Arundel formation ironstone

[val horn]

Went hunting in the Arundel formation an east coast US early cretaceous terrestrial/lacustrine   facies and found this odd rock.  My first thought was that it was a broken piece of machinery .  It is not magnetic, though it is heavy for its size ,a flat bottomed triangular piece 2 cm on its longest axis.  I have shown it to a number of people, one suggested lungfish tooth plate and several just felt that it seems to be ironstone and therefore cannt be a fossil.  Lungfish toothplates have been (rarely) found in this formation.  I would appreciate any help in understanding this.  The last photo is a picture of my rock on a photo of a palatal lungfish toothplate.

 

[patelinho7]

Hard to tell, definitely looks like something. Could you give some more focused photos of the top and side? Did you find this at the park or in an exposure elsewhere? Definitely could be the lungfish tooth plate

[Rockwood]

The texture of a lungfish tooth is quite different from that of ironstone. 

[val horn]

It was not found at the park, but at another local site.  I have tried to use the magnifier on the phone, I hope these are better.  

[val horn]

In the next to last photo it starts to look like there might be a partial  smooth layer over a grainier basal layer-- or that could be wishful thinking

[JohnJ]

Lungfish tooth plates have a distinctive internal structure and texture that are not visible on your piece.

[Rockwood]

With ironstone an incomplete shape could be a little like the monkeys on a typewriter/ given enough tries,  thing. That would make calling it a natural cast a super long shot. 

[patelinho7]

@WhodamanHD @EMP 

 

calling in some people with experience with Potomac Group

[EMP]

It's kind of hard to tell from the photos, the second one makes me think fossil, but the last set has me leaning towards "just" ironstone. I've found gastropods from the Arundel before, and I know people have found bivalves in it, so maybe it's the mold of a bivalve ligament? Could also be some kind of trace fossil, though not like the ones I mostly find. Any idea how large it is and roughly where it was found? That would help narrow down the possibilities.

 

For context, the ironstone concretions are usually globular and random in how they're positioned, so the ring-like structure of this is unique at the very least. 

[val horn]

It was found near hanover md /bwi  and is about 2 cm at its longest dimension

[EMP]

15 hours ago, val horn said:

It was found near hanover md /bwi  and is about 2 cm at its longest dimension

 

The MGS has a couple of species of bivalves listed in the systematic paleontology: Lower Cretaceous : Maryland Geological Survey : Free Download, Borrow, and Streaming : Internet Archive

 

Cyrena marylandica and Unio patapscoensis:

 

 

The size is too large for either species. The ligament area wouldn't represent the whole length of the hinge, but in your specimen it would have to in order to stay within the range of all but the absolute largest of U. patapscoensis. 

 

 

Looking at some pictures online (since the MGS plates don't really show the ligament area) it doens't look like it could be from a Unio:

 

 

This photo of one from Wikipedia doesn't show any structures along the hinge that I was thinking of. 

 

I'm leaning more towards it being some kind of weird ironstone relic. For a second it looked like maybe the ribbing off an ammonite or something, but unless you're way further southeast of BWI, that area is mostly Potomac Group bedrock, which doesn't have ammonites.

 

At the very least, though, you did find an exposure of Cretaceous rocks, so there's a good chance you will find some cool fossils.