Thomas is 11 and lives in Lancashire. He sent photos of these two objects and wanted to find out more about them. This is what Rockwatch Ambassador Michael thinks Thomas has found.
Thanks for sending me the two photographs. I can tell you about both of them: The brown pebble-like object is not actually a fossil, but is called a Septarian Concretion (or Nodule). These formed when a patch of lime concentrated around something in a mudstone sediment (often a shell) a short distance below the seafloor, during the time of deposition of the sediment. These concretions shrank internally as they hardened and small cracks formed in their interior. These filled with mineral, which in your case looks like calcite (lime). You have a very good example of a small but complete one of one of these concretions which has been slightly abraded to reveal the inner structure. Because of the way they formed, it is not unusual for a decent, uncompressed fossil to be found inside them. though it may have a “crazy paving” appearance due to the changes that occurred in the concretion as it became more indurated. The brown colouration is probably due to an iron compound, such as Iron Carbonate (Siderite) Because of their superficial resemblance, they are sometimes called Turtle Stones.
The larger pebble is probably Carboniferous Limestone, containing the disaggregated remnants of a crinoid (sea-lily). These abounded at times in the Early Carboniferous seas, some 330-350 million years ago. Although informally called “lilies”, they are actually echinoderms – related to sea urchins and starfish but were a cylindrical stem anchored to the sea floor by a holdfast which fed by waving its feathery pinnules (on top) in the sea water to catch and filter out any microscopic food particles. Look again and you’ll probably start seeing all sorts of fossils in rocks derived from the same limestone in your area.
You can find good illustrations of all these online with a simple search. I shall be pleased to send an opinion on any more from your collection. It is how I started at about the same age and nearly 60 years later I have a massive amount of material collected from all over the world. A tip for photos is to include a scale (a ruler will be ideal) and for most, 2 or 3 shots from different angles. I look forward to hearing from you and maybe meeting on one of our Rockwatch trips in Northern England once the lockdown is over. Keep up the good work.
Dear Mr Oates , thank you for replying so quickly, thanks for explaining about my finds. I found it very interesting and I now see from looking closer that it’s a sea lilies fossil . I look forward to meeting you one day on a field trip. I am fascinated about how old my turtle stone is, it’s amazing. From Thomas