Final answer:
During the Early Cretaceous, avian diversity was significant with groups like Enantiornithes and Ornithurae. Enantiornithes had distinct anatomy and did not survive after the period, whereas Ornithurae are part of modern birds' lineage. The mass extinction at the Cretaceous-Paleogene boundary affected many species, but birds descended from theropods survived.
Step-by-step explanation:
During the Early Cretaceous period, there was a significant diversification of avian species. Groups like Enantiornithes and Ornithurae were distinct from modern birds. The Enantiornithes were the dominant bird group then, with notable features like teeth in their jaws and a shortened tail with tail feathers, distinct from modern birds' anatomy, specifically in how certain shoulder bones joined. These birds did not survive past the Cretaceous.
Meanwhile, the Ornithurae group had a short, fused tail called a pygostyle, and this clade is part of the evolutionary line of modern birds. This diversity indicates that the bird lineage was robust and evolving along several branches during the Cretaceous period. Despite the mass extinction event at the Cretaceous-Paleogene boundary, which led to a significant loss of diversity among many forms of life, birds, descendants of theropod dinosaurs, managed to survive and continue evolving into the diverse forms we see today.