EXTINCTION AND SURVIVAL OF BIRDS (AVES) AT THE CRETACEOUS-TERTIARY BOUNDARY: EVIDENCE FROM WESTERN NORTH AMERICA

By the end of the Cretaceous, the coastal plain along the Western Interior Seaway was home to a large number of bird species both within and outside of crown-group birds (Neornithes). The diversity of birds in the Maastrichtian Hell Creek and Lance Formations of Montana and Wyoming approaches that of the sympatric mammals. The avian fossils from these formations include multiple species of avisaurid enantiornithines and freshwater hesperornithiforms, as well as a few other non-crown-group species. However, the coastal plain faunas are dominated by neognathous neornithines, including fossils that appear to be relatives of the Galliformes (fowl), Anseriformes (waterfowl), Charadriiformes (shore birds), and other neoavian orders.

These birds were likely as diverse ecologically as they are systematically. The birds along the coastal plain of the western interior were predominantly omnivores who dove, waded, swam, walked, and flew in search of food. The presence of numerous juvenile and possibly nestling bird bones in the area indicate that Montana and Wyoming provided suitable habitat for some breeding bird populations prior to the close of the Cretaceous.

The stratigraphic distribution of birds in the Lance, Hell Creek, and lower Fort Union Formations provides some clarification of the pattern of avian extinction at the end of the Cretaceous. At the grossest scale, the clades outside of crown-group birds went extinct, while crown-group birds survived. The enantiornithines do not appear to have survived close to the K-T boundary, in contrast with the hesperornithiforms. The extinction of the non-crown clades appears to be part of a long term decline of those clades in coastal areas that began in the Campanian. All of the neornithine lineages present in the late Cretaceous of western North America survived the mass extinction. Their survival includes the perseverance of at least one breeding species of neognath.