Trilobite from Pembrokeshire?

[Yuanls]

Hi all,

I collected this piece of rock from Saundersfoot beach last week. Because it was loose on the beach, I don't have much indication of formation or age, but I'm aware that the majority of rocks in that area are Carboniferous, and I'm also aware that Carboniferous chert does exist in the Pembrokeshire region. I'm not sure where it could have came from otherwise. 

 

The fossil in question is a scoop-shaped, hemispherical mold in chert which features two discontinuous troughs running down its centre.

 

Upon closer inspection, the surface of the mold furthest from the camera is lined with dimples (seen on the second and third image)

 

 

 

 

The near side of the mold is also textured with dimples which are somewhat larger and double-ringed, however they do not show up well in images.

I've made a cast of them using blu tack, which better shows their texture.

 

To me, the dimples are reminiscent of either a trilobite cephalon or an echinoid test, though the shape of the overall fossil doesn't seem to match.

Any ID or suggestions would be much appreciated!

[Fossildude19]

Definitely not a trilobite. 

Could be an internal mold of a brachiopod or bivalve, but I am not positive about this. 

There has been some weathering, and that could have distorted the mold. 

 

 

[Yuanls]

2 hours ago, Fossildude19 said:

Definitely not a trilobite. 

Could be an internal mold of a brachiopod or bivalve, but I am not positive about this. 

There has been some weathering, and that could have distorted the mold. 

 

 

 

Thank you very much for your suggestion. A brachiopod mold was certainly the first thing I thought of when I found the fossil, but the presence of the pits and dimples, which do not seem to originate from the weathering of the rock, made me doubtful that the fossil was truly a brachiopod. Are any brachiopods known to have pits like this fossil shows?

[JohnJ]

Research the type of echinoid steinkerns found in the area.  The dimples suggest evidence of a fragmented echinoid.