Is this a fossil split in half? Quarry hill park, Rochester, MN USA (Platteville Limestone, Ordovician)

[Wafflero27]

Hi everyone. I went fossil hunting for the first time to this very nice park in Rochester, MN. It is called Quarry Hill and it holds some Ordovician limestone in the Platteville sedimentary rock. I had this one finding, which I am unsure if it is a fossil or not. Other fossils that have been found in this area include gastropods, cephalopods, trilobites, brachiopods, etc. 

 

I split a limestone rock and then I noticed this pattern within (which is symmetric in both halves, as you can see). I also included some close-up pictures using my 10X loupe. I am aware it may be just a mineral vein but I figured it is worth asking. The structure is approximately 21mm long. Thanks! 

 

I included 4 images which are: Image of the specimen, close-up image of the specimen, image of the specimen in situ, and image of the quarry. 

 

 

[Rockwood]

Arm from an ophiuroid? 

[Carl]

Very hard to say with what's exposed. It could be a wire-shaped thing or a slice through a sheet-shaped thing.