Biology Reference
In-Depth Information
Along the western trail, compression forces generated later at the con-
verging northern plate boundaries have not yet formed the spectacular
mountain scenery in places like Denali National Park, Alaska. The landscape
is low-lying, and much of it is damp and swampy. The Brooks Range inland
from the northern coast, just being assembled from terranes transported
from the west and south, rises above the sea to an elevation of about 1000 m.
To the south, the Alaska Range is forming from an island arc that is col-
liding with the continental margin; at 74 Ma during the Campanian-
Maestrichtian period it will be lifted to about sea level (Ridgeway et al.
2002). Another period of uplift will occur between 65 and 60 Ma, establish-
ing moderately elevated highlands, while the intervening area between the
Brooks and Alaska ranges will remain submerged or just above sea level.
Southward, along the Rocky Mountains, Cretaceous uplands extend
intermittently to about central Colorado; then even lower hills continue
to the northern proto-Sierra Madres in Mexico. Beyond about 20°N, just
north of Mexico City, this trail ends. Substantial expanses of land will not
be seen again until the Guiana and Brazilian shields bordering the swamps
of the Amazon Basin.
The Cretaceous fauna in the far north includes dinosaurs. One of the
most conspicuous is the hadrosaur, or duck-billed,
Edmontosaurus
, a non-
migrating, nonhibernating, presumably cold-blooded plant eater that lives
in groups or small herds. At its maximum it can reach some three meters
tall, twelve meters long, and weigh up to three tons. Other hadrosaurs
include
Kritosaurus
and the crested
Lambeosauri
. There are plant-eating,
horned (ceratopsian) dinosaurs like
Pachyrhinosaurus
and
Anchiceratops
,
the thick, domed-skulled
Pachycephalosaurus
, and the small and fast
Thes-
celosaurus
, which runs on two legs. Meat-eaters are
Albertosaurus
,
Tyran-
nosaurus
,
Troodon
,
Dromaeosaurus
, and
Sauronitholestes
. These twelve di-
nosaurs, together with others living elsewhere in Alaska and preserved in
deposits around the present-day Colville River region, refl ect the warm
moist climate (Clemens and Nelms 1993; Rich et al 2002) and the lush
and diverse vegetation necessary to support them. Warm conditions must
have prevailed in the Arctic throughout most of the year, because certainly
the smaller, fl esh-eating
Troodon
and
Dromaeosaurus
and their young could
hardly have migrated far enough seasonally to reach signifi cantly different
light regimes, climate, or vegetation.
The plant formations are different from modern ones, because most of
the individual lineages, associations, and their ecological requirements have
not differentiated into familiar present-day forms. For example, deciduous
gymnosperms of the families Taxodiaceae (today including
Glyptostrobus
,
