Leaves of salicaceae (Salix and Populus) from Oligocene sediments near Tepexi de Rodriguez, Puebla, Mexico

Eleven new salicaceous taxa based on fossil leaves, seven Salix and four Po pulus species, are described from the Oligocene Los Ahuehuetes locality Pie de Vaca Formation, near Tepexi de Rodriguez in Puebla, Mexico. They are ch aracterized by the presence of typical salicoid teeth, camptodromous second ary venation, and elliptic or lanceolate shape. Morphological differences b etween these new fossil plants and those reported previously from the Unite d States and elsewhere indicate that diversification among these taxa occur red early during their differentiation. Furthermore, the limited morphologi cal similarities of the fossil and extant plants growing naturally in Mexic o also indicate a high rate of evolution. Documenting the expansion of high -latitude North American taxa into lower latitudes at the time of depositio n of the Pie de Vaca Formation confirms that at least some of the typical b oreal taxa were part of dry tropical communities in low-latitude North Amer ica and underscores the need to reevaluate the origin of this southern vege tation.