"A pair of parasarolophus (parasarolophi?). I recently read an interesting article on why certain kinds of dinosaurs may have had external ears. [link]
His theory is mostly about iguanadon, but I started comparing hadrosaur skulls, and they tend to have giant ear holes like horses and cows do. Now, parasarolophus stumped me because it doesn’t seem to have ear holes at all. I wished I could have picked up a skull and turned it around, because I couldn’t find any decent reference. Anyway, here’s a pic of what they might have looked like with ears.