Mystery Fossil

Data:
Hello,
I was searching the Internet to see if I could find help identifying the fossil pictured below (and pictures attached in case the inserted pictures don't show). I am not a collector just happened receive this from the estate of friend who passed away recently and thought it would be nice to know what it is.

My best guess is Jurassic Ammonite but I really don't have a clue other than general circular shape. Any chance you can help?

It measures 10 inches across at the widest point and he colors below are darker in the picture than actual, but hoped the detail might help.

I hope you can advise or direct me to help figure this out.

Sincerely,

Mark T.

Send Ideas to: Mark T.

 

 

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Discussions:

  • It certainly is an ammonite. You can see the typical suture lines on it. Your mention of an "internal cast" brings to mind a question I never thought of before. Ammonite fossils are almost always "solid", that is, the interior of the shell appears to be completely filled with solid material. However, were not ammonites compartmentalized internally, just like modern Nautilus shells? If so, how could sediment get into the innermost compartments of the shell? ... Paul M.

  • Some field observations.

  • The interior of nautiloid shells fill in by sediment sifting in through the interior canal that the animal used to balance gas within its chambers in life. Additionally holes in the shell itself - such as those created by sulphur sponges are a common pathway for sediments. As the original shell is often dissolved, evidence of such perforations are lost.

    In my expereince collecting Eocene fossils in NC it is not ucommon to recover nautilods where the innermost chambers (as well as nuclear whorls in large gastropods) are not filled in with any sediment, thus their molds are often missing the upper most chambers or whorls. In large nautiloids the absence of those preserved chambers may not be evident as they are obscured by subsequent chambers. There is often evidence of sulphur sponge borings on the molds of gastrobods including nautiloids. The challenge is to find that rare fossil where sediments accumulated all the way to the nuclear whorls before the shell became buried but before the shell disintegrated as well.

    Many ammonites I have seen sectioned are hollow but were filling in with crystaline minerals at the time they were found.

    I think about how much sand one can dump out of long dead helmet shells found along North Carolina beaches to imagine the process in action. ...John T.

  • Great info John -
    Consider trees. They as well as other items left in the land, absorb dissolved silicon. The
    massive amount of silicon in many tropic trees is one concept. This is what makes crystalline
    mud balls that turn into rock with crystals within.
    I would think finding a hole in a dead or dying shell depends
    on who drilled one (killer shell) and what the shell rubbed against or what chemical mix it got into.

    Might be Calcium Carbonate replacement process as well. Trapped in a clay layer or under
    sand that gets heated by a local up-lift. (Like the now receding Appalation Mountains as
    they aged. In their hay day they were tall. So a lot of local heating was done.)

    I think a chemical process engineer would be handy in the group. ...Martin H.

  • The shape of the suture lines will give you an identification. I'm not up on the details, but Placenticeras (Cretaceous) might be worth looking up. They're relatively big and flat. ... Dr. David C.

  • It’s an internal mold of an ammonite, most likely from the Cretaceous Period of the Western Interior of the US. I looked through the Treatise volume and compared it to a similar-sized specimen on my shelf and it looks very much like Greenhornoceras, a member of the family Vascoceratidae. The suture pattern, which is used for ID of ammonite genera, looks to be about the same as that on my specimen and the published illustrations (at least the same family). The vascoceratids were basically tropical ammos that migrated north into the seaway during the Turonian portion of the Late Cretaceous, and are fairly “common” in the Greenhorn Limestone in central Colorado. I did my doctorate on such ammos from the Cretaceous Western Interior in an attempt to show that their shell morphotypes can be used for environmental interpretations (turned out to be quite successful – they in general did not drift post-mortem). Greenhornoceras was one of the most common ones I encountered in my field work ...- Rick B.

  • Up here in Western New York we have some large Devonian coiled nautiloids (Nephriticeras magister) that get to be more than 30 cm diameter, and in several specimens I've encountered the innermost whorls are not well preserved if at all. Often the inner ones when they are preserved are geodized - crystals growing inward from the chamber walls but still leaving an open cavity. Makes it really hard to work out a good specimen from the concretion they are found in. In the Cretaceous ammonites I've worked with it seems either the siphuncle or breaks (from borers, etc) must have provided conduits for fine sediments to enter the shell - especially in the finer deposits of chalky sediments - and even then the smallest innermost whorls may be absent.
    As for the ammonite specimen, probably not a placenticeratid - sutures are way too simple and the sutural amplitude index (ratio of suture pattern height to length, a measure of how deep the critters could live) is way too small in placenticeratids compared to this specimen. Placenticeras stuck to shallow bottom areas (usually close to shore) - the complex sutures of that ammo were not related to depth but rather to prevent the shell from breaking too much should a mosasaur decide it wanted some calamari for dinner but not quite succeed. ...- Rick B.

  • It could be a middle Triassic Flemingitidae, going by whorl-profile, ribbing and sutures, but I can't see the sutures quite well enough (a clear closeup image of a good section of suture would've been useful). Beyrichitidae is a related family also possible. But it could also be a Jurassic or Cretaceous beast. It certainly is not Placenticeras.

    It certainly is an ammonite. You can see the typical suture lines on it. Your mention of an "internal cast" brings to mind a question I never thought of before. Ammonite fossils are almost always "solid", that is, the interior of the shell appears to be completely filled with solid material. However, were not ammonites compartmentalized internally, just like modern Nautilus shells? If so, how could sediment get into the innermost compartments of the shell?

    When an ammonoid is buried and sedimentation continues, pressure due to the column of new rock above results in pressure being applied to the specimen. It's remarkable how this can inject sediment grains through the tiny siphuncular openings in the septa (these are ventral in Triassic & later ammonoids ie they are at the "outer edges" of the whorls). Also, being thinwalled, the siphuncles and even septal walls are prone to rupturing under pressure, especially ammonoids with their very thin septal walls. I have sectioned intact Triassic nautiloids (Proclydonautilus) to find loose and broken septa well within.

    The interior of nautiloid shells fill in by sediment sifting in through the interior canal that the animal used to balance gas within its chambers in life. Additionally holes in the shell itself - such as those created by sulphur sponges are a common pathway for sediments. As the original shell is often dissolved, evidence of such perforations are lost.

    It is doubtful that borings etc pierce the shells to allow sediment to enter; dredgings of long-dead shells from the continental shelves show that most shells are unbored even after lying on the seafloor for as long as much as a million years; if a steinkern resulted from such a shell, the boring (or hole broken into the shell wall) would result in a matrix continuation from surrounding rock to steinkern, and I have never seen an ammonoid or nautiloid with such a a clearly-defined boring. Ammonoids were also much more thinwalled than nautiloids, and so made a poor substrate for borers.

    In my expereince collecting Eocene fossils in NC it is not ucommon to recover nautilods where the innermost chambers (as well as nuclear whorls in large gastropods) are not filled in with any sediment, thus their molds are often missing the upper most chambers or whorls. In large nautiloids the absence of those preserved chambers may not be evident as they are obscured by subsequent chambers. There is often evidence of sulphur sponge borings on the molds of gastrobods including nautiloids. The challenge is to find that rare fossil where sediments accumulated all the way to the nuclear whorls before the shell became buried but before the shell disintegrated as well.

    I have collected Eocene nautiloids (Aturia, Hercoglossa) locally in a horrid slimy muddy siltstone which was not injected into the inner whorls. nautiloids are present in the hundreds, up to about 30cm diameter or a bit more. The shell is preserved and still nacreous and no borings are present. The unfilled chambers have collapsed under sedimentary compaction, resulting in difficult-to-prepare (and collect!) rather unesthetic specimens. The collapse was not plastic (distiorsion) but resulted in shattering of the shell material.

    Note that ammonoids evolved from nautiloids, but that they became very different. The shells remained similar in may ways, but it seems that ammonoid animals were much more coeloid (squidlike). ...Andrew G.

 

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