What is "cimolopterygidae"

Cimolopterygidae is a group of extinct birds known from the late Cretaceous Period. Remains attributed to cimolopterygids have been found in the Frenchman Formation of Saskatchewan, the Lance Formation of Wyoming, the Fox Hills Formation of Colorado, the Hell Creek Formation of Montana, and the Allen Formation of Rio Negro, Argentina. Most date to the end of the Maastrichtian age, about million years ago, though a much earlier species has also been identified from the Campanian-aged Dinosaur Park Formation of Alberta, about 75 million years ago.