Biology Reference
In-Depth Information
1993; I, 61-64). The North American-European-Asian biotic interchanges
of the early and middle Eocene (55-50 Ma) are the most extensive of the
entire 20-million-year interval of the Eocene (Frederiksen 1988), and pos-
sibly for any time in the Cenozoic. Included in the interchanges were the
mesic
Platycarya
(Juglandaceae) and
Eucommia
(Eucommiaceae, now of
Asia); tropical cashews (
Anacardium
, now of Central and South America)
also known from the middle Eocene of Messel, Germany (Manchester,
Wilde, and Collinson 2007; see also Manchester, Xiang, and Xiang 2007),
and an extensive associated fauna. Animal assemblages from Ellesmere Is-
land near latitude 79°N contain alligator and an arboreal prosimian primate
distantly related to modern fl ying foxes (lemurs). There is evidence to sug-
gest that many animal groups (family Hyaenodontidae; orders Primates,
Artiodactyla, Perissodactyla) were fi rst present in Asia in the Paleocene
and arrived in North America near the Paleocene/Eocene boundary (Beard
2002; Bowen et al. 2002). Mild conditions across the northern migration
route are further documented in northwestern Europe where the MAT of
the London Clay fl ora is estimated at 25°C (77°F) compared to the pres-
ent 10°C. Spreading along the Mid-Atlantic Ridge extended progressively
northward and climates started to cool from the middle Eocene. At about
the same time, the North Atlantic land bridge became disrupted, and inter-
changes, especially of tropical elements and those with limited dispersal
capacities, diminished.
Along the Gulf Coast, leaf size in Paleocene and early Eocene fl oras is
almost always larger than in the Late Cretaceous, MAT is estimated at 27°C
(Wolfe and Dilcher 2000), and about half the leaves have drip tips. The
increase in moisture resulted in a replacement of the somewhat seasonally
dry tropical forest by what was essentially a lowland neotropical rain for-
est. Fossils of plants typical of that formation increase in abundance and
include the Anacardiaceae, Bombacaceae, Lauraceae, Fabaceae (Legumino-
sae), Rubiaceae, and Rutaceae, in addition to families with numerous lianas
(Icacinaceae, Menispermaceae, and Vitaceae). The lowland neotropical
rain forest ecosystem had its origin in the lower latitudes, and it dates from
the middle to late Paleocene. It will be discussed further with reference to
the Amazon Basin region, where it now occupies its greatest extent. In the
interval of the LPTM/EECL, however, it extended far north and south into
what are now the cold-temperate latitudes.
In the western part of the midcontinent region, there is a series of fl oras
spanning the interval between the early Paleocene and the early Eocene—
Fort Union, Sentinel Butte, and Golden Valley, including the Bear Den and
Camels Butte members (Hickey 1977; I, fi g. 5.7). These and other fl oras
