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, 208 (3), 287-308

Insight Into the Evolution of Avian Flight From a New Clade of Early Cretaceous Ornithurines From China and the Morphology of Yixianornis Grabaui

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Insight Into the Evolution of Avian Flight From a New Clade of Early Cretaceous Ornithurines From China and the Morphology of Yixianornis Grabaui

Julia A Clarke et al. J Anat.

Abstract

In studies of the evolution of avian flight there has been a singular preoccupation with unravelling its origin. By contrast, the complex changes in morphology that occurred between the earliest form of avian flapping flight and the emergence of the flight capabilities of extant birds remain comparatively little explored. Any such work has been limited by a comparative paucity of fossils illuminating bird evolution near the origin of the clade of extant (i.e. 'modern') birds (Aves). Here we recognize three species from the Early Cretaceous of China as comprising a new lineage of basal ornithurine birds. Ornithurae is a clade that includes, approximately, comparatively close relatives of crown clade Aves (extant birds) and that crown clade. The morphology of the best-preserved specimen from this newly recognized Asian diversity, the holotype specimen of Yixianornis grabaui Zhou and Zhang 2001, complete with finely preserved wing and tail feather impressions, is used to illustrate the new insights offered by recognition of this lineage. Hypotheses of avian morphological evolution and specifically proposed patterns of change in different avian locomotor modules after the origin of flight are impacted by recognition of the new lineage. The complete articulated holotype specimen of Yixianornis grabaui, from the Early Cretaceous Jiufotang Formation of Liaoning Province, in north-eastern China, arguably the best-preserved basal ornithurine specimen yet discovered, provides the earliest evidence consistent with the presence of extant avian tail feather fanning.

Figures

Fig. 1
Fig. 1
Map of Liaoning Province, China, showing the Yixianornis grabaui holotype specimen locality near the town of Qianyang, approximately 3 km south-west of Yixian City.
Fig. 2
Fig. 2
The Yixianornis grabaui holotype specimen (IVPP V 13631) with preservation of primary wing and tail feathers. Some body feathers are also preserved close to the ventral edge of the thoracic region and near the neck.
Fig. 9
Fig. 9
Details of the tail feathering preserved in Yixianornis grabaui. Numbers placed at the distal ends of feathers correspond to the eight identified rectrices. Arrows and ‘?’ are explained in the text.
Fig. 3
Fig. 3
The skull of Yixianornis grabaui exposed in ventral view. See Appendix 1 for anatomical abbreviations.
Fig. 4
Fig. 4
Presacral vertebrae and pectoral girdle of Yixianornis grabaui. See Appendix 1 for anatomical abbreviations.
Fig. 5
Fig. 5
Sacral vertebrae, pelvic girdle and limb of Yixianornis grabaui. See Appendix 1 for anatomical abbreviations. Dashed regions in the line drawing indicate broken or crushed areas.
Fig. 6
Fig. 6
Furcula of Yixianornis grabaui prior to damage. See Appendix 1 for anatomical abbreviations.
Fig. 10
Fig. 10
Strict consensus cladogram of the two most parsimonious trees resulting from analysis of the characters and taxa in Appendices 2 and 3[length: 422, CI: 0.63, RI 0.82, RC 0:51 (PIC only)]. Bootstrap support for those nodes recovered in greater than 50% of the 1000 replicates performed and Bremer (1988) support values are reported to the right of the node to which they apply (Format: Bootstrap/Bremer). Mesozoic avialans with tail feathering known are pictured, Protopteryx is shown for Enantiornithes as it is basally placed within that group. No Enantiornithes are known to possess more than two elongate rectrices, although some possess none at all (Zhang & Zhou, 2004). Pygostyle morphology is illustrated for Sapeornis, Confuciusornis, Iberomesornis and Yixianornis and shown in insets. One unambiguous synapomorphy supports the clade Yixianornis + Songlingornis (Node A). Monophyly of new clade of Chinese ornithurines (Node B) is supported by four unambiguously optimized synapomorphies (numbers refer to characters and states listed in Appendix 2): 53:0, cervical vertebrae not completely heterocoelous; 94:1, presence of a procoracoid process; 99:1, coracoid without groove at medial opening of n. supracoracoideus foramen; 79:2, medial posterior process of sternum joined to sternal midline to enclose sternal fenestra; 85:1, base of furcula with a truncate or squared base. Seven unambiguous synapomorphies support placement of the clade as more closely related to Aves than Patagopteryx: 55:2, ten or fewer thoracic vertebrae; 95:1, intermuscular line on ventral surface of the coracoid; 107:1, humeral head domed or globose; 143:1, metacarpal III narrow compared with diameter of metacarpal II; 152:1, phalanx II.1 strongly dorsoventrally compressed; 160:1, antitrochanter posterodorsally located with respect to acetabulum; 193:1, metatarsal III displaced posterior to II and IV proximally. Apsaravis is placed more closely to Aves than this new lineage by six unambiguous synapomorphies: 62:4, ten sacral vertebrae; 67:1, non contacting pre- and postzygopophyses in tail; 146:1, metacarpal I articulation developed as a shelf; 159:2, ischium and pubis subparallel and appressed; 169:1, pubis compressed mediolaterally; 188:2, posterior portion of distal articular surface of tibiotarsus for tibial cartilage with well-developed wings.
Fig. 7
Fig. 7
Pectoral limb of Yixianornis grabaui. See Appendix 1 for anatomical abbreviations. Dashed regions in the line drawing indicate broken or crushed areas. The dashed area near the sternal margin in the left arm has been repaired with glue.
Fig. 8
Fig. 8
The feet of Yixianornis grabaui: the right is exposed in ventral view (top) and the left is exposed in dorsolateral view (bottom). See Appendix 1 for anatomical abbreviations.
Fig. 11
Fig. 11
Skeletal reconstruction of Yixianornis grabaui by G. S. Paul.

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