FRACTAL TRACE FOSSILS: FOOD FOR THOUGHT

Graphoglyptids (deep-sea trace fossils that exhibit ornate burrow geometries) represent benthic feeding patterns that have been interpreted as mining, grazing, and/or farming. In this study, several different graphoglyptid trace fossils were analyzed quantitatively using fractal analysis in an attempt to determine which of these three feeding modes is most likely. Graphoglyptid burrows readily lend themselves to fractal analysis, because they commonly exhibit the essential fractal characteristics of scale-invariance and self-similarity. Fractal analysis is a useful tool for analyzing geometric configurations, because it combines shape complexity and space usage into one number, the fractal dimension. Fractal dimensions of graphoglyptid burrows (Paleodictyon, Spirorhaphe, etc.) were compared with those of known mining burrows (Zoophycos) and grazing trails (Scolicia) all from Zumaia, Spain. Results indicate that the deposit-feeding (mining and grazing) burrows illustrate a high fractal dimension, as would be expected from a deposit-feeding optimal foraging strategy. In contract, graphoglyptids illustrate a consistently lower fractal dimension than the deposit feeding burrows, thus providing evidence against the suggestion that they represent efficient deposit-feeding behaviors. This observation supports the hypothesis that graphoglyptids represent farming behavior, where burrowing animals harvest a microbial or fungal food supply growing on tunnel walls.