Mystery marks on stone

[Jim Ford]

A lot of thought and earnest discussion going on here, though I reckon when you take it to a museum they won't say anything - just shrug and chuck it in the bin!

Jim

[stripee]

Originally Posted by Cotham Marble

I'm pretty sure it's a fragment of fossil. Although in its present form the object looks as though the grooving has been made on the edge of the stone, I think what has actually happened is that the fracturing of the parent stone body has in effect horizonatally 'sliced' through an ovoid form fossil - perhaps a brachiopod or other mollusc shell, leaving a sliver of the outward face of the shell 'edging' the stone fragment.

I certainly would not guarantee the identy of the fossil, the look of the stone and west Wales location suggests a sandstone - withou specificsthat could be anything from Ordovician to Pennant (carboniferous date) series, and a number of plant and animal species are possibilities, as also are sea/lake floor markings, although the later would involve some complicated processes given the apparent bedding of the stone. Anyway, should give the museum staff something to ponder over. And worth looking for more fragments.

CM

This could be what it is. Have spent a happy hour looking at fossils on the internet - fascinating

[ChrisJB]

Another reason I don't think it is a fossil, is the fact the grooves are very apparent. In a medium grained sedimentary rock like a sandstone, preservation detail is usually relatively poor. Fine mudtones and siltstones show the greatest detail on account of their finer grains being able to show finer details. Especially with the fossilised remains of smaller fossils, like brachiopods, bivalves and trilobites, etc.

Regards, Chris

[Cotham Marble]

Originally Posted by ChrisJB

In a medium grained sedimentary rock like a sandstone, preservation detail is usually relatively poor.

As general observation this is true, however care needs to be taken when considering the relative impacts of deposition environment, infill and matrix material and mineralisation process.

The poor preservation of fossils in sandstone is often a function of the deposition environment, that is the sands, whether marine, alluvial or windblown are deposited in active environments where the fossilised elements have been subject to damage as part of the deposition process. Similar damage is often seen in limestone fossils deposited in high energy environments such as shallow tidal lagoons.

The size of grain of the matrix material that may infill voids in a fossil certainly has an effect on the degree of definition left in any fossil 'cast', however mineralisation processes in sandstone draw almost no contribution from the silica which makes up the majority of a sandy matrix. Fossil elements such as shell and bone are chemically altered (petrified) with contributions of other more reactive elements present in the sands, silica being notably chemically unreactive.

So, in circumstances where an organism becomes deposited in sands which have for some reason ceased to be subject to the energetic effects of tides, floods etc, and in which there is a favourable chemical suite to support mineralisation, there is every possibility that fine detail will be preserved, albeit that this will be a rare occurence.

Not that this gives any absolute identification of the specimen in question.

CM

[stripee]

I will get the stone to the museum, but have a couple of busy weeks coming up. So will post with any more on this stone eventually when they have looked at it. Thanks for all comments!

[sometimes]

Personally I don't think that this is evidence of a fossil. to me it looks like some sort of wear, maybe from a metal wire that shifted, perhaps wire holding a gate post, something like this.
I've taken the liberty of enlarging the image a little, hope you don't mind. I think the two areas arrowed are particular difficult to 'legitimise' mentally as a fossil.

[Hedge Witch]

Originally Posted by sometimes

Personally I don't think that this is evidence of a fossil. to me it looks like some sort of wear, maybe from a metal wire that shifted, perhaps wire holding a gate post, something like this.
I've taken the liberty of enlarging the image a little, hope you don't mind. I think the two areas arrowed are particular difficult to 'legitimise' mentally as a fossil.

I really like this explanation - it's so simple and explains a lot of things.

[ChrisJB]

Originally Posted by Cotham Marble

So, in circumstances where an organism becomes deposited in sands which have for some reason ceased to be subject to the energetic effects of tides, floods etc, and in which there is a favourable chemical suite to support mineralisation, there is every possibility that fine detail will be preserved, albeit that this will be a rare occurence.

CM

In my experience, where this occurs, and you are right, it is rare, it is often iron deposits (presumably a by product of diagenesis - (processes involved in the turning of soft sediment into rock)) and may occur as the nucleus of a concretion. I'm not seeing evidence of that here. If this is indeed a fossil (which I still doubt), it appears more like traditional infill or cast development.

Regards, Chris