Biology Reference
In-Depth Information
the Old World, with interchange facilitated both by the warm climates and
continuity of the continents that readily allowed migration across the lands
of the North Atlantic. The picture is complicated by the fact that some west
coastal lands (e.g., Wrangellia) have been transported by plate movement
from the south (Stamatakos et al. 2001). Jack Wolfe presents a classifi ca-
tion of this unique Cretaceous and Paleogene vegetation of northern North
America. The system is discussed below and used in fi gure 2.10. Wolfe calls
the plant formation a polar broad-leaved deciduous forest. It is character-
ized as follows:
MAT 7°C-8°C to 15°C, distinct growth rings often present; an extinct micro-
thermal to mesothermal forest type composed of deciduous gymnosperms
and angiosperms with leaves of the angiosperms large, thin-textured, and
without drip tips.
This forest was part of a taxonomically and ecologically generalized veg-
etation widely distributed across the north polar regions of the world that
in aggregate Wolfe (1975) calls a boreotropical fl ora. This conceptualized
assemblage was not a random intermingling of ecological types, however,
because, as noted, there probably was some differentiation between the
tropical, warmer coastlands and the more temperate, cooler inland and
moderately elevated uplands. Even so, it is a useful concept for envisioning
one source and one direction of migration for some members of the proto-
deciduous forest. Later these elements would move southward and eventu-
ally coalesce with others evolving in warm-temperate to tropical regions to
form the deciduous forest formation. It would become better defi ned and
more widespread with the cooling and greater seasonal variation that devel-
oped after about the middle Eocene and especially after the early Miocene.
The Cretaceous polar broad-leaved deciduous forest ecosystem had
present-day Asian and New World components growing in tropical to
warm-temperate environments. The forest consisted of small to moderate-
sized deciduous gymnosperms and angiosperms to about 15 m, tall shrubs,
and an understory of ferns and herbs. Also present was an early version of
an aquatic ecosystem (Nymphaeales,
Ceratophyllum
). Clemens and Allison
(1985) report that large dinosaurs lived on the Arctic Slope, and on Axel
Heiberg Island (80°N) between 92 and 86 Ma there were champsosaurs
(cold-blooded, long-snouted, extinct crocodilian reptiles; Vandermark et al.
2007). Over Tertiary time, the polar broad-leaved deciduous forest ecosys-
tem gradually disappeared through extinction, elimination of present-day
Asian species, migration of many deciduous angiosperms southward into
