Either something small got in the cage. A vole is a meat eater and about the same size as a mouse.
The 1x2 wire would let small critters into that section.
I found a mess of chick wings breeks and feet and a very upset broody mom one morning many years ago and it turned out to be a ferel cat in...
Picture of this 4x4 cage would help us .
Of there is a gap around the door or access, or if the door flexes with little effort, it could have been a weasel or a rat.
Eating everything but the skull would sound more to me like the leavings of a rat. A weasel would not have eaten it all and made...