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Despite extensive discoveries across the globe over the past two centuries, little phylogenetic work has been done on the Hesperornithiformes. Spanning the late Early to Late Cretaceous, hesperornithiforms are one of the most diverse groups of Mesozoic birds in terms of both their geographical distribution and the wide differences in body size and diving specializations. This study presents the first phylogenetic analysis of the Hesperornithiformes that includes a majority of the described taxa, enabling the first detailed look at evolutionary relationships within the clade. The results of this study support the monophyly of the Hesperornithiformes, which is recovered as the sister clade to the avian crown group, Neornithes. Within the Hesperornithiformes, the Brodavidae and Hesperornithidae are monophyletic while the Baptornithidae are polyphyletic. Little evidence of species-level taxonomic differentiation is found within Hesperornis, with many species indistinguishable from Hesperornis regalis. Evoluti...

A new fossil bird is described from the Early Eocene Fur Formation of Denmark. Pellornis mikkelseni gen. et sp. nov. is represented by a single specimen that consists of three-dimensionally preserved elements of the wing and pectoral girdle together with associated parts of the left hindlimb. Comparisons based on general morphology and particular characters of the wing and most elements of the pectoral girdle indicate that the new specimen is morphologically similar to the extinct taxon Messelornithidae (Messel rails). This similarity is also expressed by a phylogenetic analysis, which supports a close relationship between the new fossil and Messel rails. The morphology of the sternum, in particular, shows that the new fossil is distinguishable from other Messelornithidae. An interesting aspect of its morphology is the presence of limb specializations such as the extensive ossification of tendons—well known among running birds—that suggest the new Danish fossil may have been a ground bird with cursorial h...

Enantiornithines are the most speciose avian clade in the Mesozoic, with a fossil record that nearly spans the Cretaceous; however, with less than half of known taxa preserving skull material, our understanding of their cranial morphology remains incomplete. Here we present a comprehensive overview of the current knowledge of enantiornithine skull anatomy and discuss the range of morphologies known for each of the main cranial elements. The typical enantiornithine skull retains numerous ancestral features such as the absence of fusion among bones, the presence of a postorbital bone, a primitive quadrate with a single headed otic process, an unforked dentary, and teeth. The postorbital in at least one taxon is unreduced, suggesting the existence of a complete infratemporal fenestra and thus an unmodified diapsid skull as in confuciusornithids. The rostrum is well known and shows considerable variation, typical of theropods; however, in terms of rostral proportions, enantiornithines are extremely limited wi...

The ability to identify the ecological role an organism plays is fundamental to modern ecology and is an important goal of palaeontological studies. Morphometrics has the potential to be a statistically robust means of furthering this goal, identifying ecomorphological trends within a group of organisms. The dramatic morphological and ecological diversity of modern and fossil birds makes them ideal for ecomorphological studies, and the vast increase in known Mesozoic avian diversity opens numerous possibilities for applying modern ecomorphology to enhance our understanding of avian diversification in the Mesozoic. This study presents a large morphometric database of modern birds analysed using a multivariate statistical approach, in order to identify correlations between ecology and morphology. These relationships were also examined in a number of Mesozoic birds in order to investigate both the ecological diversification of early birds as a whole and the specific ecological roles individual taxa may have ...

Confuciusornis sanctus stands out among the remarkable diversity of birds of the Jehol biota (Lower Cretaceous, Liaoning Province, China). Its basal position in the phylogenetic tree of birds, combined with the exceptional number of well-preserved, largely complete and articulated specimens, makes it a perfect model system for studying the variation, development and life history of early Mesozoic birds. A comprehensive morphometric study (measurements of humerus, ulna, radius, femur and tibia) previously identified two distinct size classes of C. sanctus, while demonstrating the lack of statistical support of the association between this size dimorphism and the characteristic pair of long tail feathers present in some specimens. Four plausible explanations were discussed to account for the resultant size classes: the existence of more than one species in the sample; sexual size dimorphism; two size classes corresponding to attritional death assemblages; and/or a particular growth pattern similar to that i...

Synopsis We report on a new species of enantiornithine bird from the Late Cretaceous of the Mongolian Gobi Desert, which adds to the meager record of birds from the latest part of the Mesozoic in this region. The exceptional, three‐dimensional bones of the new fossil increase the known anatomical diversity of enantiornithine birds, providing information about previously unreported morphologies such as a pneumatised furcula. Several synapomorphies nest the new bird within an enantiornithine clade of proficient fliers, but the anatomy and proportions of its flight apparatus imply significant aerodynamic restrictions. The combination of phylogeny and our functional interpretation suggests that this new fossil bird is a representative of a flightless lineage. This discovery thus provides the first evidence of a trend towards more limited flying capabilities among Enantiornithes, a group of Cretaceous birds otherwise believed to be represented by competent fliers.