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445 Views 7 Replies Last post: Jan 7, 2014 10:24 PM by MikeHardman RSS
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Jan 3, 2014 6:41 PM

Shell fragment from Highcliffe id please.

I found this shell at Highcliff (part of the Barton Clay formation). It is about 3.2cm long. What particularly interests me are the three quite regular holes at its base. How were they caused?

 

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    Jan 5, 2014 5:04 PM (in response to Minmi)
    Re: Shell fragment from Highcliffe id please.

    I don't know the species but I think it could be a modern shell.  The holes will be made from water.  I don't think it's a fossil because it looks very delicate and it doesn't seem to contain any minerals which made it a fossil.  If it's a fossil I don't think it will be any older than Oligocene age, but it will almost Surtainly be modern.

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        Jan 7, 2014 4:52 PM (in response to Minmi)
        Re: Shell fragment from Highcliffe id please.

        It is modern because even though it was found in the clay, it probably won't have always been in there. I'll post some more reasons when I have looked up the geology of the location where it was found.

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        Jan 7, 2014 5:35 PM (in response to Minmi)
        Re: Shell fragment from Highcliffe id please.

        Hi again.  I am just about sure it is the water which has made the holes.  The water often makes perfectly round holes

        (It was definietly made after the animal died.)

         

         

         

        I think it's modern because the sea will have washed it up in to the clay, or maybe someone found it and then dropped it in the clay.  It looks too fragile to be a fossil and them holes would have been filled up with rock if it was fossilised.

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          Jan 7, 2014 9:41 PM (in response to Dan)
          Re: Shell fragment from Highcliffe id please.

          Sorry Dan, but no - water won't have made those holes.

          They were probably made by a predator, when the oyster was still alive.

           

          One such predator is the oystercatcher (!).

           

          You will note that the holes are near the hinge. That's not a coincidence. The oystercatchers know that's the location of the muscle that holds the two halves of the shell together. ...So wrecking that muscle enables them to pry the halves of the shell open, and hence to get at the body of the oyster.

           

          Carnivorous gastropods such as Ecphora also made holes in bivalves, then inseted their rasp-like radula through the hole to scrape the flesh from the living animal within

          - http://books.google.co.uk/books?id=V0kmGpaq-iIC&pg=PA41&lpg=PA41&dq=bivalve+holes&source=bl&ots=rBzYd2pckR&sig=n20BgVGw4i6G-KCsrFD3qEyfuHc&hl=en&sa=X&ei=RXHMUvqjC6WL0AW-7IGADw&ved=0CE0Q6AEwCA#v=onepage&q=bivalve%20holes&f=false

           

          Moon shells (naticids) often made a pair of holes in their prey's shell.

          (Kelley, P.H., Hansen, T.A., Graham, S.E. and Huntoon, A.G., 2001. Temporal patterns in the efficiency of naticid gastropod predators during the Cretaceous and Cenozoic of the United States Coastal Plain; Palaios, v. 166, p. 165-176."

           

          There are other creatures, such as some bivalves (eg. piddocks), that bore holes in rocks, as described with photos by Jessica Winder here

          - http://natureinfocus.wordpress.com/2011/04/09/pebbles-with-holes-made-by-sea-creatures/

          If the rocks happen to contain fossils (such as oysters), the borings would probably go through them, too.

          Later, if such a fossil became separated from the rock, it could look just like your specimen.

          However, as I have said, the position of the holes in your specimen is probably not a coincidence.

           

          To decide whether the specimen is  modern or fossil, one should consider various factors, such as:

          - presence of soft tissue, even if only minute remains (hand lens possibly required)

          - location, if found in situ: was it in rock or modern soil/deposits

          - composition: a fossil oyster may be all-calcite ("Adult oyster shells are composed mainly of calcite, but there are five small areas of aragonite: the resilium, the two pads at which the adductor muscle is inserted, and the two pads at which Quenstedt's muscles are inserted" - http://www.ncbi.nlm.nih.gov/pubmed/17834841)

          - grains if host rock cemented-on to the specimen? if so: fossil

          - genus and species: if it can be ID'd, it might rule out one or other of modern/fossil.

           

          I can't tell for sure whether your specimen is modern or fossil, but Oysters somewhat like yours are known from the Barton Beds; see photos of Cubitostrea towards the end

          - http://www.discoveringfossils.co.uk/bartononsea.htm

           

          Mike

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