Monday, July 12, 2010

Large spiriferids from the Mahantango

There is a borrow pit in Saville township, Perry County, PA that exposes a part of the Mahantango that commonly contains large Spiriferid fauna like that which I've only found in the Silica Formation of Ohio. The problem is the shell material is often dissolved away by ground water sometime in the past. What is left are external shell casts and internal shell molds. See this prior post for what these brachiopods would look like if the shell was preserved better.

This first one is fairly easy to ID as Spinocyrtina Mediospirifer audaculus (edit 01/19/14) due to the flattened brachial valve as well as the wide, flat interarea and deltidium.

This next specimen is either Mediospirifer or Platyrachella Spinocyrtina (edit 01/19/14) but I can't say for sure due to damage. I base this ID as the pediclar valve is curved. Something neat to notice are the large muscle scars that have been preserved by the mold. The third picture shows these very well.

This specimen seems to fit the Paraspirifer type as it's large and nut shaped. The two valves have a roughly similar shape and you can see the same kind of muscle scar as on the above Mediospirifer/Platyrachella.

I'm not sure what this one is/was as it's been distorted as some point in the past. My instincts want to say that it's a Paraspirifer but I probably will never know.

This is another mold that has been severely distorted and thus I can't be sure what it was. I wonder how these fossils became warped when the rocks themseleves show no obvious tectonic distortions. My guess is that an animal burrowed near the shell after it was buried and the pressure broke it or (if it was softened by decay processes) pushed it out of shape.

Last is this specimen which I found with the matrix still intact enough to salvage. You can see the shell pops out of the matrix cleanly and still has some remnant shell material left. It's a little distorted but I think this is another Spinocyrtina based on the interarea size and shape.


  1. I feel dumb to post this, but I think that brachiopod only occurs in the lower Devonian (i.e. the Oriskanyan and Helderbergian) as I've never found it in rocks younger than that. this all comes from the Needmore and Mandata Shale exposures in Bethel. The Mandata has Mediospirifer in abundace, but the Needmore lacks it.

  2. I can teel you a bit about the environment it lived in too. I've found it in rocks that were deposited in near shore areas, at the edges of reefs that fringed the coasts. it would have needed many cobbles to grasp on to as it would've had difficulty seperating water. Overall it means that the area had shallow water depth and rocks.

    1. Thanks for the comment and you should not feel dumb to ask a questions or posit an opinion. I find Mediospirifer in Middle Devonian rocks in NY within the Windom shale of the Moscow formation as well as the Centerfield member of the Ludlowville Formation. So it definitely exists until the Givetian stage. the Mahantango formation is equivalent in age to those formations and was closer to shore (probably behind the reefs).