Jingmai K. O'Connor

A Nearly Modern Amphibious Bird from the Early Cretaceous of Northwestern China

Three-dimensional specimens of the volant fossil bird Gansus yumenensis from the Early Cretaceous Xiagou Formation of northwestern China demonstrate that this taxon possesses advanced anatomical features previously known only in Late Cretaceous and Cenozoic ornithuran birds. Phylogenetic analysis recovers Gansus within the Ornithurae, making it the oldest known member of the clade. The Xiagou Formation preserves the oldest known ornithuromorph-dominated avian assemblage. The anatomy of Gansus, like that of other non-neornithean (nonmodern) ornithuran birds, indicates specialization for an amphibious life-style, supporting the hypothesis that modern birds originated in aquatic or littoral niches.

Phylogenetic Support for a Specialized Clade of Cretaceous Enantiornithine Birds with Information from a New Species

A new species of enantiornithine bird from the Lower Cretaceous Yixian Formation of northeastern China is reported. The new taxon, Shanweiniao cooperorum, possesses several enantiornithine synapomorphies as well as the elongate rostral morphology (rostrum equal to or exceeding 60% the total length of the skull) of the Chinese early Cretaceous enantiornithines, Longipteryx chaoyangensis and Longirostravis hani. The discovery of this new specimen highlights the existence of a diverse clade of trophically specialized enantiornithines, Longipterygidae, for which we present phylogenetic support in a new comprehensive cladistic analysis of Mesozoic birds. Shanweiniao provides new information on the anatomy of longipterygids, and preserves a rectricial morphology previously unknown to enantiornithines, with at least four tail feathers closely arranged. This supports the hypothesis that enantiornithines were strong fliers and adds to the diversity of known tail morphologies of these Cretaceous birds.

A redescription of Chaoyangia beishanensis (Aves) and a comprehensive phylogeny of Mesozoic birds

We review the enigmatic Chaoyangia beishanensis, one of the earliest birds described from the Jiufotang Formation, north-eastern China, and the first to be identified as an ornithurine (Aves: Ornithothoraces) and thus a member of the clade that includes living birds. A complete discussion of the validity of this taxon, which once included the holotype of Songlingornis, is provided, along with a revised diagnosis. The morphology of Chaoyangia is described, including extensive comparison with better known, recently discovered ornithurines as well as several other groups of Mesozoic birds (Confuciusornithiformes, Sapeornithiformes, Enantiornithes). Although preserved information is limited, the large number of fused sacral vertebrae and presence of a distal dorsal process on the ischium are among the features supporting early hypotheses that the only known specimen of Chaoyangia represents an ornithurine. Unique among ornithurines, Chaoyangia possesses two dorsal processes on the ischium, and thus remains a valid taxon. We include this taxon in a cladistic analysis to test morphological hypotheses regarding its systematic position. Although the results of the analysis are highly resolved and support the referral of Chaoyangia and Zhongjianornis to Ornithurae, support for the tree overall is very low. Recently discovered taxa have blurred the once clear morphological gap separating the two ornithothoracine clades (Ornithurae and Enantiornithes), and thus the increase in taxonomic diversity has caused a decrease in the stability of hypothetical&break; relationships.

Preservation of ovarian follicles reveals early evolution of avian reproductive behaviour

The two groups of archosaurs, crocodilians and birds, form an extant phylogenetic bracket for understanding the reproductive behaviour of dinosaurs. This behaviour is inferred from preserved nests and eggs, and even gravid individuals. Data indicate that many ‘avian’ traits were already present in Paraves—the clade that includes birds and their close relatives—and that the early evolution of the modern avian form of reproduction was already well on its way. Like living neornithine birds, non-avian maniraptorans had daily oviposition and asymmetrical eggs with complex shell microstructure, and were known to protect their clutches. However, like crocodilians, non-avian maniraptorans had two active oviducts (one present in living birds), relatively smaller eggs, and may not have turned their eggs in the way that living birds do. Here we report on the first discovery of fossilized mature or nearly mature ovarian follicles, revealing a previously undocumented stage in dinosaur reproduction: reproductively active females near ovulation. Preserved in a specimen of the long bony-tailed Jeholornis and two enantiornithine birds from the Early Cretaceous period lacustrine Jehol Biota in northeastern China, these discoveries indicate that basal birds only had one functional ovary, but retained primitive morphologies as a result of their lower metabolic rate relative to living birds. They also indicate that basal birds reached sexual maturity before skeletal maturity, as in crocodiles and paravian dinosaurs. Differences in follicular morphology between Jeholornis and the enantiornithines are interpreted as forming an evolutionary gradient from the reproductive condition in paravian dinosaurs towards neornithine birds. Furthermore, differences between the two enantiornithines indicate that this lineage might also have evolved advanced reproductive traits in parallel to the neornithine lineage.

The completeness of the fossil record of mesozoic birds: implications for early avian evolution.

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.

A New Enantiornithine from the Yixian Formation with the First Recognized Avian Enamel Specialization

ABSTRACT We report on a new enantiornithine bird, Sulcavis geeorum, gen. et sp. nov., from the Jehol Group of northeastern China. The fossil preserves robust teeth with longitudinal grooves radiating from the occlusal tip preserved in the enamel on the lingual surface. This is the first known occurrence of specialized tooth enamel within Aves. Compared with other Mesozoic groups, stomach contents are hardly ever preserved within enantiornithine specimens; therefore, this new tooth morphology reveals new evidence regarding the diversity of trophic niches occupied by the clade. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

A New Ornithuromorph (Aves: Ornithothoraces) Bird from the Jehol Group Indicative of Higher-Level Diversity

ABSTRACT Basal Ornithuromorpha, until recently, was one of the most poorly documented segments of early avian evolution. The known species diversity of the ornithuromorph clade has increased rapidly with the addition of new discoveries from the Early Cretaceous deposits of northeastern China. Reported in this paper is the discovery of a new bird from the Lower Cretaceous Yixian Formation, Liaoning Province, China. The specimen represents a new species, Longicrusavis houi, but bears similarities to Hongshanornis longicresta from the same formation of Inner Mongolia. The two birds are comparable in size and share an unusual sigmoid mandible and elongate hindlimbs relative to their forelimbs. Together these taxa represent a clade (Hongshanornithidae, new taxon) of specialized ‘shorebirds’ whose elongate hindlimbs indicate ecological adaptations different from those of other Jehol ornithuromorphs. Phylogenetic relationships of Mesozoic birds are discussed based on the results of a comprehensive cladistic analysis. New morphological information on Ornithuromorpha is provided through the detailed description of the new taxon together with new information on Hongshanornis.

Anatomy of the basal ornithuromorph bird Archaeorhynchus spathula from the Early Cretaceous of Liaoning, China

ABSTRACT As one of the earliest-known beaked ornithuromorph birds, Archaeorhynchus spathula is important for understanding the early evolution of this derived avian clade. Recently, two new complete and articulated specimens of Archaeohyrnchus spathula were collected from Lower Cretaceous deposits of Jianchang, Liaoning, northeastern China. These specimens preserve new anatomical information, particularly in regards to the morphology of the skull (including the inner structure of the occiput), forelimbs, hind limbs, and sternum. The sternum is similar to that of an unnamed ornithuromorph from the Xiagou Formation, Gansu Province. These new specimens are subadult: together with the holotype of Archaeorhynchus, they constitute the entire Early Cretaceous record of subadult ornithuromorphs and reveal important information regarding the ontogeny of this group. The caudal vertebrae are complete in the two referred specimens and provide information about the development of the pygostyle in basal ornithuromorphs. At least four vertebrae form the pygostyle of Archaeorhynchus and fusion progresses distoproximally. The preservation of gastroliths in all known specimens of Archaeorhynchus signifies that it was likely herbivorous.

Insight into the early evolution of the avian sternum from juvenile enantiornithines

The sternum is one of the most important and characteristic skeletal elements in living birds, highly adapted for flight and showing a diverse range of morphologies. New exceptional material of young juvenile specimens from the Early Cretaceous Jehol Group in northeastern China reveals the unique sequence of development in the sternum of Enantiornithes, the dominant clade of Cretaceous birds. We recognize six ossifications that together form the sternum, three of which were previously unknown. Here we show that although basal living birds apparently have retained the dinosaurian condition in which the sternum develops from a bilateral pair of ossifications (present in paravian dinosaurs and basal birds), the enantiornithine sternal body primarily develops from two unilateral proximo-distally arranged ossifications. This indicates that although superficially similar, the sternum formed very differently in enantiornithines and ornithuromorphs, suggesting that several ornithothoracine sternal features may represent parallelism. This highlights the importance of ontogenetic studies for understanding homology and the evolution of skeletal features in palaeontology.

Anatomy of the Early Cretaceous Enantiornithine Bird Rapaxavis pani

The exquisitely preserved longipterygid enantiornithine Rapaxavis pani is redescribed here after more extensive preparation. A complete review of its morphology is presented based on information gathered before and after preparation. Among other features, Rapaxavis pani is characterized by having an elongate rostrum (close to 60% of the skull length), rostrally restricted dentition, and schizorhinal external nares. Yet, the most puzzling feature of this bird is the presence of a pair of pectoral bones (here termed paracoracoidal ossifications) that, with the exception of the enantiornithine Concornis lacustris, are unknown within Aves. Particularly notable is the presence of a distal tarsal cap, formed by the fusion of distal tarsal elements, a feature that is controversial in non-ornithuromorph birds. The holotype and only known specimen of Rapaxavis pani thus reveals important information for better understanding the anatomy and phylogenetic relationships of longipterygids, in particular, as well as basal birds as a whole.