Biology Reference
In-Depth Information
MEXICO, THE ANTILLES, CENTRAL AMERICA
Widespread seasonally drying climates, expressed primarily as less winter
rainfall, are evident from sediments in northern Baja California (Peterson
and Abbott 1979). The paleosols show a sequence from quartz-kaolinite
and cation-depleted soils in the Paleocene and early Eocene, indicating a
humid tropical climate (MAP 1250-1900 mm), to caliche, vermiculite,
and smectite clays in the late Eocene, characteristic of drier and seasonal
climates (MAP 630 mm). The present MAP is 250 mm. These trends in
the later Eocene were toward drier and more seasonal climates. Climati-
cally, conditions were not arid, but local arid habitats existed because of
the augmenting effects of topography and soil. Drying climates intensifi ed,
and dry to arid vegetation continued to develop and expand, especially in
middle Miocene through Pliocene times, and during each of the dry glacial
intervals of the Quaternary.
There are few extensive and recently studied fossil fl oras available from
the middle Eocene through the early Miocene in northern Latin America,
especially ones integrated into multifaceted investigations most effec-
tive for reconstructing and tracing the history of ecosystems. The Eocene
fl oras, in particular, often contain marginally preserved fossils that are dif-
fi cult to identify accurately. Two sites have provided information on Oli-
gocene vegetation of northern Latin America. One is the Los Ahuehuetes
fl ora from the Pié de Vaca Formation in the state of Puebla, Mexico. It is
probably late Oligocene to possibly early Miocene in age. The plant mac-
rofossils are currently under study by Susana Magallón-Puebla and Sergio
Cevallos-Ferriz, and the microfossils by Enrique Martínez-Hernández and
colleagues at UNAM (e.g., Magallón-Puebla and Cevallos-Ferriz 1994;
Martínez-Hernández and Ramírez-Arriaga 1999). The fl ora contains at
least one present-day temperate Asian plant familiar from fossil fl oras to
the north. The genus
Eucommia
is native to western and central China, and
its presence in southern Mexico documents the extension of present-day
Asian elements into northern Latin America during the Tertiary. In addi-
tion to this and other mesic species in the fl ora, drier vegetation is indicated
by
Haplorhus
,
Cercocarpus
,
Prosopis
(mesquite), and
Sophora
. The fl ora was
deposited near the confl uence of the southern Sierra Madre Oriental and
the just-rising eastern Transvolcanic Belt in a region of at least moderate
topographic diversity. In the same area today, there is mesic vegetation, as
well as drier communities resulting from edaphic conditions, slope, expo-
sure, and rain shadow habitats such as the
Nolina-Hechtia-Agave
near desert
on the border of Puebla and Veracruz. The fossil fl ora documents the con-
