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A reappraisal of Boluochia zhengi (Aves: Enantiornithes) and a discussion of intraclade diversity in the Jehol avifauna, China

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A careful reappraisal of the only known specimen of the poorly understood fossil enantiornithine bird Boluochia zhengi reveals numerous morphological similarities that suggest this taxon is closely related to the well-known Longipteryx chaoyangensis, and so is assignable to the most diverse recognized clade of Early Cretaceous enantiornithines, the Longipterygidae. This new study of the holotype of B. zhengi reveals new longipterygid synapomorphies and expands our knowledge of the temporal and geographical ranges and diversity of the clade. We suggest that the trophic specialization that characterizes longipterygids may have been a major factor contributing to the success of this clade.
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... Not to scale. Zhang, 2003b;Novas and Pol, 2005;Xu et al., 2008;Yuan, 2008;O'Connor et al., 2011b;Agnolin and Novas, 2013;Godefroit et al., 2013b;Cau et al., 2017;Novas et al., 2018). Features shared by Rahonavis and avialans, but not by dromaeosaurids or unenlagiids, include large and prominent acromion on the scapula, twisted distal end of the ulna, and several pelvic and pedal features . ...
... Xu et al. (2011) considered that Archaeopteryx, Anchiornis, and Xiaotingia shared with traditionally recognized deinonychosaurs some unique features unknown in any other theropod group, including: (1) large promaxillary fenestra, (2) T-shaped lacrimal, (3) lateral, posteriorly expanding longitudinal groove on the dentary, (4) manual phalanx IV-2 shorter than IV-1, (5) short ischium, (6) ischium with distally located obturator process and posterodistal process, and (7) extensible pedal phalanx II-2. However, Agnolin and Novas (2013) claimed that these traits are diagnostic of Paraves, and that some of them (e.g., characters 1, 2, 4, 6) are shared with basal birds (e.g., Confuciusornis, Sapeornis, Jeholornis; Chiappe et al., 1999;Zhang, 2002, 2003a,b;O'Connor et al., 2011b;Lee and Worthy, 2012). In agreement with this interpretation, Xu et al. (2011) reported a large number of typical deinonychosaurian features that are wanting in Archaeopteryx, Anchiornis, and Xiaotingia, including lateral exposure of the splenial, muscle scar on the deltopectoral crest, and enlarged, raptorial ungual on pedal digit II. ...
... Others interpreted tail feathers as having assisted with maintaining body balance when perching (Chuong et al., 2013;Agnolin et al., 2017), or with stabilizing the trunk when climbing (Hu et al., 2015). Still other authors suggested that elongate tail feathers might have been associated with sexual display or visual communication (Chiappe et al., 1999Zhang and Zhou, 2000;Clarke et al., 2006;Li et al., 2006Li et al., , 2012Zheng et al., 2007;O'Connor et al., 2011bO'Connor et al., , 2012O'Connor et al., , 2013Padian and Horner, 2011). These possibilities are not mutually exclusive, and it seems reasonable to suppose that different tail feather functions complemented and interacted with one another in the lives of these animals. ...
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Recent years witnessed the discovery of a great diversity of early birds as well as closely related non-avian theropods, which modified previous conceptions about the origin of birds and their flight. We here present a review of the taxonomic composition and main anatomical characteristics of those theropod families closely related with early birds, with the aim of analyzing and discussing the main competing hypotheses pertaining to avian origins. We reject the postulated troodontid affinities of anchiornithines, and the dromaeosaurid affinities of microraptorians and unenlagiids, and instead place these groups as successive sister taxa to Avialae. Aiming to evaluate previous phylogenetic analyses, we recoded unenlagiids in the traditional TWiG data matrix, which resulted in a large polytomy at the base of Pennaraptora. This indicates that the TWiG phylogenetic scheme needs a deep revision. Regarding character evolution, we found that: (1) the presence of an ossified sternum goes hand in hand with that of ossified uncinate processes; (2) the presence of foldable forelimbs in basal archosaurs indicates widespread distribution of this trait among reptiles, contradicting previous proposals that forelimb folding driven by propatagial and associated tendons was exclusive to the avian lineage; (3) in basal paravians and avialans (e.g., Archaeopteryx, Anchiornis) the wings are relatively large and wide, with relatively short rectricial feathers, a rounded alar contour, and a convex leading margin. These taxa exhibit restricted forelimb folding capability with respect to more derived birds, their hands being preserved at angles of flexion (with respect to the radius/ulna) of no less than 90°. In more derived birds, however, the rectrices are notably elongate and the angle between the hand and forearm is much less than 90°, indicating not only increased forelimb folding capability but also an increased variety of wingbeat movements during flight. Because of the strong similarities in pectoral girdle configuration between ratites and basal avialans and paravians, it is possible to infer that the main forelimb movements were similar in all these taxa, lacking the complex dorsoventral wing excursion characteristic of living neognathans.
... Longipterygidae is a consistently recovered clade within Enantiornithes [11] comprising six genera [11][12][13][14] from the Yixian and Jiufotang formations (≈125-120 Ma) [11] of the Jehol Group of north-eastern China. The most conspicuous features of longipterygids are their elongate rostra and rostrally restricted dentition [12]. ...
... Longipterygidae is a consistently recovered clade within Enantiornithes [11] comprising six genera [11][12][13][14] from the Yixian and Jiufotang formations (≈125-120 Ma) [11] of the Jehol Group of north-eastern China. The most conspicuous features of longipterygids are their elongate rostra and rostrally restricted dentition [12]. Longipteryx has been suggested to have fed on fish in reference to its large and recurved teeth and its more robust rostrum than other longipterygids [15,16]. ...
... [202], with scripts available from [203]. Additional files 2, 3,4,5,6,7,8,9,10,11,12,13,14 and 15 also include interactive HTML-based three-dimensional graphs of all multivariate analysis results and two-dimensional multivariate FEA graphs annotated with contour plots of each jaw. Both were made using a package from Plotly (Plotly, Canada) for R [204], version 4.9.4.1. ...
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Background Birds are key indicator species in extant ecosystems, and thus we would expect extinct birds to provide insights into the nature of ancient ecosystems. However, many aspects of extinct bird ecology, particularly their diet, remain obscure. One group of particular interest is the bizarre toothed and long-snouted longipterygid birds. Longipterygidae is the most well-understood family of enantiornithine birds, the dominant birds of the Cretaceous period. However, as with most Mesozoic birds, their diet remains entirely speculative. Results To improve our understanding of longipterygids, we investigated four proxies in extant birds to determine diagnostic traits for birds with a given diet: body mass, claw morphometrics, jaw mechanical advantage, and jaw strength via finite element analysis. Body mass of birds tended to correspond to the size of their main food source, with both carnivores and herbivores splitting into two subsets by mass: invertivores or vertivores for carnivores, and granivores + nectarivores or folivores + frugivores for herbivores. Using claw morphometrics, we successfully distinguished ground birds, non-raptorial perching birds, and raptorial birds from one another. We were unable to replicate past results isolating subtypes of raptorial behaviour. Mechanical advantage was able to distinguish herbivorous diets with particularly high values of functional indices, and so is useful for identifying these specific diets in fossil taxa, but overall did a poor job of reflecting diet. Finite element analysis effectively separated birds with hard and/or tough diets from those eating foods which are neither, though could not distinguish hard and tough diets from one another. We reconstructed each of these proxies in longipterygids as well, and after synthesising the four lines of evidence, we find all members of the family but Shengjingornis (whose diet remains inconclusive) most likely to be invertivores or generalist feeders, with raptorial behaviour likely in Longipteryx and Rapaxavis. Conclusions This study provides a 20% increase in quantitatively supported fossil bird diets, triples the number of diets reconstructed in enantiornithine species, and serves as an important first step in quantitatively investigating the origins of the trophic diversity of living birds. These findings are consistent with past hypotheses that Mesozoic birds occupied low trophic levels.
... Commonly, Longipteryx is placed together with Boluochia, Rapaxavis, Shanweiniao, and Longirostravis, forming the clade Longipterygidae (O'Connor et al., 2016;Wang et al., 2018;. This clade is characterized by rostra that account for 60% or more of the total skull length ( Figure 1), a dentition that is restricted to the premaxilla and rostral tip of the dentary, and a proportionately large pygostyle (O'Connor et al., 2009;O'Connor et al., 2011a). Among these taxa, Boluochia and Longipteryx form a clade (Longipteryginae), united by the presence of a unique tarsometatarsus morphology in which metatarsal IV projects distally farther than metatarsal III (O'Connor, 2019;O'Connor et al., 2011a). ...
... This clade is characterized by rostra that account for 60% or more of the total skull length ( Figure 1), a dentition that is restricted to the premaxilla and rostral tip of the dentary, and a proportionately large pygostyle (O'Connor et al., 2009;O'Connor et al., 2011a). Among these taxa, Boluochia and Longipteryx form a clade (Longipteryginae), united by the presence of a unique tarsometatarsus morphology in which metatarsal IV projects distally farther than metatarsal III (O'Connor, 2019;O'Connor et al., 2011a). Of the longipterygids, only Longipteryx is known from multiple specimens, and these are among the best preserved of the entire clade Wang et al., 2015;Zhang et al., 2001). ...
Article
While the morphology and evolution of the quadrate among early birds and through the evolutionary origin of birds is not well known, we add to knowledge about that past diversity through description of the morphology of the quadrate in the unusually elongate skull of the Cretaceous enantiornithine bird Longipteryx chaoyangensis. The lateral and caudal surfaces of the quadrate are well exposed in two specimens revealing morphologies typical of early birds and their dinosaurian close relatives like a small otic head and two mandibular condyles. However, both skeletons exhibit quadrates with a unique, enlarged lateral crest that has not been previously described among Mesozoic birds. It is possible that the rostral surface of this lateral expansion served as the origination site for enlarged jaw musculature in a manner similar to the enlarged subcapitular tubercle in extant galloanserine birds. The caudally concave surface of the quadrate likely reflects some aspect of cranial pneumaticity, with its shape and position reminiscent of quadrates found in close non‐avialan maniraptoran relatives. It is possible that this lateral crest has a wider distribution among enantiornithines and other early birds and that the crest has been misidentified as the orbital process in some more damaged specimens. In addition, the enlarged lateral mandibular condyle (relative to the medial condyle) differs from the condition typically reported among enantiornithines and could indicate a difference in jaw function or mechanics in this bird with an elongated rostrum, or simply misinterpretations of morphology. Further examination of the quadrate in temporally early and phylogenetically stemward birds, along with their close outgroups, could greatly impact the study of several different aspects of bird biology including assessment of phylogenetic relationships, interpretation of the function and kinematics of the skull, reconstruction of foraging paleoecology, and evolution of skull morphological diversity among Mesozoic birds. Study of the quadrate of the extinct enantiornithine bird Longipteryx documents new morphologies among birds including an extensive lateral crest and caudolateral fossa that may have been related to evolutionary changes in their bite, and may provide new material for phylogenetic assessment across stemward birds. Our work also highlights an increase in morphological character variability among enantiornithine quadrates, and indicates some potential areas of misinterpretation in previous studies.
... The cranial anatomy of Early Cretaceous enantiornithines is more diverse (Fig. 3;O'Connor and Chiappe, 2011), in particular, several long-snouted genera (families Longipterygidae and Longirostravisidae: Zhang et al., 2001;Hou et al., 2004;O'Connor et al., 2009O'Connor et al., , 2011aO'Connor et al., , 2011c and also large-headed, with robust dentition Bohaiornithidae (M. Wang et al., 2014b) have been described. ...
... This may be opposition of the additional toe turned posteriorly (zygodactyly in Bucconidae and Cuculidae or heterodactyly in Trogonidae) or formation of a syndactyl foot, with the closely positioned digits united by a ligamentous cover (Alcedinidae). The majority of enantiornithines lack similar changes in the foot structure (the so-called heterodactyl foot of Dalingheornis cannot be accepted; O'Connor, 2009); only in Longipteryx and Boluochia, the ends of metatarsals II-IV are at the same level (O'Connor et al., 2011c), as in extant forest birds (Stegmann, 1958). In other Early Cretaceous enantiornithines, the ends of metatarsals II and IV are located more or less proximal to metatarsal III (see, e.g., Wang and Liu, 2016, text- fig. ...
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New data on the taxonomic and morphological diversity of Early Cretaceous Enantiornithes are reviewed. A new hypothesis concerning the phylogenetic position of Pengornithidae is proposed. These birds traditionally treated as primitive enantiornithines may in fact be more closely related to Ornithuromorpha. This phylogenetic placement implies that the fan-shaped tail and modern-type humeral joint was formed once in the early evolution of birds. Hence, the similarity between Pengornithidae and other enantiornithines may be plesiomorphic. The ecology of Early Cretaceous enantiornithines is discussed. The increased mobility of neck in Holbotia is possibly accounted for by the cranioinertial swallowing mechanism, as in modern ratites. The hypotheses of scansorial adaptations in Parapengornis and Fortunguavis are criticized. In addition, the phylogenetic position of Mystiornis is discussed.
... Similarly, the more highly recurved, compressed and enlarged teeth of Longipteryx compared to other enantiornithines suggest a relatively hypercarnivorous diet, further supported by the discovery of small crenulations on the distal margin of the premaxillary teeth (Fig. 8D). The curvature of the teeth has led to suggestions Longipteryx was piscivorous (O'Connor et al., 2011a), although this is unsupported by direct evidence in any of the several dozen known specimens. The diversity of dental morphologies without the presence of gastroliths suggests strong partitioning of relatively soft available invertebrate faunas. ...
Article
Although direct associations that reveal diet are extremely rare in the fossil record, the rich Lower Cretaceous Jehol Lagerstätte has produced dozens of specimens preserving ingested items, which together reveal important information regarding the early evolution of the avian alimentary canal. Direct evidence indicates Jeholornis and Sapeornis ate seeds and like living granivores utilized a gastric mill although only the more derived Sapeornis possessed a crop for food storage. Despite their smaller numbers in the Early Cretaceous, most direct evidence pertains to the Ornithuromorpha, indicating a structurally and functionally modern alimentary canal was present in even the earliest members of this clade. Similar evidence is altogether lacking in the Enantiornithes suggesting this clade was characterized by a primitive alimentary canal, potentially factoring into the ultimate extinction of this successful Cretaceous lineage. The role of the gizzard in the loss of teeth in Aves is more complex than in non-avian theropod lineages. As in neornithines, the presence of gastroliths in basal ornithuromorphs may not be entirely indicative of herbivory although this diet does appear to correlate with complete tooth loss in this clade. Positive selection for specialized tooth morphologies persists throughout the evolution of Aves and in the Ornithuromorpha appears to be linked to piscivory. Although represented by the greatest numbers, no direct indicator of diet preference is preserved in any confuciusornithiform or enantiornithine suggesting these clades utilized different nutritional strategies. The absence of teeth and gastroliths in all confuciusornithiforms suggest this lineage may have secondarily switched to a non-herbivorous, soft diet. Similarly, the absence of gizzard stones in enantiornithines also suggests a soft diet while their small body size suggests Early Cretaceous species would have fed on invertebrates. The diversity of recognized dental patterns including enamel specializations observed in Jehol enantiornithines suggests effective resource partitioning of available invertebrate faunas and that teeth had an active participation in enantiornithine feeding strategies.
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Birds are some of the most diverse organisms on Earth, with species inhabiting a wide variety of niches across every major biome. As such, birds are vital to our understanding of modern ecosystems. Unfortunately, our understanding of the evolutionary history of modern ecosystems is hampered by knowledge gaps in the origin of modern bird diversity and ecosystem ecology. A crucial part of addressing these shortcomings is improving our understanding of the earliest birds, the non‐avian avialans (i.e. non‐crown birds), particularly of their diet. The diet of non‐avian avialans has been a matter of debate, in large part because of the ambiguous qualitative approaches that have been used to reconstruct it. Here we review methods for determining diet in modern and fossil avians (i.e. crown birds) as well as non‐avian theropods, and comment on their usefulness when applied to non‐avian avialans. We use this to propose a set of comparable, quantitative approaches to ascertain fossil bird diet and on this basis provide a consensus of what we currently know about fossil bird diet. While no single approach can precisely predict diet in birds, each can exclude some diets and narrow the dietary possibilities. We recommend combining (i) dental microwear, (ii) landmark‐based muscular reconstruction, (iii) stable isotope geochemistry, (iv) body mass estimations, (v) traditional and/or geometric morphometric analysis, (vi) lever modelling, and (vii) finite element analysis to reconstruct fossil bird diet accurately. Our review provides specific methodologies to implement each approach and discusses complications future researchers should keep in mind. We note that current forms of assessment of dental mesowear, skull traditional morphometrics, geometric morphometrics, and certain stable isotope systems have yet to be proven effective at discerning fossil bird diet. On this basis we report the current state of knowledge of non‐avian avialan diet which remains very incomplete. The ancestral dietary condition in non‐avian avialans remains unclear due to scarce data and contradictory evidence in Archaeopteryx. Among early non‐avian pygostylians, Confuciusornis has finite element analysis and mechanical advantage evidence pointing to herbivory, whilst Sapeornis only has mechanical advantage evidence indicating granivory, agreeing with fossilised ingested material known for this taxon. The enantiornithine ornithothoracine Shenqiornis has mechanical advantage and pedal morphometric evidence pointing to carnivory. In the hongshanornithid ornithuromorph Hongshanornis only mechanical advantage evidence indicates granivory, but this agrees with evidence of gastrolith ingestion in this taxon. Mechanical advantage and ingested fish support carnivory in the songlingornithid ornithuromorph Yanornis. Due to the sparsity of robust dietary assignments, no clear trends in non‐avian avialan dietary evolution have yet emerged. Dietary diversity seems to increase through time, but this is a preservational bias associated with a predominance of data from the Early Cretaceous Jehol Lagerstätte. With this new framework and our synthesis of the current knowledge of non‐avian avialan diet, we expect dietary knowledge and evolutionary trends to become much clearer in the coming years, especially as fossils from other locations and climates are found. This will allow for a deeper and more robust understanding of the role birds played in Mesozoic ecosystems and how this developed into their pivotal role in modern ecosystems. Video abstract
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