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1728 Views 3 Replies Last post: Aug 9, 2013 11:10 AM by orion2001 RSS
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Jul 23, 2013 8:47 PM

can anyone explain the parts of a echinoid ?

i have found lots of echinoid fossils over the years but have never worked out what parts i'm looking at, can anyone help? I seem to have the same type of echinoids but showing different details or layers? can anyone explain what parts are internal or external, and how a convex imprint could be the same as a concave?


for instance


whole "internal"? cast with holes also showing hint of plates (convex) whole cast showing protruding plant looking parts as opposed to holes (convex) detail of plates only (no holes) (convex) concave imprint, but showing same details as 1st picture, (holes where I would have expected to find protusions)


I know it's a complicated subject, but to me it would make sense that using basic mold making techniques an internal cast would be convex, and an external one concave, and both would be very different. But I seem to have both convex and concave showing the same imprints?

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    Jul 25, 2013 12:43 PM (in response to orion2001)
    Re: can anyone explain the parts of a echinoid ?

    Dear Orion -


    Yes, this can be tricky, I hope the following explanation helps.


    A Natural mould of an organism (in this case an echinoid)  is an impression of it preserved in the matrix. A natural cast is an infill of a natural mould or other replacement of the shell - this could be the inside of the echinoid where the shell itself has been used as the mould.


    The details you mention are complicated because the position of all the major features of the external surface of the echinoid test are composed of single plates of calcite which abut one another. They are perforated at the ambulacra by the pore-pairs and at the apical disc by the ocular and genital pores.  (Fossils Explained 30: Facrofossils in flint, Geology Today, 2000)




















    The Echinoid Directory -


    Thank you for your enquiry - where did you find your fossils?


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    Jul 26, 2013 2:18 PM (in response to orion2001)
    Re: can anyone explain the parts of a echinoid ?


    Just thought it may help you if i point out a few things.


    These flint echinoids are missing there tests (shell) these have worn away on the beach as its quite soft compared to the flint, so any detail that is remaining is a copy of the inside surface of the test.


    I can see where you are coming from with the one in the last image, but i think that looks like it could  be part of the calcite test attached to the flint & we are looking at the inside surface of the test again, if so, the detail that you are expecting to see could have been on the missing portion?

    Also worth remembering is that fine details like this will be erased quickly by the sea & this will change the overall appearance  depending how tumbled the specimen becomes  . Really these flint casts are not the best

    examples to study but if you can find a flint filled echinoid thats still encased in chalk then it will often be a different story, they usually have the most perfect preservation to them & with a hand lens or microscope you can see all the minute details.

    Hope that helps.




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