Hillichnus lobosensis igen. et isp. nov., a complex trace fossil produced by tellinacean bivalves, Paleocene, Monterey, California, USA

A highly complex trace fossil has been well exposed and long known at Lobos Point, south of Monterey, central California but has defied interpretation. The sediments are Paleocene deep-sea canyon fill. Improvement of our knowledge of deposit-feeding bivalves has allowed a reasonable interpretation of the trace fossil as the work of a subsurface deposit-feeding tellinacean bivalve. The trace fossil is designated Hillichnus lobosensis igen. et isp. nov. Excursions of the animal’s inhalant siphon have created feather-like and spreite-like structures to either side of a basal axial tube complex. Siphonal excursions to the seafloor have left an array of upward-curving tubes following a straight or somewhat zigzag course, comprising the uppermost level of the structure. The length of these tubes indicates that feeding was taking place well below oxygenated sediments. This in turn suggests that, in addition to deposit feeding, chemosymbiosis with sulfide-oxidizing bacteria may have been practised. On the other hand, the continuous forward movement of the animal rather conflicts with this interpretation. Some individuals show a clustered grouping of the rising tubes, which suggests short-term feeding at the seafloor.