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40 new specimens of Ichthyornis provide unprecedented insight into the postcranial morphology of crownward stem group birds

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Abstract

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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... The three-dimensional retrodeformation method presented herein has been developed and tested on two case studies. It was first developed for and applied to several cervical vertebrae (CV) of the diplodocine sauropod Galeamopus pabsti, based on a single individual (SMA 0011/NMZ 1000011; Tschopp and Mateus, 2017), and further refined to reconstruct the sternum of the crownward stem-bird Ichthyornis dispar (Marsh, 1880;Clarke, 2004;Field et al., 2018;Benito et al., 2022). This refinement combined information from multiple individuals preserving different aspects of each bone in 2D and 3D, allowing an idealised composite reconstruction. ...
... The basis for the reconstruction and retrodeformation of the sternum of Ichthyornis were several 2D and 3D preserved specimens (Figure 2), described by Benito et al. (2022), showing extensive overlap and thus allowing individual well preserved parts to be combined into an idealised composite reconstruction, hereafter referred to as a "prototype"; see Table 1. The terminology describing the preservation of the sterna follows Baumel and Witmer (1993) and Livezey and Zusi (2006). ...
... The different specimens of Ichthyornis were compared, and individual well preserved regions, such as the crossed coracoid sulci in FHSM VP-18702 ( Supplementary Figure 2A) or the craniolateral process and ventral lip of NHMUK PV A 905 (Supplementary Figure 2B), were detached from their original retopology meshes and combined into a new composite reconstruction. The well preserved coracoids KUVP 2281, ALMNH PV 1993.2.133, andYPM VP.001733 (Clarke, 2004;Benito et al., 2022) were used to assist in the shape reconstruction of the individual asymmetrical coracoid sulci while keeping their posterior margins symmetrical. The general shape and curvature of the lateral sternal plates were guided by sterna of comparable extant taxa, such as Anas discors, Ardea alba, Burhinus oedicnemus, and Sterna hirundo (Marsh, 1880;Clarke, 2004;Benito et al., 2022), while ensuring that the outline of the Ichthyornis sternum remained consistent with the preserved specimens (Supplementary Figure 2C). ...
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... If this material indeed belongs to a lithornithid, it would provide compelling evidence that the clade survived across the boundary. However, it should be noted that several Mesozoic stem ornithurines also have a hooked acromion that approaches the condition seen in Lithornithidae [64,81,82]. Thus, the identity of this fossil remains uncertain, and more material needs to be recovered from both this formation and other contemporaneous localities to clarify which groups of total-clade palaeognaths persisted across the K-Pg boundary. ...
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Our analysis of vertebrate remains collected from a Cenomanian microsite in the Lincoln Limestone, the basal-most member of the Greenhorn Formation in Russell County, Kansas, identifies ornithurine (sensu Chiappe, 1996) avian fossils. The specimens presented here are slightly younger than the oldest avian remains known from North America (from the Woodbine Formation, Texas) and of a similar age as the avians from the Asheville Formation in Saskatchewan, Canada. This find thus connects the extensive geographic range of the earliest North American birds from Saskatchewan south through central Kansas and on to Texas. The specimens discussed here are fragmentary, yet show features definitive of ornithurine birds. One specimen is attributable to Ichthyornis, whereas another preserves teeth characteristic of ornithurine birds. In addition to bird bones, the microsite yielded numerous bony fish remains, shark teeth, coniasaur vertebrae, and other lizard bones.
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HERE WE REPORT ON THREE NEW SPECIES OF ORNITHUROMORPH BIRDS FROM THE LOWER CRETACEOUS XIAGOU FORMATION IN THE CHANGMA BASIN OF GANSU PROVINCE, NORTHWESTERN CHINA: Yumenornis huangi gen. et sp. nov., Changmaornis houi gen. et sp. nov., and Jiuquanornis niui gen. et sp. nov.. The last of these is based on a previously published but unnamed specimen: GSGM-05-CM-021. Although incomplete, the specimens can be clearly distinguished from each other and from Gansus yumenensis Hou and Liu, 1984. Phylogenetic analysis resolves the three new taxa as basal ornithuromorphs. This study reveals previously unrecognized ornithuromorph diversity in the Changma avifauna, which is largely dominated by Gansus but with at least three other ornithuromorphs. Body mass estimates demonstrate that enantiornithines were much smaller than ornithuromorphs in the Changma avifauna. In addition, Changma enantiornithines preserve long and recurved pedal unguals, suggesting an arboreal lifestyle; in contrast, Changma ornithuromorphs tend to show terrestrial or even aquatic adaptions. Similar differences in body mass and ecology are also observed in the Jehol avifauna in northeastern China, suggesting niche partitioning between these two clades developed early in their evolutionary history.
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Mesozoic remains of embryonic and early juvenile birds are rare. To date, a handful of in ovo embryos and early juveniles of enantiornithines from the Early Cretaceous of China and Spain and the Late Cretaceous of Mongolia and Argentina have comprised the entire published record of perinatal ontogenetic stages of Mesozoic birds. We report on the skeletal morphology of three nearly complete early juvenile avians from the renowned Early Cretaceous Yixian Formation of Liaoning Province in northeastern China. Evidence of the immaturity of these specimens is expressed in the intense grooving and pitting of the periosteal surfaces, the disproportionately small size of the sterna, and the relative size of the skull and orbits. Size notwithstanding, anatomical differences between these three specimens are minimal, leaving no basis for discriminating them into separate taxa. Numerous osteological synapomorphies indicate that they are euenantiornithine birds, the most diverse clade of Enantiornithes, but their identification as members of a particular euenantiornithine taxon remains unclear. Their early ontogenetic stage, however, provides important information about the postnatal development of this specious clade of Cretaceous birds. The presence of pennaceous wing feathers suggests that fledging occurred very early in ontogeny, thus supporting a precocial or highly precocial strategy for enantiornithine hatchlings. The morphology of these new early-stage juveniles is also significant in that they allow a better understanding of the homologies of several avian compound bones because the components of these skeletal compounds are preserved prior to their coossification. The general morphology of the wrist and ankle of these juveniles highlights once again the striking similarity between nonavian theropods and early birds.
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Avian fossils from the Upper Cretaceous of Mongolia provide significant scientific insight into the evolution of early birds, primarily due to the scarcity of continental interiors with a well-documented avifauna in the Cretaceous record. This paper describes in detail the anatomy and histology of a new taxon of early ornithuromorph bird, Hollanda luceria, from the Barun Goyot Formation at Khermeen Tsav in the Gobi Desert of Mongolia. The new taxon is represented exclusively by hindlimb elements, and is characterized by having elongated hindlimbs with an extremely reduced metatarsal IV and an unusual tibiotarsal-femoral articulation centered on a highly peaked lateral articular facet of the tibiotarsus. Cladistic and ecospace analyses were also carried out in order to infer evolutionary relationships and ecology of this primitive bird. These analyses indicate that the new taxon is a previously undescribed lineage of basal ornithuromorph and an outgroup of Ornithurae (sensu Chiappe, 2002), which may have had a cursorial lifestyle similar to that of the modern roadrunner, Geococcyx californianus.
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Many palaeobiological analyses have concluded that modern birds (Neornithes) radiated no earlier than the Maastrichtian, whereas molecular clock studies have argued for a much earlier origination. Here, we assess the quality of the fossil record of Mesozoic avian species, using a recently proposed character completeness metric which calculates the percentage of phylogenetic characters that can be scored for each taxon. Estimates of fossil record quality are plotted against geological time and compared to estimates of species level diversity, sea level, and depositional environment. Geographical controls on the avian fossil record are investigated by comparing the completeness scores of species in different continental regions and latitudinal bins. Avian fossil record quality varies greatly with peaks during the Tithonian-early Berriasian, Aptian, and Coniacian-Santonian, and troughs during the Albian-Turonian and the Maastrichtian. The completeness metric correlates more strongly with a 'sampling corrected' residual diversity curve of avian species than with the raw taxic diversity curve, suggesting that the abundance and diversity of birds might influence the probability of high quality specimens being preserved. There is no correlation between avian completeness and sea level, the number of fluviolacustrine localities or a recently constructed character completeness metric of sauropodomorph dinosaurs. Comparisons between the completeness of Mesozoic birds and sauropodomorphs suggest that small delicate vertebrate skeletons are more easily destroyed by taphonomic processes, but more easily preserved whole. Lagerstätten deposits might therefore have a stronger impact on reconstructions of diversity of smaller organisms relative to more robust forms. The relatively poor quality of the avian fossil record in the Late Cretaceous combined with very patchy regional sampling means that it is possible neornithine lineages were present throughout this interval but have not yet been sampled or are difficult to identify because of the fragmentary nature of the specimens.
Article
There is growing evidence of developmental plasticity in early branching dinosaurs and their outgroups. This is reflected in disparate patterns of morphological and histological change during ontogeny. In fossils, only the osteohistological assessment of annual lines of arrested growth (LAGs) can reveal the pace of skeletal growth. Some later branching non-bird dinosaur species appear to have followed an asymptotic growth pattern, with declining growth rates at increasing ontogenetic ages. By contrast, the early branching sauropodomorph Plateosaurus trossingensis appears to have had plastic growth, suggesting that this was the plesiomorphic condition for dinosaurs. The South African sauropodomorph Massospondylus carinatus is an ideal taxon in which to test this because it is known from a comprehensive ontogenetic series, it has recently been stratigraphically and taxonomically revised, and it lived at a time of ecosystem upheaval following the end-Triassic extinction. Here, we report on the results of a femoral osteohistological study of M. carinatus comprising 20 individuals ranging from embryo to skeletally mature. We find major variability in the spacing of the LAGs and infer disparate body masses for M. carinatus individuals at given ontogenetic ages, contradicting previous studies. These findings are consistent with a high degree of growth plasticity in M. carinatus .
Article
Yanornithidae is an Early Cretaceous ornithuromorph clade that preserves the oldest direct evidence indicative of a piscivorous diet in avian evolution. The family hitherto contained only a single genus Yanornis and two putative species. The yanornithids are readily distinguishable from other Mesozoic avians in having a long rostrum packed with numerous teeth. Despite the discoveries of dozens of specimens, little is known about the taxonomical and ecomorphological diversity of this clade due to a lack of comparative studies. Here, we describe two new yanornithid taxa, Similiyanornis brevipectus gen. et sp. nov. and Abitusavis lii gen. et sp. nov., based on two nearly complete skeletons from the Early Cretaceous Jehol Biota. We perform a taxonomical reassessment of yanornithids and rectify the assignments of published putative Yanornis-like materials. The results show that Yanornithidae contains three genera and three species, which demonstrate a wide range of limb proportions that exceed the degree seen in other L early ornithuromorph clades. http://zoobank.org/urn:lsid:zoobank.org:pub:64C621AF-48B7-499E-9F41-B6023B2E5F78
Article
We describe a new enantiornithine specimen from the Lower Cretaceous Jiufotang Formation of the Jehol Biota, northeastern China, which can be assigned to the species Piscivorenantiornis inusitatus. The new specimen confirms the presence of the unusual articulation of the cervical vertebrae that characterizes P. inusitatus. The disarticulated bony elements in the new specimen, some of which are preserved in different views or completely missing from the holotype, enable a more comprehensive morphological reconstruction and a refined phylogenetic assessment of P. inusitatus. The new fossil demonstrates that the occipitals are recessed by a pair of fossae lateral to the foramen magnum, a feature otherwise unknown among Mesozoic and extant birds. Our phylogenetic analysis tentatively resolves P. inusitatus as the sister taxon to the clade consisting of Pterygornis dapingfangensis and Mirusavis parvus, which is recovered deeply nested within the Enantiornithes. We hypothesize that the craniolateral processes of the sternum are a derived feature in enantiornithines.
Article
The evolutionary diversification of birds has been facilitated by specializations for various locomotor modes, with which the proportion of the limb skeleton is closely associated. However, recent studies have identified phylogenetic signals in this system, suggesting the presence of historical factors that have affected its evolutionary variability. In this study, in order to explore potential roles of ontogenetic integration in biasing the evolution in the avian limb skeleton, evolutionary diversification patterns in six avian families (Anatidae, Procellariidae, Ardeidae, Phalacrocoracidae, Laridae, and Alcidae) were examined and compared to the postnatal ontogenetic trajectories in those taxa, based on measurement of 2641 specimens and recently collected ontogenetic series, supplemented by published data. Morphometric analyses of lengths of six limb bones (humerus, ulna, carpometacarpus, femur, tibiotarsus, and tarsometatarsus) demonstrated that: 1) ontogenetic trajectories are diverse among families; 2) evolutionary diversification is significantly anisotropic; and, most importantly, 3) major axes of evolutionary diversification are correlated with clade‐specific ontogenetic major axes in the shape space. These results imply that the evolutionary variability of the avian limbs has been biased along the clade‐specific ontogenetic trajectories. It may explain peculiar diversification patterns characteristic to some avian groups, including the long‐leggedness in Ardeidae and tendency for flightlessness in Anatidae.
Article
The morphology and function of all muscles controlling the pigeon hand were analyzed. Muscle action was determined in situ by inducing contraction via silver wire electrodes in anesthetized birds. EMG electrodes were implanted in the test muscle and an adjacent muscle to monitor contraction and volume conduction respectively. Results indicate that pigeons have fine control of hand and digit movements. However, the directions of movement are restricted. Movements have been eliminated or severely limited in those directions that experience strong stress during flight. Such restrictions may reduce the amount of muscular activity required for stabilization of the hand and its components. Mobility is retained in directions not subject to large stresses and where movement is essential for the kinematics of flight to be executed properly. © 1995 Wiley-Liss, Inc.
Article
A taxonomic illustrated review of the Late Cretaceous fossil birds known from the Argentine locality of El Brete (Salta Province, Patagonia, Argentina) is presented here for the first time. Although some of these specimens were first reported in the early 1980s, and then a handful more were presented in literature thoughout the 1990s, this important collection of fossil birds has largely remained undescribed since it was gathered 30 years ago. This is in spite of the fact that the El Brete collection provided the basis of C.A. Walker’s insight that enantiornithine birds are anatomically—and phylogenetically—distinct from all other living and extinct avians. A catalogue of the known specimens from this site is also presented, and many elements are illustrated for the first time. We revise the anatomy of taxa from El Brete and provide complete lists of referred specimens, based on casts of the collection held in the Natural History Museum, London, and Walker’s original manuscript. Six valid euenantiornithine birds are currently known from the El Brete locality—Enantiornis Walker 1981, Lectavis Chiappe 1993, Soroavisaurus Chiappe 1993, Yungavolucris Chiappe 1993, and Martinavis Walker et al. 2007—alongside a new taxon described and documented in this paper (Elbretornis). This new fossil euenantiornithine is remarkable because its humerus is highly pneumatised, more extensively than any other known Mesozoic fossil bird. The physiological and evolutionary implications of this bird and the other El Brete taxa are discussed.
Article
Partial reconstruction of hindlimb myology and syndesmology of Hesperornis regalis, based on fossil and extant comparative anatomical material, is provided. Possessing a combination of morphological features characteristic of modern loons and grebes, H. regalis was somewhat closer to loons in overall hindlimb anatomy. The acetabular foramen indicates that the femoral head of Hesperornis had a more restricted excursion in the acetabulum compared with modern Aves. The development of femoral retractors m. iliofemoralis and m. caudofemoralis exceeded that of modern foot-propelled diving birds, showing greater protractive forces applied to the femur during the propulsive stroke. The origin of intermediate and lateral portions of m. gastrocnemius reached its maximum in Hesperornis, extending proximally beyond the greater trochanter and migrating onto the cranial surface of the femur. Ligaments of the knee joint, which show typical avian morphology, allowed a high degree of inward–outward rotation of the tibiotarsus, which was held close to the body and was probably enclosed in the thickly feathered skin of the body wall. Flexion–extension movements in the knee joint were highly reduced by the action of well-developed mm. flexores cruris, m. iliofibularis, the postacetabular portion of m. iliotibialis lateralis and m. femorotibialis medialis. The intertarsal joint of Hesperornis possessed a high degree of rotational freedom. In this way it resembled that of loons, showing a loon-like manner of tarsometatarsal movements. This was, however, combined with grebe-like movements of the toes, which undoubtedly had asymmetrical lobes. Hesperornis regalis shows its own independent way of mastering underwater locomotion, which led to the appearance of what became the most specialized avian foot-propelled diver ever known.
Article
Current classification of birds recognizes three subclasses which are morphologically distinct: the Archaeornithes for Archaeopteryx, the Odontornithes for the Hesperornithiformes and the Ichthyornithiformes, and the Neornithes for all modern birds and their extinct immediate relatives. (Some authorities1 prefer different names for some of these taxa.) I have examined new material recently discovered in the Upper Cretaceous rocks of Argentina which indicates the existence of a group of birds having features so different from those of the currently recognized subclasses that they seem to represent a fourth subclass, here named the Enantiornithes (‘opposite birds’). I describe unique features of the Enantiornithes which include a reduced outer metatarsal, in some forms an extreme modification of the remaining elements of the tarsometatarsus, a highly modified pectoral girdle, and sometimes a characteristic perforation in the proximal end of the humerus.
Article
Manuscript copy. Thesis--University of Florida. Vita. Bibliography: leaves 213-217.
Article
A new avian genus and species, Zhongjianornis yangi gen. et sp. nov., is reported from the Lower Cretaceous lacustrine deposits of the Jiufotang Formation in Liaoning, northeast China. The new taxon is characterized by possessing the following combination of features: upper and lower jaws toothless, snout pointed, humerus with large and robust deltopectoral crest, second phalanx of the major manual digit longer than the first phalanx, unguals of the alular and major digits of similar length and significantly shorter than the corresponding penultimate phalanges, tibiotarsus slender and more than twice the length of the tarsometatarsus, and metatarsal IV longer than the other metatarsals. Phylogenetic analysis indicates that Zhongjianornis is phylogenetically basal to Confuciusornis and the dominant Mesozoic avian groups, Enantiornithes and Ornithurae, and therefore provides significant new information regarding the diversification of birds in the Early Cretaceous. It also represents the most basal bird that completely lacks teeth, suggesting that tooth loss was more common than expected in early avian evolution and that the avian beak appeared independently in several avian lineages, most probably as a response to selective pressure for weight reduction. Finally, the presence of a significantly enlarged humeral deltopectoral crest suggests that Zhongjianornis shares with other basal birds such as Jeholornis, Sapeornis and Confuciusornis a distinctive mode of adaptation for flight contrasting with that seen in more advanced birds, which instead possess an elongated sternum and a prominent keel.
The most complete enantiornithine from 2818
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An Early Cretaceous enantiornithine (Aves) preserving an unlaid egg and probable 2824 medullary bone
An Early Cretaceous enantiornithine (Aves) preserving an unlaid egg and probable 2824 medullary bone. Nature Communications 10: 1275 DOI: 10.1038/s41467-019-09259-2825 x.
A species-level phylogeny of the Cretaceous 2839
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Aves: Ornithuromorpha): implications for body size evolution 2840 amongst the earliest diving birds
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Morphometric comparison of the 2848
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Anatomical variation and avian anatomy
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Macroevolutionary dynamics of dentition in Mesozoic birds 2865 reveal no long-term selection towards tooth loss
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Brocklehurst N, Field DJ. 2021. Macroevolutionary dynamics of dentition in Mesozoic birds 2865 reveal no long-term selection towards tooth loss. iScience 24: 102243 DOI: 2866 10.1016/j.isci.2021.102243.
A femur of the Late Cretaceous giant bird Gargantuavis from 2871
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Cruzy (southern France) and its systematic implications
Cruzy (southern France) and its systematic implications. Palaeovertebrata 42: e3 2872 DOI: 10.18563/pv.42.1.e3.
Development and evolution of regionalization within the avian 2874 axial column
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The phylogenetic affinities of the bizarre Late 2887
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RegNum -The international clade names repository
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The oldest Antarctic bird
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Chatterjee S. 1989. The oldest Antarctic bird. Journal of Vertebrate Paleontology 9(3): 16A.
The morphology and systematics of Polarornis, a Cretaceous loon 2896 (Aves: Gaviidae) from Antarctica
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Chatterjee S. 2000. The morphology and systematics of Polarornis, a Cretaceous loon 2896 (Aves: Gaviidae) from Antarctica. In Proceedings of the 5th Symposium of the Society 2897 of Avian Paleontology and Evolution, Beijing (Vol. 1: 125-155).