Slow worms are considered lizards as opposed to snakes, both are reptiles. Now I get that there are traits that distinguish them (eye lids, ears ...). But snake species themselves vary already quite a lot.
The Wikipedia article on snakes states:
Based on comparative anatomy, there is consensus that snakes descended from lizards.
But how comes slow worms are considered lizards despite the loss of their limbs during evolution, whereas the same evidently has happened to snakes?
What's the defining factor here? The time frame when it happened, that it happened independently (like the evolution of, say, an eye)?
Note: I couldn't find any detailed descriptions about the alleged evolution of slow worms, whereas for snakes it is easy to find material.