Eocene Bursera (Burseraceaea) in La Carroza Formation, Mexico: a dry tropical flora member

Abstract

History of plant diversity in Mexico might be sought in detailed descriptions and comparisons of fossil taxa. Fossil plants allow the recognition of plant communities and their history, especially if they include key elements. In Mexico, Bursera is correlated with dry tropical habitats, and its presence in the fossil record certainly helps to trace the history of both the taxon and the habitat. Eocene material from the La Carroza Formation, La Popa basin, in Nuevo León, Mexico contains a diverse leaf assemblage among which certain leaves resemble two morphotypes, both members of Bursera. A brief description of the leaf architecture of Burseraceae is presented in order to unify criteria and establish comparative parameters that allow identification of fossil leaflet compressions and impressions. Burseraceae has variable leaf architecture, and a single character can change in a single leaf and/or leaflet. For example, the characteristic brochidodromous condition of the family may change to craspedodromous, semi-craspedodromous or even eucamptodromous, in a single leaf. Both Eocene species that are recognized here suggest that the sections Bursera and Bullockia, were differentiated around Middle Eocene; however, previous studies suggest an earlier differentiation of these sections. The two new fossil species in La Popa basin support the presence of members of the extant dry tropical forest in northwestern Mexico, but not necessarily the presence of a dry community. This community developed at a time of geologic/physiographic changes in the area, promoting drought and the establishment of a dry flora.Keywords: Burseraceae leaves; Eocene; La Carroza Formation; leaf architecture; Nuevo León, Mexico. Resumen La historia de la diversidad vegetal en México podría ser buscada en descripciones detalladas y comparaciones de los taxa fósiles. Las plantas fósiles permiten el reconocimiento de las comunidades vegetales y su historia