Hesperornithes

Hesperornithes is an extinct group of specialized aquatic avialans very closely related to modern birds. They are known to have inhabited bodies of freshwater and saltwater of the Northern Hemisphere, in the form of strong predatory swimmers and divers. Many hesperornitheans are flightless, although some members still retain flying abbilities. Hesperornitheans were among the very first avialans to colonize the marine environment. However, these unique animals became extinct in the K-Pg extinction event.