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Late Cretaceous birds of southern South America: Anatomy and systematics of Enantiornithes and Patagopteryx deferrariisi

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... The acromion is subrectangular in shape with a blunt end; it is largely aligned with the main axis of the blade, only slightly diverging craniodorsally. There is no evidence of the pit-like fossa that characterizes enantiornithines such as those from the Late Cretaceous of El Brete, Argentina [i.e., see Chiappe (1996) for an example of this trait in Enantiornithes]. The length of the acromion exceeds that of the humeral articular facet (Fig. 7). ...
... The acetabulum has a ratio of 0.1 with respect to the total length of the ilium; it is defined by a craniocaudally broad pubic pedicel and a well-developed antitrochanter. Like in other enantiornithinesdparticularly well-visible in three-dimensional specimens (e.g., Chiappe, 1996;Walker and Dyke, 2009)dthe antitrochanteric facet faces anteroventrally. At the dorsal edge of the ilium, at the level of the acetabulum, there is a protuberance that establishes the general boundary between the preacetabular and postacetabular wings (Fig. 8) [i.e., the dorsal antitrochanter of Sereno et al. (2002), and supratrochanteric process of Hu et al. (2015)]. ...
... This structure is not visible on the left femur. The proximolateral portions of both femora are sufficiently well preserved to be certain about the absence of a shelf-like posterior trochanter, a structure that is common in many other enantiornithines (Chiappe, 1996) including the bohaiornithids Zhouornis . The lateral condyle of the femur lacks a fibular condyle, a condition shared with other enantiornithines (Chiappe and Walker, 2002). ...
Article
Despite the abundant number of enantiornithine fossils from the Jehol Biota, the cranial anatomy of these birds remains only superficially known. Similarly, data on dental replacement within this clade, and among toothed birds in general, is largely lacking. Here we describe a new and exquisitely preserved specimen of a bohaiornithid enantiornithine from the Lower Cretaceous Jiufotang Formation in Liaoning Province, northeastern China. The new specimen provides novel information on the cranial anatomy of these birds and unprecedented data on their tooth resorption, implantation, and replacement pattern, mostly visualized through computed laminography (CL) images. The new information demonstrates the presence of alternating tooth replacement with symmetrical signaling control, a pattern that is possibly shared by other enantiornithines. We also test the monophyly of Bohaiornithidae, one of the most species-rich groups of Jehol enantiornithines, through the addition of the new specimen. While our phylogenetic results support a monophyletic clade composed of several taxa traditionally included in this group (i.e., Bohaiornis, Sulcavis, Zhouornis, Longusunguis, and Parabohaiornis), it excludes Shenqiornis mengi, a species used to phylogenetically defined Bohaiornithidae. The fact that Shenqiornis is not resolved among the above-mentioned clade raises concerns about the usage of Bohaiornthidae as originally defined (i.e., most recent common ancestor of Shenqiornis mengi and Bohaiornis guoi, and all its descendants).
... Sacral count among theropods is still a topic of debate. However, most authors agree in that increase in the number of sacral vertebrae is diagnostic of derived birds (Chiappe, 1996;Gauthier, 1986). Gauthier (1986) indicates that the presence of five fused sacral vertebrae may represent the plesiomorphic condition for Theropoda. ...
... Gauthier (1986) indicates that the presence of five fused sacral vertebrae may represent the plesiomorphic condition for Theropoda. The sacral count increases in selected genera of Ornithomimidae and Oviraptorosauria, in which six sacral vertebrae are identified (Chiappe, 1996;Rauhut, 2003). In most troodontids and dromaeosaurids, the plesiomorphic number of five sacrals is retained, and the ilium also hides (in lateral view) the last dorsal and first caudal vertebrae, as can be observed in Sinovenator, Mei, Deinonychus, and Bambiraptor (MCZ 4371; Ostrom, 1976a;Norell & Makovicky, 1997;Burnham et al., 2000;Xu, 2002;Gao et al., 2012). ...
... However, an increase in sacral vertebrae can be observed. In basal ornithurines, a count of more than seven vertebrae has invariably been reported (Chiappe, 1996;Chiappe, Ji, Ji, & Norell, 1999;Zhou & Zhang, 2003a). In Euenantiornithes, the sacral number in most species is no less than 8, whereas in the basal form Iberomesornis, the number of synsacral elements is six, as in Archaeopteryx and Jeholornis. ...
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Unenlagia comahuensis was originally described as a phylogenetic link between nonavian dinosaurs and birds. Later it was interpreted by some authors as belonging to the deinonychosaurian clade Dromaeosauridae, and more recently as phylogenetically closer to birds than to dromaeosaurids. The only known specimen is represented by an incomplete skeleton, including vertebrae, incomplete scapular girdle, pelvis, and limbs, coming from Upper Cretaceous beds of Neuquén province, Patagonia, Argentina. The aim of the present paper is to include a detailed anatomical description of Unenlagia (currently only known by preliminary descriptions). Detailed analysis of Unenlagia anatomy resulted in the recognition of one possible additional Unenlagiidae synapomorphy (i.e., the presence of cup‐like iliac articulation on ischium). We recognize derived anatomical traits that Unenlagia and kin share with birds, lending support to the interpretation that unenlagiids are stem‐Avialae. Particularly, some appendicular features (e.g., scapula with subtriangular and relatively reduced acromion, poor outward projection of the glenoid and glenoidal lips on the scapula, lateral orientation of scapular glenoid, craniolaterally oriented deltopectoral crest of humerus) may be related to the acquisition of anatomical novelties that in birds are associated with flight. The present contribution on Unenlagia provides new data regarding the early evolution of avian features.
... In extinct non-pygostylian archosaurs, the osteological correlate for this ligament (the fovea) is located between the AMT and the PMT off the tip of the femoral head long axis [15], whereas it is at the centre of the femoral head apex since early pygostylians (Confuciusornis [69]; enantiornithines [70][71][72][73]; and early euornithines [74,80,81] ) (figure 4a-h). Thus, during the evolution to pygostylians, the fovea seemingly relocated to the centre of the femoral head [82] as though the ligament itself shifted its insertion (figure 4e versus f; electronic supplementary material, SI.4). However, instead of ligamentous repositioning, it is more consistent to attribute this change to the evolutionary shift of femoral head morphogenesis from torsion to mediad growth: the proximal end including the fovea changed its orientation during ontogeny in the former but not in the latter (figure 5c,d versus e,f ). ...
Article
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Significant evolutionary shifts in locomotor behaviour often involve comparatively subtle anatomical transitions. For dinosaurian and avian evolution, medial overhang of the proximal femur has been central to discussions. However, there is an apparent conflict with regard to the evolutionary origin of the dinosaurian femoral head, with neontological and palaeontological data suggesting seemingly incongruent hypotheses. To reconcile this, we reconstructed the evolutionary history of morphogenesis of the proximal end of the femur from early archosaurs to crown birds. Embryological comparison of living archosaurs (crocodylians and birds) suggests the acquisition of the greater overhang of the femoral head in dinosaurs results from additional growth of the proximal end in the medial-ward direction. On the other hand, the fossil record suggests that this overhang was acquired by torsion of the proximal end, which projected in a more rostral direction ancestrally. We reconcile this apparent conflict by inferring that the medial overhang of the dinosaur femoral head was initially acquired by torsion, which was then superseded by mediad growth. Details of anatomical shifts in fossil forms support this hypothesis, and their biomechanical implications are congruent with the general consensus regarding broader morpho-functional evolution on the avian stem.
... However, in R. americana, contrasting with volant birds it is not caudoventrally oriented. As pointed out by Chiappe (1996) and Woolley (2000), The shape and inclination of the condylus dorsalis and ventralis of ...
Article
The Greater Rhea (Rhea americana, Rheidae) is a flightless paleognath with a wide geographical distribution in South America. The morphology of its shoulder girdle and wings are different from those of flying birds and some characteristics are similar to basal birds and paravian theropods. We present a detailed osteological, myological, and functional study of the shoulder and the wing of the Greater Rhea. Particular features of the anatomy of the pectoral girdle and wing of Rhea include the lack of triosseal canal, reduced origin area of the mm. pectoralis p. thoracica and supracoracoideus and the lack of a propatagium. The wing muscle mass is markedly reduced, reaching only 0.89% of total body mass (BM). Forelimb muscles mass values are low compared to those of flying birds and are congruent with the non-use of wings for active locomotion movements. R. americana does not flap the wings dorso-ventral as typical for flying birds, but predominantly in cranio-caudal direction, following a craniolateral to caudomedial abduction-adduction arc. When the wings are fully abducted, they are inverted L-shaped, with the inner surface caudally faced, and when the wings are folded against the body, they do not perform the complete automatic wing folding nor the circumduction of the manus, a movement performed by extant volant birds. This study complements our knowledge of the axial musculature of the flightless paleognaths and highlights the use of the Greater Rhea as a model, which may help understand the evolution of Palaeognathae, as well as the origin of flapping flight among paravian theropods.
... Although Late Cretaceous enantiornithines have been collected from nearly every continent except for Antarctica, specimens are far fewer and typically very fragmentary. Very few articulated specimens are known (only Neuquenornis, Parvavis and Elsornis) (Chiappe & Calvo, 1994;Chiappe et al. 2006;Wang et al. 2014) and the only notable cranial material is that of Gobipteryx from Mongolia (Elzanowski, 1976;Chiappe et al. 2001), Neuquenornis from Argentina (Chiappe, 1996) and the recently described Falcatakely from the uppermost Cretaceous deposits of Madagascar . In contrast to Early Cretaceous specimens, Late Cretaceous enantiornithines are more commonly preserved in three dimensions (Chiappe & Calvo, 1994;Elzanowski, 1976;Chiappe et al. 2001;Atterholt et al. 2018). ...
Article
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A new enantiornithine bird is described on the basis of a well preserved partial skeleton from the Upper Cretaceous Qiupa Formation of Henan Province (central China). It provides new evidence about the osteology of Late Cretaceous enantiornithines, which are mainly known from isolated bones; in contrast, Early Cretaceous forms are often represented by complete skeletons. While the postcranial skeleton shows the usual distinctive characters of enantiornithines, the skull displays several features, including confluence of the antorbital fenestra and the orbit and loss of the postorbital, evolved convergently with modern birds. Although some enantiornithines retained primitive cranial morphologies into the latest Cretaceous Period, at least one lineage evolved cranial modifications that parallel those in modern birds.
... PC1 correlates negatively with all size-corrected residues (electronic supplementary material, table S1), suggesting that it refers to appendicular elongation and thus correlates with body size to some degree; the eigenvector coefficients of forelimb measurements are all greater than those of hindlimbs, indicating that PC1 describes whether the increase of limb length largely results from the elongation of the forelimb or the hindlimb. For instance, the Late Cretaceous ornithuromorph Patagopteryx, a flightless taxon [38], is the largest taxon analysed here and has the proportionately shortest forelimb (less than half the hindlimb) and has the highest PC1 score royalsocietypublishing.org/journal/rspb Proc. R. Soc. ...
Article
The origin of birds from non-avian theropod dinosaurs is one of the greatest transitions in evolution. Shortly after diverging from other theropods in the Late Jurassic, Mesozoic birds diversified into two major clades—the Enantiornithes and Ornithuromorpha—acquiring many features previously considered unique to the crown group along the way. Here, we present a comparative phylogenetic study of the patterns and modes of Mesozoic bird skeletal morphology and limb proportions. Our results show that the major Mesozoic avian groups are distinctive in discrete character space, but constrained in a morphospace defined by limb proportions. The Enantiornithines, despite being the most speciose group of Mesozoic birds, are much less morphologically disparate than their sister clade, the Ornithuromorpha—the clade that gave rise to living birds, showing disparity and diversity were decoupled in avian history. This relatively low disparity suggests that diversification of enantiornithines was characterized in exhausting fine morphologies, whereas ornithuromorphs continuously explored a broader array of morphologies and ecological opportunities. We suggest this clade-specific evolutionary versatility contributed to their sole survival of the end-Cretaceous mass extinction.
... M. puboischiofemoralis internus retained an insertion on the preacetabular fossa in Nothronychus, similar to other maniraptorans (Hutchinson & Gatesy, 2000;Norell & Makovicky, 1999), but is not as severely reduced as in Ornithurae (Chiappe, 1996). Therefore, modification of this structure can be regarded as intermediate in theropod evolution. ...
Article
Full-text available
Therizinosaurs are highly modified, probably herbivorous, theropods from the Upper Cretaceous of Asia and North America. They are characterized by an extensively pneumatized axial skeleton, and in the derived forms, an incipiently opisthopubic pelvis. The evolution of such a pelvis is expected to be associated with extensive modification of the muscular system. The muscular system is reconstructed using observed muscle scars, reconstructions of the theropods Staurikosaurus and Tyrannosaurus, the ornithischian Maiasaura, and extant crocodilians and birds. The results indicate convergence with birds and ornithischian dinosaurs, such that the retroverted pubis in some maniraptorans can be regarded as analogous with the postacetabular bar in ornithischians. Functional implications also make derived therizinosaurs, such as Nothronychus, in some respects convergent with birds as the pubis is retroverted, becoming fused with the ischium, a laterally flaring synsacrum, and an avian‐like pes.
... The fibula bears a craniolaterally directed tubercle for the insertion of m. iliofibularis at the level approximately one-quarter of the distance from the proximal end of the tibiotarsus. In contrast, that tubercle is laterally directed in the basal ornithuromorphs Patagopteryx deferrariisi and Vorona berivotrensis (Forster et al., 1996), but caudally or caudolaterally oriented in Ornithurae (Chiappe, 1996). The distal end of the fibula is missing, precluding assessment of its total length. ...
Article
Confuciusornithidae is the clade of Early Cretaceous birds most rich in materials and plays a central role in our understanding of the evolution of avian horny beaks and pygostyles. A handful of specimens demonstrate that this avian group is distinguishable from other basal birds by their robust, toothless upper and lower jaws, a fused scapulocoracoid and a tiny claw on the middle manual digit, among other features. Here, we report a new taxon of Confuciusornithidae, Yangavis confucii gen. et sp. nov., from the Early Cretaceous Jehol Biota, northeastern China. This new bird, however, has a normal-sized major digit claw, as in other basal birds, which was probably regained independently as Confuciusornithidae evolved, based on our phylogenetic study. Unfortunately, the biological significance of this trait is unclear owing to a lack of analogues in modern birds (manual claws are completely lost in adults). Yangavis confucii is differentiated from other confuciusornithids by its proportionally much longer forelimb. Our morphometric analysis indicates that the morphospace of Confuciusornithidae, with the addition of Y. confucii, is greatly broadened to a degree that it overlaps with the Early Cretaceous Ornithuromorpha and Enantiornithines, indicating that the biological diversity of confuciusornithids is greater than previously thought.
... The open extensor sulcus is deep, and (Fig. 5B), B. advenus (Fig. 4E), F. hoffmani (Fig. 5E), Parahesperornis alexi (Fig. 5F), and Hesperornis regalis (Fig. 5G). In one of the basal ornithuromorph Patagopteryx deferrariisi, the width of the distal epiphysis is approximately equal to that of the mid-shaft ( Fig. 5A; Chiappe, 1996). In Brodavis varneri, both medial and lateral edge of the distal epiphysis are flared, giving it a triangular appearance (Fig. 5D). ...
Article
Asian hesperornithiforms are extremely rare in contrast to North American records; thus, their diversity in Asia during the Cretaceous is unclear. Maastrichtian hesperornithiform materials have been reported from both fluvial and marine deposits in North America but only from fluvial deposits in Asia. Asian hesperornithiforms from Maastrichtian deposits have been considered as freshwater taxa because of their occurrence from fluvial sediments and their histological features. Here, we report the first hesperornithiform record from marine Maastrichtian deposits in Asia. It is represented by an isolated left tibiotarsus from the inter-arc basin deposit of the Kita-ama Formation (lower Maastrichtian), Izumi Group of southwest Japan. It has a shallow tibial incision, fibular crest extending to the mid shaft, and laterally angled lateral articular surface. Although its phylogenetic position within Hesperornithiformes is ambiguous, these characters are similar to non-hesperornithid hesperornithiforms. Unossified proximal and distal epiphyses indicate that this individual was immature. A remarkably thick cortical area of the tibiotarsus suggests that this hesperornithiform was a sea-dwelling bird and that the habitat of this group during the Maastrichtian extended to both terrestrial and marine environments in Asia and North America.
Preprint
Ichthyornis has long been recognized as a pivotally important fossil taxon for understanding the latest stages of the dinosaur-bird transition, but little significant new postcranial material has been brought to light since initial descriptions of partial skeletons in the 19th Century. Here, we present new information on the postcranial morphology of Ichthyornis from 40 previously undescribed specimens, providing the most detailed morphological assessment of Ichthyornis to date. The new material includes four partially complete skeletons and numerous well-preserved isolated elements, enabling new anatomical observations such as muscle attachments previously undescribed for Mesozoic euornitheans. Among the elements that were previously unknown or poorly represented for Ichthyornis , the new specimens include an almost-complete axial series, a hypocleideum-bearing furcula, radial carpal bones, fibulae, a complete tarsometatarsus bearing a rudimentary hypotarsus, and one of the first-known nearly complete three-dimensional sterna from a Mesozoic avialan. Several pedal phalanges are preserved, revealing a remarkably enlarged pes presumably related to foot-propelled swimming. Although diagnosable as Ichthyornis , the new specimens exhibit a substantial degree of morphological variation, some of which may relate to ontogenetic changes. Phylogenetic analyses incorporating our new data and employing alternative morphological datasets recover Ichthyornis stemward of Hesperornithes and Iaceornis , in line with some recent hypotheses regarding the topology of the crownward-most portion of the avian stem group, and we establish phylogenetically-defined clade names for relevant avialan subclades to help facilitate consistent discourse in future work. The new information provided by these specimens improves our understanding of morphological evolution among the crownward-most non-neornithine avialans immediately preceding the origin of crown group birds.
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