ENCRUSTERS OF THE CALIFORNIA PLIO-PLEISTOCENE: A PALEOECOLOGICAL PERSPECTIVE

Encrusters are suspension-feeding animals that live permanently attached to a substrate. As a group they are highly diverse, containing representatives from all the major marine invertebrate phyla. Despite the abundance and wide distribution of encrusters in both fossil and modern environments, they are often overlooked in terms of paleoecological importance within paleocommunities. Indeed, fossil encrusters are often omitted from faunal lists and are routinely removed from their host organisms by people working on the hosts.

The Plio-Pleistocene of California provides ample opportunity for examination of the role of encrusters in marine ecosystems. The Pleistocene Palos Verdes Sand cropping out in Newport Beach, CA represents deposition in a shallow marine setting above storm wave base. It contains a diverse molluscan fauna dominated by the bivalve Chione. Shell encrusters present include bryozoans and spirorbid worms. The Pleistocene Nestor Terrace cropping out at Point Loma, San Diego, CA was deposited in a shallower marine environment than the Palos Verdes Sand. It is also dominated by molluscs, particularly the gastropod Tegula and several species of limpet. Spirorbid worms are abundant on many of the shells in the assemblage, and bryozoans, vermetid gastropods, and barnacles are also present in small numbers. A third depositional facies is represented by the Pliocene Imperial Formation in San Diego County, CA. There, oysters encrusted by bryozoans, barnacles, and anomiid oyster holdfasts occur in laterally extensive fossil concentrations interbedded with thick packages of mud. Despite clear differences among the three environments, encrusters are important components of taxonomic and ecological diversity in each one. While the taxonomic component of diversity is roughly the same in each setting, relative abundances of encruster types vary considerably among the three.