Slugs, or earwigs. Go out after dark with a flashlight and inspect...

You could also look for caterpillars, and if you find same, you might want to consider if there are some ordinary violets you could spare as forage. Violets are primary food for almost all the fritillary butterflies - don't know which of them you might have in your area.
(Yes, I know - I'd better duck - it's not kind to suggest that one's favorite flower is a preferred snack for a caterpillar, and then say "keep the caterpillar".)

Hmmm...
Earwigs make the flowers tatty; I've not known them to eat leaves.
Slugs & snails give the game away with the slime trails.
Caterpillars: probably too early for fritillary larvae in your area; moth larvae slightly more likely, but still not a top contender, I would have thought.
Don't rule out mice this time of year. I've just has the tops of some seedlings mown off (in the cold greenhouse), and that's what I suspect.

Gardenpaws is right and there is one fritillary in particular which is endangered and should be conserved--Speyeria diana.