I'm going to go with the 1-stack, but it's been a while since I have seen one. Many bones, but no skull.

The last set of bones that I found was oriented such that it looked like it could have head-planted, so I actually considered digging down to see if a skull was down there, but figured the chances were pretty slim.

I did stumble across a Queen, near my base, which is convenient, especially since I don't use a map, and finding the bosses can be challenging.