Weathered Shale From Lusters Gate Yields Fossils!

[Shamalama]

During my stop over at Lusters Gate VA, during my spring trip, I brought back some shale from the exposure to let it weather and see if I could find anything. The shale from Lusters Gate is the Liberty Hall formation which is middle Ordovician (Mohawkian stage) in age. From what I can tell, it is roughly equivalent to the Shippensburg and Chambersburg Limestones in PA; it's below the Lexington Limestone of Kentucky and is considered a part of the Black River-Trenton group in NY and Ontario. Most of what I found there were Climatograptus sp. graptolites. Today I was pawing through some of the pieces I'd left out in the weather these last few months and came across these three fossils:

Here is a brachiopod internal mold

This looks like a gastropod of some sort

And this is a cephalon to a Trilobite!!!!!

As you can see, all are very small and isolated from any other fossils. Likely washed into the sediment basin during strong currents or storms. Any ID help is appreciated!

[piranha]

Hi Dave,

The trilobite appears to be a trinucleid in the family: Raphiophoridae. Ampyxina is a good possibility on this one.

Attached is photo of Ampyxina scarabeus from an untitled bulletin of Ordovician fossils of Virginia.

[Bullsnake]

The brachiopod looks like orbiculoidea.

Sorry, can't get to species level.

Edited July 22, 2012 by Bullsnake

[Shamalama]

Thanks for the help, guys. I think Ampyxina is correct for the Trilo but I'm less sure about Orbiculoidea for the Brach.

[FossilDAWG]

Both Ampyxina and Paratrinucleus can be found at Luster's Gate, but in my experience Paratrinucleus acervulus is many times more common. The cephalons are very hard to tell apart with the exception that Paratrinucleus has a frill around the front margin (like in Cryptolithus). In your specimen the front of the cephalon, where the frill would be, is still buried in matrix. If you remove some shale from in front, I think the odds are pretty good you'll find a frill. However, the frill and associated genal spines may be lost from molted specimens. Here's a photo from this web site.

Another trilobite that is often cited, and sometimes photographed, from Luster's Gate, is Dionide holdeni. It's not clear to me how to distinguish this trilobite from Paratrinucleus.

Don

[piranha]

Paratrinucleus is exluded by virtue of two distinct morphological features. Paratrinucleus has pitted features that extend to the genal angles and by contrast Ampyxina does not have any pitting on the cephalon. Additionally, Ampyxina does not have the glabellar furrows. Attached are a series of photos to better illustrate the differences and clearly shows Dave's trilobite is an Ampyxina. Further research appears to indicate that Ampyxina scarabeus is in fact synonymous with Ampyxina powelli.

[FossilDAWG]

Thanks Scott. I agree about the Ampyxina. Which species name has priority?

Don

[piranha]

Thanks Scott. I agree about the Ampyxina. Which species name has priority?

Don

Looks like recent consensus places it as: Ampyxina powelli (Raymond, 1920)

[squalicorax]

Nice one dave!

[Shamalama]

Scott and Don, good discussion and info. Thanks for the confirmation that the Trilobite is Ampyxina powelli (Raymond, 1920)!

Nathan - Not bad for some random chunks of shale pulled out of a roadcut.

[Herb]

Dave, I think I'll go with Trematis sp. on the brac. Best guess on what I can see on the gastropod is Sinuites sp.

Edited July 25, 2012 by Herb