Biology Reference
In-Depth Information
and partially studied fl oras, and even more Quaternary sites. These fossil
fl oras and faunas range throughout the 100 Ma interval covered in this text,
and provide one of the most extensive databases in the world for tracing
biotic and environmental history.
MEXICO
The Cretaceous sea that covered the interior of northern North America
extended along the coastal plains of Mexico to the base of the protoeastern
and -western sierras, and as a mostly shallow sea through central Mexico,
across the Tropic of Cancer, to the Isthmus of Tehuantepec and the Yucatán
Peninsula (fi gs. 2.15). The waters were deep along the Chihuahuan Trough
(Carciumaru and Ortega 2008), shallow over the Yucatán Peninsula, and
there were barely emergent lands like the Tamaulipas Peninsula extending
south from Big Bend National Park in Texas. There were reef-forming corals,
and near Sabinas in the state of Coahuila, there are deposits of coal, indicat-
ing shallow seas, swamps, a low-lying landscape, and warm-temperate to
subtropical environments. To the west were hills of the early Sierra Madre
Occidental (fi g. 2.16). These mountains have a history that is similar to and
contemporaneous with the Rocky Mountains, but the exact relations are
not well understood. The region is the most extensive geologic province in
Mexico, over 1300 km long, and one of the world's largest volcanic fi elds.
Activity continues into modern times as shown by the spectacular lava fl ows
from Volcán Ceboruco in Nayarit through which Mexico Highway 15 is cut.
Uplift was intense in the Cretaceous and waned in the latest Cretaceous and
early in the Tertiary. This change in regional orogeny is shown with great
effect in panoramas exposed in the canyons of northern Mexico. There are
upturned, deep-water, ammonite-containing limestones over 300 m thick
in Huasteca Canyon southwest of Monterrey (II, fi g. 2.4). In contrast, later
strata are fl at-lying and have remained undisturbed (II, fi g. 2.5). Thus, the
landscape of Mexico at 100 Ma consisted of a line of low mountains on
either side of a mostly submerged central plateau. The hills were covered
with ferns, early conifers, and a few angiosperms that were still in their
early stages of radiation. The Cretaceous fl oras of Mexico are mostly Late
Cretaceous or Maestrichtian in age (71-65 Ma) and from the northern part
of the country. They include the La Misión and Rosario assemblages of Baja
California, the Huepac Chert in Sonora, the Olmos fl ora in Coahuila, and
the Piedras Negras fl ora along the Río Grande in Coahuila (chap. 5). Like
their Cretaceous counterparts elsewhere, they tell of warm, equable cli-
mates that extended along low thermal gradients and over discontinuous,
