Saturday, December 31, 2011

Emanuella praeumbonata from the Mahantango formation

Ambocoelia umbonata is a fairly common species in Hamilton group aged rocks but I hadn't encountered it in the Mahantango until my visit Thanks to Jack K. for pointing out that the fossils below, that I found at the pit near Roseburg mentioned here, are actually Emanuella praeumbonata. Below are pictures of a slab of calcareous shale from the pit with impressions and whole shells embedded in it.

Note the slight distortion of the shells and how they all distort in the same direction. This is evidence that the rocks stretched slightly when they were folded during the formation of the Appalachian mountains. This makes sense since they are located on the bending edge of a syncline (a "U" shaped fold of rocks). Below are impressions of the pedicle valves of Ambocoelia umbonata Emanuella praeumbonata.

These are whole shells still embedded within the shale. Note that there is one oriented with the pedicle valve down (with relation to the bedding plane of the rock) and one with the brachial valve oriented down. This tells me that the shells are not likely in life position but likely were tumbled from their original location and buried during a storm event. The fact that the shells are articulated (meaning they have both valves attached together still) is yet more evidence for that scenario.

A profile view of one of the shells within the rock. The pedicle valve is pointed down thus the brachial valve is up. Note then that the brachial valve is convex, as is typical in Emanuella praeumbonata, instead of concave to flat as it would be in Ambocoelia umbonata.

These specimens were (probably) collected from the Mahantango formation, Tully member which are middle Devonian (Givetian stage) in age. This is my first known example of Emanuella praeumbonata from the Mahantango formation.

I've found Ambocoelia umbonata before from the Widder formation (Hungry Hollow member) at Arkona, Ontario and the Ludlowville formation (Wanakah shale) at Lake View, NY and the general shape and size of those specimens are what lead me to jump to my earlier conclusion that the specimens I'd found at the Roseburg pit were the same genera.

Friday, December 30, 2011

Pustulatia pustulosa from the Mahantango fm?

From the same pit that I found the Dechenella trilobite I also found these brachiopods that I think are Pustulatia pustulosa. They are slightly distorted due to geologic forces but seem to match the shape of the brachial valve of specimens that I've found in the Kashong shale of New York as seen in This Post.

This is the first time I've found this species within the Mahantango. It is a known (and supposedly common) species within the Mahantango based on it's inclusion on one of the Devonian plates within the book "Fossil collecting in Pennsylvania" by Donald Hoskins (PA Geological Survey publication G40).

Thursday, December 29, 2011

Basidechenella from the Mahantango

While exploring a newly discovered pit near Roseburg, PA I found a new genera of Trilobite for my collection. I think it's a Basidechenella but some details are missing. Below is the fossil which is a cast of the original fossil since the calcitic shell has long since dissolved away.

I'm positive it's not the more common genera Phacops because the glabella (the "nose" like part between the eyes) is much narrower and lacks the sensory bumps. The rest of the cephalon (the head) displays a margin around the edge which is also lacking in Phacops.

The fossil was found in rocks from the Mahantango formation which is middle Devonian (Givetian) in age. At this locality the Mahantango transitions into the upper Devonian (Fransian) aged Trimmers Rock formation (Equivalent to Lockhaven formation elsewhere in PA and the Wiscoy formation in NY). As the fossil was found in a rubble pile I can't say for sure that it is definitely from the Mahantango but then again, I can;t for sure say where the division line is between the Givetian and Fransian aged rocks as there is no unconformity. Normally I'd look for the Tully Limestone but that is generally not present in the Hamilton group rocks of Central PA.

Wednesday, December 28, 2011

Roseburg Quarry

Over the Thanksgiving holiday I had the chance to do some exploring of borrow pits and quarries north of Harrisburg. One of the sites I visited was a borrow pit near Roseburg, PA. I actually passed it while headed somewhere else but turned around to see if it had anything good.

Here is a picture of the pit facing north. You can see that there is a gentle slope to the rock and it actually dips to the southeast (right side of the picture).

Here are a couple of the more common fossils that I found

A large Rhipidomella penelope brachiopod

A flattened Bembexia gastropod

Some fossil "hash"

I did find some other fossils there that were new finds for me from PA and from the area that I will detail in subsequent posts.

I suspected that the rock was part of the Hamilton group and more specifically the Mahantango formation and I was right. However what surprised me was that when I looked at the geologic map most of the quarry is shown to be part of the Trimmers Rock formation which is upper Devonian and part of the Chemung group.

Here is a stratigraphic diagram of the Devonian rocks in Pennsylvania from the PA Geological Survey website HERE.

Here is a screen capture of the local geology of the pit from Google Earth. I am using the geologic map overlay available from the USGS website HERE.

The orange color represents the Mahantango formation and the green color is the Trimmers Rock formation. The main areas I was exploring were on the northeast side to the central part of the pit although most of the fossils seemed to be in the layers that were on the bottom of the pit. This map together with the dip of the rocks and my knowledge of local stratigraphy tells me that the layers in the bottom of the pit are most likely from the Mahantango formation while the top most layers are from the Trimmers Rock formation. That still doesn't tell me for sure where the fossils I found in the gravel piles that were pushed around came from but I'm fairly confident they are from the Mahantango.

Below is a wider geologic map view to put the regional geology into perspective. The layers I was exploring are on the outer edge of a large synclinorium (the Minersville Synclinorium) that has Mississippian rocks at it's center (actually Pennsylvanian rocks outcrop farther to the northeast but Mississippian is the youngest visible in this picture). A synclinorium is a large group of layered rocks that generally bend downward into a relaxed "U" shape but has smaller folds within it that may bend into individual tighter anticlines ("A" shapes) and synclines ("U" shapes). Not far to the north and east the synclinorium bends up into an anticlinorium (the Tuscorora Anticlinorium) which exposes older rocks from the Ordovician. The thumbtack shows the general location of the pit.

This same pattern is repeated all over the ridge and valley providence that the Appalachian mountains compose. With the Google maps and my knowledge of the stratigraphy I am better able to look for other potential sites and have a good idea what to expect. So to bring it all back together, the pit has fossils from the Mahantango and possibly the Trimmers Rock formations. Also while the rock layers are only gently dipping there may have been some geologic effects visible in the fossils (as you'll read in subsequent posts).

Monday, December 26, 2011

Holocystites from the Waldron shale

Among the fauna found within the Waldron Osgood shale is a Cystoid called Holocystites. Specimens of this odd echinoderm were the most common fossil I found when I had the opportunity to collect the Waldron Osgood shale a few months ago. It could have been the layer or the particular area that was exposed but I was finding them very easily.

They are about the size of a Quail egg or small Chicken egg and that is the general shape that one looks for. Cleaned off you can see a pebble textured surface that is broken into small plates.

Here are several next to each other to show you variety of shape and size.

This is a chunk of the Waldron Osgood shale with a partial Holocystites embedded in it. I say partial because the plate is thin and the fossil does not go all the way through.
This occurrence was not uncommon and some of the loose specimens even displayed this same feature. My theory is that during storm events some Holocystites were buried quickly but some did not get completely covered. Those specimens were exposed partially above the surface and subject to scavenging/erosion. So in essence we are looking at the underside of the bedding plane in the above picture. Since the piece was not found in situ I am only speculating.

The Waldron Osgood shale is Silurian (Telychian to Sheinwoodian stage) in age and is roughly equivalent to the Rochester shale in New York.

*updated 4/24/14*

Saturday, December 24, 2011

Possible Hexactinellid sponge from the Keyser Formation

Another strange item that I've found in the Keyser formation are these fossilized "threads". I've found several instances of these fossils and they do not look like a geologic phenomena. When viewed normally they look almost like grass, leaves or wood but that is not possible as the Keyser formation is upper Silurian to lower Devonian in age and plants were not yet that developed.

A closer look reveals a definite fibrous structure and lack of layering. The "threads", as I call them, overlap and diverge from one another seemingly at random.

Another specimen with good examples. This has been polished a little by my Dremel tool to see if any other details emerge. I was starting to think they were fossilized algae or trace fossils.

I'm truly stumped by this one so I went to the Fossil Forum to see if anyone else recognized what they were. The general consensus is that they could be some root structures from a Hexactinellid sponge. There is a lack of good information on the fauna of the Keyser formation and so I'm still trying to determine if there are any known Hexactinellid sponges that have been found within it. I've found Stromatoporid sponges in the Keyser formation before and blogged about them Here and Here, but they don't have root structures; instead preferring to encrust and form mound like shapes. If anyone has any ideas please let me know.

Thursday, December 22, 2011

Possible Cystoids from the Keyser formation

While searching an exposure of the Keyser formation near Mt. Union, PA I found some odd rounded objects.

I think they may be Cystoids but when I found them they had no discernible markings. Thinking them to be chert nodules I soaked them overnight in some vinegar and that is why they look a little etched. It wasn't until I got a closer look at the etched pieces that I noticed what looked like a pattern of plates. You can see what I am talking about in the specimen below.

As you can tell from the pictures above it seems to have a diamond shaped cross section so maybe that represents a top and bottom. I do not see any clear evidence of a stem or arms though. At this point it looks like the pattern may be an internal mold of a Cystoid or Crinoid but I doubt I'll ever be able to ID it since there it little to go on.

The Keyser formation is upper Silurian to lower Devonian in age (Pridolian to Lochkovian stage).

Tuesday, December 20, 2011


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).

Sunday, December 18, 2011

Eospirifer eudora from the Keyser formation

Below is the sole example of a fossil that I found in the Keyser formation near Mt. Union, PA. I think it's an Eospirifer sp. as the genera is known from the Helderberg fauna which is upper Silurian to lower Devonian.

Pedicle valve


Brachial valve



The Keyser formation spans the boundary between the Silurian and Devonian (Pridoli to Lochkovian stage) and thus it is hard to date exactly when a fossil came from. Based on the stratigraphic location of the beds I was searching being closer to the underlying Tolonoway formation (Silurian, Pridoli stage) than the overlying Old Port formation (Devonian, Lochkovian stage), I am betting this fossil came from the very latest Silurian.

Friday, December 16, 2011

Bryozoan "Garden" from Lockport, NY

A final piece from the many that I have collected from the Irondequoit limestone Rochester Shale near Lockport, NY. This is a slab of limestone that has several species of Bryozoan on it and is a good example of the "Fossil Bryozoan Gardens" that are well known from the Irondequoit limestone Rochester Shale.



Of the species that are present on the slab I can ID Chilotrypa ostiolata, Chasmatopora asperato-striata, and Fenestella elegans. Then there are a couple of new forms that I am not yet familiar with.

Mat like Fistuloporids?

These could be Callopora or Hallopora

I will visit the site where I found these fossils again and hopefully I will find some more species to add to my collection.

The Irondequoit limestone Rochester Shale is Silurian in age (Wenlock series, Sheinwoodian stage).