Thomas A. Stidham J. Howard Hutchison


In addition to the two described Avisaurus species (A. archibaldi Brett-Surman and Paul and A. gloriae Varrichio and Chiappe), undescribed material from the Maastrichtian contributes important morphologic, biostratigraphic, and biogeographic data on North American avisaurids. A juvenile Avisaurus specimen of an undescribed species from the Maastrichtian Hell Creek Formation of Montana reveals the presence of distal tarsals in enantiornithines and an ontogenetic stage shared with non-avian dinosaurs. All known Avisaurus fossils were deposited in a variety of habitats along the Western Interior Seaway of North America and areas more interior. The presence of other clades of birds in the Hell Creek Formation indicates that avisaurids formed only part of a diverse avifauna at the close of the Cretaceous in North America. Phylogenetic analysis of the new Maastrichtian material places these new specimens as the sister to the other known Avisaurus species within Avisauridae. This hypothesis of avisaurid relationships points to a South American origin of this clade with a dispersal or vicariant event to North America before or early in the Campanian. This event was penecontemporaneous with the Laurasian movement of alvarezsaurids and sauropods. These Montanan enantiornithine specimens represent the northern-most record of this bird clade in North America. These Maastrichtian species and specimens are from very late in the Maastrichtian and are probably the youngest known enantiornithines. The stratigraphic position of these birds with respect to the rest of the fauna indicate that their extinction was nearly synchronous with the extinction of other toothed bird clades and other vertebrates at the end of the Cretaceous.KEY WORDS. Aves. Enantiornithes. Avisaurus. Biogeography


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