The Hell Creek Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 157

The Hell Creek
Formation

B ACKGROUND : THE K / T
BOUNDARY AND THE EXTINC TION
OF THE DINOSAURS
The onset of the Cretaceous Period saw the
continued break-up of the supercontinent
of Pangea, owing to movements deep in
the Earth's mantle. Initially North America
and Europe pulled apart, opening up the
North Atlantic Ocean, and by the end of
the early Cretaceous South America and
Africa also began to cleave apart to form
the South Atlantic. Meanwhile the Tethys
Ocean extended westwards so that the
northern continents were divided from
those in the south. This loss of land
bridges and migration routes led to a
diversification of animal and plant life on
separated continents.
In particular, as the continents divided,
the dinosaurs began to diverge, and in
North America significant evolutionary
changes were to occur in these dominant
land animals before their final extinction

at the end of the period. For example, at
the start of the Cretaceous many
dinosaurs were able to migrate between
North America and Europe, but by the
middle of the period, although
Iguanodon continued to be the dominant
ornithopod in Europe, its place in North
America was taken by Tenontosaurus .
Likewise, while the massive sauropods and
stegosaurs, so successful in the Jurassic
(see Chapter 9), continued to dominate
in South America, these groups quickly
declined in northern continents, their
place taken by smaller, ornithischian
herbivores, which moved in vast herds
across the Cretaceous landscape. They
included the first ceratopsids, such
as Psittacosaurus, Protoceratops , and
Triceratops , the ankylosaurs, and the
pachycephalosaurs. At the same time the
iguanodonts gave rise to the duck-billed
hadrosaurs, such as Maiasaura and
Edmontosaurus . Among the carnivores the