Bryozoan-Hermit Crab-Gastropod Associations from the Fernando (?) Formation (Late Pliocene, Southern California) and Salada Formation (Middle Miocene, Western Baja California)

Penny A. Morris, Judith T. Smith

Trochid and naticid gastropod shells have been altered by bryozoan-hermit crab associations to produce subspherical to ovate balls 3-6 cm in diameter. Hermit crab occupation can be determined by the location and development of bryozoans, presence of other incrusting organisms, worn shells and altered apertures. In this material, the apertures are reduced in size (some are only 3-4 mm) and oval in shape.

Silty to sandy sediments and associated fossils suggest shallow water conditions at both the late middle Miocene Salada Formation of western Baja California and the late Pliocene Fernando (?) Formation at 6th and Flower Streets, Los Angeles, California. At the present, four genera of bryozoans have been found to be incrusting these shells. Bryozoans from the Salada Formation include Antropora, Cauloramphus (?) and Hippothoa hyalina. Although the first two species appear to be extinct, the latter is extant, found on nontrochid-naticid gastropod shells and does not incrust heavily. The bryozoan Hippopodinella adpressa, found in the Fernando (?) Formation, lives only as far north as Mexico today.

Investigations are underway to determine whether single or multiple bryozoan taxa incrust particular shells and whether hermit crabs were able to enlarge their living spaces by absorbing shell material. Such alteration of the shell coupled with anatomical adaptation of the crab's abdomen, as m modern examples, could enable fossil hermit crabs to occupy relatively scarce shells for a longer period of time. SEM micrographs are expected to verify bryozoan identifications and indicate whether they have the same faunal affinities as the associated mollusks: Tertiary-Caribbean for the Salada Formation and California to Pacific-Panamic for the Fernando (?) Formation.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995