Possible Monoplacophoran fossil from the Ludlowville formation

Sometimes you make good finds not in the field but at home when you are sorting through your finds. The fossil below is such an example.When I picked it up I probably thought it was an inarticulate brachiopod but as I cleaned it up and looked more closely it seems to me that it might be something different. When you look at it in profile the very tip of the shell appears to point backwards rather than straight up. If this were an inarticulate brachiopod I would expect the tip to be straight or very muted. There also appears to be a hole in the tip of the shell which could indicate that the tip broke off or maybe it is supposed to be that way (anal opening?). My instincts are telling me this could be a Monoplacophoran which is a mollusk similar to a gastropod or snail but different enough to not be included with them. Still I am unsure as I can not find any information on Monoplacophorans being found within the Hamilton group.

I have found Monoplacophorans before but they came from the Ordovician of the Verulam formation.

Archinacella patelliformis from the Verulam fm.

On my trips up to Canada I make a point to get up to the Brechin area and visit the Ordovician aged rock quarries. On one such trip I found the piece below which is rife with a Monoplacophoran called Archinacella patelliformis. Monoplacophorans (latin for "bearing one plate") are gastropod like mollusks which have a cap shaped shell rather than a spiral shape. They are considered very primitive and there is thought that they are representative of the original ancestor of all mollusks.

Upon a wide view one doesn't see too much but the eye is drawn to the light colored oval resting on the darker colored fossil (possibly a bit of Trilobite skeleton).

A closer view reveals that the oval is actually a shell with growth rings.

Here is another view with the light angled so you can see the shell details a bit better.

As I looked over the rock some more I found several more specimens of this unusual fossil. I think I found seven examples of varying sizes on this same piece of rock.

Here is another specimen that I found on a different trip. It's from a different layer as you can see from the way the limestone has weathered. Again you don't really notice it unless your looking for something that doesn't fit. I picked up this piece intitially because of the Sowerbyella brachial valve and the partial Flexicalymene cephalon.

Upon closer inspection I saw the cap like shell of a Monoplacophoran.

An even closer view. Note that the very top of the shell is nearly flat.

The specimens above were found in the Verulam formation which is middle Ordovician in age (Katian stage, equivalent to the lower Caradocian stage in Europe and the Mohawkian stage in the US).  I based my ID of the fossils on the book "South-Central Ontario Fossils, A Guide to the Ancient Marine Life of the Region" by William A. Hessin, 2009. Thanks to my friends Kevin and Joe for taking me to these quarries in the first place.

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).