Hadrosaurs, or duck-billed dinosaurs, are a hugely successful group of ornithopod dinosaurs that lived in the Late Cretaceous on all northern continents as well as in South America. Besides having toothless duck-like bills, hadrosaurs had large heads with hundreds of teeth packed into what's called a dental battery, large bodies, and the ability to walk on both two legs or on all fours. Additionally, many hadrosaurs had elaborate crests on their heads, and include spike-crested Saurolophus, dinner-plate crested Corythosaurus, and tube-crested Parasaurolophus. Hadrosaurs are part of a group of dinosaurs called the ornithopods that not only included duck-bills, but also spike-thumb Iguanodonts, and superficially kangaroo-shaped hypsilophodonts. The iguanodonts can be considered to be something of an evolutionary transition between hypsilophodonts and hadrosaurs; their bodies were fairly similar-looking to hadrosaurs although they generally lacked crests.
The dental battery of hadrosaurs didn't come out of nowhere – it has a well-documented fossil record. Ornithopods, like most reptiles, continually replaced their teeth throughout their lifetime. Hypsilophodonts, the smaller kangaroo-shaped ornithopods, don't have dental batteries. Instead, their teeth are all somewhat leaf-shaped or triangular, and like most vertebrates the portions of the teeth that are out of the gums are covered in enamel, and the core of the tooth is filled with living pulp tissue. Iguanodont teeth, as you might have guessed, show a transitional condition. Their teeth are very large, shaped like a long diamond with a rounded apex, and closely packed, eliminating the gum tissue between tooth positions. Additionally, the enamel on the teeth is asymmetrically deposited, being thicker on the cheek-side of the upper teeth, and the tongue-side of the lower teeth.
The condition that's seen in hadrosaurs is similar to that seen in iguanodonts, only more extreme. Hadrosaur teeth are still closely packed, eliminating spaces between them, but are substantially smaller in comparison to the jaw size, with enamel solely on one side of each tooth. Each tooth position, or "tooth family" holds as many as five stacked teeth continually growing and ready to erupt. The closely-packed diamond-shaped teeth are locked into a battery, which as a whole, resembles pineapple skin.