most of the upper Cretaceous period, the Western Interior
portion of North America was covered by a large, shallow (some
100 to 300 meters of maximal depth), epicontinental seaway.
This vast seaway stretched from the Gulf of Mexico all the
way to the Arctic Ocean. It was during the late Maastrichtian
stage that the Fox Hills Seaway retreated, leaving the Fox
Hills formation. During the Late Cretaceous, fluctuations
sea level led to various bouts of exposure of some regions.
As a result, there is today some inter-tonguing of terrestrial
deposits, the most famous of which is the Hell
Creek Formation, home of T. rex. The famous and younger
Creek Formation overlies the Fox Hills Formation in Montanaand,
and is the youngest formation of the Cretaceous period in
this western region. The formation dates to approximately
69 to 70 million years old.
Hills Formation exposures occur in south-central and southwestern
North Dakota, and mostly comprise sandstones and siltstones
that were deposited in shallow marine and lagoon environments.
The fossil biota is enormously diverse. Including plant, cartilaginous
and ray-finned fishes, land and aquatic reptiles (turtles,
crocs, salamanders, mosasaurs), and dinosaurs , including
Troodon formosus, the Dromaeosaurid Struthiomimus, Nodosaurids,
Hadrosaurids like Edmontosaurus, and Ceratopsids like Triceratops.
The vertebrates from the formation, however, are mainly known
from teeth and other bones, with complete or nearly complete
fossils rarely found.
the faunal diversity, the Fox Hills Formation is most famous
for incredibly beautiful ammonoid fossils. The high degree
of preservation of their natural mother-of-pearl shell makes
them appear to be preserved as opal, often called ammolite.
This is a consequence of light passing through the various
layers of aragonite and conchiolin deposited by the ammonite
in life. Conchiolin is secreted by various other mollusks,
including oysters, and is a complex protein.
The chemical composition of ammolite is variable, depending
on the chemical composition of the sea water at the time of
deposition. Besides aragonite, calcite, silica, pyrite, or
other minerals. The shell itself may contain a number of trace
elements, including: aluminium; barium; chromium; copper;
iron; magnesium; manganese; strontium; titanium; and vanadium
could be present. Ammolite, like pearls are considered biogenic
gemstone, and can exhibite a rainbow of collors that varies
with angle perspective.
Hills Formation ammonoids normally occur in concretions, which
are cracked to reveal the ammonoids (and other sea life such
as bivalves). Modern-day cephalopods are known to come together
for mating, only to die soon thereafter. This seems to be
the reason that Fox Hills’ concretions often contains
a death assemblage of ammonoids, testament to the fact that
they mated together and then died together.