Telmatosauridae is a small family of basal hadrosauroids -- early relatives of the duck-billed dinosaurs -- known from the Late Cretaceous of southeastern Europe, roughly 70 million years ago. At the time, this region was a scatter of islands in the Tethys Sea, and telmatosaurids show clear signs of insular dwarfism: members grew to only about 5 meters long, notably smaller than mainland hadrosauroid relatives, in a pattern consistent with the limited resources of island habitats. Like other hadrosauroids, they had a duck-like beak suited to cropping vegetation and the beginnings of the dental batteries that later, more derived hadrosaurids would develop much further.
The type genus, Telmatosaurus, from the Hațeg Basin of Romania, is the best known member and one of the most completely described dinosaurs from Cretaceous Europe, including growth-stage data drawn from its limb bones. Related genera Tethyshadros (Italy) and the more recently named Kryptohadros (Romania) have shown that basal hadrosauroids were more diverse across these Late Cretaceous European islands than once thought, representing an endemic evolutionary side-branch that developed in relative isolation from the larger hadrosaur radiation happening across Asia and North America at the same time.