Small beetle that I'm sad I dropped

Okay, I generally don't try to do this, but I'm very curious about a beetle I found, dropped and could not find again, and then died a little inside.

Description: 3-5mm. Small head. Round/slightly oval elytra. The elytra gave a seamless transition to the pronotum, continued same curvature of elytra, however, the pronotum was very small. the overall effect was a tear-drop shaped beetle with the pronotum and abdomen hard to separate with a head attached.

Habitat: Found in the dirt under a large log. It was crawling around/in/near a very rotten mushroom that had been growing in the dirt under the logs. The log was situated about 100 meters from the ocean.

I have the sinking suspicion that when and if someone figures out what this was, I'm going to be very irritated. I dropped a piece of dirt into the jar and dropped the beetle instead of the other way around. Thanks for any help!