Morphological corollaries and ecological implications of flightlessness in the kakapo (Psittaciformes:Strigops habroptilus)

Transformation of the pectoral girdle in pennaraptorans: critical steps in the formation of the modern avian shoulder joint

Important transformations of the pectoral girdle are related to the appearance of flight capabilities in the Dinosauria. Previous studies on this topic focused mainly on paravians yet recent data suggests flight evolved in dinosaurs several times, including at least once among non-avialan paravians. Thus, to fully explore the evolution of flight-related avian shoulder girdle characteristics, it is necessary to compare morphology more broadly. Here, we present information from pennaraptoran specimens preserving pectoral girdle elements, including all purportedly volant taxa, and extensively compare aspects of the shoulder joint. The results show that many pectoral girdle modifications appear during the evolution from basal pennaraptorans to paravians, including changes in the orientation of the coracoid body and the location of the articulation between the furcula and scapula. These modifications suggest a change in forelimb range of motion preceded the origin of flight in paravians. During the evolution of early avialans, additional flight adaptive transformations occur, such as the separation of the scapula and coracoid and reduction of the articular surface between these two bones, reduction in the angle between these two elements, and elongation of the coracoid. The diversity of coracoid morphologies and types of articulations joining the scapula-coracoid suggest that each early avialan lineage evolved these features in parallel as they independently evolved more refined flight capabilities. In early ornithothoracines, the orientation of the glenoid fossa and location of the acrocoracoid approaches the condition in extant birds, suggesting a greater range of motion in the flight stroke, which may represent the acquisition of improved powered flight capabilities, such as ground take-off. The formation of a new articulation between the coracoid and furcula in the Ornithuromorpha is the last step in the formation of an osseous triosseal canal, which may indicate the complete acquisition of the modern flight apparatus. These morphological transitions equipped birds with a greater range of motion, increased and more efficient muscular output and while at the same time transmitting the increased pressure being generated by ever more powerful flapping movements in such a way as to protect the organs. The driving factors and functional adaptations of many of these transitional morphologies are as yet unclear although ontogenetic transitions in forelimb function observed in extant birds provide an excellent framework through which we can explore the behavior of Mesozoic pennaraptorans.

Embryonic Development of the Avian Sternum and Its Morphological Adaptations for Optimizing Locomotion

The sternum is part of the forelimb appendicular skeleton found in most terrestrial vertebrates and has become adapted across tetrapods for distinctive modes of locomotion. We review the regulatory mechanisms underlying sternum and forelimb development and discuss the possible gene expression modulation that could be responsible for the sternal adaptations and associated reduction in the forelimb programme found in flightless birds. In three phylogenetically divergent vertebrate lineages that all undertake powered flight, a ventral extension of the sternum, named the keel, has evolved independently, most strikingly in volant birds. In flightless birds, however, the sternal keel is absent, and the sternum is flattened. We review studies in a variety of species that have analysed adaptations in sterna morphology that are related to the animal’s mode of locomotion on land, in the sky and in water.

Ancestral transoceanic colonization and recent population reduction in a nonannual killifish from the Seychelles archipelago

Whether freshwater fish colonize remote islands following tectonic or transoceanic dispersal remains an evolutionary puzzle. Integrating dating of known tectonic events with phylogenomics and current species distribution, we find that killifish species distribution is not explained by species dispersal by tectonic drift only. Investigating the colonization of a nonannual killifish (golden panchax, Pachypanchax playfairii) on the Seychelle islands, we found genetic support for transoceanic dispersal and experimentally discovered an adaptation to complete tolerance to seawater. At the macroevolutionary scale, despite their long-lasting isolation, nonannual golden panchax show stronger genome-wide purifying selection than annual killifishes from continental Africa. However, progressive decline in effective population size over a more recent timescale has probably led to the segregation of slightly deleterious mutations across golden panchax populations, which represents a potential threat for species preservation in the long term.

Waxing and Waning of Wings

A major challenge to Darwinian evolution is explaining 'rudimentary' organs. This is particularly relevant to birds: rudimentary wings occur in fossils, as well as in developing, molting, and flight-impaired birds. Evidence shows that young birds flap small wings to improve locomotion and transition to flight. Although small wings also occur in adults, their potential role in locomotion is rarely considered. Here we describe the prevalence of rudimentary wings in extant birds, and how wings wax and wane on many timescales. This waxing and waning is integral to the avian clade and offers a rich arena for exploring links between form, function, performance, behavior, ecology, and evolution. Although our understanding is nascent, birds clearly show that rudimentary structures can enhance performance and survival.

Evidence for a giant parrot from the Early Miocene of New Zealand

Insular avifaunas have repeatedly spawned evolutionary novelties in the form of unusually large, often flightless species. We report fossils from the Early Miocene St Bathans Fauna of New Zealand that attests to the former existence of a giant psittaciform, which is described as a new genus and species. The fossils are two incomplete tibiotarsi from a bird with an estimated mass of 7 kg, double that of the heaviest known parrot, the kakapo Strigops habroptila. These psittaciform fossils show that parrots join the growing group of avian taxa prone to giantism in insular species, currently restricted to palaeognaths, anatids, sylviornithids, columbids, aptornithids, ciconiids, tytonids, falconids and accipitrids.

Anatomy of the forelimb musculature and ligaments of Psittacus erithacus (Aves: Psittaciformes)

Parrots (order Psittaciformes) are a rather homogeneous group of birds that can be easily distinguished by the notably modified morphology of the skull and hindlimb. Detailed description of the forelimb morphology in these birds has never been provided, though parrots are often used as model objects in flight studies. Parrots are also considered the closest living relatives of the perching birds (Passeriformes), and thus knowledge of the wing morphology in Psittaciformes is important for understanding the evolution of the locomotor apparatus on the way to the most speciose group of birds. Here we provide a comprehensive illustrated description of the wing morphology (musculature and ligaments) of the African grey parrot (Psittacus erithacus) and compare it with several closely related taxa of the high clade Eufalconimorphae and more distantly related outgroups (based on personal dissections and literature data). We note a general similarity of the wing musculature between P. erithacus and Falconidae. A number of features common with the outgroup Columbidae supports a generally plesiomorphic structure of the forelimb in parrots as compared with the Passeriformes. Nevertheless, the wing of the Psittaciformes displays a series of structural (likely autapomorphic) modifications, which can be explained in terms of adaptations for flight with vertical body. An analysis of the anatomical data for parrots (ratio of wing elevators and highly unusual development of the M. supracoracoideus), which is based on the current experiment-based knowledge of the flapping flight in birds, allows us to hypothesize that parrots are able to produce useful aerodynamic force during the upstroke, which is also known for pigeons and hummingbirds. This supposed ability of vertical flight and the zygodactyl foot together link the origin of parrots with the dense (likely tropical) forests.

Systematics and phylogeny of the Zygodactylidae (Aves, Neognathae) with description of a new species from the early Eocene of Wyoming, USA

Zygodactylidae are an extinct lineage of perching birds characterized by distinct morphologies of the foot and wing elements. Although the clade has a complex taxonomic history, current hypotheses place Zygodactylidae as the sister taxon to Passeriformes (i.e., songbirds). Given the rather sparse fossil record of early passeriforms, the description of zygodactylid taxa is important for inferring potentially ancestral states in the largest radiation of living birds (i.e., the ∼6,000 species of extant passeriforms). Despite the exceptional preservation of many specimens and considerable species diversity in Zygodactylidae, the relationships among species have not been previously evaluated in a phylogenetic context. Herein, we review the fossil record of Zygodactylidae from North America and describe five new well-preserved fossils from the early Eocene Green River Formation of Wyoming. Two specimens are identified as representing a new species and the first records of the taxon Zygodactylus outside Europe. Anatomical comparisons with previously named taxa and the results of phylogenetic analysis including newly described specimens and previously named zygodactylid taxa provide the first hypothesis of the species-level relationships among zygodactylids. The monophyly of Zygodactylidae is supported in these new analyses. However, the monophyly of Primozygodactylus and the taxonomic distinction between Zygodactylus and Eozygodactylus remain unresolved and would likely benefit from the description of additional specimens.

Myology of the forelimb of Majungasaurus crenatissimus (Theropoda, Abelisauridae) and the morphological consequences of extreme limb reduction

Forelimb reduction occurred independently in multiple lineages of theropod dinosaurs. Although tyrannosaurs are renowned for their tiny, two-fingered forelimbs, the degree of their reduction in length is surpassed by abelisaurids, which possess an unusual morphology distinct from that of other theropods. The forelimbs of abelisaurids are short but robust and exhibit numerous crests, tubercles, and scars that allow for inferences of muscle attachment sites. Phylogenetically based reconstructions of the musculature were used in combination with close examination of the osteology in the Malagasy abelisaurid Majungasaurus to create detailed muscle maps of the forelimbs, and patterns of the muscular and bony morphology were compared with those of extant tetrapods with reduced or vestigial limbs. The lever arms of muscles crossing the glenohumeral joint are shortened relative to the basal condition, reducing the torque of these muscles but increasing the excursion of the humerus. Fusion of the antebrachial muscles into a set of flexors and extensors is common in other tetrapods and occurred to some extent in Majungasaurus. However, the presence of tubercles on the antebrachial and manual elements of abelisaurids indicates that many of the individual distal muscles acting on the wrist and digits were retained. Majungasaurus shows some signs of the advanced stages of forelimb reduction preceding limb loss, while also exhibiting features suggesting that the forelimb was not completely functionless. The conformation of abelisaurid forelimb musculature was unique among theropods and further emphasizes the unusual morphology of the forelimbs in this clade.

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

The geometry of feather barbs (barb length and barb angle) determines feather vane asymmetry and vane rigidity, which are both critical to a feather's aerodynamic performance. Here, we describe the relationship between barb geometry and aerodynamic function across the evolutionary history of asymmetrical flight feathers, from Mesozoic taxa outside of modern avian diversity (Microraptor, Archaeopteryx, Sapeornis, Confuciusornis and the enantiornithine Eopengornis) to an extensive sample of modern birds. Contrary to previous assumptions, we find that barb angle is not related to vane-width asymmetry; instead barb angle varies with vane function, whereas barb length variation determines vane asymmetry. We demonstrate that barb geometry significantly differs among functionally distinct portions of flight feather vanes, and that cutting-edge leading vanes occupy a distinct region of morphospace characterized by small barb angles. This cutting-edge vane morphology is ubiquitous across a phylogenetically and functionally diverse sample of modern birds and Mesozoic stem birds, revealing a fundamental aerodynamic adaptation that has persisted from the Late Jurassic. However, in Mesozoic taxa stemward of Ornithurae and Enantiornithes, trailing vane barb geometry is distinctly different from that of modern birds. In both modern birds and enantiornithines, trailing vanes have larger barb angles than in comparatively stemward taxa like Archaeopteryx, which exhibit small trailing vane barb angles. This discovery reveals a previously unrecognized evolutionary transition in flight feather morphology, which has important implications for the flight capacity of early feathered theropods such as Archaeopteryx and Microraptor. Our findings suggest that the fully modern avian flight feather, and possibly a modern capacity for powered flight, evolved crownward of Confuciusornis, long after the origin of asymmetrical flight feathers, and much later than previously recognized.

Ecogeographical variation in the Southern Giant Petrel (Macronectes giganteus)

The Southern Giant Petrel ( Macronectes giganteus (Gmelin, 1789); Aves, Procellariiformes), ranging from New Zealand to the Graham Peninsula and Patagonia and also from coastal Antarctica to Gough Island, displays significant geographic variation throughout its range. Six breeding provinces were identified, which show significant interpopulational variation. The formerly proposed subspecies Macronectes giganteus giganteus and Macronectes giganteus solanderi were confirmed. Macronectes giganteus solanderi has smaller homologuous parts than M. g. giganteus. Two of the provinces belong to the subspecies M. g. solanderi. Females are smaller and have shorter bills than males. In M. g. giganteus, outer appendages are longer in the sub-Antarctic than in the Antarctic, which is consistent with Allen’s rule. Moreover, an east-to-west cline shows a gradual decrease in body size. Within M. g. solanderi, the Chubut River Estuary (Argentina) and the Falkland Islands form the southern province, and Gough Island the northern province. The birds have shallower bills in Argentina than in the Falklands or Gough, but tarsi are longest in Gough. Macronectes giganteus giganteus is morphologically polymorphic but taxonomically stable, whereas M. g. solanderi, which is biometrically less variable, could undergo taxonomical differentiation. This paper gives arguments for further morphometric and genetic studies on the taxon.

Higher-order phylogeny of modern birds (Theropoda, Aves: Neornithes) based on comparative anatomy. II. Analysis and discussion

In recent years, avian systematics has been characterized by a diminished reliance on morphological cladistics of modern taxa, intensive palaeornithogical research stimulated by new discoveries and an inundation by analyses based on DNA sequences. Unfortunately, in contrast to significant insights into basal origins, the broad picture of neornithine phylogeny remains largely unresolved. Morphological studies have emphasized characters of use in palaeontological contexts. Molecular studies, following disillusionment with the pioneering, but non-cladistic, work of Sibley and Ahlquist, have differed markedly from each other and from morphological works in both methods and findings. Consequently, at the turn of the millennium, points of robust agreement among schools concerning higher-order neornithine phylogeny have been limited to the two basalmost and several mid-level, primary groups. This paper describes a phylogenetic (cladistic) analysis of 150 taxa of Neornithes, including exemplars from all non-passeriform families, and subordinal representatives of Passeriformes. Thirty-five outgroup taxa encompassing Crocodylia, predominately theropod Dinosauria, and selected Mesozoic birds were used to root the trees. Based on study of specimens and the literature, 2954 morphological characters were defined; these characters have been described in a companion work, approximately one-third of which were multistate (i.e. comprised at least three states), and states within more than one-half of these multistate characters were ordered for analysis. Complete heuristic searches using 10 000 random-addition replicates recovered a total solution set of 97 well-resolved, most-parsimonious trees (MPTs). The set of MPTs was confirmed by an expanded heuristic search based on 10 000 random-addition replicates and a full ratchet-augmented exploration to ascertain global optima. A strict consensus tree of MPTs included only six trichotomies, i.e. nodes differing topologically among MPTs. Bootstrapping (based on 10 000 replicates) percentages and ratchet-minimized support (Bremer) indices indicated most nodes to be robust. Several fossil Neornithes (e.g. Dinornithiformes, Aepyornithiformes) were placed within the ingroup a posteriori either through unconstrained, heursitic searches based on the complete matrix augmented by these taxa separately or using backbone-constraints. Analysis confirmed the topology among outgroup Theropoda and achieved robust resolution at virtually all levels of the Neornithes. Findings included monophyly of the palaeognathous birds, comprising the sister taxa Tinamiformes and ratites, respectively, and the Anseriformes and Galliformes as monophyletic sister-groups, together forming the sister-group to other Neornithes exclusive of the Palaeognathae (Neoaves). Noteworthy inferences include: (i) the sister-group to remaining Neoaves comprises a diversity of marine and wading birds; (ii) Podicipedidae are the sister-group of Gaviidae, and not closely related to the Phoenicopteridae, as recently suggested; (iii) the traditional Pelecaniformes, including the shoebill (Balaeniceps rex) as sister-taxon to other members, are monophyletic; (iv) traditional Ciconiiformes are monophyletic; (v) Strigiformes and Falconiformes are sister-groups; (vi) Cathartidae is the sister-group of the remaining Falconiformes; (vii) Ralliformes (Rallidae and Heliornithidae) are the sister-group to the monophyletic Charadriiformes, with the traditionally composed Gruiformes and Turniciformes (Turnicidae and Mesitornithidae) sequentially paraphyletic to the entire foregoing clade; (viii) Opisthocomus hoazin is the sister-taxon to the Cuculiformes (including the Musophagidae); (ix) traditional Caprimulgiformes are monophyletic and the sister-group of the Apodiformes; (x) Trogoniformes are the sister-group of Coliiformes; (xi) Coraciiformes, Piciformes and Passeriformes are mutually monophyletic and closely related; and (xii) the Galbulae are retained within the Piciformes. Unresolved portions of the Neornithes (nodes having more than one most-parsimonious solution) comprised three parts of the tree: (a) several interfamilial nodes within the Charadriiformes; (b) a trichotomy comprising the (i) Psittaciformes, (ii) Columbiformes and (iii) Trogonomorphae (Trogoniformes, Coliiformes) + Passerimorphae (Coraciiformes, Piciformes, Passeriformes); and (c) a trichotomy comprising the Coraciiformes, Piciformes and Passeriformes. The remaining polytomies were among outgroups, although several of the highest-order nodes were only marginally supported; however, the majority of nodes were resolved and met or surpassed conventional standards of support. Quantitative comparisons with alternative hypotheses, examination of highly supportive and diagnostic characters for higher taxa, correspondences with prior studies, complementarity and philosophical differences with palaeontological phylogenetics, promises and challenges of palaeogeography and calibration of evolutionary rates of birds, and classes of promising evidence and future directions of study are reviewed. Homology, as applied to avian examples of apparent homologues, is considered in terms of recent theory, and a revised annotated classification of higher-order taxa of Neornithes and other closely related Theropoda is proposed. (c) 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 149, 1-95.

A phylogenetic analysis of the Gruiformes (Aves) based on morphological characters, with an emphasis on the rails (Rallidae)

The order Gruiformes, for which even familial composition remains controversial, is perhaps the least well understood avian order from a phylogenetic perspective. The history of the systematics of the order is presented, and the ecological and biogeographic characteristics of its members are summarized. Using cladistic techniques, phylogenetic relationships among fossil and modern genera of the Gruiformes were estimated based on 381 primarily osteological characters; relationships among modern species of Grues (Psophiidae, Aramidae, Gruidae, Heliornithidae and Rallidae) were assessed based on these characters augmented by 189 characters of the definitive integument. A strict consensus tree for 20,000 shortest trees compiled for the matrix of gruiform genera (length = 967, CI = 0.517) revealed a number of nodes common to the solution set, many of which were robust to bootstrapping and had substantial support (Bremer) indices. Robust nodes included those supporting: a sister relationship between the Pedionomidae and Turnicidae; monophyly of the Gruiformes exclusive of the Pedionomidae and Turnicidae; a sister relationship between the Cariamidae and Phorusrhacoidea; a sister relationship between a clade comprisingEurypygaandMesselornisand one comprisingRhynochetosandAptornis; monophyly of the Grues (Psophiidae, Aramidae, Gruidae, Heliornithidae and Rallidae); monophyly of a clade (Gruoidea) comprising (in order of increasingly close relationship)Psophia,Aramus,Balearicaand other Gruidae, with monophyly of each member in this series confirmed; a sister relationship between the Heliornithidae and Rallidae; and monophyly of the Rallidae exclusive ofHimantornis. Autapomorphic divergence was comparatively high forPedionomus,Eurypyga,Psophia,HimantornisandFulica; extreme autapomorphy, much of which is unique for the order, characterized the extinct, flightlessAptornis.

In the species–level analysis of modern Grues, special efforts were made to limit the analytical impacts of homoplasy related to flightlessness in a number of rallid lineages. A strict consensus tree of 20,000 shortest trees compiled (length = 1232, CI = 0.463) confirmed the interfamilial relationships resolved in the ordinal analysis and established a number of other, variably supported groups within the Rallidae. Groupings within the Rallidae included: monophyly of Rallidae exclusive ofHimantornisand a clade comprisingPorphyrio(includingNotornis) andPorphyrula; a poorly resolved, basal group of genera includingGymnocrex,Habroptila,Eulabeornis,Aramides,CanirallusandMentocrex; an intermediate grade comprisingAnurolimnas,Amaurolimnas, andRougetius; monophyly of two major subdivisions of remaining rallids, one comprisingRallina(paraphyletic),Rallicula, andSarothrura, and the other comprising the apparently paraphyletic ‘long–billed’ rails (e.g.Pardirallus,Cyanolimnas,Rallus,GallirallusandCabalusand a variably resolved clade comprising ‘crakes’ (e.g.Atlantisia,LaterallusandPorzana, waterhens (Amaurornis), moorhens (Gallinulaand allied genera) and coots (Fulica). Relationships among ‘crakes’ remain poorly resolved;Laterallusmay be paraphyletic, andPorzanais evidently polyphyletic and poses substantial challenges for reconciliation with current taxonomy. Relationships among the species of waterhens, moorhens and coots, however, were comparatively well resolved, and exhaustive, fine–scale analyses of several genera (Grus,Porphyrio,Aramides,Rallus,LaterallusandFulica) and species complexes (Porphyrio porphyrio–group,Gallirallus philippensis–group andFulica americana–group) revealed additional topological likelihoods. Many nodes shared by a majority of the shortest trees under equal weighting were common to all shortest trees found following one or two iterations of successive weighting of characters. Provisional placements of selected subfossil rallids (e.g.Diaphorapteryx,AphanapteryxandCapellirallus) were based on separate heuristic searches using the strict consensus tree for modern rallids as a backbone constraint.

These analyses were considered with respect to assessments of robustness, homoplasy related to flightlessness, challenges and importance of fossils in cladistic analysis, previously published studies and biogeography, and an annotated phylogenetic classification of the Gruiformes is proposed.