Biology Reference
In-Depth Information
fl owering plants were fi rst becoming widespread over the Earth. The next
criterion—the time at which seas had retreated to expose a greater expanse
of continental lowlands—was in the early to middle Paleocene around
65-60 Ma. Finally, the most propitious time climatically was during the
LPTM/EECL (60 Ma to 55 or 50 Ma), and it was in this interval in the
middle to late Paleocene that the above physical, biological, and climatic
factors combined to produce a lowland neotropical rain forest recognizable
in the modern sense (Morley 2000; Ziegler et al.,2003; Burnham and John-
son 2004; Jaramillo and others 2006; Wing et al., 2009). It is thus one of
the oldest of the modern forested ecosystems of the New World.
Argentina offers the most extensive sequence of Cretaceous through
early Eocene and younger fl oras in South America. The Argentinean fl oras
have been studied by Ana and Sergio Archangelsky, Vivian Barreda, Ed-
gardo Romero, and colleagues at the Museo Argentino de Ciencias Natura-
les “Bernardino Rivadavia” (Buenos Aires) and other museums, universi-
ties, and institutes in the country (Archangelsky et al. 2009). There is an
Early Cretaceous fl ora in the Springhill Formation of the Magellanes Basin
of southern Argentina and Chile (Baldoni and Archangelsky 1983; Villar de
Seoane 2001). It contains familiar gymnosperms (e.g.,
Brachyphyllum
) and
other plant fossils of widespread distribution. For example,
Otozamites
is a
cycadlike plant known in Jurassic and Cretaceous deposits from Antarctica
to Oaxaca, Mexico. The microfossils are mostly gymnosperms (saccate or
winged conifer pollen 30-50 percent; nonsaccate 15-17 percent), spores
of ferns and related groups (30-43 percent); and in the Early Cretaceous
Kachaike Formation, there are rare angiosperm leaf fragments. In the Late
Cretaceous, ferns and gymnosperms are still abundant, as shown by the
Anfi teatro fl ora of Santa Cruz Province radiometrically dated at 120 Ma
(Aptian), and there are fragmentary leaves of the Nymphaeaceae, water lily,
type (Romero and Archangelsky 1986). These angiosperm leaf fossils, to-
gether with those from the Aptian-Albian Crato Formation of Brazil, are the
oldest ones known from South America (Barreda and Archangelsky 2006).
By the Middle Cretaceous, in the Cañadon Seco Formation of central Pa-
tagonia, there are eleven types of angiosperm pollen.
Nothofagus
pollen is
present by the Campanian in Antarctica and Australia; angiosperm pollen
reaches 63-79 percent in the Paleocene at some localities, Casuarinaceae
is present in Chubut, Argentina, by the early Eocene (Zamaloa et al. 2006),
and Proteaceae and
Nothofagus
forests (as opposed to scattered trees) are
evident in the middle Eocene. Collectively, these fl oras show a fern- and
gymnosperm-dominated vegetation being supplemented by a diversifying
assemblage of fl owering plants. The early Eocene Laguna del Hunco fl ora
