Barb geometry of asymmetrical feathers reveals a transitional morphology in the evolution of avian flight

Functional constraints on the number and shape of flight feathers

As a fundamental ecological aspect of most organisms, locomotor function significantly constrains morphology. At the same time, the evolution of novel locomotor abilities has produced dramatic morphological transformations, initiating some of the most significant diversifications in life history. Despite significant new fossil evidence, it remains unclear whether volant locomotion had a single or multiple origins in pennaraptoran dinosaurs and the volant abilities of individual taxa are controversial. The evolution of powered flight in modern birds involved exaptation of feathered surfaces extending off the limbs and tail yet most studies concerning flight potential in pennaraptorans do not account for the structure and morphology of the wing feathers themselves. Analysis of the number and shape of remex and rectrix feathers across a large dataset of extant birds indicates that the number of remiges and rectrices and the degree of primary vane asymmetry strongly correlate with locomotor ability revealing important functional constraints. Among these traits, phenotypic flexibility varies reflected by the different rates at which morphological changes evolve, such that some traits reflect the ancestral condition, whereas others reflect current locomotor function. While Mesozoic birds and Microraptor have remex morphologies consistent with extant volant birds, that of anchiornithines deviate significantly providing strong evidence this clade was not volant. The results of these analyses support a single origin of dinosaurian flight and indicate the early stages of feathered wing evolution are not sampled by the currently available fossil record.

An evaluation of the effect of hydrofluoric acid (HF) treatment on keratins

Hydrofluoric acid (HF) is commonly used in geological and paleontological research to extract organic fossils for morphological and chemical studies. However, during HF treatment, organic matter can also be altered, which raises concerns that HF-treated organic matter may not be representative of the original organic matter. To provide reference data for protein studies on fossils, herein, we use Fourier transform infrared (FTIR) spectroscopy to investigate the effect of HF (21.3 M) treatment on keratins, with treatment durations ranging from 2 to 48 h. Results show that the FTIR spectra of HF-treated samples are overall similar to that of the untreated sample, while curve fitting shows that HF treatment has led to alteration of the secondary structure in all the HF-treated samples and the effect is time-dependent. The 2- and 4-h treatment mainly reduced the content of the random coils, α-helix, and intermolecular β-sheet. From 8h onwards, the content of random coils greatly increased at the expense of other structures. Our results imply that for protein detection in fossils using FTIR spectroscopy, the negative effect of HF treatment is not substantial, as the bands characteristic of proteins, that is, amide A, amide B, amide I, amide II, and amide III, are still present after the 48-h treatment. If the target is a secondary structure, the effect of HF treatment should be considered. When HF treatment is necessary, limiting the treatment duration to less than 4h may be a choice.

Incipient wing flapping enhances aerial performance of a robotic paravian model

The functional origins of bird flight remain unresolved despite a diversity of hypothesized selective factors. Fossil taxa phylogenetically intermediate between typical theropod dinosaurs and modern birds exhibit dense aggregations of feathers on their forelimbs, and the evolving morphologies and kinematic activational patterns of these structures could have progressively enhanced aerodynamic force production over time. However, biomechanical functionality of flapping in such transitional structures is unknown. We evaluated a robot inspired by paravian morphology to model the effects of incremental increases in wing length, wingbeat frequency, and stroke amplitude on aerial performance. From a launch height of 2.8 m, wing elongation most strongly influenced distance travelled and time aloft for all frequency-amplitude combinations, although increased frequency and amplitude also enhanced performance. Furthermore, we found interaction effects among these three parameters such that when the wings were long, higher values of either wingbeat frequency or stroke amplitude synergistically improved performance. For launches from a height of 5.0 m, the effects of these flapping parameters appear to diminish such that only flapping at the highest frequency (5.7 Hz) and amplitude (60°) significantly increased performance. Our results suggest that a gliding animal at the physical scale relevant to bird flight origins, and with transitional wings, can improve aerodynamic performance via rudimentary wing flapping at relatively low frequencies and amplitudes. Such gains in horizontal translation and time aloft, as those found in this study, are likely to be advantageous for any taxon that engages in aerial behavior for purposes of transit or escape. This study thus demonstrates aerodynamic benefits of transition from a gliding stage to full-scale wing flapping in paravian taxa.

Feather function and the evolution of birds

The ability of feathers to perform many functions either simultaneously or at different times throughout the year or life of a bird is integral to the evolutionary history of birds. Many studies focus on single functions of feathers, but any given feather performs many functions over its lifetime. These functions necessarily interact with each other throughout the evolution and development of birds, so our knowledge of avian evolution is incomplete without understanding the multifunctionality of feathers, and how different functions may act synergistically or antagonistically during natural selection. Here, we review how feather functions interact with avian evolution, with a focus on recent technological and discovery-based advances. By synthesising research into feather functions over hierarchical scales (pattern, arrangement, macrostructure, microstructure, nanostructure, molecules), we aim to provide a broad context for how the adaptability and multifunctionality of feathers have allowed birds to diversify into an astounding array of environments and life-history strategies. We suggest that future research into avian evolution involving feather function should consider multiple aspects of a feather, including multiple functions, seasonal wear and renewal, and ecological or mechanical interactions. With this more holistic view, processes such as the evolution of avian coloration and flight can be understood in a broader and more nuanced context.

Coracoid strength as an indicator of wing-beat propulsion in birds

Birds generate a propulsive force by flapping their wings. They use this propulsive force for various locomotion styles, such as aerodynamic flight, wing-paddle swimming and wing-assisted incline running. It is therefore important to reveal the origin of flapping ability in the evolution from theropod dinosaurs to birds. However, there are no quantitative indices to reconstruct the flapping abilities of extinct forms based on their skeletal morphology. This study compares the section modulus of the coracoid relative to body mass among various extant birds to test whether the index is correlated with flapping ability. According to a survey of 220 historical bird specimens representing 209 species, 180 genera, 83 families and 30 orders, the section modulus of the coracoid relative to body mass in non-flapping birds was significantly smaller than that of flapping birds. This indicates that coracoid strength in non-flapping birds is deemphasised, whereas in flapping birds the strength is emphasised to withstand the contractile force produced by powerful flapping muscles, such as the m. pectoralis and m. supracoracoideus. Therefore, the section modulus of the coracoid is expected to be a powerful tool to reveal the origin of powered flight in birds.

Fossil basicranium clarifies the origin of the avian central nervous system and inner ear

Among terrestrial vertebrates, only crown birds (Neornithes) rival mammals in terms of relative brain size and behavioural complexity. Relatedly, the anatomy of the avian central nervous system and associated sensory structures, such as the vestibular system of the inner ear, are highly modified with respect to those of other extant reptile lineages. However, a dearth of three-dimensional Mesozoic fossils has limited our knowledge of the origins of the distinctive endocranial structures of crown birds. Traits such as an expanded, flexed brain, a ventral connection between the brain and spinal column, and a modified vestibular system have been regarded as exclusive to Neornithes. Here, we demonstrate all of these ‘advanced’ traits in an undistorted braincase from an Upper Cretaceous enantiornithine bonebed in southeastern Brazil. Our discovery suggests that these crown bird-like endocranial traits may have originated prior to the split between Enantiornithes and the more crownward portion of avian phylogeny over 140 Ma, while coexisting with a remarkably plesiomorphic cranial base and posterior palate region. Altogether, our results support the interpretation that the distinctive endocranial morphologies of crown birds and their Mesozoic relatives are affected by complex trade-offs between spatial constraints during development.

Analysis and comparison of protein secondary structures in the rachis of avian flight feathers

Avians have evolved many different modes of flying as well as various types of feathers for adapting to varied environments. However, the protein content and ratio of protein secondary structures (PSSs) in mature flight feathers are less understood. Further research is needed to understand the proportions of PSSs in feather shafts adapted to various flight modes in different avian species. Flight feathers were analyzed in chicken, mallard, sacred ibis, crested goshawk, collared scops owl, budgie, and zebra finch to investigate the PSSs that have evolved in the feather cortex and medulla by using nondestructive attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). In addition, synchrotron radiation-based, Fourier transform infrared microspectroscopy (SR-FTIRM) was utilized to measure and analyze cross-sections of the feather shafts of seven bird species at a high lateral resolution to resolve the composition of proteins distributed within the sampled area of interest. In this study, significant amounts of α-keratin and collagen components were observed in flight feather shafts, suggesting that these proteins play significant roles in the mechanical strength of flight feathers. This investigation increases our understanding of adaptations to flight by elucidating the structural and mechanistic basis of the feather composition.

Feathered dinosaurs

Feathers are the most complex integumentary structures in the animal world. They come in a variety of forms, the most familiar of which are remiges (flight feathers). Flight feathers are composed of a central shaft made up of a hollow calamus (quill), which is inserted into the skin, and a more distal rachis. Hundreds of parallel barbs branch from the sides of the rachis. In turn, smaller hooked barbules branch off the barbs, allowing them to interlock in a tight zipper-like fashion to form vanes. Variations in rachis, barb and barbule morphology result in other feather types such as contour feathers, bristles and down feathers. Feathers have a remarkable array of functions - they form airfoils and elaborate display structures, they serve to camouflage and insulate, to generate and help detect sound, and even to disintegrate into powder to condition other feathers.

Morphological characterization of flight feather shafts in four bird species with different flight styles

Variation in rachis (central shaft) morphology in individual remiges (flight feathers) within and among species reflects adaptations to requirements imposed by aerodynamic forces, but the fine-scale variation of feather morphology across remiges is not well known. Here we describe how the shape of the rachis, expressed by the height/width ratio, changes along the longitudinal and lateral axis of the wing in four bird species with different flight styles: flapping-soaring (white storks), flapping-gliding (common buzzards), passerine-type (house sparrows) and continuous flapping (pygmy cormorants). Overall, in each wing feather, irrespective of species identity, rachis shape changed from circular to rectangular, from the base towards the feather tip. The ratio between the height and width of the calamus was similar across remiges in all species, whereas the ratio at the base, middle and tip of the rachis changed among flight feathers and species. In distal primaries of white storks and common buzzards, the ratio decreased along the feather shaft, indicating a depressed (wider than high) rachis cross section towards the feather tip, whereas the inner primaries and secondaries became compressed (higher than wide). In house sparrows, the rachis was compressed in each of the measurement points, except at the distal segment of the two outermost primary feathers. Finally, in pygmy cormorants, the width exceeds the height at each measurement point, except at the calamus. Our results may reflect the resistance of the rachis to in-plane and out-of-plane aerodynamic forces that vary across remiges and across study species. A link between rachis shape and resistance to bending from aerodynamic forces is further indicated by the change of the second moment of areas along the wing axes.

Volant Fossil Vertebrates: Potential for Bioinspired Flight Technology

Animal flight is ecologically important and has a long evolutionary history. It has evolved independently in many distantly related clades of animals. Powered flight has evolved only three times in vertebrates, making it evolutionarily rare. Major recent fossil discoveries have provided key data on fossil flying vertebrates and critical insights regarding the evolution and different arrangements of animal flight surfaces. Combined with new methodologies, these discoveries have paved the way for potentially expanding biomimetic and biologically inspired designs to incorporate lessons from fossil taxa. Here, we review the latest knowledge and literature regarding flight performance in fossil vertebrates. We then synthesise key elements to provide an overview of those cases where fossil flyers might provide new insights for applied sciences.

Specialized Feathers Produce Sonations During Flight in Columbina Ground Doves

The shape of remiges (primary and secondary feathers) is constrained and stereotyped by the demands of flight, but members of the subfamily of New World ground doves (Peristerinae) possess many atypical remex shapes with which they produce sonations of alarm. Within the genus Columbina specifically, the seventh primary feathers (P7) have elongated barbs that create a protrusion on the trailing vane which varies in size and shape between species. These feathers are hypothesized to have been coopted to produce communicative sounds (i.e., sonations) during flight, but the mechanism of this sound production is unknown. We tested the sound-producing capabilities of spread wing specimens from three species of ground doves (C. inca, C. passerina, and C. talpacoti) in a wind tunnel. High speed video and audio analyses indicated that all wings of adult birds produced buzzing sounds in the orientation and flow velocity of mid-upstroke. These buzzing sounds were produced as the protrusion of elongated barbs fluttered and collided with adjacent P6 feathers at a fundamental frequency of 200 and 400 Hz, respectively. Wings from juvenile C. inca produced significantly quieter buzzes and most (three of four individuals) lacked the elongated barbs that are present in adults. Buzzing sounds produced in the wind tunnel were similar to those produced by wild birds indicating that these P7 feathers have been coopted to produce acoustic signals (sonations) during flight. The shape and mechanism of sound production described here in Columbina appear to be unique among birds.

Flight, symmetry and barb angle evolution in the feathers of birds and other dinosaurs

There has been much discussion over whether basal birds (e.g. Archaeopteryx and Confuciusornis) exhibited active flight. A recent study of barb angles has suggested they likely could not but instead may have exhibited a gliding phase. Pennaceous primary flight feathers were proposed to show significant shifts in barb angle values of relevance to the inference of flight in these extinct taxa. However, evolutionary trends in the evolution of these barb angle traits in extant volant taxa were not analysed in a phylogenetic frame. Neither the ancestral crown avian condition nor the condition in outgroup dinosaurs with symmetrical feathers were assessed. Here, we expand the fossil sample and reanalyse these data in a phylogenetic frame. We show that extant taxa, including strong flyers (e.g. some songbirds), show convergence on trailing barb angles and barb angle asymmetry observed in Mesozoic taxa that were proposed not to be active fliers. Trailing barb angles in these Mesozoic taxa are similar to symmetrical feathers in outgroup dinosaurs, indicating that selective regimes acted to modify primarily the leading-edge barb angles. These trends inform dynamics in feather shape evolution and challenge the notion that barb angle and barb angle ratios in extant birds directly inform the reconstruction of function in extinct stem taxa.

Ultraviolet light illuminates the avian nature of the Berlin Archaeopteryx skeleton

The question of whether the iconic avialan Archaeopteryx was capable of active flapping flight or only passive gliding is still unresolved. This study contributes to this debate by reporting on two key aspects of this fossil that are visible under ultraviolet (UV) light. In contrast to previous studies, we show that most of the vertebral column of the Berlin Archaeopteryx possesses intraosseous pneumaticity, and that pneumatic structures also extend beyond the anterior thoracic vertebrae in other specimens of Archaeopteryx. With a minimum Pneumaticity Index (PI) of 0.39, Archaeopteryx had a much more lightweight skeleton than has been previously reported, comprising an air sac-driven respiratory system with the potential for a bird-like, high-performance metabolism. The neural spines of the 16th to 22nd presacral vertebrae in the Berlin Archaeopteryx are bridged by interspinal ossifications, and form a rigid notarium-like structure similar to the condition seen in modern birds. This reinforced vertebral column, combined with the extensive development of air sacs, suggests that Archaeopteryx was capable of flapping its wings for cursorial and/or aerial locomotion.

Behavioral neuroimaging in birds using PET

BACKGROUND

Birds comprise the most diverse group of terrestrial vertebrates. This success likely is related to the evolution of powered flight over 75 mya. Modern approaches for studying brain function, however, have yet to be fully adapted and applied to birds, especially as they relate to specific behaviors including flight. New method: We have developed a comprehensive set of in vivo experimental methods utilizing PET imaging with F-18 labeled fluorodeoxyglucose (FDG) to study regional changes in metabolism specifically related to flight, yet applicable to other behaviors as well. It incorporates approaches for selection of species, behavioral/imaging paradigm, animal preparation, radiotracer injection route, image quantification, and image analysis via an enhanced brain atlas. We also carried out preliminary modeling studies to better understand tracer kinetics.

RESULTS

The methods were successful in identifying brain regions statistically associated with flight using only 8 animals. Peak brain uptake of FDG between birds and rodents is similar despite much higher blood glucose levels in birds. We also confirmed that brain uptake of FDG steadily decreases after the initial peak and provide evidence that it may be related to greater dephosphorylation of FDG phosphate than that observed in mammals. Comparison with existing methods: FDG PET has been used in only a few studies of the bird brain. We introduce a new species, more realistic flight behavior, paired (test/retest) design, and improved quantification and analysis approaches.

CONCLUSIONS

The proposed imaging protocol is non-invasive yet sensitive to regional metabolic changes in the bird brain related to behavior.

Fluid⁻Structure Interaction for Biomimetic Design of an Innovative Lightweight Turboexpander

Inspired by bird feather structures that enable the resistance of powerful aerodynamic forces in addition to their lower weight to provide stable flight, a biomimetic composite turbine blade was proposed for a low-temperature organic Rankine cycle (ORC) turboexpander that is capable of producing lower weight expanders than that of stainless steel expanders, in addition to reduce its manufacturing cost, and hence it may contribute in spreading ORC across nonconventional power systems. For that purpose, the fluid-structure interaction (FSI) was numerically investigated for a composite turbine blade with bird-inspired fiber orientations. The aerodynamic forces were evaluated by computational fluid dynamics (CFD) using the commercial package ANSYS-CFX (version 16.0) and then these aerodynamic forces were transferred to the solid model of the proposed blade. The structural integrity of the bird-mimetic composite blade was investigated by performing finite element analysis (FEA) of composite materials with different fiber orientations using ANSYS Composite PrepPost (ACP). Furthermore, the obtained mechanical performance of the composite turbine blades was compared with that of the stainless steel turbine blades. The obtained results indicated that fiber orientation has a greater effect on the deformation of the rotor blades and the minimum value can be achieved by the same barb angle inspired from the flight feather. In addition to a significant effect in the weight reduction of 80% was obtained by using composite rotor blades instead of stainless steel rotor blades.

A fully feathered enantiornithine foot and wing fragment preserved in mid-Cretaceous Burmese amber

Over the last three years, Burmese amber (~99 Ma, from Myanmar) has provided a series of immature enantiornithine skeletal remains preserved in varying developmental stages and degrees of completeness. These specimens have improved our knowledge based on compression fossils in Cretaceous sedimentary rocks, adding details of three-dimensional structure and soft tissues that are rarely preserved elsewhere. Here we describe a remarkably well-preserved foot, accompanied by part of the wing plumage. These body parts were likely dismembered, entering the resin due to predatory or scavenging behaviour by a larger animal. The new specimen preserves contour feathers on the pedal phalanges together with enigmatic scutellae scale filament (SSF) feathers on the foot, providing direct analogies to the plumage patterns observed in modern birds, and those cultivated through developmental manipulation studies. Ultimately, this connection may allow researchers to observe how filamentous dinosaur 'protofeathers' developed-testing theories using evolutionary holdovers in modern birds.

Flightless birds are not neuroanatomical analogs of non-avian dinosaurs

Background

In comparative neurobiology, major transitions in behavior are thought to be associated with proportional size changes in brain regions. Bird-line theropod dinosaurs underwent a drastic locomotory shift from terrestrial to volant forms, accompanied by a suite of well-documented postcranial adaptations. To elucidate the potential impact of this locomotor shift on neuroanatomy, we first tested for a correlation between loss of flight in extant birds and whether the brain morphology of these birds resembles that of their flightless, non-avian dinosaurian ancestors. We constructed virtual endocasts of the braincase for 80 individuals of non-avian and avian theropods, including 25 flying and 19 flightless species of crown group birds. The endocasts were analyzed using a three-dimensional (3-D) geometric morphometric approach to assess changes in brain shape along the dinosaur-bird transition and secondary losses of flight in crown-group birds (Aves).

Results

While non-avian dinosaurs and crown-group birds are clearly distinct in endocranial shape, volant and flightless birds overlap considerably in brain morphology. Phylogenetically informed analyses show that locomotory mode does not significantly account for neuroanatomical variation in crown-group birds. Linear discriminant analysis (LDA) also indicates poor predictive power of neuroanatomical shape for inferring locomotory mode. Given current sampling, Archaeopteryx, typically considered the oldest known bird, is inferred to be terrestrial based on its endocranial morphology.

Conclusion

The results demonstrate that loss of flight does not correlate with an appreciable amount of neuroanatomical changes across Aves, but rather is partially constrained due to phylogenetic inertia, evident from sister taxa having similarly shaped endocasts. Although the present study does not explicitly test whether endocranial changes along the dinosaur-bird transition are due to the acquisition of powered flight, the prominent relative expansion of the cerebrum, in areas associated with flight-related cognitive capacity, suggests that the acquisition of flight may have been an important initial driver of brain shape evolution in theropods.

Electronic supplementary material

The online version of this article (10.1186/s12862-018-1312-0) contains supplementary material, which is available to authorized users.

Early Evolution of Modern Birds Structured by Global Forest Collapse at the End-Cretaceous Mass Extinction

The fossil record and recent molecular phylogenies support an extraordinary early-Cenozoic radiation of crown birds (Neornithes) after the Cretaceous-Paleogene (K-Pg) mass extinction [1-3]. However, questions remain regarding the mechanisms underlying the survival of the deepest lineages within crown birds across the K-Pg boundary, particularly since this global catastrophe eliminated even the closest stem-group relatives of Neornithes [4]. Here, ancestral state reconstructions of neornithine ecology reveal a strong bias toward taxa exhibiting predominantly non-arboreal lifestyles across the K-Pg, with multiple convergent transitions toward predominantly arboreal ecologies later in the Paleocene and Eocene. By contrast, ecomorphological inferences indicate predominantly arboreal lifestyles among enantiornithines, the most diverse and widespread Mesozoic avialans [5-7]. Global paleobotanical and palynological data show that the K-Pg Chicxulub impact triggered widespread destruction of forests [8, 9]. We suggest that ecological filtering due to the temporary loss of significant plant cover across the K-Pg boundary selected against any flying dinosaurs (Avialae [10]) committed to arboreal ecologies, resulting in a predominantly non-arboreal post-extinction neornithine avifauna composed of total-clade Palaeognathae, Galloanserae, and terrestrial total-clade Neoaves that rapidly diversified into the broad range of avian ecologies familiar today. The explanation proposed here provides a unifying hypothesis for the K-Pg-associated mass extinction of arboreal stem birds, as well as for the post-K-Pg radiation of arboreal crown birds. It also provides a baseline hypothesis to be further refined pending the discovery of additional neornithine fossils from the Latest Cretaceous and earliest Paleogene.

A flattened enantiornithine in mid-Cretaceous Burmese amber: morphology and preservation

Cretaceous amber from Myanmar (∼99 Ma Burmese amber) has become a valuable supplement to the traditional skeletal record of small theropod dinosaurs preserved in sedimentary rocks, particularly for coelurosaurs and enantiornithines. The specimens recovered from this deposit preserve skeletal material and soft tissues in unmatched detail. This provides opportunities to study three-dimensional preservation of soft tissues, microstructure, and pigmentation patterns that are seldom available elsewhere in the fossil record. Ultimately, this line of research provides insights into life stages that are difficult to preserve, the ecology and appearance of the groups involved, and the evolutionary-development of structures such as feathers. Here we describe the most recent discovery from Burmese amber, an articulated skeleton of an enantiornithine bird. This individual has been sectioned along the coronal plane, providing a unique view inside multiple body regions. Osteological observations and plumage patterns support placement within the Enantiornithes, and suggest that the animal may have been a juvenile at the time of death. The specimen has a complex taphonomic history that includes exposure at the surface of a resin flow prior to encapsulation, and may include scavenging by some of the insects trapped within the same amber piece. The chemical composition observed along surface exposures and shallowly buried regions of the body indicate that the specimen has not undergone significant exchange with its surroundings. High iron concentrations are present in regions that preserve soft tissues as carbon films, and calcium distribution corresponds to regions where bones breach the surface of the amber.

Aerobic performance in tinamous is limited by their small heart. A novel hypothesis in the evolution of avian flight

Some biomechanical studies from fossil specimens suggest that sustained flapping flight of birds could have appeared in their Mesozoic ancestors. We challenge this idea because a suitable musculoskeletal anatomy is not the only requirement for sustained flapping flight. We propose the “heart to fly” hypothesis that states that sustained flapping flight in modern birds required an enlargement of the heart for the aerobic performance of the flight muscles and test it experimentally by studying tinamous, the living birds with the smallest hearts. The small ventricular size of tinamous reduces cardiac output without limiting perfusion pressures, but when challenged to fly, the heart is unable to support aerobic metabolism (quick exhaustion, larger lactates and post-exercise oxygen consumption and compromised thermoregulation). At the same time, cardiac growth shows a crocodilian-like pattern and is correlated with differential gene expression in MAPK kinases. We integrate this physiological evidence in a new evolutionary scenario in which the ground-up, short and not sustained flapping flight displayed by tinamous represents an intermediate step in the evolution of the aerobic sustained flapping flight of modern birds.

Flight aerodynamics in enantiornithines: Information from a new Chinese Early Cretaceous bird

We describe an exquisitely preserved new avian fossil (BMNHC-PH-919) from the Lower Cretaceous Yixian Formation of eastern Inner Mongolia, China. Although morphologically similar to Cathayornithidae and other small-sized enantiornithines from China's Jehol Biota, many morphological features indicate that it represents a new species, here named Junornis houi. The new fossil displays most of its plumage including a pair of elongated, rachis-dominated tail feathers similarly present in a variety of other enantiornithines. BMNHC-PH-919 represents the first record of a Jehol enantiornithine from Inner Mongolia, thus extending the known distribution of these birds into the eastern portion of this region. Furthermore, its well-preserved skeleton and wing outline provide insight into the aerodynamic performance of enantiornithines, suggesting that these birds had evolved bounding flight-a flight mode common to passeriforms and other small living birds-as early as 125 million years ago.

Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates

Harnessing flight strategies refined by millions of years of evolution can help expedite the design of more efficient, manoeuvrable and robust flying robots. This review synthesizes recent advances and highlights remaining gaps in our understanding of how bird and bat wing adaptations enable effective flight. Included in this discussion is an evaluation of how current robotic analogues measure up to their biological sources of inspiration. Studies of vertebrate wings have revealed skeletal systems well suited for enduring the loads required during flight, but the mechanisms that drive coordinated motions between bones and connected integuments remain ill-described. Similarly, vertebrate flight muscles have adapted to sustain increased wing loading, but a lack of in vivo studies limits our understanding of specific muscular functions. Forelimb adaptations diverge at the integument level, but both bird feathers and bat membranes yield aerodynamic surfaces with a level of robustness unparalleled by engineered wings. These morphological adaptations enable a diverse range of kinematics tuned for different flight speeds and manoeuvres. By integrating vertebrate flight specializations-particularly those that enable greater robustness and adaptability-into the design and control of robotic wings, engineers can begin narrowing the wide margin that currently exists between flying robots and vertebrates. In turn, these robotic wings can help biologists create experiments that would be impossible in vivo.

Multi-cored vortices support function of slotted wing tips of birds in gliding and flapping flight

Slotted wing tips of birds are commonly considered an adaptation to improve soaring performance, despite their presence in species that neither soar nor glide. We used particle image velocimetry to measure the airflow around the slotted wing tip of a jackdaw (Corvus monedula) as well as in its wake during unrestrained flight in a wind tunnel. The separated primary feathers produce individual wakes, confirming a multi-slotted function, in both gliding and flapping flight. The resulting multi-cored wingtip vortex represents a spreading of vorticity, which has previously been suggested as indicative of increased aerodynamic efficiency. Considering benefits of the slotted wing tips that are specific to flapping flight combined with the wide phylogenetic occurrence of this configuration, we propose the hypothesis that slotted wings evolved initially to improve performance in powered flight.

Mosaic evolution in an asymmetrically feathered troodontid dinosaur with transitional features

Asymmetrical feathers have been associated with flight capability but are also found in species that do not fly, and their appearance was a major event in feather evolution. Among non-avialan theropods, they are only known in microraptorine dromaeosaurids. Here we report a new troodontid, Jianianhualong tengi gen. et sp. nov., from the Lower Cretaceous Jehol Group of China, that has anatomical features that are transitional between long-armed basal troodontids and derived short-armed ones, shedding new light on troodontid character evolution. It indicates that troodontid feathering is similar to Archaeopteryx in having large arm and leg feathers as well as frond-like tail feathering, confirming that these feathering characteristics were widely present among basal paravians. Most significantly, the taxon has the earliest known asymmetrical troodontid feathers, suggesting that feather asymmetry was ancestral to Paraves. This taxon also displays a mosaic distribution of characters like Sinusonasus, another troodontid with transitional anatomical features.

Troodontids were theropod dinosaurs closely related to birds. Here, Xu and colleagues describe a new, feathered troodontid species, Jianianhualong tengi, dating from the Lower Cretaceous period in China that provides insight into troodontid mosaic evolution and paravian feathering.

Functional roles of Aves class-specific cis-regulatory elements on macroevolution of bird-specific features

Unlike microevolutionary processes, little is known about the genetic basis of macroevolutionary processes. One of these magnificent examples is the transition from non-avian dinosaurs to birds that has created numerous evolutionary innovations such as self-powered flight and its associated wings with flight feathers. By analysing 48 bird genomes, we identified millions of avian-specific highly conserved elements (ASHCEs) that predominantly (>99%) reside in non-coding regions. Many ASHCEs show differential histone modifications that may participate in regulation of limb development. Comparative embryonic gene expression analyses across tetrapod species suggest ASHCE-associated genes have unique roles in developing avian limbs. In particular, we demonstrate how the ASHCE driven avian-specific expression of gene Sim1 driven by ASHCE may be associated with the evolution and development of flight feathers. Together, these findings demonstrate regulatory roles of ASHCEs in the creation of avian-specific traits, and further highlight the importance of cis-regulatory rewiring during macroevolutionary changes.

Rachis morphology cannot accurately predict the mechanical performance of primary feathers in extant (and therefore fossil) feathered flyers

It was previously suggested that the flight ability of feathered fossils could be hypothesized from the diameter of their feather rachises. Central to the idea is the unvalidated assumption that the strength of a primary flight feather (i.e. its material and structural properties) may be consistently calculated from the external diameter of the feather rachis, which is the only dimension that is likely to relate to structural properties available from fossils. Here, using three-point bending tests, the relationship between feather structural properties (maximum bending moment, M_max and Young's modulus, E_bend) and external morphological parameters (primary feather rachis length, diameter and second moment of area at the calamus) in 180 primary feathers from four species of bird of differing flight style was investigated. Intraspecifically, both E_bend and M_max were strongly correlated with morphology, decreasing and increasing, respectively, with all three morphological measures. Without accounting for species, however, external morphology was a poor predictor of rachis structural properties, meaning that precise determination of aerial performance in extinct, feathered species from external rachis dimensions alone is not possible. Even if it were possible to calculate the second moment of area of the rachis, our data suggest that feather strength could still not be reliably estimated.

Diverse feather shape evolution enabled by coupling anisotropic signalling modules with self-organizing branching programme

Adaptation of feathered dinosaurs and Mesozoic birds to new ecological niches was potentiated by rapid diversification of feather vane shapes. The molecular mechanism driving this spectacular process remains unclear. Here, through morphology analysis, transcriptome profiling, functional perturbations and mathematical simulations, we find that mesenchyme-derived GDF10 and GREM1 are major controllers for the topologies of rachidial and barb generative zones (setting vane boundaries), respectively, by tuning the periodic-branching programme of epithelial progenitors. Their interactions with the anterior-posterior WNT gradient establish the bilateral-symmetric vane configuration. Additionally, combinatory effects of CYP26B1, CRABP1 and RALDH3 establish dynamic retinoic acid (RA) landscapes in feather mesenchyme, which modulate GREM1 expression and epithelial cell shapes. Incremental changes of RA gradient slopes establish a continuum of asymmetric flight feathers along the wing, while switch-like modulation of RA signalling confers distinct vane shapes between feather tracts. Therefore, the co-option of anisotropic signalling modules introduced new dimensions of feather shape diversification.

Flying starlings, PET and the evolution of volant dinosaurs

Birds have evolved behavioral and morphological adaptations for powered flight. Many aspects of this transition are unknown, including the neuroanatomical changes that made flight possible [1]. To understand how the avian brain drives this complex behavior, we utilized positron emission tomography (PET) scanning and the tracer (18)F-fluorodeoxyglucose (FDG) to document regional metabolic activity in the brain associated with a variety of locomotor behaviors. FDG studies are typically employed in rats [2] though the technology has been applied to birds [3]. We examined whole-brain function in European Starlings (Sturnus vulgaris), trained to fly in a wind tunnel while metabolizing the tracer. Drawing on predictions from early anatomical studies [4], we hypothesized increased metabolic activity in the Wulst and functionally related visual brain regions during flight. We found that flight behaviors correlated positively with entopallia and Wulst activity, but negatively with thalamic activity.

Synchrotron imaging of dentition provides insights into the biology of Hesperornis and Ichthyornis, the "last" toothed birds

Background

The dentitions of extinct organisms can provide pivotal information regarding their phylogenetic position, as well as paleobiology, diet, development, and growth. Extant birds are edentulous (toothless), but their closest relatives among stem birds, the Cretaceous Hesperornithiformes and Ichthyornithiformes, retained teeth. Despite their significant phylogenetic position immediately outside the avian crown group, the dentitions of these taxa have never been studied in detail. To obtain new insight into the biology of these ‘last’ toothed birds, we use cutting-edge visualisation techniques to describe their dentitions at unprecedented levels of detail, in particular propagation phase contrast x-ray synchrotron microtomography at high-resolution.

Results

Among other characteristics of tooth shape, growth, attachment, implantation, replacement, and dental tissue microstructures, revealed by these analyses, we find that tooth morphology and ornamentation differ greatly between the Hesperornithiformes and Ichthyornithiformes. We also highlight the first Old World, and youngest record of the major Mesozoic clade Ichthyornithiformes. Both taxa exhibit extremely thin and simple enamel. The extension rate of Hesperornis tooth dentine appears relatively high compared to non-avian dinosaurs. Root attachment is found for the first time to be fully thecodont via gomphosis in both taxa, but in Hesperornis secondary evolution led to teeth implantation in a groove, at least locally without a periodontal ligament. Dental replacement is shown to be lingual via a resorption pit in the root, in both taxa.

Conclusions

Our results allow comparison with other archosaurs and also mammals, with implications regarding dental character evolution across amniotes. Some dental features of the ‘last’ toothed birds can be interpreted as functional adaptations related to diet and mode of predation, while others appear to be products of their peculiar phylogenetic heritage. The autapomorphic Hesperornis groove might have favoured firmer root attachment. These observations highlight complexity in the evolutionary history of tooth reduction in the avian lineage and also clarify alleged avian dental characteristics in the frame of a long-standing debate on bird origins. Finally, new hypotheses emerge that will possibly be tested by further analyses of avian teeth, for instance regarding dental replacement rates, or simplification and thinning of enamel throughout the course of early avian evolution.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0753-6) contains supplementary material, which is available to authorized users.

The wings before the bird: an evaluation of flapping-based locomotory hypotheses in bird antecedents

BACKGROUND

Powered flight is implicated as a major driver for the success of birds. Here we examine the effectiveness of three hypothesized pathways for the evolution of the flight stroke, the forelimb motion that powers aerial locomotion, in a terrestrial setting across a range of stem and basal avians: flap running, Wing Assisted Incline Running (WAIR), and wing-assisted leaping.

METHODS

Using biomechanical mathematical models based on known aerodynamic principals and in vivo experiments and ground truthed using extant avians we seek to test if an incipient flight stroke may have contributed sufficient force to permit flap running, WAIR, or leaping takeoff along the phylogenetic lineage from Coelurosauria to birds.

RESULTS

None of these behaviours were found to meet the biomechanical threshold requirements before Paraves. Neither was there a continuous trend of refinement for any of these biomechanical performances across phylogeny nor a signal of universal applicability near the origin of birds. None of these flap-based locomotory models appear to have been a major influence on pre-flight character acquisition such as pennaceous feathers, suggesting non-locomotory behaviours, and less stringent locomotory behaviours such as balancing and braking, played a role in the evolution of the maniraptoran wing and nascent flight stroke. We find no support for widespread prevalence of WAIR in non-avian theropods, but can't reject its presence in large winged, small-bodied taxa like Microraptor and Archaeopteryx.

DISCUSSION

Using our first principles approach we find that "near flight" locomotor behaviors are most sensitive to wing area, and that non-locomotory related selection regimes likely expanded wing area well before WAIR and other such behaviors were possible in derived avians. These results suggest that investigations of the drivers for wing expansion and feather elongation in theropods need not be intrinsically linked to locomotory adaptations, and this separation is critical for our understanding of the origin of powered flight and avian evolution.

Mummified precocial bird wings in mid-Cretaceous Burmese amber

Our knowledge of Cretaceous plumage is limited by the fossil record itself: compression fossils surrounding skeletons lack the finest morphological details and seldom preserve visible traces of colour, while discoveries in amber have been disassociated from their source animals. Here we report the osteology, plumage and pterylosis of two exceptionally preserved theropod wings from Burmese amber, with vestiges of soft tissues. The extremely small size and osteological development of the wings, combined with their digit proportions, strongly suggests that the remains represent precocial hatchlings of enantiornithine birds. These specimens demonstrate that the plumage types associated with modern birds were present within single individuals of Enantiornithes by the Cenomanian (99 million years ago), providing insights into plumage arrangement and microstructure alongside immature skeletal remains. This finding brings new detail to our understanding of infrequently preserved juveniles, including the first concrete examples of follicles, feather tracts and apteria in Cretaceous avialans.

Theory of the development of curved barbs and their effects on feather morphology

Feathers exhibit an extraordinary diversity of shapes, which are used by birds to accomplish a diverse set of functions. Pennaceous feathers have a double branched morphology that develops from a tube of epidermis, and variation in branch geometry determines feather shape. Feather development is both complex (i.e., a simple developmental modification can have multiple effects on mature feather shape), and redundant (i.e., different developmental modifications can create the same shape). Due to this, it is not readily apparent how different feather shapes develop. In many feathers, barbs are not straight, but instead curve in toward, or away, from the feather tip. Barb curvature can affect the shape of mature feathers but the development of curved barbs is unknown. Previous research has hypothesized that barb curvature could develop either during the helical growth of barb ridges in the tubular feather germ, or during barb angle expansion as the feather unfurls from the sheath. To better understand the development of curved barbs and their effects on mature feathers we present a theoretical model of curved barb development and test the model with empirical investigations of feathers. We find that curved barbs affect many aspects of feather morphology including vane width, barb length, and barb spacing. In real feathers, curved barbs can develop both during helical barb ridge growth and during barb angle expansion, with most of the observed curvature due to barb angle expansion. Our results demonstrate that barb angle expansion as a feather unfurls from the sheath is a complex and dynamic process that plays an important role in determining the shape and structure of mature feathers. Curved barbs create heterogeneity in barb geometry within the feather vane, which could have important implications for aerodynamic function and the development of within feather pigmentation patterns. J. Morphol. 277:995-1013, 2016. © 2016 Wiley Periodicals, Inc.

Specialized primary feathers produce tonal sounds during flight in rock pigeons (Columba livia)

For centuries, naturalists have suggested that the tonal elements of pigeon wing sounds may be sonations (non-vocal acoustic signals) of alarm. However, spurious tonal sounds may be produced passively as a result of aeroelastic flutter in the flight feathers of almost all birds. Using mechanistic criteria emerging from recent work on sonations, we sought to: (1) identify characteristics of rock pigeon flight feathers that might be adapted for sound production rather than flight, and (2) provide evidence that this morphology is necessary for in vivo sound production and is sufficient to replicate in vivo sounds. Pigeons produce tonal sounds (700±50 Hz) during the latter two-thirds of each downstroke during take-off. These tones are produced when a small region of long, curved barbs on the inner vane of the outermost primary feather (P10) aeroelastically flutters. Tones were silenced in live birds when we experimentally increased the stiffness of this region to prevent flutter. Isolated P10 feathers were sufficient to reproduce in vivo sounds when spun at the peak angular velocity of downstroke (53.9-60.3 rad s(-1)), but did not produce tones at average downstroke velocity (31.8 rad s(-1)), whereas P9 and P1 feathers never produced tones. P10 feathers had significantly lower coefficients of resultant aerodynamic force (CR) when spun at peak angular velocity than at average angular velocity, revealing that production of tonal sounds incurs an aerodynamic cost. P9 and P1 feathers did not show this difference in CR These mechanistic results suggest that the tonal sounds produced by P10 feathers are not incidental and may function in communication.

Paleophysiology: From Fossils to the Future

Future environments may resemble conditions that have not existed for millions of years. To assess the adaptive options available to organisms evolving under such circumstances, it is instructive to probe paleophysiology, the ways in which ancient life coped with its physical and chemical surroundings. To do this, we need reliable proxies that are based on fundamental principles, quantitatively verified in living species, and observable in fossil remains. Insights have already come from vertebrates and plants, and others will likely emerge for marine animals if promising proxies are validated. Many questions remain about the circumstances for the evolution of environmental tolerances, metabolic rates, biomineralization, and physiological responses to interacting species, and about how living organisms will perform under exceptional conditions.

Sexual Dimorphism and Population Differences in Structural Properties of Barn Swallow (Hirundo rustica) Wing and Tail Feathers

Sexual selection and aerodynamic forces affecting structural properties of the flight feathers of birds are poorly understood. Here, we compared the structural features of the innermost primary wing feather (P1) and the sexually dimorphic outermost (Ta6) and monomorphic second outermost (Ta5) tail feathers of barn swallows (Hirundo rustica) from a Romanian population to investigate how sexual selection and resistance to aerodynamic forces affect structural differences among these feathers. Furthermore, we compared structural properties of Ta6 of barn swallows from six European populations. Finally, we determined the relationship between feather growth bars width (GBW) and the structural properties of tail feathers. The structure of P1 indicates strong resistance against aerodynamic forces, while the narrow rachis, low vane density and low bending stiffness of tail feathers suggest reduced resistance against airflow. The highly elongated Ta6 is characterized by structural modifications such as large rachis width and increased barbule density in relation to the less elongated Ta5, which can be explained by increased length and/or high aerodynamic forces acting at the leading tail edge. However, these changes in Ta6 structure do not allow for full compensation of elongation, as reflected by the reduced bending stiffness of Ta6. Ta6 elongation in males resulted in feathers with reduced resistance, as shown by the low barb density and reduced bending stiffness compared to females. The inconsistency in sexual dimorphism and in change in quality traits of Ta6 among six European populations shows that multiple factors may contribute to shaping population differences. In general, the difference in quality traits between tail feathers cannot be explained by the GBW of feathers. Our results show that the material and structural properties of wing and tail feathers of barn swallows change as a result of aerodynamic forces and sexual selection, although the result of these changes can be contrasting.