SAY IT AIN'T SOW? -- A CRITICAL RE-EXAMINATION OF AGRICHNIA

Seilacher in 1977 proposed that the complex traces known as graphoglyptids (e.g. Paleodictyon) represent animals cultivating microbes in open burrows in the deep sea, akin to leaf-cutter ants growing fungi. Ekdale et al. (1984) proposed that the ethological category agrichnia, or farming traces, be applied to these forms. This concept needs a critical assessment in the context of our knowledge of organisms that “farm.” We consider farming to be the active generation of useful food from less nourishing precursors. Trapping of food is distinct from farming because food is gathered and consumed directly, perhaps with a delay, but is not used as feedstock. The clearest modern examples of animal farming are terrestrial social insects; less developed versions may exist in the shallow photic zone; e.g., damselfish. Mueller et al. (2005), in a review of agriculture in insects, established four criteria necessary for an organism to be considered to farm its food: habitual planting, cultivation, harvesting and nutritional dependency. These criteria have yet to be convincingly demonstrated in any proposed agrichnial trace. Planting or cultivation may not leave direct fossil evidence; their presence relies heavily on indirect inference. For example, it would be difficult to distinguish deliberate cultivation of microbes on burrow walls from incidental growth. Harvesting, because it might generate distinctive bioglyphs, may be preserved. Nutritional dependency should not leave direct evidence. We suggest that the usage of “agrichnia” be limited until more definite evidence is found of its preservation in ichnofossils.