Barsbold, R. et al. Science 346, 1253293 (2014). & Zhang, F. Largest bird from the Early Cretaceous and its implications for the earliest avian ecological diversification. To investigate how pygostyle relative size evolved throughout the avian lineage, the mean pygostyle length/femur length was calculated for the seven extinct and extant clades; note that stem Ornithuromorpha is paraphyletic. 45131 (Harvard Univ Nuttall Ornithological, 1993). . 16, all the characters equally weighted, and 35 characters ordered (Supplementary Notes). They are bowed slightly laterally and cranially as in Confuciusornis20 and Sapeornis30 and bear a short neck that separates a round head from the trochanteric region, as in Confuciusornis20. Species. https://doi.org/10.6084/m9.figshare.9869537, https://doi.org/10.19615/j.cnki.1000-3118.180530, https://doi.org/10.1093/zoolinnean/zly045/5066665, http://creativecommons.org/licenses/by/4.0/, Case study of the convergent evolution in the color patterns in the freshwater bivalves, A new confuciusornithid bird with a secondary epiphyseal ossification reveals phylogenetic changes in confuciusornithid flight mode, Distal spinal nerve development and divergence of avian groups, Cancel 6i). Huxley, T. H. Further Evidence of the Affinity between the Dinosaurian Reptiles and Birds. ac Skeletal reconstructions of FPDM-V-9769 in cranial (a), dorsal (b), and left-lateral (c) views. Internet Explorer). Nature 403, 155156 (2000). Forelimb elements. volume8, Articlenumber:9014 (2018) In modern birds, the rectrices attach to these. NGMC is the abbreviation for the National Geological Museum of China. Article The total evidence from Cretaceous pygostylians strongly suggests late stage formation of the pygostyle was common, similar to extant birds. We employed a non-destructive anatomical analysis with a phase-contrast synchrotron micro-CT technique. Unfused juvenile pygostyles in extant and Cretaceous birds. Four-winged dinosaurs from China. 5c). 6b; Supplementary Table1), a condition shared with Archaeopteryx and confuciusornithids, but different from most volant birds24. Lee, M. S. & Worthy, T. H. Likelihood reinstates Archaeopteryx as a primitive bird. Dinosaur Mus. & Sullivan, C. A bizarre Jurassic maniraptoran from China with elongate ribbon-like feathers. Xu, X. et al. Except for the Late Jurassic Archaeopteryx, non-ornithothoracine birds had previously been known only from the Jehol Biota and contemporary deposits in northern Korean Peninsula. Zool. Overall, this study demonstrates that further exploratory efforts in the Early Cretaceous sediments, where the occurrence of avian skeletons is limited to none, are important to increase our understanding of the Early Cretaceous birds. The rendered three-dimensional model for FPDM-V-9769 is available at https://doi.org/10.6084/m9.figshare.9869537. Clark, C. J. This discovery provides important insights into how birds refined their early flight capabilities before the appearance of the keeled sternum, pygostyle and triosseal canal. The furcula is robust and boomerang-shaped (Fig. By five months of age, the core pygostyle vertebrae are fully fused. Cranial and selected axial elements except caudal vertebrae. Commun. Frozen emu carcasses at 3.5 and 5.5 months post-hatch were obtained from the Montana Emu Farm (Kalispell, Montana). Evidence for post hatch pygostyle fusion is found in subadult Sapeornithiformes specimens28. Simplified strict consensus tree of 1264 most parsimonious trees with a length of 1323, resulting from analyses with an addition of FPDM-V-9769 to a matrix on fossil birds16. MOR is the abbreviation for the Museum of the Rockies. Next, the mean pygostyle length/femur length was calculated for the three major extant clades. The ulna is slender, weakly bowed caudally, and slightly shorter than the humerus (Fig. Dana J. Rashid. A box indicates where a sample was taken for osteohistological analyses. Zhongornis haoae, from the Yixian Jehol Biota, dating to 125Ma, was postulated to be a short-tailed bird lacking a pygostyle14. Get the most important science stories of the day, free in your inbox. Acta Geologica Sinica - English Edition 80, 631635 (2006). A shortened tail is advantageous for flight, as evidenced by reduced tails in other flying vertebrates such as pterosaurs and bats, and by analyses indicating shorter tails are more aerodynamically favorable11. 2G, white arrowheads). 7a). Five to seven micron sections were cut using a Jung RM2035 microtome, and transferred to glass slides. 108 (degree of ossification of the sternum), Char. The sections were transferred to a concave glass slide, and von Kossa stained according to the kit instructions (Silver staining kit acc. Both are . Am. Abbreviations: cev cervical vertebra, co coracoid, dv dorsal vertebra, fe femur, fu furcula, hu humerus, il ilium, mc metacarpal, md manual digit, py pygostyle, ra radius, sv sacral vertebra, ti tibia, ul ulna. Basal pygostylians, including the Confuciusornithidae11,12, Jinguofortisidae13,14, and Sapeornithidae15,16, exhibit pectoral and forelimb features that are less comparable to modern birds than those in the more derived clade Ornithothoraces. Osteohistological sections. PubMed (Cambridge University Press, 2007). Pygopus is a genus belonging to the family of Australian legless lizards (Pygopodidae).Members of this genus are also commonly called scaly-foot.. Article 149, 97116 (2007). Abbreviations: ivf intervertebral foramen, ns neural spine, ps paddle-like structure, tp transverse process, vf vertebral foramen. Juvenile confuciusornithiform specimens have not been collected, so pygostyle fusion at younger stages cannot be assessed for this group. 2). All elements are in cranial, caudal, medial, and lateral views from left to right. Mus. Proceedings. The juvenile nighthawk appears to have additional separate ossified bones, akin to ribs, that fuse into the parapophyses (Fig. Chiappe, L. M. Anatomy and Systematics of the Confuciusornithidae (Theropoda: Aves) from the late Mesozoic of Northeastern China. The centra of the more proximal free caudals exhibit suboval articular surfaces in which the cranial articular surface is slightly concave while the caudal articular surface is flat. 2J,K). Lefvre, U., Hu, D., Escuilli, F., Dyke, G. & Godefroit, P. A new long-tailed basal bird from the Lower Cretaceous of north-eastern China. Transverse processes are observed as dorsolateral projections in free caudal and pygostyle vertebrae (Fig. These specimens, as well as avian skeletal specimens from the Museum of the Rockies, were digitally photographed. Iberomesornis (Sanz et al. The interclavicular angle is estimated to be 70, which is most comparable to that of Chongmingia13. One such feature that is often ignored but paradoxically is critical to avian evolution is the tail. The data suggests that extension of the caudal transverse processes and parapophyses themselves occurs via epiphyseal plate-mediated ossification, which is likely a universal avian (if not vertebrate) trait, but further extension can be achieved by incorporating additional bony elements. The fibular crest extends 1/3 of the length, and a single cnemial crest is present. The proximal and distal ends of this phalanx are expanded dorsoventrally, and the ventral surface is flat to slightly concave. For this study, a transverse process is defined as a lateral process derived from the neural arch21. & Norell, M. A. Fusion between the twentieth and . 6g). Modern birds have extremely short tail skeletons relative to Archaeopteryx and nonavialian theropod dinosaurs. Communications Biology (Commun Biol) Ostrom, J. H. Archaeopteryx and the origin of birds. 1); the Lower Cretaceous Kitadani Formation (Aptian)23 (seeSupplementary Notes for further details). The anterior three sacral vertebrae have wide transverse processes that join to form a lateral bar that abuts the anterior ilium (Fig. The elongated pygostyle is also characteristic of more typical enantiornithine birds, but is not found in Mesozoic Ornithurines except for Baptornis ( Martin and Tate, 1976 ). Department of Cell Biology and Neuroscience, Montana State University, Bozeman, MT, 59717, USA, Honors College, Montana State University, Bozeman, MT, 59717, USA, Dinosaur Institute, Los Angeles County Museum of Natural History, Los Angeles, CA, 90007, USA, Section of Ornithology, Los Angeles County Museum of Natural History, Los Angeles, CA, 90007, USA, Keck School of Medicine, University of Southern California, Los Angeles, CA, 90033, USA, Department of Pathology and Anatomy, University of Missouri, Columbia, MO, 65211, USA, Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, 10010, China, Department of Biological Sciences, Clemson University, Clemson, SC, 29634, USA, Chapman University, Orange, CA, 92866, USA, You can also search for this author in Biol. However, the distribution of the vascular canals decreases in density toward the periosteal region, implying that the growth rate was declining prior to death. 3c). The Early Cretaceous basal birds were known largely from just two-dimensionally preserved specimens from north-eastern China (Jehol Biota), which has hindered our understanding of the early evolution of birds. Avian tail ontogeny, pygostyle formation, and interpretation of juvenile Mesozoic specimens. Chiappe, L. M., Shuan, J., Qiang, J. https://doi.org/10.1038/s41598-018-27336-x, DOI: https://doi.org/10.1038/s41598-018-27336-x. Commun Biol 2, 399 (2019). Nature 475, 465470 (2011). The specimen was interpreted as non-avian, based on feather morphology, its age (99Ma), the absence of vertebral processes, and the concave ventral furrow in the vertebrae, not seen in the long-tailed birds Archaeopteryx and Jeholornis. Requests for materials can be directed to the corresponding author. At eight days post-hatching, all four core pygostyle vertebrae are separate and distinct. Morphologically, the furcula of F. prima is most similar to that of Confuciusornis20 in bearing a caudal projection with a rounded area at each omal end. In extant birds, this is typically associated with a fleshy structure called the rectricial bulb that secures the tail feathers (rectrices) [1]. 3D,E). Notably, the ossification front between the centrum and the tips presents as an epiphyseal plate, with the distal cartilage serving as the zone of reserve or resting cartilage. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. and Z.Z. (C) Adult Wilsons snipe distal tail. Presumably, these vertebrae represent cranial ones. The yellow arrow in this and subsequent panels indicates the spinal cord channel. 51, 625639 (2008). Turner, A. H., Makovicky, P. J. The furcula is in contrast to those of Ornithothoraces; ornithothoracine furcula is deep-U-shaped or V-shaped and slender. The midshaft of the radius is nearly as robust as that of the ulna, in contrast to other non-ornithothoracines in which the ulna is substantially stouter15,35. Xu, X. et al. b Schematic line drawing of the skeletons. The search for potential intermediate species has been extensive, and one possible candidate was identified. 6e), but not to the extent seen in Jeholornis and Chongmingia13. Serrano, F. J., Palmqvist, P. & Sanz, J. L. Multivariate analysis of neognath skeletal measurements: implications for body mass estimation in Mesozoic birds. Zhou, Z. The pygostyle is proportionately robust and subequal in length to the tarsometatarsus as in most. Archiv F. Zool. There are eight free caudal vertebrae and an extremely large pygostyle, estimated to be composed of 10 to 15 fused vertebrae (Sanz and Bonaparte 1988, 1992). 6a). For the bird skin specimens, extraneous surrounding tissue was pared away digitally, to reveal tail vertebrae anatomy. Mohiuddin, Y. S. Intervertebral disc development in chickens. Based on the preservation, the ilium (and the rest of the pelvic girdle) of FPDM-V-9769 is presumably unfused to each other nor are they fused to the synsacrum, as in other Early Cretaceous birds and most non-avian theropods38. The lack of vertebral processes in the amber-embedded specimen may be due to its juvenile state, as seen in the unmineralized processes in the juvenile extant bird tails analyzed in this study. Unlike most other Early Cretaceous birds, the specimen is three-dimensionally preserved, and exhibits several autapomorphies, leading to erect a new taxon, Fukuipteryx prima gen. et sp. For emu specimens, we heartily thank Don Collins at the Montana Emu Farm in Kalispell, MT for his donation of frozen carcasses, and Yellowstone Wild Game Processing in Bozeman, MT for processing those carcasses. This creature was named Archaeopteryx lithographica: ancient wing of the Lithographic limestone. Palasiat. These cartilages, while fiber-rich, are not to be confused with the fibrocartilage of the ligaments that attach to the tips. 55, 126 (2017). 5a, b)13. Gao, C. et al. designed the project and performed the research, T.I., S.K. A pygostyle from a non-avian theropod. It is unknown whether the deltopectoral crest is perforated as in confuciusornithids and Sapeornis. Guo, X., Li, X. This palaeogeographic bias hinders not only our understanding of the global distribution of these most-basal clades but also of the evolution of flight-related features in different environments during this critical temporal span. LACM is the abbreviation for the Los Angeles County Museum of Natural History, followed by the specimen number. Bareggi, R., Grill, V., Zweyer, M., Narducci, P. & Forabosco, A. Thus, such features may have already been present in basalmost pygostylians as suggested in that of F. prima. In addition, the postzygapophyses are craniocaudally longer than the associated centrum. Nat. Further exploration of the Early Cretaceous fossil birds outside East Asia are greatly needed to clarify the palaeogeographical distribution of these basal birds. Since pygostyle fusion in juveniles is a regular feature of extant birds, we sought to determine whether the same phenomenon occurred in Cretaceous avians. To obtain This specialized tail is present throughout the entire diversity of living birds, albeit with many modifications for clade-specific behaviors. Although skeletally immature, FPDM-V-9769 exhibits several skeletal morphologies that cannot be exclusively ascribed to ontogenetic variation. Imai, T., Azuma, Y., Kawabe, S. et al. Like modern birds, it had: Feathered wings and a feathered tail Backwards-pointing pubis Backwards-pointing pedal digit I, located at the bottom of the metatarsus BUT, it was very primitive compared to other birds in that it had: While the surangular is incomplete and it is difficult to determine the rostral extent of the foramen, its position and circular shape are reminiscent of the caudal mandibular fenestra present in confuciusornithids11,24,25,26 as well as in paravian Anchiornis27 (described as a surangular foramen). Alizarin red is a calcium-sensitive dye that stains mineralized tissue red, and von Kossa is a silver-based stain that is phosphate sensitive, and stains mineralized tissue black. 5 shows the mean total pygostyle length/femur length of specimens within each bird group with indicated error bars (the bar graph and the error bars were created using Microsoft Excel). These data call for a reinterpretation of specimens such as Zhongornis14, a juvenile proposed to be an intermediate in the long- to short-tailed avian transition, and the recent discovery of the coelurosaur tail embedded in amber15. J. M. Yoshida at Kobe Design University created skeletal reconstruction and life restoration of the specimen. CAS The dorsal centra are procoelous with subcircular to subrectangular cranial articular surfaces, and relatively flat caudal articular surface (Fig. Vertebr. An unusual bird (Theropoda, Avialae) from the Early Cretaceous of Japan suggests complex evolutionary history of basal birds, https://doi.org/10.1038/s42003-019-0639-4. 1a). These have previously been described as transverse processes22, but because they are not neural arch-derived, are termed here as parapophyses (Fig. These include: the U-shaped furcula without a hypocleidium as in Archaeopteryx, Jeholornis, confuciusornithids, and jinguofortithids (Fig. The oldest record of Ornithuromorpha from the Early Cretaceous of China. The Early Cretaceous Jeholorniformes is the next stemward avialan 9, 10 and lacks the pygostyle, a compound element formed by a series of fused and shortened distal caudal vertebrae. Higher magnification images were obtained with a Zeiss Axioscope A.1 in conjunction with a Jenoptik ProgRes C14 Plus digital camera and accompanying software. You are using a browser version with limited support for CSS. The unfused metacarpals and metatarsals may further indicate the immature state of FPDM-V-9769. Note that Fukuipteryx prima represents the only Early Cretaceous non-ornithothoracine avialan collected outside of the Jehol Group. The postacetabular wing is dorsoventrally low, very mediolaterally compressed, and craniocaudally long. The pygostyle of C. shifan appears to be exposed in left lateral view (Fig. USA 114, 1147011475 (2017). MicroCT scanning of a Confuciusornis sanctus pygostyle (NGMC 98-8-2;18) indicates that in this subadult, approximately eight vertebrae ankylosed into the pygostyle, evident by the number of unfused neural arch components (Fig. Assuming that vertebral length remained approximately constant across clades, a greater number of vertebrae incorporated into the pygostyle was therefore more likely in confuciusornithids and enantiornithines. The centra are craniocaudally longer than mediolaterally wide, and slightly dorsoventrally compressed caudally. In the extant birds analyzed in this study, the greatest number of pygostyle vertebrae was five, estimated for the Humboldt penguin (see Supplementary Fig. supervised the excavation and initial preparation of the specimen, and T.I., M.W. PubMed The analysis was performed with the default setting except for the following: maximum number for trees in memory=equal to 10,000. The First Mesozoic Heterodactyl Bird from China. In enantiornithines, unfused pygostyle vertebrae in juveniles is evident in IVPP 15564 (Fig. In modern . Blom, J. 1.5mm thick) with a razor blade. Maxwell, E. E. Comparative embryonic development of the skeleton of the domestic turkey Meleagris gallopavo and other galliform birds. Takuya Imai. The pygostyle is long, robust, and rod-shaped as in confuciusornithids and enantiornithine longipterygids33 (Supplementary Table1). These structures were not observed in other juveniles in this study (Fig. To verify unfused mineralized pygostyle vertebrae after hatching, juvenile chicken pygostyles were further analyzed by microCT scanning (Fig. To obtain 1988; Sanz and Bonaparte 1992) is in many ways intermediate between Archaeopteryx and birds with modern tail morphology [ILLUSTRATION FOR FIGURE 5 OMITTED]. 2E,F). 1C). The humeral shaft is bowed dorsally (Fig. Its designation as avian marks the somewhat contested emergence of true flight, but the increasing discoveries of additional paravians blurs that line. It is noted that these canals are also visible in CT images of other long bones, such as the humerus, and exhibit a similar condition (Fig. 8a). a Surangular in medial and lateral views. The error bars represent the upper 99% CI. Their cranial articular surface exhibits incipient heterocoelous condition, whereas the caudal articular surface appears slightly concave. photographed IVPP specimens and contributed to their analyses. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate. Thus, the current phylogenetic position of F. prima may indicate a more complex evolutionary pattern in the body plan of early birds, as demonstrated in another potentially-non-pygostylian bird Chongmingia13. The mineralization state of the vertebral processes notwithstanding, the size and shape of processes on pygostyle vertebrae vary significantly in both extant and Cretaceous birds28,29,30. Forster, C. A., Sampson, S. D., Chiappe, L. M. & Krause, D. W. The theropod ancestry of birds: new evidence from the Late Cretaceous of Madagascar. PubMed The bar graph in Fig. 40, 731747 (2003). Intervertebral foramina are recognizable as shallow depressions ventral to the neural spines. Correspondence to The osteocyte lacunae are concentrated in the inner region and semi-circular in shape (Fig. Gazi Medical Journal 6, 5558 (1995). 4c). a Left femur. This paddle-like structure is ridged along the midline, forming a triangular transverse section, and gradually becomes dorsoventrally thinner toward the distal end. A boomerang-shaped furcula is found in juvenile and subadult specimens of Sapeornis43,44, and the furcular shape does not seem to change dramatically during ontogeny in enantiornithines31. The major metacarpal (Fig. A Feathered Dinosaur Tail with Primitive Plumage Trapped in Mid-Cretaceous Amber. Reconciling slow growth in Archaeopteryx. Furthermore, as described above, F. prima exhibits numerous primitive features comparable to non-ornithothoracine birds but not to the more crownward taxa. 5 and in Supplementary Table1. The value of n (sample size) represents the number of specimens measured, noted in SupplementaryTable1. Due to the state of preservation, it is unclear whether uncinate processes are present during life. American Museum Novitates, 146 (2007). Google Scholar. Stained tissue slices were subsequently imaged with either a Unitron TCS tablet mounted on a Zeiss Stemi 200c dissecting microscope for lower resolution images, or a Zeiss Stemi SV11 stereoscope with a Jenoptik ProgRes C14 camera using associated software for higher resolution imaging. PubMed Gatesy, S. M. & Dial, K. P. From frond to fan: Archaeopteryx and the evolution of short-tailed birds. Rep. 8, 112 (2018). Relevant to Mesozoic specimens, the lack of a pygostyle in juveniles and a variable number of relatively featureless caudal vertebrae can blur the distinction of transitional species. Bremer values were calculated as the support indices. Also, the much reduced vertebral processes on juvenile caudal vertebrae can obscure the differentiation between taxa, as seen for the coelurosaur tail in amber. Zhou, Z. Elzanowski, A., Manegold, A. This specimen is reinterpreted here as possibly avian. 2D). Scale bars equal 5mm in ac, 2mm in d, and 5mm in e. The pygostyle is present and ends with a paddle-like structure (ps in Fig. For microCT scanning, skin and skeletal specimens were selected from the ornithology collections of the Los Angeles County Museum of Natural History. . Additional changes in the avian axial skeleton, including substantial fusion of sacral vertebrae and bone fusions in the extremities have persisted in modern birds, and are a testament to their adaptive advantages. 32, 11031112 (2012). For von Kossa staining, a juvenile chicken pygostyle dissected free of skin and surrounding tissue was sliced by hand in transverse sections (approx. In Ichthyornis. TWO fossils from northeast China are poised to knock Archaeopteryx, the earliest known bird, right off its perch. Results from these experiments reveal that the pygostyle, rectrices, rectricial bulbs, and bulbi rectricium musculature form a specialized fanning mechanism. It is slightly bowed medially, possibly due to post-depositional deformation. Archaeopteryx, a Jurassic dinosaur generally recognized as the first bird, exhibited a mosaic suite of features both avian- and non-avian-like 4 . Zhou, Z. Introduction Modern. Fusion of chicken pygostyle vertebrae therefore requires 5 months for completion, and occurs in the distal to proximal direction. Evolutionary radiation of the Jehol Biota: chronological and ecological perspectives. Journal of Vertebrate Paleontology 33, 141152 (2013). The discovery of F. prima further increases the geological distribution of non-ornithothoracine birds. 113, 790804 (2014). With this arrangement, growth occurs in the outward direction only, allowing for elongation of ossified processes during this later-stage event. A.B. Additionally, whereas the pygostyle has been suggested as one of the key flight adaptations in the early evolution of birds23, a recent review argues that the early pygostyle is merely a by-product of the tail reduction to the extreme and unrelated to the flight adaptation10. Very recently, several new genera and species of non-ornithothoracine avialans have been described from north-eastern China10,13,14,24. (A) Adult emu pygostyle. Pygostyles differ between subadult and adult confuciusornithids, however; foramina present in subadult pygostyles are lost with ontogeny, indicating remodeling similar to that in neornithines. 154 (degree of fusion between the semilunate carpal and metacarpals), Char. Google Scholar. The mean pygostyle length/femur length for the seven extinct and extant clades was subsequently calculated and plotted by bar graph. Archaeopteryx is the basalmost bird with a tail composed of 21-22 . (I) Chicken 4+ years old, pygostyle transverse section, alcian blue and picrosirius red. Barsbold, R. et al. This condition is shared in the Jinguofortithidae, whilst their manual morphology is relatively derived (i.e., strongly curved minor metacarpal, and reduced minor digits)14. In Handbook of Avian Anatomy: Nomina Anatomica Avium (eds Baumel, J. J. et al.) 8b). OConnor, J. K. A Systematic Review of Enantiornithes (Aves: Ornithothoraces) (University of Southern California, 2009). (A) 4.5 month old emu transverse process from a free caudal vertebra, transverse paraffin section, stained with alcian blue and picrosirius red. Successively aged chicken tails from late embryonic stages to six months post hatching were collected and analyzed by histology (Fig. Carvalho, I. et al. This specimen consists of a preserved tail with ten vertebrae and attached feathers. The CT experiments were conducted at BL20-B2 of SPring-8 (RIKEN/JASRI), Sayo, Hyogo, Japan, with the following acquisition parameters: energy=70keV or 113keV, number of projections=900 per 180, propagation distance=4.2m, exposure time=1.03.0s per projection, and pixel size=12.9224.1m. 5b), slender and relatively short ulna as in Archaeopteryx and confuciusornithids (Fig. c Left coracoid in medial, lateral, cranial, and caudal views from top to bottom. Avian tail ontogeny, pygostyle formation, and interpretation of juvenile Mesozoic specimens. Cite this article. The major and minor metacarpals are separated throughout their length. Assuming the ossification sequence observed in the chicken was conserved in the Mesozoic, caudal vertebrae in young juveniles likely had reduced calcified processes. In fact, the 75 million-year-old Rahonavis ostromi from Madagascar33, with a long skeletal tail preserving 13 vertebrae (the length of the distal-most indicating a much greater caudal series), has at times been considered a non-avian dinosaur34,35,36,37 and at other times a bird2,33,38.
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