I went out to my backyard today, and what had seemed to be big healthy lettuce plants, have nearly all been munched up. Whatever it is starts around the outer edges of the leaves on the outside of the plant, leaving jagged edges of the leaves (if very much of the leaf at all). I am growing romaine and buttercrunch lettuce.

Is this caterpillar damage or something? It happened all of a sudden, and hasn't touched my tomato, spinach, broccolli, or strawberry plants.