Biology Reference
In-Depth Information
cal lowlands, and where a few plants of northern origin like
Alnus
and
Quer-
cus
grew, as they do at present.
The uplift of the Panama land bridge, connecting two continents that
had been separated for about 67 million years (circa 70-3 Ma), is a water-
shed event in the biotic history of the New World. Details of this history
are important not only because they allow a better understanding of the
biogeography of the region, but because of the evolutionary implications,
as many plants and animals intermingled and competed for the fi rst time.
Thus, determining the time of uplift is important. This can be established
by comparing the similarity of terrestrial fl oras and faunas of progressively
younger ages on either side of the bridge (chap. 9). It can be further esti-
mated through study of fossil marine faunas from the Caribbean and the Pa-
cifi c Ocean because, as Wendell Woodring (1966) has noted, a land bridge
is also a sea barrier. Even with several kinds of paleobiological information
available, however, the history is complex, and it is useful to have evidence
from independent lines of inquiry. One such line is the chemical analysis of
pelagic (open-ocean) sediments (Donnelly 1989).
Surface waters of the Atlantic and Pacifi c oceans contain relatively little
silica because it is taken up by microscopic planktic (fl oating) marine or-
ganisms to construct their shells. When the organisms die, these shells drift
down through intermediate depths, dissolve in deep waters, and the silica
accumulates on the ocean fl oor. Upwelling brings the silica back to the sur-
face, where it is removed again by marine organisms. This cycle means
that surface waters are low in silica while intermediate and deep waters are
richer. Water that fl ows from the Pacifi c to the Atlantic through the South-
ern Ocean is mostly surface water low in silica, and that fl owing from the
Atlantic to the Pacifi c is mostly intermediate-depth water rich in silica. As
a result, bottom and intermediate waters of the Pacifi c have a higher silica
signature than those of the Atlantic Ocean.
When deep-sea cores from the Atlantic, Pacifi c, and Caribbean Sea were
analyzed for silica content, they showed a multipart history. In the latest
Cretaceous and early in the Tertiary, all the waters have a similar silica
content indicating open ocean circulation through the isthmus. In the late
Eocene, the silica content between the Atlantic and Pacifi c becomes differ-
ent, refl ecting the rise of the proto-Greater Antilles separating these ocean
basins. Then mid-depth and deep waters of the Atlantic and Pacifi c oceans
become different in silica content from the Caribbean at about 15 Ma, cor-
related with the continuing rise of the Lesser Antilles and the development
of a submerged sill in the Isthmian region, beginning the eventual forma-
