Geology/Unique Fossil Identification
I have a fossil that my father passed onto me and I am curious about what it exactly is and contains. I took it into the University of Alaska Museum and a grad student helped to identify some clam and invertebrate structures, but I would appreciate a full identification. I will include several good pictures of it I took this morning. I am in need of money for bills right now, and am curious as to what the value of this fossil is. My dad was a collector so I believe that it is quite rare. Anyways, I appreciate any and all knowledge. Thanks for your time
You have what looks like a limestone concretion containing ammonite and possibly pelecepod shells. I say possibly pelecepod shells The cobble has been rounded by water, either by being worked in a stream or on a beach. The opalescence seen in the second photo is from the way the shells have been "cut" by the abraiding of the rock.
Assemblages like these were formed when a large number of the animals died and sank and were concentrated in one spot either by currents or the number that died all at once. The collection of shells were then cemented together by the deposition of limestone.
You may have seen limestone facing in buildings or in public bathrooms in older buildings where "ghost" shells are evident. The "ghost" patterns are caused by the various orientations the shells were in when the blade of the rock saw cut through them when the rock was slabbed. That is kind of the phenomena you are seeing.
As to its value. That is really up to the purchaser. I would not expect it to be worth more than $100 if that since it is small and the fossils are not distinct.
This link is to a pdf file of a USGS paper on a formation in Alaska of Triassic age that contains a high concentration of ammonite shells. It might be the formation from which your cobble came, but without notes from you father specifically stating where he found it one cannot be sure without some lab work correlating it to the rock of the formation. The shape of the cut shells in your specimen make me believe they are ammonites (gastropods) due to the oblong or oblate profiles of the shells, pelecypods leave a different set of shapes when "cut" at angles. In my experience Pelecypods do not have opalescence in their shells.