Sunday, March 13, 2016

Dicoelosia varica brachiopod from the Bois d'Arc formation

A highlight of my collecting trip to the Bois d'Arc formation was finding the tiny brachiopod Dicoelosia varica. It is somewhat common in certain horizons of the formation and I probably picked up a dozen or more specimens loose plus a number of specimens on matrix. such as the one below with three specimens.

Dicoelosia varica is a member of Order Orthida and is very easy to identify. It is a small shell shaped like the letter "V" with two distinct. rounded lobes. Between the two lobes the shell pinches down and the margin retreats towards the umbo. There are very few brachiopods that have this kind of shell structure with Pygites sp. being the only other similar looking species I can think of. Both valves of the shell have thin radial striae and thin concentric growth lines. The pedicle and brachial valves both extend past the margin at the posterior and then double back at an angle to meet at the hinge line. The pedicle valve has a triangular shaped delthyrium through which the pedicle muscle once extended.

Brachial valve
Pedicle valve
Posterior (pedicle valve on top)
Posterior (pedicle valve on bottom)

Compare these specimens with the smaller sized specimens I have from the Haragan formation.

I found the specimen 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.

"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

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