Mississippian Bear Gulch Coiled Cephalopod Fossil
Order Goniatida, Family Anthracoceratidae
Time: Mississippian (~320 m.y.a.)
mm = 1 inch): Fossil is 70 mm 25 mm across living chamber Matrix: 225
mm by 165 mm
Heath Shale Formation, Bear Gulch Limestone, Fergus County, Montana
The Bear Gulch Limestone is a deposit of some 70 square km in extent
and 30 m in depth that has been a source of one of the most diverse
assemblages of fossil fish with some 110 species having been described
over the past 30 years. Most were new to science, and provided
a unique view of the marine environment of Mississippian times.
Fine preservation of both fish and invertebrates is a hallmark
of these deposits, presumably due to an anoxic depositional environment.
This specimen is a goniatitic ammonite of the genus Anthracoceras.
The Ammonidea have a rich evolutionary history which began in the
Upper Silurian/Lower Devonian. The order began an extensive radiation
in the Triassic and Jurassic, and became most diverse during the
Cretaceous, only to become extinct during the late Cretaceous extinction
event which also saw the demise of the non-avian dinosaurs. The
Goniatite forms became extinct during the Permian. Specimens from
the Mississippian are not often seen, making this example most
unusual. This is the largest example I have had in seven years,
some three times the size of typical specimens.
fossil pictures to enlarge