What The Heck Is This?

[ElizabethHarris]

I found these 2 rocks in a bed of river rocks at an apartment complex in Denver Co. I found the biggest piece 1st and continued looking for more interesting rocks when I found the small piece. I was surprised that they fit together. The big piece weighs approximately 2 lbs and the small piece approximately 1/2 lb. If it's not a fossil then I need  to know how it formed. 

[Kane]

I'm not seeing any cancellous texture to suggest bone. I would say these are sedimentary rocks with a mineralization concretion, but no fossils that I can see.