Hello there,
I apologize if this is already a topic, but I haven't seen anything regarding the topic - so I made this! I apologize once more! Any who -- I just recently finished a couple Gravedigger games (a couple on Easy, a couple on Medium), and while playing.. I managed to find a couple of the skeletons/skulls you can find for achievements. Well, while playing, I managed to find the Winged Skelly, Kumos Skelly, and the Wizard's Skull (half of it hidden behind a casket). and I was dumb enough NOT to screencap two of these findings, until I found the Winged Skelly and thought 'oh, maybe I should screenshot it for evidence'. But I do hope that you can take my word for it!
I do have this hidden under this spoiler, so here it is. :) PS: I added the achievements section to show that I never received it, in case!
Screenshot

I had refreshed the page a couple times, and even went to my achievements page to see if perhaps it was just delayed or something. I went back to the game multiple times and replayed it just to see if, perhaps it wasn't registering? I am not sure if it's a glitch or something, but I figured I'd mention something.
Also!! The browser I am using is Google Chrome and it is up to date!!! Thank you. :)
- I'm pretty sure you have to fully uncover a skeleton in order for it to count. In your screenshot, you haven't uncovered the skeleton's legs.
That also means that if a casket it blocking part of a skeleton, there's no way to fully uncover it and it won't count (as was the case with the Wizard's skull you found).

Speiro is correct, the whole thing has to be uncovered and cannot have a coffin over part of it. Sorry you ran into that. I had that happen 2 times with the mortiking.
Dexter by ❤