Quantitative functional imaging of the pigeon brain: implications for the evolution of avian powered flight.

The evolution of flight is a rare event in vertebrate history, and one that demands functional integration across multiple anatomical/physiological systems. The neuroanatomical basis for such integration and the role that brain evolution assumes in behavioural transformations remain poorly understood. We make progress by (i) generating a positron emission tomography (PET)-based map of brain activity for pigeons during rest and flight, (ii) using these maps in a functional analysis of the brain during flight, and (iii) interpreting these data within a macroevolutionary context shaped by non-avian dinosaurs. Although neural activity is generally conserved from rest to flight, we found significant increases in the cerebellum as a whole and optic flow pathways. Conserved activity suggests processing of self-movement and image stabilization are critical when a bird takes to the air, while increased visual and cerebellar activity reflects the importance of integrating multimodal sensory information for flight-related movements. A derived cerebellar capability likely arose at the base of maniraptoran dinosaurs, where volumetric expansion and possible folding directly preceded paravian flight. These data represent an important step toward establishing how the brain of modern birds supports their unique behavioural repertoire and provide novel insights into the neurobiology of the bird-like dinosaurs that first achieved powered flight.

A theropod trackway providing evidence of a pathological foot from the exceptional locality of Las Hoyas (upper Barremian, Serranía de Cuenca, Spain).

We describe a trackway (LH-Mg-10-16) occurring in laminated carbonated limestones of the Las Hoyas locality, Serranía de Cuenca, Spain. It is unmistakably a large theropod dinosaur trackway encompassing two unusual aspects, namely, wide-steps, and a set of equally deformed left footprints (with a dislocated digit). The layer also preserves other vertebrate trails (fish Undichna) and different impressions in the sediment. To address these complex settings, we devised a multidisciplinary approach, including the ichnological and taphonomical descriptions, characterisation of the rock lithofacies using thin-sections, 3D structured-light digitalisation with a high precision of 200-400 µm, and a geometric morphometric comparison with a large sample of bipedal dinosaur trackways. Sedimentary analyses showed that the trackway was produced in a humid, benthonic microbial mat, the consistency and plasticity of which enabled the preservation of the details of the movement of the animal. The results of the geometric analysis indicate that the "wide-steps" of the trackway is not unusual compared to other trackways, providing evidence that it was made by a single individual with an estimated hip height approximately 2 m. Analogous pathologies in extant archosaurs that yield the combination of wide steps and deformed digits in the same trackway were considered. All results mutually support the hypothesis that a large theropod dinosaur, with a pathological foot, generated the trackway as it crossed an area of shallow water while slowly walking towards the main water source, thus stepping steadily over the benthonic mat over which multiple fish were swimming.

Ontogenetic niche shifts in the Mesozoic bird Confuciusornis sanctus.

Paleontological evidence reveals that the rapid growth characteristic of living birds evolved close to the origin of the crown-group Neornithes, as more stemward birds experienced protracted growth until becoming fully grown.1 Research on Mesozoic confuciusornithids, the earliest divergence of fully beaked birds, has revealed a complex life cycle in which these birds experienced multiple growth phases.2-4 Such a life-history pattern calls for the exploration of the role that ontogenetic niche shifts may have played in size-structuring confuciusornithid populations.5,6 Here, by analyzing the skeletal morphometrics of a dense sample of fossil individuals of Confuciusornis sanctus (n = 171, all fledged), we show that the youngest individuals of this confuciusornithid species experienced a precocious burst of beak growth, probably facilitating access to novel food resources that helped them meet the high energetic demands of their initial growth spurt. Such an early burst of facial (i.e., snout) growth resembles that of young crocodilians.7 However, in these reptiles, facial growth slows down soon thereafter, and the matching of snout scaling between mid-sized and larger individuals instigates demographic competence and the dispersion of the former.8 In contrast, our results reveal that beak growth in C. sanctus continued steadily. We hypothesized that the protracted facial growth of older individuals led to ontogenetic niche shifts by dietary segregation among size classes within populations. Our study thus confirms that the life cycle of C. sanctus was notably different from that of modern birds, and it reveals that beak size allometry may have facilitated population cohesiveness between coinhabiting age classes.

Beyond the beak: Brain size and allometry in avian craniofacial evolution.

Birds exhibit an enormous variety of beak shapes. Such remarkable variation, however, has distracted research from other important aspects of their skull evolution, the nature of which has been little explored. Key aspects of avian skull variation appear to be qualitatively similar to those of mammals, encompassing variation in the degree of cranial vaulting, cranial base flexure, and the proportions and orientations of the occipital and facial regions. The evolution of these traits has been studied intensively in mammals under the Spatial Packing Hypothesis (SPH), an architectural constraint so-called because the general anatomical organization and development of such skull parts makes them evolve predictably in response to changes in relative brain size. Such SPH predictions account for the different appearances of skull configurations across species, either in having longer or shorter faces, and caudally or ventrally oriented occiputs, respectively. This pattern has been morphometrically and experimentally proven in mammals but has not been examined in birds or other tetrapods, and so its generality remains unknown. We explored the SPH in an interspecific sample of birds using three-dimensional geometric morphometrics. Our results show that the dominant trend of evolutionary variation in the skull of crown-group birds can be predicted by the SPH, involving concomitant changes in the face, the cranial vault and the basicranium, and with striking similarities to craniofacial variation among mammals. Although craniofacial variation is significantly affected by allometry, these allometric effects are independent of the influence of the SPH on skull morphology, as are any effects of volumetric encephalization. Our results, therefore, validate the hypothesis that a general architectural constraint underlies skull homoplasy evolution of cranial morphology among avian clades, and possibly between birds and mammals, but they downplay encephalization and allometry as the only factors involved.

Craniofacial development illuminates the evolution of nightbirds (Strisores).

Evolutionary variation in ontogeny played a central role in the origin of the avian skull. However, its influence in subsequent bird evolution is largely unexplored. We assess the links between ontogenetic and evolutionary variation of skull morphology in Strisores (nightbirds). Nightbirds span an exceptional range of ecologies, sizes, life-history traits and craniofacial morphologies constituting an ideal test for evo-devo hypotheses of avian craniofacial evolution. These morphologies include superficially 'juvenile-like' broad, flat skulls with short rostra and large orbits in swifts, nightjars and allied lineages, and the elongate, narrow rostra and globular skulls of hummingbirds. Here, we show that nightbird skulls undergo large ontogenetic shape changes that differ strongly from widespread avian patterns. While the superficially juvenile-like skull morphology of many adult nightbirds results from convergent evolution, rather than paedomorphosis, the divergent cranial morphology of hummingbirds originates from an evolutionary reversal to a more typical avian ontogenetic trajectory combined with accelerated ontogenetic shape change. Our findings underscore the evolutionary lability of cranial growth and development in birds, and the underappreciated role of this aspect of phenotypic variability in the macroevolutionary diversification of the amniote skull.

Tempo and Pattern of Avian Brain Size Evolution.

Relative brain sizes in birds can rival those of primates, but large-scale patterns and drivers of avian brain evolution remain elusive. Here, we explore the evolution of the fundamental brain-body scaling relationship across the origin and evolution of birds. Using a comprehensive dataset sampling> 2,000 modern birds, fossil birds, and theropod dinosaurs, we infer patterns of brain-body co-variation in deep time. Our study confirms that no significant increase in relative brain size accompanied the trend toward miniaturization or evolution of flight during the theropod-bird transition. Critically, however, theropods and basal birds show weaker integration between brain size and body size, allowing for rapid changes in the brain-body relationship that set the stage for dramatic shifts in early crown birds. We infer that major shifts occurred rapidly in the aftermath of the Cretaceous-Paleogene mass extinction within Neoaves, in which multiple clades achieved higher relative brain sizes because of a reduction in body size. Parrots and corvids achieved the largest brains observed in birds via markedly different patterns. Parrots primarily reduced their body size, whereas corvids increased body and brain size simultaneously (with rates of brain size evolution outpacing rates of body size evolution). Collectively, these patterns suggest that an early adaptive radiation in brain size laid the foundation for subsequent selection and stabilization.

The consequences of craniofacial integration for the adaptive radiations of Darwin's finches and Hawaiian honeycreepers.

The diversifications of Darwin's finches and Hawaiian honeycreepers are two text-book examples of adaptive radiation in birds. Why these two bird groups radiated while the remaining endemic birds in these two archipelagos exhibit relatively low diversity and disparity remains unexplained. Ecological factors have failed to provide a convincing answer to this phenomenon, and some intrinsic causes connected to craniofacial evolution have been hypothesized. The tight coevolution of the beak and the remainder of the skull in diurnal raptors and parrots suggests that integration may be the prevalent condition in landbirds (Inopinaves). This is in contrast with the archetypal relationship between beak shape and ecology in Darwin's finches and Hawaiian honeycreepers, which suggests that the beak can adapt as a distinct module in these birds. Modularity has therefore been proposed to underpin the adaptive radiation of these groups, allowing the beak to evolve more rapidly and freely in response to ecological opportunity. Here, using geometric morphometrics and phylogenetic comparative methods in a broad sample of landbird skulls, we show that craniofacial evolution in Darwin's finches and Hawaiian honeycreepers seems to be characterized by a tighter coevolution of the beak and the rest of the skull (cranial integration) than in most landbird lineages, with rapid and extreme morphological evolution of both skull regions along constrained directions of phenotypic space. These patterns are unique among landbirds, including other sympatric island radiations, and therefore counter previous hypotheses by showing that tighter cranial integration, not only modularity, can facilitate evolution along adaptive directions.

Osteohistology and Life History of the Basal Pygostylian, Confuciusornis sanctus.

More than a thousand specimens of Confuciusornis sanctus have been recovered from the Early Cretaceous Jehol Group of Northeastern China. Here, we investigate the bone microstructure of 33 long bones sampled from 14 C. sanctus specimens in an attempt to assess the life history patterns of this basal pygostylian bird. Analysis of the histology of various skeletal elements (femur, humerus, tibia, radius, and ulna) revealed differences in the histology of their bone walls. Based on the osteohistology, we coded the examined specimens into five histology age classes. We found that histological age was not strictly correlated with body size. The variability in the histology of multiple bones from single skeletons suggests differences in the growth rate of the skeleton in response to allometry, functional demands, and pathology. We show that although fibrolamellar bone is widespread across birds, the extent and duration of this rapid phase of bone deposition is highly variable. Comparisons among Mesozoic birds confirm that early ontogenetic growth was rapid, but that later post-hatching growth was strongly influenced by the ontogenetic age of the individual, body size, and local environment, as well as taxonomy. Our findings indicate that C. sanctus experienced rapid growth from early ontogeny until almost fully grown, and thereafter transitioned to slow, episodic growth (for at least 3-4 years) to reach skeletal maturity. Anat Rec, 303:949-962, 2020. © 2019 American Association for Anatomy.

A novel approach for the metric analysis of fern fronds: Growth and architecture of the Mesozoic fern Weichselia reticulata in the light of modern ferns.

The architecture of primary and secondary pinnae of the fossil fern Weichselia reticulata (C. Stokes et Webb) Fontaine is studied based on 28 large specimens from the upper Barremian La Huérguina Formation of the fossil locality Las Hoyas, Cuenca, Spain. The study of the specimens is performed through a morphometric analysis consisting in a reformulation of the Branching Algorithms method of shape description including measurements (insertion angle, distance between pinnae, first segment length and rachis width) and ratios (interval ratio, branching ratio and tapering ratio). A protocol to relocate isolated fragments of fossil pinnae is also established using the interval ratio (distance between pinnae/ previous distance between pinnae) and insertion angle of the pinnae. All specimens show a similar architecture, having elliptic primary pinnae with a sinuous apically tapering primary rachis and triangular secondary pinnae with pinnules of different morphologies. The analysis of the architecture allows to propose that the position of the frond was plagiotropic and that the frond growth was basiplastic for the petiole head and acroplastic for the primary pinnae. The metric method is applied to explore the architecture of four living fern species (Angiopteris evecta (Forst.) Hoffm., Matonia pectinata R.Br., Sphaeropteris cooperi (F.Muell.) R.M.Tryon, and Woodwardia unigemmata (Makino) Nakai). Weichselia architecture results extremely ordered and regular in comparison with the primary pinnae variation of the living species.

The multifactorial nature of beak and skull shape evolution in parrots and cockatoos (Psittaciformes).

BACKGROUND: The Psittaciformes (parrots and cockatoos) are characterised by their large beaks, and are renowned for their ability to produce high bite forces. These birds also possess a suite of modifications to their cranial architecture interpreted to be adaptations for feeding on mechanically resistant foods, yet the relationship between cranial morphology and diet has never been explicitly tested. Here, we provide a three-dimensional geometric morphometric analysis of the developmental and biomechanical factors that may be influencing the evolution of psittaciformes' distinctive cranial morphologies. RESULTS: Contrary to our own predictions, we find that dietary preferences for more- or less- mechanically resistant foods have very little influence on beak and skull shape, and that diet predicts only 2.4% of the shape variation in psittaciform beaks and skulls. Conversely, evolutionary allometry and integration together predict almost half the observed shape variation, with phylogeny remaining an important factor in shape identity throughout our analyses, particularly in separating cockatoos (Cacatuoidea) from the true parrots (Psittacoidea). CONCLUSIONS: Our results are similar to recent findings about the evolutionary trajectories of skull and beak shape in other avian families. We therefore propose that allometry and integration are important factors causing canalization of the avian head, and while diet clearly has an influence on beak shape between families, this may not be as important at driving evolvability within families as is commonly assumed.

Fully fledged enantiornithine hatchling revealed by Laser-Stimulated Fluorescence supports precocial nesting behavior.

Laser-Stimulated Fluorescence (LSF) is used to identify fully fledged feathering in the hatchling enantiornithine bird specimen MPCM-LH-26189, supporting precocial nesting behavior in this extinct group. The LSF results include the detection of a long pennaceous wing feather as well as cover feathers around the body. The LSF technique showed improved detection limits over and above synchrotron and UV imaging which had both been performed on this specimen. The findings underscore the value of using a wide range of analytical techniques.

The evolutionary relationship among beak shape, mechanical advantage, and feeding ecology in modern birds.

Extensive research on avian adaptive radiations has led to a presumption that beak morphology predicts feeding ecology in birds. However, this ecomorphological relationship has only been quantified in a handful of avian lineages, where associations are of variable strength, and never at a broad macroevolutionary scale. Here, we used shape analysis and phylogenetic comparative methods to quantify the relationships among beak shape, mechanical advantage, and two measures of feeding ecology (feeding behavior and semiquantitative dietary preferences) in a broad sample of modern birds, comprising most living orders. We found a complex relationship, with most variables showing a significant relationship with feeding ecology but little explanatory power. For example, diet accounts for less than 12% of beak shape variation. Similar beak shapes are associated with disparate dietary regimes, even when accounting for diet-feeding behavior relationships and phylogeny. Very few lineages optimize for stronger bite forces, with most birds exhibiting relatively fast, weak bites, even in large predatory taxa. The extreme morphological and behavioral flexibility of the beak in birds suggests that, far from being an exemplary feeding adaptation, avian beak diversification may have been largely contingent on trade-offs and constraints.

A diminutive perinate European Enantiornithes reveals an asynchronous ossification pattern in early birds.

Fossils of juvenile Mesozoic birds provide insight into the early evolution of avian development, however such fossils are rare. The analysis of the ossification sequence in these early-branching birds has the potential to address important questions about their comparative developmental biology and to help understand their morphological evolution and ecological differentiation. Here we report on an early juvenile enantiornithine specimen from the Early Cretaceous of Europe, which sheds new light on the osteogenesis in this most species-rich clade of Mesozoic birds. Consisting of a nearly complete skeleton, it is amongst the smallest known Mesozoic avian fossils representing post-hatching stages of development. Comparisons between this new specimen and other known early juvenile enantiornithines support a clade-wide asynchronous pattern of osteogenesis in the sternum and the vertebral column, and strongly indicate that the hatchlings of these phylogenetically basal birds varied greatly in size and tempo of skeletal maturation.

Geometric morphometrics reveals sex-differential shape allometry in a spider.

Common scientific wisdom assumes that spider sexual dimorphism (SD) mostly results from sexual selection operating on males. However, testing predictions from this hypothesis, particularly male size hyperallometry, has been restricted by methodological constraints. Here, using geometric morphometrics (GMM) we studied for the first time sex-differential shape allometry in a spider (Donacosa merlini, Araneae: Lycosidae) known to exhibit the reverse pattern (i.e., male-biased) of spider sexual size dimorphism. GMM reveals previously undetected sex-differential shape allometry and sex-related shape differences that are size independent (i.e., associated to the y-intercept, and not to size scaling). Sexual shape dimorphism affects both the relative carapace-to-opisthosoma size and the carapace geometry, arguably resulting from sex differences in both reproductive roles (female egg load and male competition) and life styles (wandering males and burrowing females). Our results demonstrate that body portions may vary modularly in response to different selection pressures, giving rise to sex differences in shape, which reconciles previously considered mutually exclusive interpretations about the origins of spider SD.

Open data and digital morphology.

Over the past two decades, the development of methods for visualizing and analysing specimens digitally, in three and even four dimensions, has transformed the study of living and fossil organisms. However, the initial promise that the widespread application of such methods would facilitate access to the underlying digital data has not been fully achieved. The underlying datasets for many published studies are not readily or freely available, introducing a barrier to verification and reproducibility, and the reuse of data. There is no current agreement or policy on the amount and type of data that should be made available alongside studies that use, and in some cases are wholly reliant on, digital morphology. Here, we propose a set of recommendations for minimum standards and additional best practice for three-dimensional digital data publication, and review the issues around data storage, management and accessibility.

The shapes of bird beaks are highly controlled by nondietary factors.

Bird beaks are textbook examples of ecological adaptation to diet, but their shapes are also controlled by genetic and developmental histories. To test the effects of these factors on the avian craniofacial skeleton, we conducted morphometric analyses on raptors, a polyphyletic group at the base of the landbird radiation. Despite common perception, we find that the beak is not an independently targeted module for selection. Instead, the beak and skull are highly integrated structures strongly regulated by size, with axes of shape change linked to the actions of recently identified regulatory genes. Together, size and integration account for almost 80% of the shape variation seen between different species to the exclusion of morphological dietary adaptation. Instead, birds of prey use size as a mechanism to modify their feeding ecology. The extent to which shape variation is confined to a few major axes may provide an advantage in that it facilitates rapid morphological evolution via changes in body size, but may also make raptors especially vulnerable when selection pressures act against these axes. The phylogenetic position of raptors suggests that this constraint is prevalent in all landbirds and that breaking the developmental correspondence between beak and braincase may be the key novelty in classic passerine adaptive radiations.

Studying avian encephalization with geometric morphometrics.

Encephalization is a core concept in comparative neurobiology, aiming to quantify the neurological capacity of organisms. For measuring encephalization, many studies have employed relative brain sizes corrected for expected allometric scaling to body size. Here we highlight the utility of a multivariate geometric morphometric (GM) approach for visualizing and analyzing neuroanatomical shape variation associated with encephalization. GM readily allows the statistical evaluation of covariates, such as size, and many software tools exist for visualizing their effects on shape. Thus far, however, studies using GM have not attempted to translate the meaning of encephalization to shape data. As such, we tested the statistical relationship between size and encephalization quotients (EQs) to brain shape utilizing a broad interspecific sample of avian endocranial data. Although statistically significant, the analyses indicate that allometry accounts for <10% of total neuroanatomical shape variation. Notably, we find that EQs, despite being corrected for allometric scaling based on size, contain size-related neuroanatomical shape changes. In addition, much of what is traditionally considered encephalization comprises clade-specific trends in relative forebrain expansion, particularly driven by landbirds. EQs, therefore, fail to capture 90% of the total neuroanatomical variation after correcting for allometry and shared phylogenetic history. Moving forward, GM techniques provide crucial tools for investigating key drivers of this vast, largely unexplored aspect of avian brain morphology.

A Cretaceous eutriconodont and integument evolution in early mammals.

The Mesozoic era (252-66 million years ago), known as the domain of dinosaurs, witnessed a remarkable ecomorphological diversity of early mammals. The key mammalian characteristics originated during this period and were prerequisite for their evolutionary success after extinction of the non-avian dinosaurs 66 million years ago. Many ecomorphotypes familiar to modern mammal fauna evolved independently early in mammalian evolutionary history. Here we report a 125-million-year-old eutriconodontan mammal from Spain with extraordinary preservation of skin and pelage that extends the record of key mammalian integumentary features into the Mesozoic era. The new mammalian specimen exhibits such typical mammalian features as pelage, mane, pinna, and a variety of skin structures: keratinous dermal scutes, protospines composed of hair-like tubules, and compound follicles with primary and secondary hairs. The skin structures of this new Mesozoic mammal encompass the same combination of integumentary features as those evolved independently in other crown Mammalia, with similarly broad structural variations as in extant mammals. Soft tissues in the thorax and abdomen (alveolar lungs and liver) suggest the presence of a muscular diaphragm. The eutriconodont has molariform tooth replacement, ossified Meckel's cartilage of the middle ear, and specialized xenarthrous articulations of posterior dorsal vertebrae, convergent with extant xenarthran mammals, which strengthened the vertebral column for locomotion.

Soft-tissue and dermal arrangement in the wing of an Early Cretaceous bird: Implications for the evolution of avian flight.

Despite a wealth of fossils of Mesozoic birds revealing evidence of plumage and other soft-tissue structures, the epidermal and dermal anatomy of their wing's patagia remain largely unknown. We describe a distal forelimb of an enantiornithine bird from the Lower Cretaceous limestones of Las Hoyas, Spain, which reveals the overall morphology of the integument of the wing and other connective structures associated with the insertion of flight feathers. The integumentary anatomy, and myological and arthrological organization of the new fossil is remarkably similar to that of modern birds, in which a system of small muscles, tendons and ligaments attaches to the follicles of the remigial feathers and maintains the functional integrity of the wing during flight. The new fossil documents the oldest known occurrence of connective tissues in association with the flight feathers of birds. Furthermore, the presence of an essentially modern connective arrangement in the wing of enantiornithines supports the interpretation of these primitive birds as competent fliers.

3D geometry and quantitative variation of the cervico-thoracic region in Crocodylia.

This study aims to interpret the axial patterning of the crocodylian neck, and to find a potential taxonomic signal that corresponds to vertebral position. Morphological variation in the cervico-thoracic vertebrae is compared in fifteen different crocodylian species using 3D geometric morphometric methods. Multivariate analysis indicated that the pattern of intracolumnar variation was a gradual change in shape of the vertebral series (at the parapophyses, diapophyses, prezygapohyses, and postzygapohyses), in the cervical (C3 to C9) and dorsal (D1-D2) regions which was quite conservative among the crocodylians studied. In spite of this, we also found that intracolumnar shape variation allowed differentiation between two sub regions of the crocodylian neck. Growth is subtly correlated with vertebral shape variation, predicting changes in both the vertebral centrum and the neural spine. Interestingly, the allometric scaling for the pooled sample is equivalently shared by each vertebra studied. However, there were significant taxonomic differences, both in the average shape of the entire neck configuration (regional variation) and by shape variation at each vertebral position (positional variation) among the necks. The average neck vertebra of crocodylids is characterized by a relatively cranio-caudally short neural arch, whereby the spine is relatively longer and pointed orthogonal to the frontal plane. Conversely, the average vertebra in alligatorids has cranio-caudally longer neural spine and arch, with a relatively (dorso-ventrally) shorter spine. At each vertebral position there are significant differences between alligatorids and crocodylids. We discuss that the delayed timing of neurocentral fusion in Alligatoridae possibly explains the observed taxonomic differences.