Orbiculoidea
Brachiopoda,
Lingulida, Discinidae
Geological
Time: Mississippian
Size: 12
mm – 26 mm across
Fossil Site:
Heath Shale Formation, Bear Gulch Limestone, Fergus County, Montana
Description:
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 is a group of inarticulate brachiopods
known as orbiculoids. Notice the preserved original shell on
some examples, as well as the growth rings. The depositional
environment must have been nonacidic in order for original shell
to be preserved as seen here.
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