Tuesday, September 1, 2020

Brevispirifer gregarius brachiopod from the Jeffersonville limestone

Perhaps the most important brachiopod know from the Jeffersonville limestone formation in Kentucky is Brevispirifer gregarius. It is a common constituent of the formation and even has a zone within named after it due to the abundance of fossils.  B. gregarius is a small Spiriferid type brachiopod that is roughly half moon shaped when viewing the dorsal surface of the brachial valve. The two valves are both convex with the pedicle valve a bit more than the brachial. The interarea of the pedicle valve is slightly recurved with a well defined "V" shaped foramen. The surface of both valves have coarse radiating ribs and a fold/sulcus feature.

I've seen exposures of the B. gregarius zone in the Jeffersonville formation that is full of dis-articulated valves. Whole specimens can be found but are generally the exception rather than the rule. These diminutive brachiopods must have been densely packed on the seafloor to have so many shells be found. Interestingly I have only ever found the dis-articulated pedicle valves to be able to collect in the colluvium.

Specimen #1 - Brachial valve with Beekite rings



Pedicle valve

Posterior view

Another posterior view with the shell rotated 180 degrees.

Specimen #2 - Brachial valve


Pedicle valve


Left profile

Right profile

 These next two pictures are of a partial, dis-articulated pedicle valve interior.

Another dis-articulated pedicle valve interior.  The difference in features could be a result of incomplete preservation and replacement by the Quartz. Both were collected from the same horizon in the same roadcut.

I collected these specimens from the Jeffersonville formation (Devonian, Eifelian stage) near Louisville, KY 

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