Caron, Scheltema, et al. (2006) had suggested that the wrinkles on the top surfaces of "Odontogriphus" specimens were caused by the rippling contractions of a mollusc-like muscular foot. Butterfield disputed this on the grounds that: a molluscan foot is also mainly composed of cellular material, which he thought unlikely to be fossilized in Burgess Shale conditions; the wrinkles were too straight and ran too precisely across the animals' bodies; the gaps between them were the same size as the gaps between the gill-like structures round the foot. Instead he argued that they were evidence of externally visible segmentation, which is found in polychaetes but not in molluscs. He concluded that "Wiwaxia" was an evolutionary "aunt" of polychaetes, while "Odontogriphus" could be an evolutionary "aunt" of polychaetes or of molluscs or of brachiopods – or even a "great aunt" of all three, as it could have been an early member of the lophotrochozoa, a "super-phylum" that includes the polychaetes, molluscs and brachiopods.