bite

So I've been reading a paperback. I'm about halfway through and I notice something on the back cover. An arc curved around the top of the spine. It's tooth impressions. Somebody bit this book. No corresponding bite marks on the front. Where was the lower jaw?

On page 199 I discovered the other half of the bite. "Maybe they needed their hands, for finding their keys?, so they held the book between their teeth to use their hands," Kent suggested.