RECOGNITION AND PALEOECOLOGIC SIGNIFICANCE OF COMPLEX ICHNOFABRICS IN THE TRACE FOSSIL RECORD: AN EXAMPLE FROM THE EOCENE OF SOUTHERN CALIFORNIA

Throughout the Phanerozoic, ichnofabrics provide evidence of the exploitation of spatial and nutrient resources by benthic communities. Complex ichnofabrics result from juxtaposition or superimposition of multiple trace fossils, usually reflecting a diverse infaunal community containing a variety of burrowing taxa. However, one individual can create a complex ichnofabric single-handedly by producing different traces representing different behaviors in the same sediment. A single bivalve can produce three distinct ichnogenera during its locomotion, resting and deposit-feeding activities, including Protovirgularia, Lockeia and Lophocteniuum, respectively. Similarly, an individual may produce a single complex trace fossil containing different components created by multiple body parts at different levels within the sediment. Sometimes the different components of a complex trace fossil may be misinterpreted as separate ichnotaxa.

The trace fossil Hillichnus lobosensis consists of several components that were produced simultaneously by different body parts (siphons, foot and shell) of a single individual of deep-burrowing bivalve with dual siphons, similar to Macoma or Tellina. The discrete activities of the incurrent and excurrent siphons during feeding, respiration and egestion, along with locomotion and resting activities of the body, creates an elaborate ichnofabric that comprises a single ichnogenus. Because of its morphologic and behavioral complexity, Hillichnus has been recognized and reported only rarely, and often tentatively, in just a few occurrences in deep-marine (turbidite) deposits from the Cretaceous and early Paleogene. A newly discovered occurrence of Hillichnus in Eocene strata near San Diego, California is preserved in shallow-marine (perhaps intertidal) facies within a highly diverse benthic community. Different levels of individual specimens of Hillichnus are preserved at multiple localities in San Diego County, which taken in isolation may be interpreted as Protovirgularia or Lockeia. However, when considered together they represent a shallow-marine Hillichnus ichnofabric. This San Diego occurrence significantly extends the paleoevironmental range and paleoecologic implications of this complex trace fossil.