The Hell Creek Formation - Fossil Ecosystems of North America: A Guide to the Sites and their Extraordinary Biotas - page 163

that the dinosaurs actually became
extinct just prior to the K/T impact
(Williams, 1994). However, Pearson et al .
(2002) have shown that this stratigraphic
interval is nearly devoid of all fossils and
that this is simply due to the absence of
suitable channel deposits in this interval
rather than to a pre-K/T extinction of the
dinosaurs.
Taphonomically, the Hell Creek
Formation can be truly regarded as a
Concentration Lagerstätte with common
bone-beds recorded at different levels
(Russell and Manabe, 2002). A monotypic
bone-bed dominated by Triceratops
occurs in the upper part of the formation
in Montana, while one dominated by
Edmontosaurus occurs in the lower part of
the formation in central South Dakota and
North Dakota. A well-known multispecies
bone-bed occurs 1 m (3.3 ft) above
the contact with the Fox Hill Sandstone
in the Ruth Mason Quarry in South
Dakota ( 164 ) and has been estimated to
contain disarticulated remains of 2,000
individuals of Edmontosaurus , along with
tyrannosaurs, small theropods, nodosaurs,
pachycephalosaurs, and ceratopsids
(Larson, 1985). Another from South
Dakota has yielded scattered elements of
small raptorial dinosaurs, oviraptosaurs,
pachycephalosaurs, and thescelosaurs
(Russell and Manabe, 2002). These have
been compared (Johnson et al ., 2002) to
the monotypic ceratopsid bone-beds
from the Judith River Group of Alberta
(see p. 183), which possibly formed by
storm surges across a low-relief coastline;
this hypothesis provides both the
mechanism of death by drowning and
the mode of burial.

There is some evidence that the
Hell Creek Formation may also be
regarded as a Conservation Lagerstätte.
Cartilaginous structures such as beaks,
extensions of vertebral spines, and ribs have
been reported along with evidence of
vascular structures. Spectacular skeletons
encased in skin impressions have long been
known to occur from the contiguous Lance
Formation and an amazing specimen of
Anatotitan has recently been reported from
the Hell Creek Formation with over 50% of
its skin intact ( 177 ). Russell and Manabe
(2002) have suggested that the relatively
organic, anoxic, and slow-moving waters of
coastal wetlands may provide unusual
opportunities for such preservation.

D ESCRIPTIONS OF THE H ELL
C REEK F ORMATION BIOTA
Tyrannosaurus rex
One of the largest-ever flesh-eating land
animals, this bipedal predatory dinosaur
was 14 m (45 ft) long and weighed up to
8 tons ( 167 - 169 ). The large skull had an
extra joint in the lower jaw to increase the
gape for biting large prey, and the jaw was
lined with 18 cm (7 in) serrated, blade-like
teeth. Powerful hind limbs supported the
three-toed clawed feet, but the forearms,
with their two-fingered hands, were so tiny
that Barnum Brown doubted that they
belonged to this animal when he collected
the first specimens of this species. Until
recently this famous dinosaur was known
from only a few specimens, but in the
1990s several remarkable finds were made
including the well-known 'Sue' and 'Stan'
both discovered by the Black Hills Institute
of Geological Research in South Dakota
(Larson and Donnan, 2002) ( 167 , 168 ).