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This lush landscape of abundant
rivers and open forests was vegetated
with angiosperm-dominated woodland
composed of small- to medium-sized trees
of laurels, sycamores, magnolias, and
palms, but lacking oaks, maples, or willows
and, of course, any grass (grassland did not
evolve until the Miocene). Feeding on this
plentiful vegetation were enormous herds
of large herbivorous dinosaurs dominated
by ceratopsids (60%) and hadrosaurs
(23%), along with lesser numbers of
ankylosaurs and pachycephalosaurs. The
most numerous carnivores in the food
chain, but feeding only on insects and
small animals, were the bird-like
ornithomimids (5%), while the smaller
dromeosaurs and trootids (3%), hunting
in packs, were possibly able to kill larger
prey. The unquestioned top predator,
Tyrannosaurus
, comprised 4% of the
dinosaur population (figures from White
et
al
., 1998).
Inhabiting the rivers were frogs and
salamanders along with turtles and
crocodiles, while the river borders were
home to lizards and rare snakes.
Pterosaurs still dominated the airways, but
large flightless birds dived in the water
bodies for a rich variety of fish. Proximity
to a remnant Western Interior Seaway is
indicated by some of the mollusks, which
are known to prefer brackish, if not
marine, conditions (Hartman and
Kirkland, 2002).
Just as in the Morrison (Chapter 9),
the small mammals occupied cryptic
niches out of sight of the predaceous
dinosaurs. Some of the early marsupials
reached the size of present-day badgers
(
Didelphodon
), and with the imminent
demise of the dinosaurs would soon have
the opportunity to diversify into the empty
niches vacated by their rivals.
65 million years old) with the slightly
older Judith River Group of Dinosaur
Provincial Park, Alberta which was
deposited approximately 10 million
years earlier during the Campanian
Stage of the late Cretaceous Period.
This group outcrops in Dinosaur
Provincial Park in Alberta, Canada (with
equivalent strata in Montana known as
the Two Medicine Formation). It is
immediately obvious that at family level
there are close similarities in the
dinosaur faunas, with tyrannosaurids,
ornithomimids, dromeosaurs, ceratopsids,
hadrosaurs, ankylosaurs, and
pachycephalosaurs represented in both
formations, but it is significant that the
Judith River dinosaur population was far
more diverse with 44 genera compared to
only 22 from Hell Creek. The statistics
from these two sites have been used by
some scientists to suggest that the
dinosaurs were already in gradual decline
before the impact event finally wiped them
out at the K/T boundary.
For example, although the ceratopsid
(horned) dinosaurs persisted right to the
end of the Cretaceous, only two genera
are known from Hell Creek (
Triceratops
and
Torosaurus
) compared to six from
Judith River, where
Chasmosaurus,
Centrosaurus
,and
Styracosaurus
were
also common. Similarly only two genera
of duckbilled hadrosaurs
(
Edmontosaurus
and
Anatotitan
) are
known from Hell Creek compared to
eleven from Judith River, including well-
known forms such as
Parasaurolophus
,
Hypacrosaurus
, and
Lambeosaurus
.
Theropods also declined in diversity
towards the end of the Cretaceous,
especially the ornithomimids and even the
tyrannosaurids. Four different
tyrannosaurids are known from the Judith
River Group with
Albertosaurus
(weighing
only 2 tons) the top predator. When
Tyrannosaurus
migrated into North
America from Asia in Hell Creek times
and replaced its smaller cousins, its reign
may have been tyrannical, but was
relatively short-lived.
C
OMPARISON OF THE
H
ELL
C
REEK
F
ORMATION WITH OTHER
C
RETACEOUS DINOSAUR SITES
Judith River Group, Alberta, Canada
It is worthwhile comparing the Hell
Creek
Formation (Maastrichtian Stage;
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