Triangular textured Ordovician fossil, Decorah Shale, MN

[Thescelosaurus]

I have a few hash slabs I use for educational purposes, and I'd done a couple of events recently, so I was taking a look at one of the slabs and noticed this small fossil. It is approximately 6.5 long by 3 mm across. The shape is roughly triangular, and the cross section may be triangular (it could be a V-shaped "sheath"); unless it is incomplete or collapsed, it cannot be four-sided. It is not perfectly symmetric, but slightly curved and lopsided (although the skew of the keel could be a preservational artifact). Both exposed surfaces have linear markings, so that it looks vaguely segmented, but on closer inspection the lines seem to mark the edges of long narrow "scales" which appear to be slightly imbricated. The hash slab was collected last spring from a spoil pile left by house construction on the Mississippi River Boulevard, St. Paul side, a few blocks south of Summit Avenue. I know it is Decorah Shale, but I can't provide more detailed stratigraphy. I assume it is from the lower third of the formation, based on geography. (The scale bar in the photo is mm-scale.)

The object doesn't look like any of the usual suspects (coral, bryozoan, brachiopod, bivalve, nautiloid, snail, trilobite, echinoderm, trace fossil). It shares some aspects with conulariids, but the texturing does not look like discrete rods, and it doesn't appear that the cross-section could be four-sided. The only other thing that came to mind is a hyolith, but do hyoliths have this kind of texturing when well-preserved?

[Thescelosaurus]

One more photo:

[Peat Burns]

It almost reminds me of a fragment of trilobite hypostome (kind of like isotelus).

But having looked at the pattern of photos online, the direction of the texture doesn't look right: https://en.m.wikipedia.org/wiki/Hypostome_(trilobite)

Edited February 13, 2016 by Peat Burns

[Thescelosaurus]

That's an interesting possibility, but as you noted the surface textures are different. The pattern of lines on the object look almost like some kind of growth lines, like it grew by adding one row at a time.

[Fossildude19]

Reminds me a bit of a conulariid, but I don't know that I have seen that pattern on them before.

Conularia images.

Hmm. Can't wait to see what others have to say.

Regards,

[The Goatlady]

I don't know what the wedge shaped thing is but I would like an id on the other fossil in the picture - it's in the lower left corner for the first picture, about the 7 or 8 o'clock position. I have the exact same thing in my Ordovician fossils. Thanks.

[Fossildude19]

I don't know what the wedge shaped thing is but I would like an id on the other fossil in the picture - it's in the lower left corner for the first picture, about the 7 or 8 o'clock position. I have the exact same thing in my Ordovician fossils. Thanks.

Those appear to be some type of Bryozoan.

Regards,

[prem]

my guess is a broken piece of echinoderm plate...the calcite in those plates will tend to form layers along the cleavage planes.

[Thescelosaurus]

I don't know what the wedge shaped thing is but I would like an id on the other fossil in the picture - it's in the lower left corner for the first picture, about the 7 or 8 o'clock position. I have the exact same thing in my Ordovician fossils. Thanks.

Those appear to be some type of Bryozoan.

Regards,

Yes, that's a small fragment of a frondose (flat and branching) bryozoan. The slab has a lot of this general form of bryozoan, along with a bunch of small bivalves among the rest of the usual Decorah stuff.

my guess is a broken piece of echinoderm plate...the calcite in those plates will tend to form layers along the cleavage planes.

Interesting! Any particular types likely? The main group here is of course crinoids. Reportedly the Decorah here also has rare sea stars, edrioasteroids, rhombiferan cystoids, and stylophorans; I can vouch for the edrioasteroids, but I don't know about the others (the reputed Decorah stylophorans I've seen looked like pleurocystitids as well).

[TqB]

I suspect it's echinoderm too. In particular, echinoid teeth show lamellae like the ones on your specimen and are known from the Ordovician.

Thanks to JohnJ for this link:

http://www.nhm.ac.uk/resources-rx/files/reichsmith09-65610.pdf

(from this page on the forum: http://www.thefossilforum.com/index.php?/topic/45006-unknown-pennsylvanian-period-fossil/page-2 )

Edited February 14, 2016 by TqB

[minnbuckeye]

Having the ability to search Minnesota Decorah shale frequently, I question the hypostome and echinoderm theory based on my local findings. . As Fossildude suggests, Conularia is the direction to lean towards unless this is a very rare find for our local Decorah shale. Hopefully it IS a rare item!! We will see what the experts say. Also, from the look of the rock, could this be Platteville instead of Decorah shale?

Edited February 14, 2016 by minnbuckeye

[TqB]

Apart from the shape, it doesn't really look like a conulariid (as both Tim and Thescelosaurus say). It looks more like stacked plates than rods.

[abyssunder]

I don't know what it is, but the "V" shaped folding pattern is not going to the pointed end like on echinoid tooth, is going to the opposite direction. I'm leaning toward an echinoderm ambulacrum, maybe.

[Thescelosaurus]

There's just enough like a conulariid (e.g., those at Show Us Your Conulariids!), and there's just enough like a hyolith (e.g., those at Hyolithes from Madison County, NY), that I keep circling those options. The structure does not appear to be solid. I hesitate to call the end an "aperture", but I've got another photo here to show how the walls come together (this area is difficult to photograph because too many things are affecting the focus at this angle). Both of the visible sides project slightly from the matrix.

[Thescelosaurus]

The focus isn't as good, but maybe the shadows are better?

[Peat Burns]

Apart from the shape, it doesn't really look like a conulariid (as both Tim and Thescelosaurus say). It looks more like stacked plates than rods.

I agree. I would be surprised if someone could demonstrate that it was a conularid.

[erose]

Some of the earliest Chitons are found in the Ordovician.

[JohnJ]

I would look for other broken fossils, on the slab, that did not present in the same way, but might show this pattern in the break.

[Auspex]

Some of the earliest Chitons are found in the Ordovician.

True, but the material in question looks more calcitic.

[Thescelosaurus]

I would look for other broken fossils, on the slab, that did not present in the same way, but might show this pattern in the break.

This side of the slab does not appear to have anything comparable. The assemblage consists of chunks of primarily frondose bryozoans, a couple of large thin pieces of brachiopod shell, a few crinoid columnals, five decent trilobite pygidia and trilobite hash, and a number of snails and probable snails, along with patches of striations such as are found on common local burrows. The other side is similar, except the bryo hash is more abundant, there are no striations, and there are numerous gray objects a few mm across, some of which appear to be small bivalves. Tangential observation: in my slabs, small identifiable pygidia seem to be most abundant in the company of snails.

[Auspex]

A bit of crinoid, perhaps?

[abyssunder]

Usually the spines of the crinoid cup (those that I've seen), the primibrachial or axillary plates are larger (12-15mm long) and have different form, generally conical (also the anal sac spines), although some of them are curved at the distal end, not straight.

[Raggedy Man]

http://www.bluffcountryfossils.net/blog/conulariid-a-fairly-rare-find/

One of our own, Bev, has a wonderful example of a conulariid on her site. The specimen is from Fillmore County, MN Ordovician- Galena Formation.

The specimens do not match, however, it does allow for visual comparison that may help you.

Best regards,

Paul

[Gen. et sp. indet.]

Machaeridian or septemchitonid are options worth thinking through.

[Thescelosaurus]

I decided to post it to my blog (http://equatorialminnesota.blogspot.com/2016/02/one-small-mystery-and-another.html). It's such a neat little thing!