Mucrosprifier mucronatus from the Moscow formation

One of the most recognizable Brachiopod genus that can be found in the middle Devonian is Mucrospirifer. Below is a specimen of M. mucronatus from the Windom shale member of the Moscow formation. It's a wide hinged shell, often with the hinge line extending farther beyond the main body out into points. This gives the shell a wing like appearance and a friend of mine once told me his daughter called them fossil butterflies. The long hinge line is thought to provide stability for the shell on muddy surfaces where there was not as much of a hard substrate to attach to.

Mucrospirifer is so named due to it's "mucronate" appearance (mucronate is an adjective that means "ending abruptly in a short sharp point or 'mucro'").  That description is based on specimens that have the margin of the shell meeting the hinge line at an acute angle rather than a rounded appearance.The specimen below illustrates that habit well on the right side of the specimen.

There are many different Mucrospirifer species that I have collected from the middle Devonian deposits such as M. arkonensis, M. prolificus, and M. thedfordensis.

But it seems nearly all of the above species were recombined in 1964 by John Tillman ("Variation in Species of Mucrospirifer from Middle Devonian Rocks of Michigan, Ontario, and Ohio", John R. Tillman, Journal of Paleontology, Vol. 38, No. 5 (Sep., 1964), pp. 952-964) into M. mucronatus with the exception of M. thedfordensis. Tillman remarked that "M. thedfordensis (Shimer & Grabau)
differs from M. mucronatus (Conrad) in having a U-shaped to V-shaped sulcus, never with a flattened floor; a low moderately convex fold, never with flattened surface."

Finally, here is an example of M. mucronatus from the Silica shale.

This specimen was found at a borrow pit on Deep Springs Road in Madison County, NY which exposes the Windom shale member of the Moscow formation (middle Devonian in age, Givetian stage.)