Hillichnus lobosensis, an unusually complex trace fossil of burrowing bivalves from the Paleogene of southern California, U.S.A.

Several specimens of the complex trace fossil Hillichnus lobosensis have been recovered from the Middle Eocene Scripps Formation in San Diego County, California. These specimens represent the second undoubted occurrence of the ichnospecies. This report extends the geographic range of the ichnospecies from central to southern California, the stratigraphic range from Late Paleocene to Middle Eocene, and the paleoenvironmental range from deep-water turbidite to shallow-marine nearshore settings. The San Diego specimens are the only known specimens of Hillichnus to be housed safely within a curated natural history collection (San Diego Natural History Museum), whereas the originally designated type specimens of H. lobosensis are subject to weathering and erosional processes at the type locality in Monterey County, California. The San Diego specimens also are important, because the trace fossils are the first of their ichnospecies that have been found in association with body fossils of the presumed bivalve (tellinacean) trace makers.