A mid-Cretaceous enantiornithine foot and tail feather preserved in Burmese amber

This work is licensed under a Creative Commons Attribution 4.0 International License.Since the first skeletal remains of avians preserved in amber were described in 2016, new avian remains trapped in Cretaceous-age Burmese amber continue to be uncovered, revealing a diversity of skeletal and feather morphologies observed nowhere else in the Mesozoic fossil record. Here we describe a foot with digital proportions unlike any previously described enantiornithine or Mesozoic bird. No bones are preserved in the new specimen but the outline of the foot is recorded in a detailed skin surface, which is surrounded by feather inclusions including a partial rachis-dominated feather. Pedal proportions and plumage support identification as an enantiornithine, but unlike previous discoveries the toes are stout with transversely elongated digital pads, and the outer toe appears strongly thickened relative to the inner two digits. The new specimen increases the known diversity and morphological disparity among the Enantiornithes, hinting at a wider range of habitats and behaviours. It also suggests that the Burmese amber avifauna was distinct from other Mesozoic assemblages, with amber entrapment including representatives from unusual small forms

The most complete enantiornithine from North America and a phylogenetic analysis of the Avisauridae

The most complete known North American enantiornithine was collected in 1992 but never formally described. The so-called “Kaiparowits avisaurid” remains one of the most exceptional Late Cretaceous enantiornithine fossils. We recognize this specimen as a new taxon, Mirarce eatoni (gen. et sp. nov.), and provide a complete anatomical description. We maintain that the specimen is referable to the Avisauridae, a clade previously only known in North America from isolated tarsometatarsi. Information from this specimen helps to clarify evolutionary trends within the Enantiornithes. Its large body size supports previously observed trends toward larger body mass in the Late Cretaceous. However, trends toward increased fusion of compound elements across the clade as a whole are weak compared to the Ornithuromorpha. The new specimen reveals for the first time the presence of remige papillae in the enantiornithines, indicating this feature was evolved in parallel to dromaeosaurids and derived ornithuromorphs. Although morphology of the pygostyle and (to a lesser degree) the coracoid and manus appear to remain fairly static during the 65 million years plus of enantiornithine evolution, by the end of the Mesozoic at least some enantiornithine birds had evolved several features convergent with the Neornithes including a deeply keeled sternum, a narrow furcula with a short hypocleidium, and ulnar quill knobs—all features that indicate refinement of the flight apparatus and increased aerial abilities. We conduct the first cladistic analysis to include all purported avisuarid enantiornithines, recovering an Avisauridae consisting of a dichotomy between North and South American taxa. Based on morphological observations and supported by cladistic analysis, we demonstrate Avisaurus to be paraphyletic and erect a new genus for “A. gloriae,” Gettyia gen. nov

Editorial: Early Avian Evolution

The study of early avian evolution—how birds evolved from dinosaurs and radiated into the most diverse group of amniotes on the planet—is one of the most dynamic areas of research in paleontology, fueled not only by the rapid rate of discovery of new specimens (see Foth et al.; Musser and Clarke; and Xing et al.) and sheer volume of available material (see Zheng et al.) but also by the innovative application of new analytical methods to key evolutionary questions (see Heers et al.; Liu et al.). Also critical to our understanding is the exceptional level of preservation of many Mesozoic and early Cenozoic bird fossils, which not uncommonly preserve soft tissues and other indicators that may provide key insights into the biology of these organisms (see articles by Clark and O’Connor; Foth et al.; Xing et al.; Zheng et al.). In putting together this research topic, our aim was to further expand our understanding of early avian evolution by gathering a body of work highlighting the diversity of research currently being undertaken in this area. As such, articles published in this topic have augmented our understanding of a variety of important areas related to early avian evolution, including the recognition of new taxonomic diversity (see Clark and O’Connor and Musser and Clarke), insights into the evolution of key avian traits such as flight (Heers et al.) and a toothless beak (see Louchart et al. and Zheng et al.), and the piecemeal evolution of crown avian biology (see Atterholt et al. and Heers et al.)

New information on the anatomy of the Chinese Early Cretaceous Bohaiornithidae (Aves: Enantiornithes) from a subadult specimen of Zhouornis hani

Enantiornithines are the most diverse avian clade in the Cretaceous. However, morphological specializations indicative of specific ecological roles are not well known for this clade. Here we report on an exquisitely well-preserved specimen from the Lower Cretaceous Jehol Group of northeastern China, which pedal morphology is suggestive of a unique ecological specialization within Enantiornithes. The morphology of the new specimen is largely indistinguishable from that of the holotype of the bohaiornithid enantiornithine Zhouornis hani, albeit the latter is somewhat larger. The new specimen provides important and previously unknown details of the skull of Zhouornis hani, which add to the limited knowledge about the cranial anatomy and evolution of enantiornithines. The information offered by the new specimen also augments our understanding of the postcranial morphology of bohaiornithid enantiornithines, a clade that has been only recently recognized. With the description of this specimen, Zhouornis hani becomes one of the most anatomically complete known enantiornithine species, which will facilitate future morphological studies

On the Preservation of the Beak in Confuciusornis (Aves: Pygostylia)

The Confuciusornithiformes represent the most stem-ward avian occurrence of an edentulous rostrum. Although a keratinous beak is widely considered to have covered the rostrum in confuciusornithiforms, this feature is almost never preserved, having been previously reported only in the holotype of Confuciusornis dui and the holotype of Eoconfuciusornis zhengi. This strongly contrasts with the widespread preservation of the keratinous sheaths that cover the manual and pedal ungual phalanges. Here, we report on a third occurrence of a preserved rhamphotheca in a specimen of Confuciusornis sanctus. We illuminated the preserved traces using laser-stimulated fluorescence. Similarly to E. zhengi, the rhamphotheca has been preserved only as a two-dimensional trace, whereas ungual sheaths are preserved in three dimensions. In contrast to the traces preserved in C. dui, the rhamphotheca in the discussed specimen of C. sanctus is straight rather than upturned. This hints towards hidden morphological diversity within the thousands of Confuciusornis specimens, in which species may be further differentiated by soft tissue features or behaviors, much like many living birds, that cannot be detected in fossils, even with exceptional preservation

Transformation of the pectoral girdle in pennaraptorans: critical steps in the formation of the modern avian shoulder joint

Important transformations of the pectoral girdle are related to the appearance of flight capabilities in the Dinosauria. Previous studies on this topic focused mainly on paravians yet recent data suggests flight evolved in dinosaurs several times, including at least once among non-avialan paravians. Thus, to fully explore the evolution of flight-related avian shoulder girdle characteristics, it is necessary to compare morphology more broadly. Here, we present information from pennaraptoran specimens preserving pectoral girdle elements, including all purportedly volant taxa, and extensively compare aspects of the shoulder joint. The results show that many pectoral girdle modifications appear during the evolution from basal pennaraptorans to paravians, including changes in the orientation of the coracoid body and the location of the articulation between the furcula and scapula. These modifications suggest a change in forelimb range of motion preceded the origin of flight in paravians. During the evolution of early avialans, additional flight adaptive transformations occur, such as the separation of the scapula and coracoid and reduction of the articular surface between these two bones, reduction in the angle between these two elements, and elongation of the coracoid. The diversity of coracoid morphologies and types of articulations joining the scapula-coracoid suggest that each early avialan lineage evolved these features in parallel as they independently evolved more refined flight capabilities. In early ornithothoracines, the orientation of the glenoid fossa and location of the acrocoracoid approaches the condition in extant birds, suggesting a greater range of motion in the flight stroke, which may represent the acquisition of improved powered flight capabilities, such as ground take-off. The formation of a new articulation between the coracoid and furcula in the Ornithuromorpha is the last step in the formation of an osseous triosseal canal, which may indicate the complete acquisition of the modern flight apparatus. These morphological transitions equipped birds with a greater range of motion, increased and more efficient muscular output and while at the same time transmitting the increased pressure being generated by ever more powerful flapping movements in such a way as to protect the organs. The driving factors and functional adaptations of many of these transitional morphologies are as yet unclear although ontogenetic transitions in forelimb function observed in extant birds provide an excellent framework through which we can explore the behavior of Mesozoic pennaraptorans

Cranial osteology of the Early Cretaceous Sapeornis chaoyangensis (Aves: Pygostylia)

We describe the detailed cranial osteology of Sapeornis chaoyangensis based on information from previously described specimens and IVPP V19058, a specimen that was recently reported with regards to the palatal elements but not fully described. The skull in this specimen is entirely preserved in disarticulation, providing the most comprehensive glimpse into the morphology of the cranial bones and their articulations for this taxon. Based on the new information extracted from this specimen, we revise some interpretations of cranial elements in previously published Sapeornis specimens and provide a comprehensive description of the cranial osteology for this basal pygostylian. The results of this study demonstrate that the postorbital bar, jugal bar, nasal, quadrate, and palate in Sapeornis show the plesiomorphic rigid articulations among birds. The comparison of cranial features across Paraves suggests that major cranial modifications evolved at the base of Ornithothoraces, and primarily in the ornithuromorph lineage

Histology of the femur of IVPP V18687: A, entire section; B, close up image of the section.

<p>Histology of the femur of IVPP V18687: A, entire section; B, close up image of the section.</p

A New Species of Pengornithidae (Aves: Enantiornithes) from the Lower Cretaceous of China Suggests a Specialized Scansorial Habitat Previously Unknown in Early Birds

<div><p>We describe a new enantiornithine bird, <i>Parapengornis eurycaudatus</i> gen. et sp. nov. from the Lower Cretaceous Jiufotang Formation of Liaoning, China. Although morphologically similar to previously described pengornithids <i>Pengornis houi</i>, <i>Pengornis</i> IVPP V18632, and <i>Eopengornis martini</i>, morphological differences indicate it represents a new taxon of the Pengornithidae. Based on new information from this specimen we reassign IVPP V18632 to <i>Parapengornis</i> sp. The well preserved pygostyle of the new specimen elucidates the morphology of this element for the clade, which is unique in pengornithids among Mesozoic birds. Similarities with modern scansores such as woodpeckers may indicate a specialized vertical climbing and clinging behavior that has not previously been inferred for early birds. The new specimen preserves a pair of fully pennaceous rachis-dominated feathers like those in the holotype of <i>Eopengornis martini</i>; together with the unique morphology of the pygostyle, this discovery lends evidence to early hypotheses that rachis-dominated feathers may have had a functional significance. This discovery adds to the diversity of ecological niches occupied by enantiornithines and if correct reveals are remarkable amount of locomotive differentiation among Enantiornithes.</p></div