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"bony thing"
"bony thing"  
Greetings, I was hoping you could help me in try to identify this rock and what on Earth this bony looking thing that is sticking out of it might be. Found in the Ohio Valley. The rock is light and absorbant with tiny shell impressions. I am at a total loss! Thanks much!

Answer
Raven:

What you have is a fossel cast.

Fossils are made in several way, replacement, mold and cast.

A replacement fossil is formed when the original composition of the organism or remains are replaced by another substance usually a mineral.  Petrified wood is an example.  The wood is replaces slowly over millions of years by ground water saturated with dissolved mineral usually silicone saturates the wood and replaces the cellulose of the wood with silica. This can preserve the actual cellular structure of the wood enabling identification of the tree species.

Fossil molds are made when an organism is trapped or buried in fine sediment like mud or silt.  The sediment hardens or lithifies over time, and original organism or shell is slowly dissolved away by the passage of water or decay.  What remains is the impression or the void that the object created in the surrounding sediment. In essence all that is left is a negative impression or a mold.  The mold can be filled with plaster or resin to create a cast of the fossil.  This process is used by museums.  They make molds of dinosaur bones and make resin casts of the bones for display.

The last is obviously the cast.  Nature can fill the void described above with another mineral, sometimes silica, or lithified silt.  Over time, the origninal sediment that created the mold is weathered away leaving only the cast.

What you have are two examples.  The elongaged one is a bryozonan.  From the photo I cannot tell if it is a replacement or a mold.  If it looks like, and it does, that the material of the fossil is different from the surrounding material, it may be the original skeletal remains of the creature.  Bryozoans are similar to coral, a colonial organism with an exoskeleton.  The creature lived in the little holes and was a filter feeder.

The other fossil is a brachiopod.  It is similar to a pelecipod, today's clam, but more primative, and had a small muscular foot that adhered to a surface holding them in place. Hence the name, Brach - arm and poda - foot.  It existed in the same environment as the bryozoan, in shallow seas.  Brachiopods die offs in the Permian - Triassic periods and were eventually out competed by the pelecipods.  Some still exist in small niches.  Both the bryozoans and brachs existed in the same reefal environment so is why they occur in the same piece of rock. Sometimes if you apply a bit of acid, to the rock  you can edtch out the fossils a bit, but only if the shell has been replaced by silica.  If not the acid will eat up the fossil impression.  It is hard to tell if you just have the impression (mold) of the shell or the original or replacement.

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Keith Patton

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I can answer questions concerning physical and historical geology, environmental geology/hydrology, environmental consulting, remote sensing/aerial photo interpretation, G&G computer applications, petroleum exploration, drilling, geochemistry, geochemical and microbiological prospecting, 3D reservoir modeling, computer mapping and drilling.I am not a geophysicist.

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I have 24 years experience split between the petroleum and environmental industries. I have served as an expert witness in remote sensing, developmental geologist, exploration geologist, enviromental project manager, and subject matter expert in geology and geophysical software development.

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Registered Geologist in State of Texas

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