Mystery Fossil

Data: Hi, I was wondering if you could help me identify these fossils. I have found literally tons of them in clay stone. They are usually in masses from 6 inches in diameter to a couple of feet. The first two pictures are of a clam like species, some still have both halves attached.

These second photos are of a different species that are long and elliptical cross section, they range in size from less than an inch to 6 inches or more. The first photo shows the kind of mass they are found in, the second is of a large specimen found inside the rock.

There are at least three specimens in this mass, probably more. Do you have any idea what these might be, and why they are in these masses? Also, both species occur in in the masses together.

Thank you for your time,

PS: These were found in N.E. Wyoming, USA. I have found some better specimens of the elongated species, I will try to e-mail some pictures later this week. Also I could mail a small specimen to anyone in the US if it would help with identification. These creatures range in size from pencil sized to as big as my arm, but I have not yet found a complete specimen.

Send Ideas to: Gary

PS: Here are some better pictures of the long shell fish, they are definitely shell fish, as most of them still have shell material on them. They are all long, elliptical, taper to a point on one end, and have separate chambers inside (I have found some that have not completely fossilized inside). Most of the outer shells are smooth, but some are wavy, like corrugated tin or such, you can see this in the picture of the short piece. The large specimen is the most complete large one I have found so far, it must have been very long.



PS Discussions:

  • The long fossils which Gary shows in his updated pictures are straight (uncoiled) ammonites, probably Baculites spp. ... Allen A.

  • Baculites and similar forms can be common upper Cretaceous fossils, likely to occur with Inoceramus. Inoceramus tends to occur in low-oxygen settings, presumably sitting on the seafloor and getting just enough oxygen while very little was able to live within the seafloor ( e.g., no burrowing bivalves). As the ammonites were active swimmers, they could swim up where there was more oxygen and would sink to the bottom when they died. I think there has been some speculation that ammonites might have been more tolerant of low oxygen than fish are and thus had an advantage in certain habitats.
    In the Cretaceous, there was an arm of the ocean extending from the Gulf of Mexico up the present-day Great Plains and the eastern part of the Rockies (which weren't around yet) all the way to the Arctic Ocean. As a relatively shallow and enclosed ocean, there was often opportunity to get a bit stagnant. The dark color of the rocks that the fossils are in suggests lower oxygen levels. ...David C.


  • I think these are "nodules". They are formed when organic materials which can be anything from a nudibranch to a fish, form an oxygen-deprived ("anoxic") layer of sediments around them as they decay, since the decay process uses up the available O2. In the resulting spheroid or oblong anoxic region, sulphates are often deposited, which results in the very dark interior of the structure. When the sediment is turned to stone, these regions form nodulules - areas of higher density and greater durability centered around the original piece of decayed organic matter which started the process in the first place. So, if you wish to find fossils in them, simply crack them down the center of the longest axis - sometimes the outline of the beastie is stil to be found inside! ...Ross M.

  • The larger masses (bottom two pictures) are indeed nodules or concretions. Various other minerals besides sulfides may occur in them. Sometimes they form by other processes besides having a fossil in the middle. is one source of geological information about Wyoming; also try university geology departments. Determining the age of the fossils will help a lot in narrowing down the options. The large bivalve in the second photo is shaped somewhat like an inoceramid, a group of medium to very large bivalves that were particularly common in the Cretaceous. I've seen them in some beds in Colorado, so I think there would be places in Wyoming to expect them as well. There are freshwater and marine beds of various ages in Wyoming, though, so there are many possibilites.
    If you are able to find a specimen showing the hinge of the shell, that would be helpful in getting a better identification. ...Dr. David C.

  • All ammonids except for straight ammonid, which is probably Baculites sp.

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