Testing ethological hypotheses of the trace fossil Zoophycos based on Quaternary material from the Greenland and Norwegian Seas

Despite the fact that the trace fossil Zoophycos has been found in rocks from most of the Phanerozoic, little consensus has been reached on how and why this intricate burrow was constructed. The Cenozoic morphotypes of Zoophycos typically include a helically coiled spreite arranged around a vertical shaft connected to the sediment surface. Morphological details and environmental preferences of the Zoophycos producer were studied using 156 cores from the Norwegian and Greenland Seas in order to test the different ethological hypothesis proposed for this trace fossil. The spreiten were constructed during interglacial or interstadial intervals and consist of a repeated alternation of lamellae consisting of fine-grained pelleted material, and coarse-grained unpelleted material, respectively. Spreiten were encountered in vigorously bioturbated sediment, in turbidite layers, and in layers dominated by coarse ice-rafted debris. This indifference to the composition of the substrate effectively rules out ethological models based on different forms of deposit-feeding, and the large size and wide spacing of the whorls of the spreiten also make the cesspit model unlikely. Rather, the observed features best agree with a cache behaviour, where the main purpose of the deep penetration was to store food and to prevent access by other burrowers, likely combined with some gardening of microbes. However, no indubitable evidence of a reworking of the cached material could be found, and probably the answer to how the cache was accessed by the producer is to be found in the marginal tube. The difference in diameter between marginal tube and spreiten lamellae, together with the presence of both open and filled marginal tubes indicate that the marginal tube is the result of a far more complex behaviour than simply a lateral shift through the sediment.