LATE PLEISTOCENE MYTILID AND PETRICOLID BIVALVES FROM THE OPEN ROCKY SHORES OF PACIFIC BAJA-CALIFORNIA (MEXICO) - UNUSUAL PRESERVATION OF MACROFOSSILS

Fossils representing species living on wave-swept, open rocky shores a re seldom found preserved in the stratigraphic record This report docu ments the separate occurrences of the bivalves Mytilus californianus a nd Petricola carditoides found associated with the Plasiatocene-Cretac wus noncontormity at Punts Cabras on the Pacific coast of northern Baj a California, Mexico. A pocket beach deeply incised in the steep andes ite coast at Punta Cabras is the repository of both a modem beach coqu ina and a 3.36-meter thick, fossil shell bank consisting predominantly of transported M. californianus. Analysis of the modem beach across a n onshore-offshore transect helps explain distinct stratigraphic cycle s in the homologous fossil deposit. These cycles may be related to mod est changes in sea level during the last interglacial period (oxygen i sotope stage 5a), to extraordinary storm events, or to local uplift. A t another nearby locality, fossils of P. carditoides are preserved in borings directly penetrating a gentle andesite ramp. The ramp is also partially encrusted with fossilized red coralline algae. The Mytilus d eposit, with its associated gastropod and barnacle species, signifies a transported death assemblage but the Petricola population represents part of an in situ community.