Tuesday, December 20, 2011

Orbiculoidea

Orbiculoidea is a genera of inarticulate brachiopods that are common throughout the paleozoic. Despite this I have not found many here in Pennsylvania. Not long ago I was exploring some abandoned limestone quarries, with permission of the current land owner, in Montour county that exposed the Keyser formation and I found some rather large examples.

All are partials but the interesting thing is that the shell is preserved as a dark color against the lighter host matrix.

Note the concentric growth rings and the (implied) terminal tip of the shell offset from center.


Specimen #2


Specimen #3 has the least complete shell but the most well defined growth rings.


Based on the shape and growth pattern these shells all look like they are inarticulate brachiopods, however could they possibly be a monoplacophoran instead? Monoplacophorans are gastropod like mollusks which have a cap shaped shell rather than a spiral shape. I'm not sure if they are known from the Keyser formation due to the lack of good information regarding the fauna found within the formation. The Keyser formation is upper Silurian to lower Devonian in age (Pridoli to Lochkovian stage).
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2 comments:

  1. AnonymousDecember 30, 2011 at 4:29 PM

    Dave - I've checked a number of references and can't find any "monoplacophoran" that are found in the Helderberg Group. For my two cents, it looks like an Orbiculoidea sp. Although it's hard to tell from the partial fossils, I'm thinking it might be "Orbiculoidea discus" which is mostly found in the New Scotland and not the Keyser FM. The "West Virginia Geological Survey - Vol XV /Devonian System" gives a fauna list which includes: "Orbiculoidea numulus" and Orbiculoidea schucherti" in Keyser LM. I can't come up with any good pictures of these two. Any possiblity that your quarry is in the New Scotland? Regards - Jack Kesling

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  2. DaveJanuary 2, 2012 at 5:58 AM

    Jack - If I understand the stratigraphy well enough the New Scotland is a member of the Old Port formation in PA. It's above the Keyser (which is equivalent to the Coeymans, Manlius, Roundont and Decker limestones of NJ in descending order). The quarry I got these from was just south of Turbotville in Montour county. Based on the geological map in Google earth, the boundary between the groups is close but geographically far enough away that the rocks should all be considered Keyser and Tolonoway. I suppose the mapping could be wrong or there is some interfingering. I'm ordering a copy of the "Devonian System" from the W Va. geological survey. It sounds like a useful reference. - Dave

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