Pterosaurs in the Late Cretaceous
Very often it is stated that pterosaurs declined in the late Cretaceous, sometimes due to competition with birds. However, this has been confirmed to not be the case for a variety of reasons:
– Pterosaur diversity merely decreased in the period between the Cenomanian and the Turonian, which indicates that, if anything, pterosaurs merely suffered amidst the mass extinctions that occured at the time, which also eliminated many terrestrial and aquatic sauropsids like brachiosaurs and pliosaurs.
– Pterosaur diversity is much higher in the post-Turonian Cretaceous than most people give it credit for. Azhdarchids are cosmopolitian and common through the Turonian all the way to the Maastrichian, while pteranodontids/nyctosaurs are common in North America all the way to the Campanian, and might have survived until the very end because undescribed nyctosaurid remains have been found in Brazil dating to the Maastrichian (previously assigned to Nyctosaurus). Finally, we have Gwawinapterus as a Campanian example of an istiodactylid, suggesting that pterosaurs are prone to Lazarus taxa situations.
– Bird diversity did not change radically in the post-Turonian Cretaceous; while earlier forms were replaced by newer forms like ornithurines and enantiornithes, the only new niches known to have been occupied by birds were those of large, dolphin like marine predators (hesperornithes), and those of large flightless omnivores (Gargantuavis), none of which previously occupied by pterosaurs, thus suggesting that birds did not exploit the rapid disappearence of pterosaur taxa. It is possible that Samrukia represents a large volant bird specimen, but we currently have nothing but its lower jaw, which is akin to those of flightless omnivorous birds; footprints atributed to a crane like bird have also been found, but again there is nothing to suggest it occupied a niche taken by pterosaurs. Anseriformes only occupied their iconic filter feeding niches after the Cretaceous, implying that ctenochasmatoid pterosaurs could still have been alive.
The conclusion is, therefore, that birds did not replace pterosaurs, and they might or might not have had significant diversity losses.