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A New Late Mesozoic confuciusornithid Bird from China

DOI:10.1080/02724634.1999.10011117
Authors:
Ji Qiang
Ji Qiang
Ji Qiang
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Luis M. Chiappe at Natural History Museum of Los Angeles County
Luis M. Chiappe
Ji ShuAn
Ji ShuAn
Ji ShuAn
  • This person is not on ResearchGate, or hasn't claimed this research yet.
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... Confuciusornithids were the earliest birds with a horny beak [2,7], occupying a critical position in early bird evolution. Many scholars have performed much work on the phylogenetic and microscopic features of different species, including the identification at the species level [13][14][15][16][17][18], growth patterns [8][9]19], body size and sex differences [20][21][22][23], feather patterns and colours [24][25][26], perching style and feeding habits [2,24,[27][28], and reproductive characteristics and pterygium structure [29]. An increasing number of findings facilitated reconstructions of the developmental characteristics of confuciusornithids, but the functional analyses of flightrelated structures were mainly based on qualitative results by analogy with the flight theory in extant bird. ...
... The seemingly simple cooperation of locomotion modules in birds affects their speed, efficiency, persistence, and mobility, which are closely related to behaviours for meeting survival needs such as grabbing food, fleeing from enemies, and pursuing mates. According to the characters of confuciusornithids, such as the pectoral girdle that differs from that in modern birds, the relatively long pygostyle, sternum with an underdeveloped carina, the relatively large hindlimb feathers, and the lack of foramen triosseum [2,14,[16][17]20,[45][46], their three locomotion modules should be different from those of extant birds in terms of the relationship between function and configuration. ...
COMPUTATIONAL FLUID DYNAMICS WILL SHED A NEW LIGHT ON FUNCTIONAL-MORPHOLOGICAL STUDIES OF CONFUCIUSORNITHIDS (AVES) IN EASTERN ASIA DURING EARLY CRETACEOUS
Article
Full-text available
Eastern Asia can be regarded as a dynamic environment during the Early Cretaceous in which the whole biota was complex and diverse, so the differentiation of early birds was also pronounced in this period. Confuciusornithids only uncovered from Eastern Asia, are an important early bird group for revealing the origin and early evolution of avian flight. However, results on confuciusornithid flight capabilities mainly come from qualitative analysis or speculation and analogism because of insufficient information from fossils, limited analogues in living birds, and no rigorous biomechanical testing. As an essential and popular tool in quantitative simulation and analysis, computational fluid dynamics (CFD) has major advantages over other techniques and has been well developed in recent years to solve general morphological-functional problems in palaeontolog-ical research. CFD is beneficial for studying the flight capability of confuciusornithids and will provide new quantitative evidence for the origin and evolution of early bird flight. In addition, we expect that interdisciplinary, exploratory, and tentative work will soon be applied in other similar studies, which will allow us to develop a quantitative method that can be applied effectively in functional-morphological analyses.
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... Mei was first described on the basis of an exquisitelypreserved skeleton with a bird-like sleeping posture, which is arguably the most complete Early Cretaceous troodontid specimen known (Xu and Norell, 2004;Pan et al., 2013). Sinusonasus, Daliansaurus, and Liaoningvenator all have a similar size as Sinovenator, and each of them were reported from a single, near com- Hou et al., 1995Hou et al., , 1996Hou et al., , 1997bHou et al., , 1999aHou et al., , 1999bHou et al., , 2002Hou et al., , 2004Hou, 1996Hou, , 1997bChiappe et al., 1999Chiappe et al., , 2007Chiappe et al., , 2014Chiappe et al., , 2019bJi et al., 1999Ji et al., , 2002aJi et al., , 2002bZhang et al., 2006Zhang et al., , 2009Zhou and Zhang, 2005, 2006a, 2006bGao et al., 2008Gao et al., , 2012O'Connor et al., 2009O'Connor et al., , 2011aO'Connor et al., , 2013O'Connor et al., , 2016cWang et al., 2013dWang et al., , 2013e, 2019c;Zheng et al., 2007Zheng et al., , 2013Zheng et al., , 2014 Dames, 1884;Heller, 1959;Wellnhofer, 1974Wellnhofer, , 1988Wellnhofer, , 1993Wellnhofer, , 2009Mayr et al., 2005;Wellnhofer and Marsh, 1872Marsh, , 1877Marsh, , 1880Martin and Tate, 1976;Martin, 1984;Clarke, 2004;Bell and Chiappe, 2015;Field et al., 2018b Belly River Group Case et al., 2007;Turner et al., 2012;Ely and Case, 2019;Cordes-Person, 2020 ...
The Fossil Record of Mesozoic and Paleocene Pennaraptorans
Article
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An unabated surge of new and important discoveries continues to transform knowledge of pen-naraptoran biology and evolution amassed over the last 150+ years. This chapter summarizes progress made thus far in sampling the pennaraptoran fossil record of the Mesozoic and Paleocene and proposes priority areas of attention moving forward. Oviraptorosaurians are bizarre, nonparavian pennaraptorans first discovered in North America and Mongolia within Late Cretaceous rocks in the early 20th century. We now know that oviraptorosaurians also occupied the Early Cretaceous and their unquestionable fossil record is currently limited to Laurasia. Early Cretaceous material from China preserves feathers and other soft tissues and ingested remains including gastroliths and other stomach contents, while brooding specimens and age-structured, single-species accumulations from China and Mongolia provide spectacular behavioral insights. Less specialized early oviraptorosaurians like Incisivosaurus and Microvenator remain rare, and ancestral forms expected in the Late Jurassic are yet to be discovered, although some authors have suggested Epidexipteryx and possibly other scansoriopterygids may represent early-diverging oviraptorosaurians. Long-armed scansoriopterygids from the Middle-Late Jurassic of Laurasia are either early-diverging oviraptorosaurians or paravians, and some have considered them to be early-diverging avialans. Known from five (or possibly six) feathered specimens from China, only two mature individuals exist, representing these taxa. These taxa, Yi and Ambopteryx, preserve stylopod-supported wing membranes that are the only known alternative to the feathered, muscular wings that had been exclusively associated with dinosaurian flight. Thus, scansoriopterygid specimens-particularly those preserving soft tissue-remain a key priority for future specimen collection. Dromaeosaurids and troodontids were first discovered in North America and Mongolia in Late Cretaceous rocks. More recent discoveries show that these animals originated in the Late Jurassic, were strikingly feathered, lived across diverse climes and environments, and at least in the case of dromaeosaurids, attained a global distribution and the potential for aerial locomotion at small size.
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... Specimens range considerably in size although no clear juveniles are known (humerus length ranging from 41.01 to 78.5 mm; Chiappe et al., 1999Chiappe et al., , 2008Zhang et al., 2009;Wang and O'Connor, 2017;Wang et al., 2019c). In all reported specimens of the basal pygostylian Confuciusornis the scapulocoracoid is described as fused (Hou, 1997;Chiappe et al., 1999;Ji et al., 1999;Hou et al., 2002;Dalsätt et al., 2006;Zhang et al., 2009;Wang et al., 2019c), in contrast to the condition in other non-ornithothoracine avians. In the subadult holotype of Eoconfuciusornis zhengi IVPP V11977, the oldest and basal-most member of Confuciusornithidae from the Huajiying Formation, the scapula and coracoid are reportedly only sutured (Zhang et al., 2008a). ...
Osteohistology of the Scapulocoracoid of Confuciusornis and Preliminary Analysis of the Shoulder Joint in Aves
Article
Full-text available
  • Apr 2021
As key components of the tetrapod pectoral girdle, the scapula and coracoid have played a significant role in the evolution of forelimb locomotion among terrestrial vertebrates. The transition from a rigid fused scapulocoracoid in ancestral non-avian theropods to a presumably more flexible separated scapula-coracoid in early birds is considered to be one of the key morphological transitions related to the rapid refinement of flight. In most Mesozoic birds (e.g., Enantiornithes and Ornithuromorpha) and crown birds the scapula and coracoid are separate (unfused), with few exceptions (e.g., flightless paleognaths). In contrast, in Confuciusornis, a basal pygostylian from the Early Cretaceous Jehol Biota known from thousands of specimens, the scapula and coracoid remain plesiomorphically fused. This raises questions regarding the influence of shoulder girdle architecture on the early evolution and refinement of avian flight. The paravian scapula-coracoid joint has never previously been investigated using histology, and thus joint morphology has only been inferred superficially. In order to better understand the evolution of this joint in Mesozoic birds, we make the first histological study of the scapulocoracoid glenoid joint in Confuciusornis. The results demonstrate that the scapula and coracoid both consist of cancellous and compact bone, with both fibrolamellar and parallel-fibered structure. A thin layer of calcified cartilage is present on the glenoid fossa surface, representing remnants of the articular surface for the humerus. Both histology and computed tomography reveal that the scapulocoracoid of Confuciusornis is fully fused, forming a synostosis. Humeral histology suggests the studied individual was nearing completion of its first year of growth, suggesting the Confuciusornis scapulocoracoid fused before skeletal maturity was achieved, as in flightless paleognaths, whereas in the plesiomorphic condition fusion occurs late in ontogeny. We hypothesize the fused scapulocoracoid of Confuciusornis is secondarily evolved and suggest the primary factor responsible for this morphology may have been a decrease in mechanical stimulation at the glenoid of Confuciusornis relative to other volant birds, linked to the unique flight style of this taxon. Further investigation into the histology of the glenoid joint in other Mesozoic paravians and extant birds will help to clarify the morphological transition of the scapula-coracoid joint in early avian evolution.
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... They were divided into 3 genera and 4 species: Confuciusornis sanctus, C. dui, Changchengornis hengdaoziensis, and Eoconfuciusornis zhengi [1][2][3][4][5][6]. As an important group of basal birds, it has experienced tremendous development both in quantity and evolution during Early Cretaceous, and has become the focus of paleornithology and paleoenvironment [2][3][4][5][6][7][8][9]. Now it was studied more detailed than many other fossil birds by reasons of its important phylogenetic position, abundant specimens, and exquisite preservation [6,[10][11][12][13][14][15][16]. ...
THE ENVIRONMENTAL BACKGROUND, RESEARCH STATUS AND PROSPECTS ON FLIGHT OF EARLY CRETACEOUS CONFUCIUSORNITHID BIRDS IN NORTH CHINA AND ITS ADJACENT REGIONS
Article
Full-text available
We investigated numerous related literatures of confuciusornithid birds and summarized in terms of environmental background, research status (includ-ing morphology, evolutionary characteristics, and flight capability) and prospects. It is expected to generate a new idea and perspective for the study of Early Cretaceous paleontology and paleoenviron-mentology. Confuciusornithidae is an important group of fossil birds, which has developed tremendously in north China and other east Asian regions during Early Cretaceous. They are studied deeper than many other birds, because of the significance of evolutionary status and many exquisite specimens unmatched by other birds in the same period. And confuciusornithid birds are also the focus of pale-ornithology and paleoenvironment in the reason of their coevolutionary relation. By now, the flight capability of Confuciusornithidae has not been proved, though it was one group of fossil birds with large feathers. There are also no extant birds with aerodynamic feathers on their hindlimbs. Therefore, the flight capability has not been knowed by accurate comparsion at present; the previous conclusions came from morphological analogy and functional speculation; and the research on the origin and evolution of bird flight is also limited by that. Fortunately , hindlimb feathers with aerodynamic characteristics have been found on the Deinonychus and other early birds in the past two decades, which strongly supports hindlimbs are important in the origin of birds flight. And the hindlimbs of birds play an irreplaceable role in aerial and ground locomotion. As a consequence, the hindlimbs aerodynamic performance of confuciusornithid birds will be a breakthrough in the future research. In addation, the proceeding of the digital revolution pleases us immensely that the advancements of computer technologies make the quantitative studies of birds flight more accessible. In the long run, the research of the life habits of Confuciusornithidae can provide valuable data for the coevolution of paleornithology and paleoenvironment in Early Cretaceous.
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... Mei was first described on the basis of an exquisitelypreserved skeleton with a bird-like sleeping posture, which is arguably the most complete Early Cretaceous troodontid specimen known (Xu and Norell, 2004;Pan et al., 2013). Sinusonasus, Daliansaurus, and Liaoningvenator all have a similar size as Sinovenator, and each of them were reported from a single, near com- Hou et al., 1995Hou et al., , 1996Hou et al., , 1997bHou et al., , 1999aHou et al., , 1999bHou et al., , 2004Hou, 1996Hou, , 1997bChiappe et al., 1999Chiappe et al., , 2007Chiappe et al., , 2014Chiappe et al., , 2019bJi et al., 1999Ji et al., , 2002aJi et al., , 2002bZhang et al., 2006Zhang et al., , 2009Zhou and Zhang, 2005, 2006bGao et al., 2008Gao et al., , 2012O'Connor et al., 2009O'Connor et al., , 2011aO'Connor et al., , 2013O'Connor et al., , 2016cWang et al., 2013dWang et al., , 2013e, 2019c;Zheng et al., 2007Zheng et al., , 2013Zheng et al., , 2014 Dames, 1884;Heller, 1959;Wellnhofer, 1974Wellnhofer, , 1988Wellnhofer, , 1993Wellnhofer, , 2009Mayr et al., 2005;Wellnhofer and Marsh, 1872Marsh, , 1877Marsh, , 1880Martin and Tate, 1976;Martin, 1984;Clarke, 2004;Bell and Chiappe, 2015;Field et al., 2018b Belly River Group Case et al., 2007;Turner et al., 2012;Ely and Case, 2019;Cordes-Person, 2020 ...
Chapter 2: The Fossil Record of Mesozoic and Paleocene Pennaraptorans
Chapter
Full-text available
  • Aug 2020
An unabated surge of new and important discoveries continues to transform knowledge of pen-naraptoran biology and evolution amassed over the last 150+ years. This chapter summarizes progress made thus far in sampling the pennaraptoran fossil record of the Mesozoic and Paleocene and proposes priority areas of attention moving forward. Oviraptorosaurians are bizarre, nonparavian pennaraptorans first discovered in North America and Mongolia within Late Cretaceous rocks in the early 20th century. We now know that oviraptorosaurians also occupied the Early Cretaceous and their unquestionable fossil record is currently limited to Laurasia. Early Cretaceous material from China preserves feathers and other soft tissues and ingested remains including gastroliths and other stomach contents, while brooding specimens and age-structured, single-species accumulations from China and Mongolia provide spectacular behavioral insights. Less specialized early oviraptorosaurians like Incisivosaurus and Microvenator remain rare, and ancestral forms expected in the Late Jurassic are yet to be discovered, although some authors have suggested Epidexipteryx and possibly other scansoriopterygids may represent early-diverging oviraptorosaurians. Long-armed scansoriopterygids from the Middle-Late Jurassic of Laurasia are either early-diverging oviraptorosaurians or paravians, and some have considered them to be early-diverging avialans. Known from five (or possibly six) feathered specimens from China, only two mature individuals exist, representing these taxa. These taxa, Yi and Ambopteryx, preserve stylopod-supported wing membranes that are the only known alternative to the feathered, muscular wings that had been exclusively associated with dinosaurian flight. Thus, scansoriopterygid specimens-particularly those preserving soft tissue-remain a key priority for future specimen collection. Dromaeosaurids and troodontids were first discovered in North America and Mongolia in Late Cretaceous rocks. More recent discoveries show that these animals originated in the Late Jurassic, were strikingly feathered, lived across diverse climes and environments, and at least in the case of dromaeosaurids, attained a global distribution and the potential for aerial locomotion at small size.
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... Confuciusornithidae: All known confuciusornithids are completely edentulous and show clear osteological correlations of rham-photheca, and CT images have revealed no sign of a vestigial tooth or alveolus in Confuciusornis (Ji et al., 1999;Zhang et al., 2008b;Wang et al., 2017a;Wang and Zhou, 2018). ...
Tooth vs. Beak: The Evolutionary Developmental Control of the Avian Feeding Apparatus
Article
Full-text available
Recent discoveries of exquisitely preserved nonavialan and avialan theropod dinosaurs have not only prompted studies of theropod tooth morphologies, but have also provided information about the origin and early evolution of avian beaks. Recent studies on beak morphologies and morpho-genesis in Darwin's finches have greatly improved our understanding of how avian beaks adapt to various ecological niches, but the question of how birds lost their teeth during the course of evolution has long been debated. Evolutionary developmental experiments performed on extant bird embryos bridge the gap between paleontological and neontological evidence, suggesting that the avian beak could have originated through heterochronic truncation of odontogenesis over evolutionary time. Here, we systematically review independently evolved regional and complete edentulism present in nonavialan and avialan theropod dinosaurs, and suggest that the tooth-reduction processes of different jaw bones are likely to be independently controlled. Through reviewing the recent advances of molecular regulations involved in tooth and avian beak morphogenic processes, we suggest that several molecules regulating the development of the avian beak also mediate the growth of keratinous rhamphothecae, and the divergence of odontogenic signalling pathways are likely to have accounted for both of these processes.
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... However, an unexpected feather morphotype reported in several lineages of nonavian and avian theropods (Zhang et al. 2008a;Zhang et al. 2008b;Ji et al. 1999;Zhang and Zhou 2000;O'Connor et al. 2012;Wang et al. 2014), known as rachisdominated feathers (RDFs), is characterized by an unusually wide and flattened rachis with a pair of lateral light stripes delimiting a darker medial stripe (O'Connor et al. 2012;Wang et al. 2014). The rachis of RDFs can be bordered laterally by pennaceous vanes that either extend throughout the length of the feather (e.g., Eopengornis [Wang et al. 2014]), are restricted to the distal end (e.g., Confuciusornis [Hou et al. 1995]), or are devoid of vanes (e.g., Epidexipteryx [Zhang et al. 2008b]). ...
Variations of Mesozoic Feathers: Insights from the Morphogenesis of Extant Feather Rachises
Article
Full-text available
The rachises of extant feathers, composed of dense cortex and spongy internal medulla, are flexible and light, yet stiff enough to withstand the load required for flight, among other functions. Incomplete knowledge of early feathers prevents a full understanding of how cylindrical rachises have evolved. Bizarre feathers with unusually wide and flattened rachises, known as “rachis‐dominated feathers” (RDFs) have been observed in fossil non‐avian and avian theropods. Newly discovered RDFs embedded in early Late Cretaceous Burmese ambers (∼99 Ma) suggest the unusually wide and flattened rachises mainly consist of a dorsal cortex, lacking a medulla and a ventral cortex. Coupled with findings on extant feather morphogenesis, known fossil RDFs were categorized into three morphotypes based on their rachidial configurations. For each morphotype, potential developmental scenarios were depicted by referring to the rachidial development in chickens, and relative stiffness of each morphotype was estimated through functional simulations. The results suggest rachises of RDFs are developmentally equivalent to a variety of immature stages of cylindrical rachises. Similar rachidial morphotypes documented in extant penguins suggest that the RDFs are not unique to Mesozoic theropods, though they are likely to have evolved independently in extant penguins. This article is protected by copyright. All rights reserved
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... Geometric Morphometric (GM) often utilizes as a significant mechanism to understand shape variations. It pertains to the quantitative analysis of form, a concept which includes size and shape [3] . GM, introduced new techniques on how to accomplish, present, classify and evaluate morphological parts of species. ...
Analysis of body shape variation in Glossogobius giuris (Hamilton 1882) sampled from Lake Mainit, Philippines, using geometric morphometrics
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Fluctuating Asymmetry (FA) is a tool to assess morphological differences from different bioindicator species. This study was conducted to evaluate the body shape variation of Glossogobius giuris sampled from Lake Mainit, Philippines, using geometric morphometrics. A total of 60 specimens was collected with 30 each sex. Landmarking techniques were established and applied to the fish morphology. Samples were subjected to tps Util and Symmetry and Asymmetry Geometric Data (SAGE) software to identify the body shape analysis. Among the factors analyzed, Procrustes ANOVA results shown a highly significant difference (P<0.0001) in the female and male samples and thus, indicating a shape differences between the sexes. While Principal Component Analysis (PCA) displayed a higher percentage in female samples (83.1614%) than of the male (79.3984%) denoting body shape variations between the populations indeed, females are highly inclined to perturbations since they need a buffer from various environmental requirements and primarily through the reproduction process. Thus utilizing geometric morphometric enables to determine morphological variations of species among and within the taxa.
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Anatomy and Systematics of the Confuciusornithidae (Theropoda: Aves) from the Late Mesozoic of Northeastern China
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  • Jul 2000
The following critiques express the opinions of the individual evaluators regarding the strengths, weaknesses, and value of the books they review. As such, the appraisals are subjective assessments and do not necessarily reflect the opinions of the editors or any official policy of the American Ornithologists' Union.
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Skulls of Gobipteryx (Aves) from the Upper Cretaceous of Mongolia
Article
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  • Jan 1977
Skulls of Gobipteryx (Aves) from the Upper Cretaceous of Mongolia Andrzej Elzanowski Palaeontologia Polonica 37 (1977), 153-165 Following a preliminary report (Elzanowski 1974), the holotype skull of Gobipteryx minuta is redescribed and another cranial specimen described for the first time. The second specimen confirms the palaeognathous pattern of the palate and (upper) jaw as reconstructed from the poorly preserved palate of the holotype specimen. The avian identity of Gobipteryx is reaffirmed despite claims to the contrary (Brokorb 1976).
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The First 85 million years of Avian Evolution
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More than half of the evolutionary history of birds is played out in the Mesozoic. A recent burst of fossil discoveries has documented a tremendous diversity of early avians. Clarification of the Phylogenetic structure of this diversity has provided clues for a better understanding of the evolution of functional, developmental and physiological characteristics of modern birds. Yet their long Mesozoic history is only beginning to be deciphered.
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The origin and early radiation of birds
Chapter
  • Aug 1983
First published in 1983 to celebrate the centennial of the American Ornithologists' Union, Perspectives in Ornithology collects together a series of essays and commentaries by leading authorities about especially active areas of research on the biology of birds. Readers will find in this collection a useful overview of many major concepts and controversies in ornithology.
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The origin and early diversification of birds
Article
  • Jan 1986
Numerical cladistic analysis of 73 cranial and postcranial characters has resulted in a highly corroborated hypothesis describing the phylogenetic pattern of early avian evolution. Using “non-avian theropod” dinosaurs as a comparative outgroup and root for the tree, the analysis confirmed Archaeopteryx to be the sister-group of all remaining avian taxa, or Ornithurae. This latter taxon is subdivided into two lineages, the Hesperornithiformes and the Carinatae. The carinates, in turn, were also resolved into two sister-groups, the Ichthyornithiformes and the modern birds, or Neornithes. This paper provides morphological data corroborating the divergence of the two basal clades of the Neornithes: the Palaeognathae (tinamous and ratites) and Neognathae (all other modern birds). The phylogenetic relationships of four important Cretaceous taxa were also investigated, but these fossil taxa were too fragmentary to determine their phylogenetic position unambiguously. Alexornis and Ambiortus are both carinates, but their relationships cannot be resolved in greater detail. The relationships of the Enantiornithes may lie within the Carinatae or these two taxa may be sister-groups. Gobipteryx is a neornithine and possibly the sister-group of the Palaeognathae. This analysis indicates that major patterns of morphological change took place at the time of origin of the ancestors of the Ornithurae and the Carinatae. Ornithurine innovations included major changes throughout the skeleton, whereas those of the carinates, while substantial, were primarily restricted to the pectoral girdle and forelimb. The phylogenetic results, in conjunction with the known ages of fossil taxa, indicate that the early lineages of birds very likely arose in the Jurassic. The early cladistic events within the neornithine lineage are also more ancient than generally recognized, and may well extend back to the early Cretaceous.
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A new and primitive bird from the Early Cretaceous of China
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A new Early Cretaceous enantiornithine bird, Eocathayornis walkeri, gen. et sp. nov. is reported from Liaoning, northeast China. It is about the size of Cathayornis but is more primitive. Teeth are present on the jaws. Claws are retained on three wing digits, but that of the minor digit is reduced. The width of the radius is nearly three-fourths that of the ulna. The sternum is relatively short, with a pair of long caudo-lateral processes and a low and caudally distributed keel. The coracoid is strut-like and caudally concave, typical of enantiornithine birds. The advanced features of the scapula and the wing suggest a powerful flapping flight capability. This bird is referred to the family Cathayornidae based on a few shared derived characters with Cathayornis.
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The Evolution of the Antorbital Cavity of Archosaurs: A Study in Soft-Tissue Reconstruction in the Fossil Record with an Analysis of the Function of Pneumaticity
Article
The most commonly cited apomorphy of Archosauriformes is an opening in the snout known as the antorbital cavity. Despite the ubiquity and prominence of the antorbital cavity, its function and importance in craniofacial evolution have been problematic. Discovering the significance of the antorbital cavity is a two step process: first, establishing the function of the bony cavity (that is, its soft-tissue relations), and second, determining the biological role of the enclosed structure. The first step is the most fundamental, and hence is examined at length. Three hypotheses for the function of the antorbital cavity have been advanced, suggesting that it housed (1) a gland, (2) a muscle, or (3) a paranasal air sinus. Thus, resolution is correctly viewed as a “soft-tissue problem,” and is addressed within the context of the extant phylogenetic bracket (EPB) approach for reconstructing the unpreserved features of fossil organisms. The soft-anatomical relations of the antorbital cavity (or any bony structure) are important because (1) soft tissues generally have morphogenetic primacy over bony tissues and (2) inferences about soft tissues are the foundation for a cascading suite of paleobiological inferences. The EPB approach uses the shared causal associations between soft tissues and their osteological correlates (i.e., the signatures imparted to the bones by the soft tissues) that are observed in the extant outgroups of the fossil taxon of interest to infer the soft-anatomical attributes of the fossil; based on the assessment at the outgroup node, a hierarchy characterizing the strength of the inference can be constructed. This general approach is applied to the problem of the function of the antorbital cavity, taking each hypothesized soft-tissue candidate—gland, muscle, and air sac—in turn, (1) establishing the osteological correlates of each soft-tissue system in the EPB of any fossil archosaur (i.e., extant birds and crocodilians), (2) formulating a hypothesis of homology based on similarities in these causal associations between birds and crocodilians, (3) testing this hypothesis by surveying fossil archosaurs for the specified osteological correlates, and (4) accepting or rejecting the hypothesis based on its phylogenetic congruence. Using this approach, fossil archosaurs can be reliably reconstructed with a Glandula nasalis, M. pterygoideus, pars dorsalis, and Sinus antorbitalis that are homologous with those of extant archosaurs; however, the osteological correlates of only the antorbital paranasal air sinus involve the several structures associated with the antorbital cavity. Additional evidence for the pneumatic nature of the antorbital cavity comes from the presence of numerous accessory cavities (especially in theropod dinosaurs) surrounding the main antorbital cavity. To address the origin of the antorbital cavity, the EPB approach was applied to basal archosauriforms; the data are not as robust, but nevertheless suggest that the cavity appeared as a housing for a paranasal air sinus. The second step in discovering the evolutionary significance of the antorbital cavity is to assess the function of the enclosed paranasal air sac. In fact, the function of all pneumaticity is investigated here. Rather than the enclosed volume of air (i.e., the empty space) being functionally important, better explanations result by focusing on the pneumatic epithelial diverticulum itself. It is proposed here that the function of the epithelial air sac is simply to pneumatize bone in an opportunistic manner within the constraints of a particular biomechanical loading regime. Trends in facial evolution in three clades of archosaurs (crocodylomorphs, ornithopod dinosaurs, and theropod dinosaurs) were examined in light of this new perspective. Crocodylomorphs and ornithopods both show trends for reduction and enclosure of the antorbital cavity (but for different reasons), whereas theropods show a trend for relatively poorly constrained expansion. These findings are consistent with the view of air sacs as opportunistic pneumatizing machines, with weight reduction and design optimality as secondary effects.
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