Characterising how variation in the tempo and mode of evolution has structured the phenotypic diversity of extant species is a central goal of macroevolution 1-3 . However, studies are typically limited to a handful of traits 4-6 , providing incomplete information. We analyse morphological diversification in living birds, an ecologically diverse group 7 , documenting structural scales from 'pan-skeletal' proportions down to the localised 3D shape changes of individual bones. We find substantial variation in evolutionary modes among avian subgroups and among skeletal parts, indicating widespread mosaicism, and possible differences in the structure of the macroevolutionary landscape across Earth's major environments. Water-linked groups, especially Aequorlitornithes (waterbirds) have repeatedly explored a large portion of their total morphospace, emphasizing variation in body proportions and in the shape of bones close to the body core, which are functionally related to the mechanics of locomotion 8 . In contrast, landbirds (Inopinaves) evolved distinct, group-specific body forms early in the aftermath of the K-Pg and subsequently emphasised local shape variation, especially in the head and distal limb bones, which interact more directly with the environment. Passerines, which comprise more than half of all bird species, show a conservative evolutionary dynamic that resulted in low disparity across all skeletal parts. Evidence for early establishment of the morphospace of living birds is clear for some skeletal parts, including beaks and the combined skeletal morphology. However, we find little evidence for early partitioning of that morphospace, contrary to more specific predictions of 'niche-filling' models 1,9 . Nevertheless, early divergence among broad environmental types may have caused an early divergence of evolutionary modes, suggesting an important role for environmental divergence in structuring the radiation of crown-group birds.

The Early Cretaceous diversification of birds was a major event in the history of terrestrial ecosystems, occurring during the earliest phase of the Cretaceous Terrestrial Revolution, long before the origin of the bird crown-group. Frugivorous birds play an important role in seed dispersal today. However, evidence of fruit consumption in early birds from outside the crown-group has been lacking. Jeholornis is one of the earliest-diverging birds, only slightly more crownward than Archaeopteryx, but its cranial anatomy has been poorly understood, limiting trophic information which may be gleaned from the skull. Originally hypothesised to be granivorous based on seeds preserved as gut contents, this interpretation has become controversial. We conducted high-resolution synchrotron tomography on an exquisitely preserved new skull of Jeholornis, revealing remarkable cranial plesiomorphies combined with a specialised rostrum. We use this to provide a near-complete cranial reconstruction of Jeholornis, and exclude the possibility that Jeholornis was granivorous, based on morphometric analyses of the mandible (3D) and cranium (2D), and comparisons with the 3D alimentary contents of extant birds. We show that Jeholornis provides the earliest evidence for fruit consumption in birds, and indicates that birds may have been recruited for seed dispersal during the earliest stages of the avian radiation. As mobile seed dispersers, early frugivorous birds could have expanded the scope for biotic dispersal in plants, and might therefore explain, at least in part, the subsequent evolutionary expansion of fruits, indicating a potential role of bird–plant interactions in the Cretaceous Terrestrial Revolution.

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Macroevolution is integral to understanding the patterns of the diversification of life. As the life sciences increasingly use big data approaches, large multivariate datasets are required to test fundamental macroevolutionary hypotheses. In vertebrate evolution, large datasets have been created to quantify morphological variation, largely focusing on particular areas of the skeleton. We provide a landmarking protocol to quantify morphological variation in skeletal elements across the head, trunk, hindlimb and forelimb using 3-dimensional landmarks and semilandmarks, and present a large pan-skeletal database of bird morphology for 149 taxa across avian phylogeny using CT scan data. This large collection of 3D models and geometric morphometric data is open access and can be used in the future for new research, teaching and outreach.

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