Can You Identify This Fossil Found in Sonora, CA?

[bikefuller]

I was walking along the shore of New Melones Resevoir in Sonora, CA last weekend and I stumbled upon this awesome fossil of some sort. PLease help me idetifty what it is and how old it might be.
THanks

 

[Sorry!]

I'm no professional but it just looks like a shell to me. I think most shell fossils found date back like 2-5 mya

 

[billiards]

I'm no palaeontologist, or even a geologist, but my first instinct was ammonite, named for the curly, spirally type shell which is reminiscent of the Egyptian ram god Amon's horns. It's a fragment of course, and most likely a mould at that.

The rock is a fine grained sediment, probably calcareous limestone of sorts. If that's the case then the fossil originates from the ocean. Ammonites used to float (or maybe swim?) around in the ocean and had infllatable soft bodies to give them buoyancy; when they died they sunk to the bottom.

I have no idea of the full time period of ammonites, but I've found the fossils myself in Jurassic and Cretaceous rocks around 100 to 200 Million years old.

 

[mgb_phys]

Probably some form of ammonite - like http://en.wikipedia.org/wiki/Nautilus (edit yes I know a nautilus isn't an ammonite)
Unfortunately they have been around for 500Myr and don't change much so they aren't all that useful for dating rocks.

Your fossil is a cast - the radial arcs standing up from the rock were the hollow chambers in the shell. When the creature died these filled with silt and then as the shell decayed the silt formed a solid rock cast of the inside of the body.

 

[B. Elliott]

Definitely an Ammonite. Looks a lot like a few of the smaller Ammonite fragments I have at home.