Meristella atoka brachiopod from the Bois d'Arc formation of Oklahoma

I've written about the brachiopod Meristella atoka from the Haragan formation of Oklahoma a few years back, but these next examples come from the Bois d'Arc formation. Both have the typical subtriangular to subpentagonal shape (this means they are roughly shaped like a triangle or pentagon when looking at a valve from the dorsal or ventral side), smooth shell with just a few concentric growth lines and a simple fold and sulcus. The latter feature is highly variable as can be seen in the two specimens below and when one compares the specimen from the Haragan formation linked above.

Brachial valve

Anterior

Pedicle valve

Posterior

Profile

Specimen #2 -  Pedicle valve (with a bit of Limonite/Pyrite coating)

Anterior

Brachial valve

Posterior - The Limonite/Pyrite coating covers the beak.

Profile

Another view of the Limonite/Pyrite coating. I don't believe this is a replacement of the shell, rather it could have been something that developed after the shell was buried in a low oxygen environment.

I think this next specimen may be a juvenile specimen of the same species. Pedicle valve

 Anterior

 Brachial valve

 Posterior

 Profile

I found the specimens shown on this page in the Bois d'Arc formation (Cravatt Member) near Clarita, OK which is Devonian in age (Lockhovian stage). The Bois d'Arc overlies the Haragan formation but both are the same age (Devonian, Lockhovian stage). They are both roughly correlative to the Helderberg fauna of New York and thus are contemporaneous with the Coyemans, Kalkberg and New Scotland formations.

References: 
"Stratigraphy and Paleontology of the Hunton Group in the Arbuckle Mountain Region, Part V - Bois d'Arc Articulate Brachiopods " Amsden, 1958, Oklahoma Geological Survey Bulletin 82