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Evolution: Taking Wing with Weak Feathers

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Evolution: Taking Wing with Weak Feathers Current Biology Vol 22 No 23 R992 Dispatches Evolution: Taking Wing with Weak Feathers Scientists long thought they knew what the wings of early birds looked like. allowing Archaeopteryx to have But new reconstructions of Archaeopteryx and its kin suggest quite different a strong wing even without strong feather arrangements on their wings with profound implications for the flight feathers. So, quantity makes up evolution of flight. for lack of quality. Interestingly, Archaeopteryx not Xing Xu surprising given the otherwise only evolved an unusual feather transitional status of this taxon. arrangement, but also had more than Archaeopteryx is an icon of evolution. Nevertheless, the traditional two wings for flight. The discovery of With feathers, toothed jaws and a long reconstruction has generally been the four-winged dinosaur Microraptor bony tail, Archaeopteryx represents considered reliable because it is based [6] inspired Nicholas Longrich to a truly transitional form [1]. However, on exceptionally-preserved feather re-examine Archaeopteryx fossils in a close look at the various specimens impressions [10], although the another study, which led to the of Archaeopteryx reveals some seemingly modern nature of the wing suggestion that Archaeopteryx also surprisingly modern features, such plumage once prompted a suggestion had wings on the hind legs, a feature as the asymmetric form of the flight that the feather impressions were in not known in modern birds [5]. The feathers, in this 150-million-year-old fact nineteenth-century forgeries [11]. slender-shafted flight feathers of bird. In fact, the flight feathers of This suggestion was never supported Archaeopteryx, as well as the large, Archaeopteryx were traditionally by substantive evidence, but the fact wing-forming leg feathers and now the considered very similar to those of remains that reconstructions of an multi-layered configuration of the arm modern birds in shape, arrangement, essentially modern feathery wing in wings, all demonstrate that the flight and distribution on the body, implying Archaeopteryx seem out of step with plumage of Archaeopteryx was that the feathery wing has remained the transitional evolutionary position significantly different from that of essentially the same in structure since of this ancient bird. modern birds [4–8]. the appearance of the earliest known Having closely examined two birds [1–3]. However, several recent Archaeopteryx specimens, Longrich Early Flight studies [4–7], including one by Nicholas and colleagues [8] believe that Inferring the flight capability of Longrich and colleagues in this issue of Archaeopteryx does have a transitional a transitional fossil species is Current Biology [8], demonstrate that feathery wing (Figure 1). Instead of a challenging task. Some studies, the flight feathers of Archaeopteryx and being mostly single-layered like e.g. [12], have suggested that its kin are different from modern ones in modern wings, the wings of Archaeopteryx was indeed able to fly, all these respects, and distinctly Archaeopteryx wings are multi-layered, based on such morphological features transitional. the additional layers being formed as the presence of flight feathers with by elongated coverts. Interestingly, asymmetrical vanes. However, other Early Wings this unusual wing configuration studies [4,13] have questioned the Birds (the Avialae) are a group of is also present in Anchiornis, flight capability of Archaeopteryx feathered animals with a unique a recently-reported Jurassic based on different lines of evidence. body-plan adapted to flight. As the Archaeopteryx-like dinosaur [7]. Much of this debate reflects the fact earliest known bird, Archaeopteryx has Longrich and colleagues [8] suggest that different researchers use different many flight-related features, including that the multi-layered configuration features to infer flight capability. Given flight feathers, long and robust arms, represents a primitive stage in wing the complex pattern of morphological and modified shoulder girdles evolution. evolution seen across the allowing the arms to move laterally. Longrich and colleagues’ hypothesis dinosaur-bird transition [14], the Interestingly, all of these features, casts new light on the results of an diversity of opinion is not surprising. even flight feathers, are now seen in earlier study [4] that examined the flight Longrich and colleagues’ study [8] theropod dinosaurs, and in particular, feathers of Archaeopteryx from the shows how new observations can in the birds’ closest relatives, the perspective of functional morphology. sometimes alter the significance of old Deinonychosauria [6]. This study found the shafts of the ones, helping to integrate them into Living birds share a common basic flight feathers of Archaeopteryx to a more coherent understanding of the wing configuration: a row of long flight be proportionally too slender to have early evolution of flight. feathers form the main part of the wing, the strength needed for flight, The relatively weak-looking flight and several rows of much shorter assuming that the traditional wing feathers of basal birds [4], then, feathers (coverts) provide limited reconstruction was roughly correct [4]. do not necessarily suggest that flight additional support [9]. Archaeopteryx Under Longrich and colleagues’ capability was poor, let alone entirely has been widely accepted to have interpretation, the presence of multiple absent. Early birds and their close had feathery wings with this modern feather layers compensates for the relatives could assemble an effective configuration [1–3], which is somewhat weakness of the individual feathers, flying wing using multiple rows of Dispatch R993 relatively weak feathers, as Oviraptorosauria Confuciusornis demonstrated by Longrich’s analysis Archaeopteryx Living birds of specimens of Archaeopteryx and Anchiornis Anchiornis [8]. However, Longrich and colleagues [8] also note that the multi-layered wing lacks a distally slotted configuration, a feature that reduces induced drag during flight [15] and is particularly important at slow speeds. This suggests that high-speed flight was a preferred mode of flight in Archaeopteryx and its relatives. A strength of this inference is that it is based on a comprehensive analysis of Avialae the relevant available data rather than a single feature. The early history of flight is highly complex. As Longrich and colleagues A [8] point out, flight capability is likely to have evolved independently Oviraptorosauria Confuciusornis on multiple occasions among Archaeopteryx Living birds Archaeopteryx and its kin. For Anchiornis example, flight feathers with asymmetrical vanes seem to have evolved independently at least twice near the dinosaur-bird transition, once near the base of the Avialae and once in the deinonychosaurs [8]. Archaeopteryx and its kin also have wings of varying shapes. The arm wings of Microraptor seem to be Deinonychosauria Avialae proportionally narrower than those of Archaeopteryx and Anchiornis, and the leg wings of Archaeopteryx and its kin are highly variable in morphology [5–7,10]. These data suggest that early B flight was probably not uniform, and Current Biology that any simple scenario for the early evolution of flight will likely be Figure 1. Different scenarios for the evolution of bird wings. inadequate. The multi-layered wings of Archaeopteryx and Anchiornis could represent a transitional stage Our understanding of how feathered in the evolution of modern wings (A) or a dead-end experiment in early flight evolution (B). dinosaurs close to the origin of birds built their wings, and of how they got into the air, has improved greatly in arrangement of the flight feathers and Anchiornis lie in Deinonychosauria, recent decades. However, there are in other transitional forms such a side-branch in bird evolution, as many issues yet to be addressed. First as Xiaotingia and Microraptor, suggested by a recent phylogenetic of all, a better understanding of the as well as in more basal taxa such as study [14], their multi-layered wings range of feather morphologies present Caudipteryx [8]. are likely to be unique to the in taxa near the dinosaur–bird Any credible evolutionary scenario Deinonychosauria [8] or an even more transition is particularly important. The must be built within a firm phylogenetic exclusive group and are more likely to flight feathers of the earliest known framework. In this case, an accurate represent an evolutionary dead end birds and their close relatives are, in theropod phylogeny will provide (Figure 1B). The wings of general, similar to those of modern the basis for reconstructing the Archaeopteryx and its kin are very birds [16,17]. However, there are also dinosaur-bird transition, and the complex and highly variable, and this various differences between early and systematic position of Archaeopteryx was probably also true of the early modern flight feathers in development, plays a key role in interpreting the evolution of flight. morphology, fine-scale arrangement, evolutionary history of avian wings [14]. and large-scale distribution on the If Archaeopteryx is an early References body [4,5,8,18]. It will be necessary in representative of the bird lineage, as 1. Wellnhofer, P. (2009). Archaeopteryx-the icon the course of future research to suggested by most phylogenetic of evolution (Mu¨nchen: Verlag Dr. Friedrich Pfeil). undertake more extensive and detailed studies [19,20], the unusual wings of 2. Heilmann, G. (1927). The Origin of Birds (New investigations of these differences, and this
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