Han Hu

An enlarged sternum with a prominent keel is a central feature of the flight apparatus of modern birds. However, sterna of near-bird dinosaurs (Pennaraptora) and early avialans are either substantially different from those of living birds or absent altogether, raising questions about how specialized sternal structures evolved in birds and how they are related to function. This remains poorly understood because of the fragmentary nature of the fossil record, and the challenges in inferring form and function from crushed fossils. We use ancestral character estimations to trace sternal trait acquisition through the bird stem group, and multivariate phylogenetic regressions to analyse relationships between sternum morphology, body mass and flight capabilities. We find that sternum evolution was episodic: basal members of Pennaraptora had proportionally small sterna, which became larger and more craniocaudally elongated in Avialae. This enlargement precedes the appearance of a midline ridge, a possible precursor of the sternal keel, in Pygostylia. Sternum size increased again in crownward Ornithuromorpha, alongside a fully formed sternal keel and enlarged caudal projections, both critical areas of flight muscle attachment. Sternal experimentation in relation to flight characteristics occurs several times throughout Pennaraptora, including within Paraves and Enantiornithes, indicating that powered flight may have evolved several times before proliferating in crown-group birds.

Here we report on the nearly complete and uncrushed 14th specimen of Archaeopteryx. Exceptional preservation and preparation guided by micro-computed tomographic data make this one of the best exemplars of this iconic taxon, preserving important data regarding skeletal transformation and plumage evolution in relation to the acquisition of flight during early avian evolution. The ventrolaterally exposed skull reveals a palatal morphology intermediate between troodontids¹ and crownward Cretaceous birds^2,3. Modifications of the skull reflect the shift towards a less rigid cranial architecture in archaeopterygids from non-avian theropods. The complete vertebral column reveals paired proatlases and a tail longer than previously recognized. Skin traces on the right major digit of the hand suggest that the minor digit was free and mobile distally, contrary to previous interpretations⁴. The morphology of the foot pads indicates that they were adapted for non-raptorial terrestrial locomotion. Specialized inner secondary feathers called tertials^5,6 are observed on both wings. Humeral tertials are absent in non-avian dinosaurs closely related to birds, suggesting that these feathers evolved for flight, creating a continuous aerodynamic surface. These new findings clarify the mosaic of traits present in Archaeopteryx, refine ecological predictions and elucidate the unique evolutionary history of the Archaeopterygidae, providing clues regarding the ancestral avian condition.

Globally, amber-rich strata are primarily found in major geological intervals, particularly the Aptian-Albian of the Early Cretaceous and the Eocene, both of which coincide with greenhouse climate conditions (Pearson et al., 2007; Katz et al., 2008; Delclòs et al., 2023). However, despite the relatively warm temperatures of the Paleocene, amber from this epoch remains exceptionally rare, with no confirmed records reported to date. This scarcity may be linked to the limited exposure of Paleocene strata worldwide. The Late Cretaceous Canadian Big Muddy amber deposit (ca. 67 Ma) which represents one of the closest known occurrences to the K-Pg boundary (Loewen et al., 2024). Here, we report the first-ever discovery of Palaeocene amber from the Sanshui Basin, China, filling a critical gap in the fossil resin record.

Jeholornis is a representative of the earliest-diverging bird lineages, providing important evidence of anatomical transitions involved in bird origins. Although ~100 specimens have been reported, its cranial morphology remains poorly documented owing to poor two-dimensional preservation, limiting our understanding of the morphology and ecology of the key avian lineage Jeholornithiformes, in addition to cranial evolution during the origin and early evolution of birds. Here, we provide a detailed description of the cranial osteology of Jeholornis prima, based primarily on high-quality, three-dimensional data of a recently reported specimen. New anatomical information confirms the overall plesiomorphic morphology of the skull, with the exception of the more specialized rostrum. Data from a large sample size of specimens reveal the dental formula of J. prima to be 0–2–3 (premaxillary–maxillary–dentary tooth counts), contrary to previous suggestions that the presence of maxillary teeth is diagnostic of a separate species, Jeholornis palmapenis. We also present evidence of sensory adaptation, including relatively large olfactory bulbs in comparison to other known stem birds, suggesting that olfaction was an important aspect of Jeholornis ecology. The digitally reconstructed scleral ring suggests a strongly diurnal habit, supporting the hypothesis that early-diverging birds were predominantly active during the day.

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.

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. Frugivorous birds play an important role in seed dispersal today, and may have done so since their origins. However, evidence of this has been lacking. Jeholornis is one of the earliest-diverging birds, only slightly more derived than Archaeopteryx , but its cranial anatomy has been poorly understood, obscuring diet-related functional interpretations. 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 was at least seasonally frugivorous, providing the earliest evidence for fruit consumption in birds, and indicating that seed dispersal was present from early in the avian radiation. As highly-mobile seed dispersers, early frugivorous birds could expand the scope for biotic dispersal in plants, and may explain, in part, the subsequent evolutionary expansion of fruits, indicating a potential role of bird-plant interactions in the Cretaceous Terrestrial Revolution.

We describe six specimens consisting of cranial remains and associated partial presacral axial series belonging to ornithuromorph birds from the Changma locality of the Lower Cretacous Xiagou Formation of northwestern Gansu Province, China. Comparison among specimens is limited by the paucity of overlapping elements, their differing exposed views, and, in some specimens, poor preservation. Despite this, three separate taxa are represented, evidenced by differences in their dentary dentitions: one specimen is edentulous, another has sharp, closely spaced, relatively high‐crowned and peg‐like teeth, and a third preserves blunt, relatively low‐crowned teeth placed in a communal groove, a morphology previously reported among adult birds only in Hesperornithiformes. We propose that the high‐crowned specimen may be referred to Gansus yumenensis based on shared similarities with the closely related Iteravis huchzermeyeri, including a very similar dentition and an edentulous premaxilla with elongate, unfused frontal processes and no palatal processes. The two other specimens are considered new taxa, for which we erect the names Meemannavis ductrix gen. et sp. nov. and Brevidentavis zhangi gen. et sp. nov. These new specimens confirm that the Changma locality is dominated by ornithuromorph birds and contribute to a better understanding of this important avifauna. The observed variation in dental morphology hints at trophic diversity like that observed in ornithuromorphs from the penecontemporaneous Jehol Group of northeastern China.

Among extant vertebrates, mammals are distinguished by having a chain of three auditory ossicles (the malleus, incus and stapes) that transduce sound waves and promote an increased range of audible—especially high—frequencies¹. By contrast, the homologous bones in early fossil mammals and relatives also functioned in chewing through their bony attachments to the lower jaw². Recent discoveries of well-preserved Mesozoic mammals have provided glimpses into the transition from the dual (masticatory and auditory) to the single auditory function for the ossicles, which is now widely accepted to have occurred at least three times in mammal evolution^3,4,5,6. Here we report a skull and postcranium that we refer to the haramiyidan Vilevolodon diplomylos (dating to the Middle Jurassic epoch (160 million years ago)) and that shows excellent preservation of the malleus, incus and ectotympanic (which supports the tympanic membrane). After comparing this fossil with other Mesozoic and extant mammals, we propose that the overlapping incudomallear articulation found in this and other Mesozoic fossils, in extant monotremes and in early ontogeny in extant marsupials and placentals is a morphology that evolved in several groups of mammals in the transition from the dual to the single function for the ossicles.

Bohaiornithidae is currently the most diverse recognized family of Early Cretaceous enantiornithines, with unique morphology of the rostrum and pedal digits. Here we describe a second specimen of the bohaiornithid Longusunguis kurochkini from the Jiufotang Formation. This specimen provides new anatomical information regarding this taxon, in particular clarifying uncertain aspects of enantiornithine cranial morphology. The rarely preserved postorbital is completely preserved on both sides of the skull, confirming the presence of a complete postorbital bar in some enantiornithines. This suggests that the plesiomorphic diapsid skull was retained by at least some basal enantiornithines and the infratemporal fenestra in Ornithothoraces may have been lost independently multiple times, providing a better understanding of cranial evolution from non-avian dinosaurs to modern birds.

Recent finding of a fossil – Oculudentavis khaungraae Xing et al. 2020, entombed in a Late Cretaceous amber – was claimed to represent a humming bird-sized dinosaur ¹ . Regardless of the intriguing evolutionary hypotheses about the bauplan of Mesozoic dinosaurs (including birds) posited therein, this enigmatic animal demonstrates various morphologies resembling lizards. If Oculudentavis was a bird, it challenges several fundamental morphological differences between Lepidosauria and Archosauria. Here we reanalyze the original computed tomography scan data of Oculudentavis . Morphological evidences demonstrated here highly contradict the avian or even archosaurian phylogenetic placement of Oculudentavis . In contrast, our analysis revealed multiple synapomorphies of the Squamata in this taxon, including pleurodont marginal teeth and an open infratemporal fenestra, which suggests a squamate rather than avian or dinosaurian affinity of Oculudentavis .