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

Resumen

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.