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| Jinguofortis perplexus
Wang, Stidham & Zhou, 2018
(artwork by Chung-Tat Cheung)
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Significance
We report the second most basal clade of the short-tailed birds (Pygostylia) from the Early Cretaceous. The new family Jinguofortisidae exhibits a mosaic assembly of plesiomorphic nonavian theropod characteristics, particularly of the fused scapulocoracoid and more derived flight-related features, further increasing the known ecomorphological diversity of basal avian lineages. We discuss the evolution of the scapula and coracoid in major tetrapod groups and early birds and hypothesize that the fused scapulocoracoid in some basal avian lineages, although rare, results from an accelerated rate of ossification and that the avian shoulder girdle likely was transformed by developmental plasticity along an evolutionary lineage leading to the crown group of birds.
Abstract
Early members of the clade Pygostylia (birds with a short tail ending in a compound bone termed “pygostyle”) are critical for understanding how the modern avian bauplan evolved from long-tailed basal birds like Archaeopteryx. However, the currently limited known diversity of early branching pygostylians obscures our understanding of this major transition in avian evolution. Here, we describe a basal pygostylian, Jinguofortis perplexus gen. et sp. nov., from the Early Cretaceous of China that adds important information about early members of the short-tailed bird group. Phylogenetic analysis recovers a clade (Jinguofortisidae fam. nov.) uniting Jinguofortis and the enigmatic basal avian taxon Chongmingia that represents the second earliest diverging group of the Pygostylia. Jinguofortisids preserve a mosaic combination of plesiomorphic nonavian theropod features such as a fused scapulocoracoid (a major component of the flight apparatus) and more derived flight-related morphologies including the earliest evidence of reduction in manual digits among birds. The presence of a fused scapulocoracoid in adult individuals independently evolved in Jinguofortisidae and Confuciusornithiformes may relate to an accelerated osteogenesis during chondrogenesis and likely formed through the heterochronic process of peramorphosis by which these basal taxa retain the scapulocoracoid of the nonavian theropod ancestors with the addition of flight-related modifications. With wings having a low aspect ratio and wing loading, Jinguofortis may have been adapted particularly to dense forest environments. The discovery of Jinguofortis increases the known ecomorphological diversity of basal pygostylians and highlights the importance of developmental plasticity for understanding mosaic evolution in early birds.
Keywords: bird, development, Mesozoic, plasticity, phylogeny
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| Jinguofortis perplexus gen. et sp. nov., IVPP V24194. Photographs of counter slab and main slab. |
Systematic Paleontology
Aves Linnaeus, 1758
Pygostylia Chiappe, 2002
Jinguofortisidae fam. nov.
Jinguofortis perplexus gen. et sp. nov.
Holotype: A complete and articulated skeleton with feathers is housed at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) under the collect number IVPP V24194 (Fig. 1 and SI Appendix, Figs. S1–S7 and Table S1).
Etymology: The generic name is derived from “jinguo” (Mandarin), referring to female warrior, and “fortis”, Latin for brave; the specific name is derived from Latin “perplexus,” referring to the combination of plesiomorphic and derived characters present in the holotype specimen.
Locality and Horizon: IVPP V24194 was collected near the village of Shixia, Weichang County, Hebei Province, China; Lower Cretaceous Dabeigou Formation of the Jehol Biota (127 ± 1.1 Ma).
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| Reconstruction of Jinguofortis perplexus, second earliest member of the short-tailed birds Pygostylia
(artwork by Chung-Tat Cheung)
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| Fig. 3. Changes to the scapula and coracoid in vertebrates with a focus on basal avian clades. A simplified tree of vertebrates (Left) shows the scapula (colored in blue) and coracoid (in green) fused into a scapulocoraocid (in gray) in most major tetrapod clades (see SI Appendix, SI Text, for details). Simplified cladogram of basal Aves (Right) shows the changes to the shoulder girdle and manus (see SI Appendix, Fig. S8, for complete phylogenetic result). Thick green lines near each clade denote temporal range with the first-appearance datum denoted. Major changes to the shoulder girdle and manus across basal avian phylogeny are summarized: (1) in most nonavian theropods, the scapula and coracoid are fused into a scapulocoracoid at an obtuse angle, and they have a manual phalangeal formula of 2–3-4; (2) scapula and coracoid become separated and form an angle of ∼90°; (3, 4) independent evolution of a fused scapulocoracoid in the Confuciusornithiformes and Jinguofortisidae; (6) minor digit is reduced resulting in a manual phalangeal formula of 2–3-2; (7) scapula and coracoid are decoupled and form an acute angle with further manual digit reduction evolving in derived ornithuromorphs; (8) alternatively, it is equally parsimonious that a fused scapulocoracoid evolved at the base of Pygostylia and was lost in pygostylians crownward of Jinguofortisidae (5). |
Min Wang, Thomas A. Stidham and Zhonghe Zhou. 2018. A New Clade of Basal Early Cretaceous Pygostylian Birds and Developmental Plasticity of the Avian Shoulder Girdle. PNAS. DOI: 10.1073/pnas.1812176115
Min Wang, Xiaoli Wang, Yan Wang and Zhonghe Zhou. 2016. A New Basal Bird from China with implications for Morphological Diversity in Early Birds. Scientific Reports. 6: 19700. DOI: 10.1038/srep19700
Chinese Cretaceous fossil highlights avian evolution eurekalert.org/e/8pkg via @EurekAlert
