Biology Reference
In-Depth Information
To some extent the development of grasslands, along with dry woody
communities, was infl uenced by a factor other than those noted above in
chapter 2. The funnel-shape confi guration of North America southward to
the Isthmus of Tehuantepec is a physiographic factor that affects moisture
availability. This means that continentality is greatest in this region and
decreases south of the Tehuantepec lowlands, where the interior is more
under the infl uence of maritime climates modifi ed by local and discontinu-
ous highlands of moderate elevation. Grasslands as a recognizable and ex-
tensive ecosystem probably developed from a savanna-like version of the
community present by the middle Miocene to one of increasingly modern
aspect during the Pliocene and Quaternary.
The mangrove community was modernized with the introduction of
Rhizophora
, probably from the Old World, in the middle Eocene.
With the introduction of grasses, probably in the early Eocene, the
beach/strand, dune community was modernized.
Aquatic communities, with an angiosperm component, have been pres-
ent since the Cretaceous.
The rain forest of the neotropical lowlands originated in the equatorial
latitudes and dates from the Paleocene.
Versions of the lower to upper montane broad-leaved forest were present
by the Late Cretaceous, but representatives were restricted to areas where
topographic diversity provided an altitudinal cline in habitats and climate,
primarily moisture. The introduction of a northern deciduous component
correlates with the cool events of the middle Miocene and later times (see
chap. 9 below).
Elements of the coniferous forest like
Abies
and
Picea
were present from
the middle Eocene in the highlands of the northern Rocky Mountains. They
probably appeared somewhat later to the south in the still-rising Sierra
Madre of northern Mexico, possibly in the late Eocene, although their exact
pathway and time of arrival cannot be determined from the fl oras available.
The present widespread pine-oak woods in the midaltitudes of Mexico and
northern Central America, as an association within the coniferous forest,
or as part of the broad-leaved deciduous forest, may also date from this time
because the oldest record of both
Pinus
(Cretaceous/Paleocene) and
Quercus
(middle Eocene) are to the north. Pine-oak was present as an association in
the late Eocene / early Oligocene Florissant fl ora of Colorado. An estimate
of the earliest time both genera intermingled as an association in Mexico
is in the Oligocene because by the Oligo-Miocene they are identifi ed indi-
vidually or together in fossil fl oras from Puebla, Chiapas, and in drill-core
sediments offshore from Guerrero (II, table 4.1).
