Ornithurae

Abstract: Charles Darwin commented that Ichthyornis, as one of the “toothed birds” from the Late Cretaceous of Kansas, offered some of “the best support to the theory of evolution” (in litt., C. Darwin to O.C. Marsh, August 31, 1880). Ichthyornis figures no less prominently today. It is one of the closest outgroups to crown clade Aves, and remains one of the only Mesozoic avialans known from more than a handful of specimens. As such, Ichthyornis is an essential taxon for analyses of deep divergences within Aves because of its influence in determining the morphologies ancestral to the crown clade. Ichthyornis, however, has languished in need of new anatomical description and taxonomic revision. Many of the best Ichthyornis specimens were largely inaccessible, plastered into Yale Peabody Museum (YPM) exhibit mounts for nearly a century. The focus of this study was the entire YPM Ichthyornis collection, the largest at any institution. The elements removed from the mounts were identified to the specimens with ...

Abstract: The effect of the Cretaceous-Paleogene (K-Pg) (formerly Cretaceous–Tertiary, K–T) mass extinction on avian evolution is debated, primarily because of the poor fossil record of Late Cretaceous birds. In particular, it remains unclear whether archaic birds became extinct gradually over the course of the Cretaceous or whether they remained diverse up to the end of the Cretaceous and perished in the K–Pg mass extinction. Here, we describe a diverse avifauna from the latest Maastrichtian of western North America, which provides definitive evidence for the persistence of a range of archaic birds to within 300,000 y of the K–Pg boundary. A total of 17 species are identified, including 7 species of archaic bird, representing Enantiornithes, Ichthyornithes, Hesperornithes, and an Apsaravis-like bird. None of these groups are known to survive into the Paleogene, and their persistence into the latest Maastrichtian therefore provides strong evidence for a mass extinction of archaic birds coinciding with the Chicxulub asteroid impact. Most of the birds described here represent advanced ornithurines, showing that a major radiation of Ornithurae preceded the end of the Cretaceous, but none can be definitively referred to the Neornithes. This avifauna is the most diverse known from the Late Cretaceous, and although size disparity is lower than in modern birds, the assemblage includes both smaller forms and some of the largest volant birds known from the Mesozoic, emphasizing the degree to which avian diversification had proceeded by the end of the age of dinosaurs.

Abstract: Current classification of birds recognizes three subclasses which are morphologically distinct: the Archaeornithes for Archaeopteryx, the Odontornithes for the Hesperornithiformes and the Ichthyornithiformes, and the Neornithes for all modern birds and their extinct immediate relatives. (Some authorities1 prefer different names for some of these taxa.) I have examined new material recently discovered in the Upper Cretaceous rocks of Argentina which indicates the existence of a group of birds having features so different from those of the currently recognized subclasses that they seem to represent a fourth subclass, here named the Enantiornithes (‘opposite birds’). I describe unique features of the Enantiornithes which include a reduced outer metatarsal, in some forms an extreme modification of the remaining elements of the tarsometatarsus, a highly modified pectoral girdle, and sometimes a characteristic perforation in the proximal end of the humerus.

Abstract: In studies of the evolution of avian flight there has been a singular preoccupation with unravelling its origin. By contrast, the complex changes in morphology that occurred between the earliest form of avian flapping flight and the emergence of the flight capabilities of extant birds remain comparatively little explored. Any such work has been limited by a comparative paucity of fossils illuminating bird evolution near the origin of the clade of extant (i.e. ‘modern’) birds (Aves). Here we recognize three species from the Early Cretaceous of China as comprising a new lineage of basal ornithurine birds. Ornithurae is a clade that includes, approximately, comparatively close relatives of crown clade Aves (extant birds) and that crown clade. The morphology of the best-preserved specimen from this newly recognized Asian diversity, the holotype specimen of Yixianornis grabaui Zhou and Zhang 2001, complete with finely preserved wing and tail feather impressions, is used to illustrate the new insights offered by recognition of this lineage. Hypotheses of avian morphological evolution and specifically proposed patterns of change in different avian locomotor modules after the origin of flight are impacted by recognition of the new lineage. The complete articulated holotype specimen of Yixianornis grabaui, from the Early Cretaceous Jiufotang Formation of Liaoning Province, in north-eastern China, arguably the best-preserved basal ornithurine specimen yet discovered, provides the earliest evidence consistent with the presence of extant avian tail feather fanning.