Biology Reference
In-Depth Information
Mexico (Axelrod 1975); that is, was the affi nity a vicariant event? Second,
might the affi nity result from periodic long-distance dispersal after uplift
of the Sierra Madre Oriental provided climatically comparable habitats at
elevational zones between 1000 and 2000 m, that is, anytime after about
the Paleocene/Eocene, and possibly into the present? Or, third, is there evi-
dence of a progressive introduction from the north with Neogene cooling?
These are not mutually exclusive possibilities, but the evidence favors the
last suggestion as a principal mode of origin.
In the southeastern United States, and in other parts of northern North
America, trees of the deciduous forest were already established in the Pa-
leocene and Eocene. Although paleobotanical information toward the south
is limited, none of these plants (with the possible exception of the wide-
spread
Ilex
) are known from the Eocene of Latin America. The same is true
through about the middle Miocene. Also, for any one period, the number
of northern deciduous forest genera in paleofl oras generally decreases from
north to south. In other words, when the relevant fossil records are plotted
with reference to northern temperate elements, the trend is for more and
earlier in the north and fewer and later toward the south.
The direction of movement would be more convincing, however, if there
were a correlated climatic event favoring a southward migration. If the fl oras
are plotted on the paleotemperature curve (fi g. 9.3), we fi nd that prior to
about the middle Miocene, northern temperate genera in Latin America are
rare to absent, with occasional
Pinus
, possibly
Picea
in the Oligo-Miocene of
Mexico, and
Ilex
. After the temperature decline at the beginning of the mid-
dle Miocene, ushering in cooler temperatures and greater seasonality, the
numbers increase to ten genera in the Paraje Solo Formation of Mexico, fi ve
genera in the Padre Miguel / Herreria fl oras of Guatemala, and one genus
(
Quercus
) in the late Miocene of Panama. The most parsimonious explana-
tion for this pattern is a progressive introduction from the north facilitated
by the temperature drop beginning in the middle Miocene (Graham 1973a,
b; II, chap. 8). The view is consistent with studies of extant genera such as
Illicium
(Morris et al. 2007).
THE BIOLOGICAL CONSEQUENCES OF CLOSURE OF THE ISTHMUS
OF PANAMA
When the last bits and pieces of land emerged from the sea and fused to
complete the Panama land bridge, the biological impact was enormous.
For the fi rst time since the Cretaceous, 80-70 Ma (Heinicke et al. 2007),
organisms were able to move overland directly between North and South
