Here in Washington we have a bunch of stones like that that sometimes hold fossilized ocean animals (crabs, shrimp, bones, etc.) they’re concretions. Basically minerals start sticking to the organic matter until it is fully encapsulated and the original animal is replaced with other minerals (agate, jasper, pyrite, opal, etc.). From the looks of the host rock, it appears to be a type of siltstone or mudstone which would also fit.
If you can, try to carefully chip away the nodule until you see something that is either a different color, material, or looks like it might be something. Then try to follow along the item as you remove the rest of the rock.
Another method that you could try, but could potentially destroy the hidden goodies depending on what mineral replaced the organic matter, is to pour liquid drain cleaner and water (50/50 mix or weaker) and see if it dissolves the rock. Vinegar works as well and is safer (weaker). Be sure to wash the whole thing with a baking soda and water paste at first and then leave it in a baking soda and water rinse until it stops bubbling.
I like this as it saves fragile details that may get destroyed when trying to chip or Dremel tool the rock away to expose the item.
Hope this helps!
That is definitely one for the experts 
