Wednesday, June 30, 2010


Mike over at Lousiville Fossils just posted a Protoleptostrophia from his local Devonian rocks so I thought I'd post specimens from the Mahantango today to keep the theme going.

This is a specimen from the Montebello sandstone member of the Mahantango and was found at Ft. Hunter, PA. It's an internal mold of the Brachial valve and you can see just a hint of shell material near the upper edge. The muscle scars are the outstanding feature of the shell as you can see the prototypical triangular shape where the pedicle muscles were attached and the stippled areas where the hinge muscles were attached. You can also see some fine ribs radiating out from the hinge of the shell. As this is an internal mold I would assume they were an internal feature as well as an external decoration.

This next specimen is from the Sherman Creek member of the Mahantango at Seven Stars, PA. It's not as complete as the prior specimen, but still shows the same muscle scars. Above it you can see a mystery shell which I used to call Devonochonetes but I am unsure of this ID anymore.

Protoleptostrophia is a member of the extinct Strophomenida order of Brachiopods and is concurrent with fellow members Strophodonta and Leptaena. While Protoleptostrophia is confined to the Devonian, the order existed from the Ordovician up into the Triassic before becoming extinct at the Triassic-Jurassic boundary. I've collected some of it's common ancestors from the Ordovician while in the Cincinnati area: Strophomena and Rafinesquina.

Monday, June 28, 2010


Greenops are fairly common trilobites in the Mahantango formation. Usually you find the feathery looking tails but not much else.

I have found a couple of examples that are prone and laid out in life position on the rock.

Greenops parts are prolific in the Widder formation. Here are some examples from my recent trek up to Arkona.

Saturday, June 26, 2010


A very common Brachiopod species in middle Devonian rocks is Chonetes. It's a small Brachiopod that doesn't get much larger than about a centimeter. Here is a specimen from the Mahantango formation.

This shell is small enough that it's often protected from ground water and thus you can find it with shell material (the white coloration you see above) still intact.

I found some of this same genera while at Arkona in the Hungry Hollow formation.

While there, friends told me that all shells of the type shown above are called Devonochonetes from that locality. This is interesting because I've been finding a shell in the Mahantango that I've been calling Devonochonetes for a while now but it's much larger than those that I call Chonetes:

Below is a photo with samples side by side so you can see the size difference for yourself.

I checked my best reference for the Mahantango formation fossils (Stratigraphy and Paleontology of the Mahantango Formation in South-central Pennsylvania. Robert Ellison, 1965) and it seems to have many examples of the smaller shell and it refers to it as Devonochonetes. Now I'm curious as to what the proper name is for the shells I'm finding. Maybe the larger shells that I have been calling Devonochonetes are another species, possibly Eodevonaria? Does anyone else have an idea?

Thursday, June 24, 2010

Modiomorpha sp. pelecypod from the Mahantango Formation

Modiomorpha is a Pelecypod from the Devonian aged, Mahantango formation of Central Pennsylvania. It's not too uncommon but getting a nice specimen can be tough as the dominant fossiliferous rock is shale/mudstone/siltstone. Below is a nice cast of the outside of one valve that shows the typical form: Oval shaped with fine ribs spaced evenly.

In the Mahantango formation it is common to find just molds and casts of the exterior and interior of shells. The rock is so porous that ground water dissolves the calcite shells long before we find the fossil. This next specimen is very interesting to me as it preserves the inside and outside of the shell in the same specimen.

The last photo above is the mold of the inside of the shell. When you look closely you can see the scar where the muscle that closed the shell was anchored.

This is the internal mold set back into the rock and it fits perfectly over the valve cast.

Here is how the rock looks when you put both halves back together without the mold.

And now with the mold inserted.

Pieces like this get me excited as it makes the animal more real to me. One day I hope to find a whole clam preserved like then and maybe I can make a replica from the casts and molds.

Tuesday, June 22, 2010


Paracyclas is a Pelecypod from the Devonian period. The Paleobiology Database says there are examples found in the upper Silurain and lower Mississippian but most of their citations come from the Devonian.

Here is an example from the Jeffersonville formation near Louisville, KY. I found this on a trip in 2007 and I thought it might be a Brachiopod at first. Once I examined it closer I found the symmetry between valves and knew this was a Pelecypod.

As a comparison, here is one that I found this past fall in a Mahantango formation deposit. It's preserved in Shale so I don't get to see both valves.

Sunday, June 20, 2010


Back to the Mahantango formation we go and here is a Goniophora. This specimen was found in the Sherman Creek member at Seven Stars.

This is a single valve of the genera that is preserved very well. Note that the rock has weathered and leached out any calcium carbonate to leave a mud stone.

This next specimen is from the same locality but is less weathered so there is still some Calcite remnant coating the shell. It's a nearly complete specimen with both valves present but missing about 1/2" of the shell. It does, however, allow and interesting diamond cross section to be seen.

One last specimen, It's a single valve and missing it's tip again but shows the characteristics that can be used to ID it. Long shell, regularly spaced and moderately raised ribs and a sharp central rib that cuts across the shell at a 45 degree angle.

Friday, June 18, 2010


Orthonota is yet another of the many Pelecypod species that can be found in the Mahantango formation of central Pennsylvania. It's a very obvious shell when spotted as it is long and narrow. In fact it look almost exactly like modern Razor Clams found in the oceans today. Typically I the shells I find are 3-4" long and about an inch wide although I've found some smaller specimens as well. Due to the large, elongated shell good specimens can be hard to find and extract from the rock.

Modern Razor Clam shells

Fossil Orthonota from the Mahantango

Wednesday, June 16, 2010

Common Spirifers of the Middle Devonian

When collecting in Devonian aged rocks you will find a lot of brachiopods from the Spiriferida order. This prolific order is abundant in the middle to late Paleozoic but is mostly decimated by the Permian mass extinction until finally disappearing from the fossil record by the early Jurassic. The most recognizable and well known member of this order is Mucrospirifer but there are several other common genera in the Middle Devonian who look very similar and are often mislabeled.

Below are four different genera who's names are often used interchangeably by some dealers and collectors who have not yet learned the differences.

This is how I tell the different genera apart in the field and in my collection.

The easiest to separate out is Spinocyrtina Mediospirifer (edit 02/02/14) which has a flat brachial valve and a deeply curved pedicle valve on the front. The back of the pedicle valve is very straight and flat so that when viewing the Brachiopod from the back it has a triangle shape.

The next commonly confused shell is that of Paraspirifer. Paraspirifer has a large, walnut shape and a very deep sulcus in the middle of the shell. when you turn the specimen around you'll notice that the Brachial and Pedicle valve curve in towards each other and nearly touch.

In the middle between Paraspirifer and Spinocyrtina is Mediospirifer Spinocyrtina (edit 02/02/14) which has the long interarea but it's sulcus is much less exaggerated. When looking at the back of the shell where the pedicle and brachial value meet, you will notice that the interarea is more elongated and has a slight curve.

Lastly we have Platyrachella which has a similar body plan and shape to Mediospirifer, but Platyrachella's hinge line is wider than Mediospirifer's and it's interarea has a more pronounced curve.

Hopefully this makes sense and I'm not too far off on my logic. There are many other species that have similar looks to the above four examples but I'm not going to get into that much detail. Have a different opinion? Let me know in the comments.