The
Bearpaw Formation , also called the
Bearpaw Shale , is a
geologic formation of
Late Cretaceous (
Campanian ) age. It
outcrops in the U.S. state of
Montana , as well as the
Canadian provinces of
Alberta and
Saskatchewan , and was named for the
Bear Paw Mountains in Montana. It includes a wide range of marine
fossils , as well as the remains of a few
dinosaurs . It is known for its fossil
ammonites , some of which are mined in Alberta to produce the
organic gemstone ammolite .
Lithology and depositional environment
Bearpaw shale being excavated to recover ammonites for ammolite production.
The formation was deposited in the Bearpaw Sea, which was part of the Western Interior Seaway that advanced and then retreated across the region during Campanian time. It is composed primarily of dark grey shales , claystones , silty claystones and siltstones , with subordinate silty sandstones . It also includes bedded and nodular concretions (both calcareous and ironstone concretions) and thin beds of bentonite . As the seaway retreated toward the southwest, the marine sediments of the Bearpaw became covered by the deltaic and coastal plain sediments of the overlying formations.
Relationship to other units
The Bearpaw Formation conformably overlies the Dinosaur Park Formation of the Belly River Group in central Alberta, and the Judith River Formation in the plains to the east and Montana. It is overlain by the Horseshoe Canyon Formation in central Alberta; by the Blood Reserve Formation and the St. Mary River Formation in southern Alberta; by the Eastend Formation in southern Saskatchewan; and by the Fox Hills Formation in Montana. To the east, it merges into the Pierre Shale .
Fauna
Color key
Notes Uncertain or tentative taxa are in small text ; crossed out taxa are discredited.
A specimen of Placenticeras ammolite from the Bearpaw Formation.
The Bearpaw Formation is famous for its well-preserved ammonite fossils. These include Placenticeras meeki , Placenticeras intercalare , Hoploscaphites , and Sphenodiscus , the baculite Baculites compressus and the bivalve Inoceramus , some of which are mined south-central Alberta to produce the organic gemstone ammolite.
Other fossils found in this formation include many types of shellfish , bony fish , sharks , rays , birds, and marine reptiles like mosasaurs such as Prognathodon overtoni and Plioplatecarpus peckensis , plesiosaurs such as Dolichorhynchops herschelensis , Albertonectes and Nakonanectes , and sea turtles . Dinosaur remains have occasionally been discovered, presumably from carcasses that washed out to sea.
Dinosaurs
Plesiosaurs
Plesiosaurs from the Bearpaw Formation
Genus
Species
Location
Member
Material
Notes
Images
Albertonectes
A. vanderveldei
Alberta
A complete, well-preserved postcranial specimen, missing only the skull.
An elasmosaurid plesiosaur . Albertonectes has the longest neck of any known plesiosaur.
Albertonectes
Nakonanectes
N. bradti
Montana
A nearly complete skeleton including the skull.
A small elasmosaurid plesiosaur with an unusually short neck.
Terminonatator
T. ponteixensis
Saskatchewan
A partially articulated incomplete skeleton, including a skull.
An elasmosaurid plesiosaur.
Dolichorhynchops
D. herschelensis
Saskatchewan
An incomplete skeleton
One of the latest known polycotylids .
Mosasaurs
Mosasaurs from the Bearpaw Formation
Genus
Species
Location
Member
Material
Notes
Images
Mosasaurus
M. missouriensis
Alberta and Montana
Several specimens, including a near complete skeleton with stomach contents
A large mosasaurine mosasaur .
Mosasaurus missouriensis
M. conodon
Saskatchewan
A large mosasaurine mosasaur.
Mosasaurus conodon
Prognathodon
P. overtoni
Alberta
Several exceptionally preserved specimens
A large mosasaurine mosasaur.
Prognathodon overtoni
Plioplatecarpus
P. primaevus
Saskatchewan
A widespread genus of plioplatecarpine mosasaur.
Plioplatecarpus
P. peckensis
Montana
Tylosaurus
T. saskatchewanensis
Saskatchewan
A single semi-complete skeleton
A large tylosaurine mosasaur.
Tylosaurus
Turtles
Turtles from the Bearpaw Formation
Genus
Species
Location
Member
Material
Notes
Images
Nichollsemys
N. baieri
Alberta
Known from skulls
A basal chelonioid sea turtle .
Nichollsemys