Aviculopecten? sp.
pectinid bivalve
specimen #1

Mississippian
Osagean Series
Fern Glen Formation
Jefferson County, Missouri

Aviculopecten sp.

The original question was ... brachiopod or bivalve?
The menu now refers to the 2 specimens in question as
Aviculopecten? sp. 1 (this specimen)
Aviculopecten? sp. 2

Viewer Comments:

Both specimens are pectinid bivalves. The large wing is
broken off on specimen #1, but is preserved on specimen #2,
giving the clinching proof of pectinid bivalve origin. When I
first saw the specimen #1 pictures, I could not be sure -- it has
ornamentation that is unclear, while the appearance of the
shell looks as much like a brach as a bivalve. However, the
interarea between the two beaks suggested a pectinid and
now specimen #2 proves it to be a pectinid.
Dr. Tom Yancey

I finally got a chance to look closely at these two Fern Glen
pectinids and compare them to some literature. They are
probably species of the genus Aviculopecten. You could list
them as Aviculopecten? sp. 1 and Aviculopecten? sp. 2. The
hinge shows they belong in the family Aviculopectinidae,
in agreement with the inequivalved condition. The arched,
ribbed valve is the left valve; the flat unribbed valve is the
right valve. Considering the size, the species probably have
been described in some of the old Missouri state or old
Illinois state reports, but it would take some time to work
through those reports to find a proper species name for either.

Dr. Tom Yancey

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Fern Glen Formation

Mississippian Fossils of Missouri