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?
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?
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)
Thank you for your enquiry - where did you find your fossils?
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.
Hi thank you both for your informative and helpfull replys. Most of my fossil collection comes from the same shoreline in Portsmouth and is mostly made up of flint echinoids with a few other oddities.
again thank you both for your help