Impression macrofloras recovered from two localities in the ~43-44 Ma Friars and lower Mission Valley Formations, San Diego County California, were deposited in a fresh or brackish water coastal lagoon during the middle Eocene sea level maximum. The newly recovered assemblages further constrain the timing and magnitude of vegetational and climatic change established from previously known middle Eocene macro and microfloras of the San Diego region. Preliminary identifications of taxa in the newly discovered floras include Acrostichum, Polypodiaceae?, Sabalites, Flacourtiaceae(?), Juglandaceae (Pterocarya?), Macclintockia, Myrtaceae, Nelumbo(?), Phytocrene sordida, Rhus, Rhizophoraceae(?), Ternstroemites, and numerous unidentified forms. Nearest living relatives and leaf physiognomy of plants in the assemblages suggest dense evergreen broadleaved forest growing under humid paratropical climatic conditions, similar to conditions interpreted from previously described floras of the 45.5 Ma Torrey Sandstone, San Diego. Overlying ~ 42-43 Ma pollen and macroflora assemblages from floodplain and fluvial facies of the Mission Valley Formation are interpreted to record open woodland vegetation growing under seasonally dry climatic conditions. The timing of the development of seasonally dry climate and corresponding vegetational change in the San Diego sequence correlates with comparable events recognized in the Clarno Formation, Central Oregon, and the Gulf Coast sequence of the southeastern US. Because middle Eocene floras from the San Diego sequence co-occur with terrestrial vertebrate faunas and are interbedded with marine macro and microfossil-bearing strata, they provide a precise benchmark for the timing and magnitude of continent-wide climatic and vegetational trends.

Key words: Macclintockia, mangrove, middle Eocene, paleobotany, paleoclimate, San Diego