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1359 Views 8 Replies Last post: Feb 5, 2013 1:29 PM by Fiona - Museum ID team RSS
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Feb 3, 2013 2:54 PM

Fossil found on Jurassic Coast - ID help needed

My daughter found this rock with a fossil impression on a beach near Weymouth, Dorset. Does anyone know what it might be or anything about it please?

 

ThankDSCF7209.JPG you!

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    Feb 3, 2013 7:07 PM (in response to DarthEagle)
    Re: Fossil found on Jurassic Coast - ID help needed

    It is the impression of a cidaroid club spine in flint.  Cidaroids are regular (displaying radial symmetry, not bilateral symmetry) sea urchins from the cretaceous period.

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      Feb 4, 2013 9:04 AM (in response to quagga)
      Re: Fossil found on Jurassic Coast - ID help needed

      Yes, a lovely cidaroid spine. If you want to know more about sea urchins you might like to visit the Echinoid Directory at:  http://www.nhm.ac.uk/research-curation/research/projects/echinoid-directory/index.html

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          Feb 4, 2013 11:13 AM (in response to DarthEagle)
          Re: Fossil found on Jurassic Coast - ID help needed

          It looks similar to the club spine of Tylocidaris known from the Upper Chalk (Cretaceous) about 100-65 million years ago.

          Chert and flint

          What are chert and flint? Chert is a mineral formed of micro-cystalline siica, which is extremely small crystals of quartz (silica) tightly packed together. 

           

          Flint is a type of chert that formed in one particular time and place, in the soft white limestone rock called the Chalk during the Late Cretaceous period, 100 to 65 million years ago, towards the end of the time of the dinosaurs.

           

          How and when was flint formed? The Chalk rock was formed in a sea which covered most of Northern and Central Europe and Britain. Tiny plankton called coccolithophores lived in the water column and over millions of years their accumulated calcium carbonate skeletons formed the Chalk.

           

          Simple filter-feeding animals called sponges were also very common on the sea floor Natural bath sponges are the skeletons of sponges composed of flexible protein material containing no mineral matter. Many sponges have a different skeleton formed of tiny needles of silica called spicules.

           

          After death, siliceous sponges were buried by later sediments, and the silica spicules dissolved under the higher pressure and temperature of burial. Chemical conditions later caused this dissolved silica to be redeposited. The redeposited silica formed the flint that replaced the Chalk rock in the layers (tabular flint) and rows of scattered lumps (nodular flint) that can be seen in white Chalk cliffs such as those at Dover.

           

          Chalk crops out at the surface in a swathe across Britain from Dover to the Yorkshire coast, but flint from the Chalk can be found all over Britain. Flint is hard and insoluble, so that when the Chalk has been dissolved or washed away by weathering the flint remains behind. Flint nodules are broken up by frost, stream or wave action, and the last two of these wear the fragments into round pebbles. Flint has been transported all over the country by the sea, rivers, glaciers in the past and by people.

           

          I suggest you keep all this information together with your specimen. If you would like me to send you specimen labels for your collections please email me at IAS2@nhm.ac.uk

          All the best,

          Fiona


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              Feb 4, 2013 3:09 PM (in response to DarthEagle)
              Re: Fossil found on Jurassic Coast - ID help needed

              Absolutely .... well done. The length of time for this process to take place is a tricky question - other than a very long time.

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