Biology Reference
In-Depth Information
forest grows immediately behind the mangroves and borders the coastal
depositional basin. Communities farther inland and higher in elevation,
such as the deciduous forest of
Quercus
and
Liquidambar
, and even the high-
altitude coniferous forest of
Abies
and
Picea
, were represented, so the rain
forest as presently constituted really was absent, or present only as a highly
modifi ed community. In its place there was the low wet phase of the lower
to upper montane broad-leaved forest. Another surprise was the presence
of
Picea
(spruce). It no longer grows in the vicinity, but it is found in the
highlands of the Sierra Madre Occidental far to the north.
The most parsimonious explanation for these patterns—the absence (or
presence of a modifi ed version) of rain forest, the good representation of
midaltitude deciduous forest and high-altitude montane coniferous for-
est, and the occurrence of spruce far south of its present range—is cooler
temperatures. However, this creates a problem regarding context informa-
tion provided by the global paleotemperature trend, and the results from
oxygen isotope and paleoecological studies of western Atlantic marine wa-
ters, which show that the mid-Pliocene between 4 and 3 Ma was a period
of warmth within an overall trend toward cooler conditions. It has been
mentioned that when such anomalies arise, especially in the modern era
when paleontological information is incorporated into such a wide variety
of taxonomic and environmental studies, it is incumbent on investigators
to (1) acknowledge an awareness of the apparent inconsistency, (2) suggest
a plausible explanation, or (3) defer conclusions pending additional data.
In this case, there is an explanation for cool conditions along the coast of
Veracruz during an otherwise warm period.
The Isthmian region of Panama began closing with the formation of a
submerged sill 9-7 Ma, and closure was essentially completed by about
3.5 Ma, when the land bridge was established. One effect was to intensify
northward fl ow of the Agulhas / Gulf Stream. When such changes in ocean
circulation occur, one consequence is upwelling of cold bottom waters
along coastal areas (e.g., Prange and Schulz 2004). The correspondence of
paleobotanical evidence for cool conditions, at the same time the isthmus
was closing, suggests upwelling as a plausible explanation. Other likely fac-
tors were higher sea levels that reduced the available real estate between the
coast and the better-drained uplands, and the barely completed Isthmian
land bridge over which were just migrating many South American taxa that
now comprise most of the tropical canopy trees of southeastern Mexico
(Wendt, 1993).
In the Antilles, a source of fossils is Dominican amber from the early
to middle Miocene, 20-15 Ma (Iturralde-Vinent 2001). The specimens
