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Age: Ordovician: Approx. 454-452 Mya Formation: Decorah Shale Location Grant County, Wi Amidst the frequent outcroppings of Platteville Limestone in Southern Wisconsin, you occasionally encounter the overlying Decorah Shale. From what I read this formation is more frequent north of Grant County, and in Minnesota as well. The outcrop itself is roughly 200 feet in length, and only extends about 6 feet from the ground. Its located in a very remote area, amongst a sea of corn fields. Here are a couple finds from my third trip to this locality. Here is a complete crinoid calyx with associated pinnule filtering appendages. Complete crinoids are much more common in Silurian/Devonian stratum. Next is a hashplate, with a few assorted brachiopods and crinoid fragments. There were also a handful of brachiopods to be found alone as well. All of these are very similar to the Ordovician Lepidocyclus perlamellosa/H. capax. Although these were much smaller in size. A single Lingula inarticulate brachiopod, which was a huge surprise since this was my first one ever. I found a mid sized orthoconic nautiloid. This one is 7 inches (17.7 cm) in length. The approximate width at its widest end is 2 3/4 (6.9 cm). Note the visible septa markings on the specimen. This is an internal mold, so structure is a little different from that of a normal mineralized specimen. Not the largest from this site, as others I've recovered push 25 inches (63.5 cm). The internal siphuncle is exhibited here in cross section: I marked the cross section and direction of the siphuncle here. Phragmocones from this locality only preserve as internal molds without any calcified structure. As a result of poor preservation they can be difficult to spot. With a trained eye, it becomes significantly easier. Here are some additional brachiopods that I dug out from this locality. A fennestellid bryozoan as well. There were a few, but none to write home about unfortunately. My favorite finds at this location are by far the large nautiloids, which have been consistently produced from here. (If only I could find one at 6 feet now ) If anyone has an i.d for that crinoid calyx, I'd be happy to post measurements or more detailed photos. Other than that I hope to run out to Indiana sometime within the month, maybe I'll see some of you out there. -Jonah
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Hello, everyone; This is a hash plate from the lower Decorah Shale of St. Paul, Minnesota. The central item in the photos is a thin dark fossil with crenulated segments. It appears that the fossil forms a nearly complete perimeter, defining a cross-section that seems to be uninterrupted on the side in the block but pinched into a bilobe on the side facing away from the block (either natural or crushed into that shape from a cylindrical or oval cross-section). Where the lobes pinch there is what appears to be a sediment-filled longitudinal gap. The scale bar has colored alternating millimeters, so we're looking at something a few millimeters across. My immediate thought was that this is phosphatic, as it is preserved similarly to trilobite fragments in the same plate (and in the Decorah in general). I can rule out most of the common groups immediately based on the apparent composition and general shape (sponges, corals, bryozoans, brachiopods, mollusks, and echinoderms). Because of the similarity to trilobite preservation, I first thought arthropod, but it certainly doesn't appear to be trilobitic. Obviously it's not an ostracode. I have very little experience with eurypterids but it doesn't seem like them, either. The other thing I've been considering is a machaeridian worm fragment, which would explain the apparent longitudinal gap and bilobed cross-section, but that crenulation thing is curious. Would any of you have any thoughts on this object?
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During our last warm spell in November ( a high of 40 may not sound warm to you southerners), I took the opportunity to collect a few buckets of Decorah Shale to process during the cold snowy month of December. Here are some of the fossils found. Nothing too exciting, but fun to find!! @Tidgy's Dad, I would be pleased if you could assist me with my attempted IDs!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! And if there are some that you do not have yet, let me know and I will see about sending. 1. I suspect Platystrophia extensa 2. Sowerbella minnesotensis. pedal valve above convex, brachial valve below concave 3. Sowerbella curdsvillensis. 4. 5. Rostricellula colei has far less costae than the one above and the costae are broad and smooth. 6. Pycnocrinus are prevalent in the Decorah, but mostly small partial stem pieces 7. Endoceras are the most common cephalopod I encounter in this formation, so the Isorthoceras was a welcome find. 8. Interior pedal valve of Strophomena filitexta, showing the well developed ridges for the large muscle attachment. 9. Unknown Strophomena interior pedal valves? At 2 cm wide and lacking the ridges of S filitexta, I am guessing Furcitella scofieldi. 10. The gumdrop bryozoan, Prasapora conoidea, has always been an enjoyment to find. They come in many sizes and shapes. The next one has me a bit confused. Normally Prasapora are NOT attached to anything and show the obvious growth rings underneath. This one obviously is attached to a Strophomena leading me to wonder if it is a sponge or another species of bryozoa instead. Thoughts are welcome. 11. The Decorah Formation yields many Hesperorthis tricenaria. The pedal valve is easy to identify with its beak and its triangular delthyrium. It is the brachial valve that I am not sure of. This is my guess to its identity. Very flat with similar ribbing to the pedal valve Here is a sample that contains what I think are both pedal and brachial valves 12. The solitary rugosa coral, L. profundum, is found every time I hunt the Decorah. Here is a nice specimen in association with a few Sowerbella. But this particular hunt provided me with two unknowns. First, is this a colonial rugosa or just 2 solitary L. profundums side by side. If colonial, I can not find any mention of a species name. The second odd coral is this one. Very small and appears worn even though the matrix it popped out of was not 13. Many species of branching bryozoa are commonly found both loose and in matrix. 14. Pionodema subaequata at the top and Doleroides pervetus below. 15. These two are 3cm by 2 cm, leaving me to think an interior and an exterior surface of a brachial valve of Strophomena septa. The size of this one is 2 cm by 1.5 cm. This pedal valve is quite raised, so I am suspicious it is Oepikina minnesotensis. 16. A possible pedicle valve of Pionodema subequata. 2 cm by 2 cm 16. 17. Brachial valve interiors of Sowerbyella curdsvillensis 18. Inarticulate brachiopods, Lingula. It is difficult to find a pristine one at the site I hunt. But I will keep trying! The name has changed to Pachyglossella eldei. 20. Gastropods are infrequently encountered in the Decorah, yet above and below this formation, they are common.
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Hunting spots unfortunately come and go. And so it will be with the site I visited this week having been one of my favorite local spot for Decorah Shale fossils it saddened me to see construction taking it away from me. So I hunted it hard and it produced fairly well. First up are a few hash plates. There was a discussion this week on whether fossils should be left in matrix or not. I decided to provide a little of both There seems to be two types of rugosa coral. One is very smooth. The other has rough ridges. The only species name I have for the Decorah is Lambeophyllum profunum, which I feel is the smooth one. I found dozens of Strophomena but have a difficult time differentiating the various species based on the look of one side of one shell. So I didn't bore you with many pictures of this genus. These are worm burrows, very common in the Decorah. I find these on occation and call them sponges. It is a ball with a stalk that seems to have attached to something. Maybe it is just a common Prasapora bryozoan. This was one of my favorite finds of the day. Gastropods in the Decorah are not pretty. The Platteville below and the Galena above produce many nice gastropods. If anyone knows an explanation, I would love to hear it!! This is your typical Prasapora bryozoan of the Decorah. Two species are mentioned without a description of how to tell them apart. This next species of Prasapora is easy to tell apart. But is uncommon to find. I was ecstatic to grab two in the same trip!! This picture speaks of how easy it is to ID a P. grandis from the others!! Now I must tearfully say goodbye to this site. But construction continues to boom in the area and new exposures will likely show up. Mike
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This was discovered yesterday on an excursion into Iowa's Ordovician Decorah Shale. Initially I thought a star shaped crinoid columnal but on close examination, no lumen can be seen. The specimen is about 5 mm in diameter. @crinus
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From the album: Bivalves of Platteville/Decorah Formation Minnesota
Darken image and no paints to enhance the natural appearances.-
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From the album: Bryozoan of Minnesota Ordovician
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I had to make a trip to the "big" city of Rochester, Mn today. As I drove by a new building site that exposed some Decorah Shale, I had to make a stop. The Decorah Shale is an impervious layer that keeps pollution from seeping into deeper rock layers and contaminating our ground water. Obviously, building permits are being obtained without adherence to the zoning which prevents interuption of this great geologic feature! I will drink my own water but collect fossils from these ill conceived sites. While visiting such a site, I discovered this tiny gastropod that I can not identify.
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Earlier today, I posted a collection of fossils from the Decorah Shale from NE Iowa. I do have a few questionable ones that I need opinions of. 1. This seems geological but is very odd. Thoughts of what it is are welcome. 2. Bryozoan encrusted gastropod? 3. Is this bryozoan or sponge? 4. This big "glob" sits atop a slab of matrix. I am perplexed.
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This will likely be my last trip to the Decorah Shale this season. I thought I would post some finds that were still in the matrix that they were found in. Many fossils of the Decorah are found separated from the rock they were hidden in. I just like fossils in matrix!!! With that said, the first picture is not in matrix. It shows a cephalopod with a Sowerbella brachiopod within it. Someone (I can't remember who) posted a similar item and I had remarked how many cephalopods have other fossils within the body chambers. Here is an example. I hope that person sees this. Secondly, @DMcLY posted a picture of a Strophomena hash plate this week. I think these pictures show the inside of the concave pedicle valve showing the muscle attachment and the exterior surface of it. It may help you visualize what you found. Now the rest are just for enjoyment:
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Couldn't help myself yesterday and returned to an outcrop of the Decorah Shale, Ordovician in NE Iowa. Found many nice specimens. (Adam, I may need to substitute some of my current finds with what is in your package) Here is something that perplexes me. I thought it was a partial trilobite cephalon/ genial spine but after looking at the species mentioned to occur in the formation, I struck out. Sooooo can anyone provide an identification? Thanks!!!!! Mike
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My last excursion to the Decorah Shale was meant to be a collection of specimens for @Tidgy's Dad, who helps me with identifications in this material. Unfortunately, the postage to Morocco has gone up exponentially (hundreds of dollars for a SMALL package) so that he will go empty handed. Here are a few things I am grappling with from that trip and hope to receive some incite from those more knowledgeable than me (so anyone!!). 1. This seems to be a cross section of a coral 3 cm in diameter. The corals frequent to the Decorah Shale are small solitary rugosa coral, having a rust color to them. Have looked for other types of coral and come up empty handed. 2. A hash plate full of crinoidal material. What are the linear objects scattered throughout the plate? Hyoliths or crinoidal? I guess while you are looking at this, any opinions on the owner to the genial spine (lower left)?
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A few weeks ago, just when we were inundated with the spring muddy season, I stopped at a site that features Decorah Shale with a little Platteville mixed in. If you haven't collected in the Decorah Shale, let me say it stays MUDDY even in a drought!! The site had been worked over for a finish grade. This means the site will soon be lost to vegetative overgrowth. So I proceeded to collect a 5 gallon bucket of mucky matrix to clean and examine at home, in case the weeds invade quickly. Here are some select finds from that bucket.
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I found this plate near in the upper decorah formation Minnesota (Ordovician period limestone). These chunks are frequently littered with trilobite bits and fragments but this lookk a little odd to me. It has a "knob" or bump structur on one side and a gradual bend on the other. Its still partially covered by coral and debris but most of it is exposed. Any ideas?
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I found this chunk of shale poking around a bluff side a while back near Lilydale Park in Minneapolis, MN. The fragment pictured is translucent and is slightly concave, it reminds me of a cheek plate but Im not sure. I'm new at this so I'm pretty clueless, I appreciate any help.
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SE Minnesota - Fillmore County Ordovician Decorah Shale These two fossils, along with many others, were found on a hunt I just hosted. This is a well known and prolific site of Decorah Shale which boosts a lot of trilobites. I believe this is a whole one as to me I believe I can identify a pygidium and cephalon, but the thorax doesn't have the distinctive 3 lobes OR the other two are still buried in the matrix. The plainess reminds me of Isotelus, a small one, but it seems too plain even for an Iso. The other fossil is an obvious imprint but I have never seen anything like it in my collecting here! Thoughts anyone? For scale Trilo top Trilo front with ceph Trilo front super cu, a bit blurred. :-( The "fan". Wet Thanks for taking a lot and any thoughts! :-D
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My last excursion into the fossiliferous rocks of SE Minnesota turned up this small plate of crinoidal material. I kept it because the one center piece looks a bit like a small crinoid cup to an uneducated eye (mine). This is TINY, about 3 mm in width. Cup?? If so, any genus/species to attach to it. Thanks for looking. @crinus Mike
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Was going to dispose of this brachiopod when I noticed what looked like some sort of epibiont on it. Any thoughts and possible ID??? Thanks, Mike
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Whenever I visit the Decorah Shale the gumdrop bryozoans called Prasapora stand out. The vast majority have a domed top to them. But once or twice a summer, I stumble on a few that are distinctively pointy. Are these separate species or just variants of the same one? The species eludes me even if they are one in the same. HELP!! Mike Normal domed ones: Pointy one:
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A nice Dictyonema flabelliforme dendroid graptolite from Oslo Fields in Norway. It's Tremadoc, Lower Ordovician in age and is thus maybe around 480 mya. Another angle :
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The Great Minnesota Brachiopod Caper of 1892 Equatorial Minnesota, Wednesday, August 31, 2016 http://equatorialminnesota.blogspot.com/2016/08/great-minnesota-brachiopod-caper.html http://equatorialminnesota.blogspot.com/2016_08_01_archive.html A couple of papers: Weiss, M. P. 1997. Falsifying priority of species names: a fraud of 1892. Earth Sciences History 16:21–32. http://earthscienceshistory.org/doi/abs/10.17704/eshi.16.1.8174541832360711 Tweet, J., 2014, Smashed rodents, false preprints, and the BBC: the paleontology of Mississippi National River and Recreation Area, Minnesota. Proceedings of the 10th Conference on Fossil Resources, Rapid City, SD May 2014. Dakoterra. Vol. 6:107–118. http://www.academia.edu/7074803/Smashed_rodents_false_preprints_and_the_BBC_the_paleontology_of_Mississippi_National_River_and_Recreation_Area_Minnesota https://independent.academia.edu/JustinTweet http://www.academia.edu/Documents/in/National_Park_Service_paleontology Merry Christmas Paul
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In the driftless areas of Minnesota, Iowa, and Wisconsin I find many Orthocone Cephalopods. Each one seems to have its unique way of being preserved, some as an outer shell, others with their internal anatomy showing. A few weeks ago Raggedy Man posted a cephalopod that looked to be a siphuncle to me. Bev on the otherhand thought it was a phragmocone. No experts chimed in so a conclusion was not determined. That following weekend, I did a little collecting in Iowa. The trip was highlighted with the finding of many cephalopods. As I cleaned the findings, my mind went back to Raggedy Man's post. The more I looked at these collections, the more I wanted to understand the anatomy exposed. I think my biggest frustrations are telling siphuncles from phragmocones. Are siphuncles always smooth surfaced? Should septa be seen in any unworn phragmacone? How does one differentiate phragmacone from the external surface of a cephalopod? Finally does anyone have a good site that ID's our local Ordovician cephalopods? Thanks for any input!!!!! Now enjoy my finds. By the way, since it IS football season, I HAD to use a BUCKEYE to size my cephalopods. If anyone is unfamiliar with a buckeye, it is a little bigger than an acorn. For any of you buckeyes, OH......
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From the album: Other Fossils
This is a large and robust Prasopora sp. from the Decorah Shale Formation of Minnesota.-
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