infaunal foragers that plow through an organic-rich horizon to feed on the sediment while on the run (Seilacher, 1967b; Raup and Seilacher, 1969). Agrichnia (e.g., Belorhaphe, Cosmorhaphe, Paleodictyon, and Spirorhaphe) are the geometrically patterned farming or trapping traces created by infauna that cultivate or ensnare their food in a web-like or labyrinthine maze of horizontal open tunnels, which may have been mucus-lined to grow bacteria or trap micro-organisms on the tunnel walls (Seilacher, 1977).