Biology Reference
In-Depth Information
locaprids, anchitherine horses, rhinos, chalicotheres, and others. A compo-
nent from South America includes sloths, anteaters, opossum, and hystri-
cognath rodents, which began crossing the Panama land bridge after about
3.5 Ma and especially after 2.7 Ma (Stehli and Webb 1985; Webb 1997,
2006; Burnham and Graham 1999).
Reconstructing the vegetation at 2 Ma is diffi cult because sediments of
that age in many instances have been covered over or removed by later gla-
ciations. Extrapolation from late Pliocene trends, analogs from later glacial
intervals, and ancillary information from land faunas, marine assemblages,
and oxygen isotope data must be used, and the interpretations assessed
with the context of trends established from these independent sources. We
will use the most recent glacial/interglacial cycle as an approximate analog
for previous ones. After crossing the northernmost zone of permafrost and
tundra, our passage would likely be impeded by the dense, boggy, and hum-
mocky environments of the boreal forest. Spruce, fi r, and hemlock grow
mixed with patches of alder, birch, willow, and poplar. At 2 Ma, this eco-
system extends south into the upper midwestern United States to about
40°N. There are some unfamiliar combinations of trees not found together
today. This is because at the beginning of the Quaternary, and after the
many reshuffl ings of later glacial times, organisms often did not have time
or opportunity to reach the full range of their ecological potential. An ex-
ample is an assemblage of spruce and black ash that grew in upper midwest-
ern North America. At 2 Ma and probably during each glacial maximum,
spruce extended well onto the plains and contributed pollen to bogs as far
south as central Texas. The central grassland ecosystem formed in the late
Miocene, developed into an essentially modern community in the Pliocene,
and especially during the cold dry intervals of the Quaternary when there
was a reduction in the number of trees. High-altitude conifers like fi r and
spruce extend farther downslope in the western mountains at 2 Ma, and al-
pine tundra grows on the higher peaks. Spruce and fi r is also present on the
highest peaks of the Appalachian Mountains, and they, too, moved down-
ward and southward into the zone of present-day deciduous forest during
all of the glacial intervals. At 2 Ma, there was probably some alpine tundra
in the northern Appalachian Mountains and less or none in the southern
Appalachians. In the middle Pliocene and in many of the glacial intervals of
the Quaternary, spruce grew in the mountains of the Transvolcanic Belt of
Mexico (Paraje Solo fossil fl ora), and even as far south as Guatemala (Padre
Miguel fossil fl ora).
In the Quaternary, the long-leaf, short-leaf, and loblolly pines of the Gulf
Coast coniferous forest intermingle with deciduous hardwood species, and
