Early Cretaceous Enantiornithine Birds (Aves, Ornithothoraces) and Establishment of the Ornithuromorpha Morphological Type

New data on the taxonomic and morphological diversity of Early Cretaceous Enantiornithes are reviewed. A new hypothesis concerning the phylogenetic position of Pengornithidae is proposed. These birds traditionally treated as primitive enantiornithines may in fact be more closely related to Ornithuromorpha. This phylogenetic placement implies that the fan-shaped tail and modern-type humeral joint was formed once in the early evolution of birds. Hence, the similarity between Pengornithidae and other enantiornithines may be plesiomorphic. The ecology of Early Cretaceous enantiornithines is discussed. The increased mobility of neck in Holbotia is possibly accounted for by the cranioinertial swallowing mechanism, as in modern ratites. The hypotheses of scansorial adaptations in Parapengornis and Fortunguavis are criticized. In addition, the phylogenetic position of Mystiornis is discussed.