I have now received a copy of Field Guide to Amphibians of Costa Rica.
Sadly, this doesn't really confirm either way as the (many) pics of different variations of Smilisca baudinii don't include this type of marking or colour (although flash isn't helping with that). My main concern is that this specimen was a small frog. About 8-9cm in length. The field guide says that Smilisca baudinii is a large frog. Of course, this could just be a young, recently metamorphosed individual. It also looks more like the 'rain frogs' in form, although that could again be because of developmental stage. When I get time, I'll have another look and maybe suggest some alternatives........
