Geoscience Reference
In-Depth Information
allosaurs (see Chapter 9) began to
decrease and were replaced by huge
theropods, such as
Tyrannosaurus
and
Albertosaurus
, and by smaller predators
such as
Deinonychus
,
Velociraptor
,and
the ostrich-like
Ornithomimus
.
The 'Age of Dinosaurs' was, however,
drawing to a close, for at the end of the
Cretaceous Period a widespread
extinction event, probably triggered by an
extra-terrestrial impact (Alvarez
et al
.,
1980), wiped out the dinosaurs along with
just about all land animals more than
1 metre (3.3 ft) long (including many
early mammals), all of the flying
reptiles, many lineages of birds, and even
one-third of higher-level plant taxa.
Marine ecosystems did not escape, and
most of the large marine reptiles
(ichthyosaurs and plesiosaurs) were lost
along with the ammonites and many
groups of brachiopods and bivalves.
The impact theory is based on the
recognition of a widespread layer at the
Cretaceous/Tertiary (or 'K/T') boundary
containing 30 times the expected amount
of iridium, a metal which is mostly extra-
terrestrial in origin being several thousand
times more abundant in meteoritic dust
than it is on the Earth's surface.
It is now 100 years since Barnum
Brown (1907) first described the
infamous Hell Creek succession in the
northern Great Plains of Montana and
the Dakotas, a succession that includes
the crucial K/T boundary. In the
intervening years these beds have yielded
some of the most amazing and
controversial dinosaurs ever found,
including the best known dinosaur of
them all,
Tyrannosaurus rex
, the 'tyrant
lizard king'! But just as with the Morrison
Formation (Chapter 9), this formation
has yielded more than just dinosaurs -
in particular important mammals,
invertebrates, and plants have con-
tributed to our increasing knowledge
of this final Cretaceous terrestrial
ecosystem; the Hell Creek Formation
has become the most thoroughly sampled
source of data to evaluate the enormous
changes in fauna and flora that occurred
across the Cretaceous/Tertiary boundary.
H
ISTORY OF DISCOVERY OF THE
H
ELL
C
REEK
F
ORMATION
Often described as the greatest dinosaur
hunter of the 20th century, it is to
Barnum Brown (1873-1963) that we
owe the 'discovery' of the Hell Creek
Formation, one of the great dinosaur
'graveyards' of the United States. Barnum
Brown, a colorful character whose field
apparel often consisted of snappy
brimmed hat and full-length fur coat,
began his career at the American
Museum of Natural History in 1897 as
assistant to Henry Fairfield Osborn and it
was under Osborn's direction that he was
sent to Montana in 1902 to examine the
beds which would eventually be named
the Hell Creek Formation. Brown's
objective was clear: to build up
the American Museum's sparse collection
of dinosaurs which was in danger of
being outdone by Andrew Carnegie's new
museum in Pittsburgh.
Search WWH ::
Custom Search