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1748 Views 13 Replies Last post: Mar 31, 2014 4:17 PM by MikeHardman RSS
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Mar 19, 2014 2:34 PM

Is this a fossil sponge encased in flint?

When I was fossil hunting in Sandown IOW as a kid my teacher said that often fossil sponges are often found encased in flints especially spheroid ones.

We found several including one that was hollow with the sponge like fossil rattling about  inside. There was a bump on the outside which we presumed was the stem of the sponge.

 

This item was found in a garden in london.

 

IMG_0675.JPG

  IMG_0676.JPG

Two different views of the same rock.

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    Mar 19, 2014 9:24 PM (in response to Gargoyle)
    Re: Is this a fossil sponge encased in flint?

    ...Could be.

     

    In some cases the sponges in flints are more nearly spherical, but being irregular does not rule out the possibility. It would be necessary to put it under a hand lens or microscope, hoping to see evidence of cellular structure and/or spicules. In other cases, there is not much left of the sponge; the centre of the flint may be 'rotten'. The process of 'rotting' (actually more of an inorganic decay that rotting) can remove much of the evidence. So even with a magnifying device, it may not be possible to see enough evidence to say one way or the other.

     

    Mike

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    Mar 19, 2014 9:33 PM (in response to Gargoyle)
    Re: Is this a fossil sponge encased in flint?

    Hi Gargoyle

    Very interesting find, I agree with Mike and think it 'could be' a sponge.

    I hope it's ok as I have attached some images of sponges/corals that also 'could be' from the Holderness coast, East Yorkshire.

     

    Tabfish

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        Mar 20, 2014 6:06 PM (in response to Gargoyle)
        Re: Is this a fossil sponge encased in flint?

        Hi Ray,

         

        Yes, they will be from chalk. Seeing as how flints come in such a wide near-randomness of shapes, the particular shapes of individual specimens usually does not warrant discussion. But there are some characteristics that crop-up commonly. They includes cavities. Some cavities/depressions are filled with chalky flour, some of them are lined with crystals (geodes). In some cases the rotten core is known or thought to be the remains of a sponge. Other fossils can be found in flints. The external form of flints in some instances arises from the siliceous material filling animal burrows before turning to flint. The processes that form flint are not perfectly understood, even today. Consequently, there are some features that we are still guessing about. Internal banding is one of those. Co-called 'banded flints' are often mistaken for fossils. If you search for 'banded flint' in NaturePlus, you'll find many examples, along with explanations.

         

        Mike

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            Mar 21, 2014 9:56 PM (in response to Gargoyle)
            Re: Is this a fossil sponge encased in flint?

            Hi Ray,

             

            Good photo, thanks.

            There seem to be shell fragments in the core; the fragmentation is curious; the original fossil could have been a sponge but I cannot see any pieces that are clearly so. Logically, one might wonder if it is coprolitic.

             

            Mike

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            Mar 23, 2014 10:22 PM (in response to Gargoyle)
            Re: Is this a fossil sponge encased in flint?

            Hi Ray

            I think your specimen could be called flint meal? these are sought after by micro fossil collecters because of there small fossil content.

            I am probably wrong although we have found a couple of similar specimens on the Holderness coast.

            You need a microscope to have a closer look.

             

            Tabfish

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              Mar 23, 2014 10:45 PM (in response to Tabfish)
              Re: Is this a fossil sponge encased in flint?

              Tabfish,

               

              I agree.

              I called it 'chalky flour', the proper term 'flint meal' not having dropped into my thought processes while I was typing! Thanks for picking me up on it. I thought the fossil fragments in Ray's specimen were on the large side for flint meal, but I guess I don't know the upper size limit.

               

              Ref on flint meal (one of several):

              - http://www.ukfossils.co.uk/guides/microfossils%20from%20flint.html

               

              Mike

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                Mar 25, 2014 6:01 AM (in response to Gargoyle)
                Re: Is this a fossil sponge encased in flint?

                Looking forward to seeing the close-ups, Ray!

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                    Mar 31, 2014 4:17 PM (in response to Gargoyle)
                    Re: Is this a fossil sponge encased in flint?

                    Hi Ray,

                     

                    Thanks for the further photos.

                     

                    The siliceous nature suggests flint itself or munged remains of a siliceous sponge. One might argue that the tiny bright (and maybe the black) lines are remnants of sponge spicules, but the general granularity of the area makes me think it unlikely such fine structures would survive.

                     

                    Mike

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