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Chicago Archaeopteryx informs on the early evolution of the avian bauplan

Nature volume 641, pages 1201–1207 (2025)Cite this article

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Abstract

Here we report on the nearly complete and uncrushed 14th specimen of Archaeopteryx. Exceptional preservation and preparation guided by micro-computed tomographic data make this one of the best exemplars of this iconic taxon, preserving important data regarding skeletal transformation and plumage evolution in relation to the acquisition of flight during early avian evolution. The ventrolaterally exposed skull reveals a palatal morphology intermediate between troodontids1 and crownward Cretaceous birds2,3. Modifications of the skull reflect the shift towards a less rigid cranial architecture in archaeopterygids from non-avian theropods. The complete vertebral column reveals paired proatlases and a tail longer than previously recognized. Skin traces on the right major digit of the hand suggest that the minor digit was free and mobile distally, contrary to previous interpretations4. The morphology of the foot pads indicates that they were adapted for non-raptorial terrestrial locomotion. Specialized inner secondary feathers called tertials5,6 are observed on both wings. Humeral tertials are absent in non-avian dinosaurs closely related to birds, suggesting that these feathers evolved for flight, creating a continuous aerodynamic surface. These new findings clarify the mosaic of traits present in Archaeopteryx, refine ecological predictions and elucidate the unique evolutionary history of the Archaeopterygidae, providing clues regarding the ancestral avian condition.

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Fig. 1: Photograph and interpretive line drawing of Archaeopteryx FMNH PA 830.
Fig. 2: Skull of Archaeopteryx FMNH PA 830 and palatal reconstructions of select paravian theropods with a cladogram depicting their relationships.
Fig. 3: Morphological details of the atlas–axis complex and distalmost caudal vertebrae in Archaeopteryx FMNH PA 830.
Fig. 4: Detail photographs of preserved skin tissue in the Chicago Archaeopteryx FMNH PA 830.

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Data availability

Raw CT data and segmented models relevant to this manuscript are available via Morphosource (Media ID 000702228; Media ID 000702231).

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Acknowledgements

We thank T. Lumbsch, B. Lauer and R. Lauer for facilitating the specimen acquisition; A. Stroup for assisting with figures; M. Colbert, J. Maisano and D. Edey for scanning the main slab; C. Wang and X. Zhang for segmentation assistance; S. Selzer for aiding preparation; D. Drummond and J. Stierberger for photographing the specimen; and B. Marks for access to extant bird specimens. M.W. is supported by the National Natural Science Foundation of China (42225201).

Author information

Authors and Affiliations

  1. Negaunee Integrative Research Center, Field Museum of Natural History, Chicago, IL, USA

    Jingmai O’Connor, Alexander Clark, Pei-Chen Kuo, Akiko Shinya & Constance Van Beek

  2. Committee on Evolutionary Biology, University of Chicago, Chicago, IL, USA

    Alexander Clark

  3. School of Zoology, Faculty of Life Sciences, Tel Aviv University, Tel Aviv, Israel

    Yosef Kiat

  4. Steinhardt Museum of Natural History, Tel Aviv University, Tel Aviv, Israel

    Yosef Kiat

  5. Center for Functional Anatomy and Evolution, Johns Hopkins University School of Medicine, Baltimore, MD, USA

    Matteo Fabbri

  6. Key Laboratory of Vertebrate Evolution and Human Origins, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China

    Jing Lu, Min Wang & Han Hu

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Contributions

J.O’C. led the research and wrote the manuscript; H.H. led the segmentation; M.F., P.-C.K. and J.L. assisted with segmentation; A.C., P.-C.K., M.F., H.H., Y.K. and M.W. assisted with research and manuscript preparation; A.S. and C.V.B. prepared the specimen.

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Correspondence to Jingmai O’Connor or Han Hu.

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Nature thanks Jessie Atterholt, Daniel Field, Johan Lindgren, Patrick O'Connor and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data figures and tables

Extended Data Fig. 1 3D reconstructed models of cranial elements based on CT data of Archaeopteryx FMNH PA 830.

a-c, premaxillae in right lateral, ventral, and caudal view; d-f, vomer in dorsal, left lateral, and ventral view; g-i, right maxilla in lateral, medial and ventromedial view; j-l, right postorbital in medial, rostral, and medial view; m-p, left ectopterygoid in ventral, dorsal, lateral, and medial view; q-v, left pterygoid in ventral, dorsal, cranial, caudal, lateral, and medial view; w-aa, left palatine in rostral, dorsal, ventral, lateral, and medial view. Scale bar equals five millimeters. cf, caudal fork; cp, choanal process; fp, frontal process; jp, jugal process; lp, lateral process; mf, maxillary fenestra; mp, maxillary process; np, nasal process; pf, pterygoid flange; pr, promaxillary fenestra; pp, premaxillary process; pt, pterygoid process; qw, quadrate wing; rp, rostral process; sp, squamosal process.

Extended Data Fig. 2 Reconstructions of the palate.

a, modified from Elzanowski (2001); b, modified from Mayr et al.13; c, current reconstruction based on new data from the Chicago Archaeopteryx (FMNH PA 830). The palatal elements are color coded: orange, vomer; green, palatine; dark orange, ectopterygoid; pink, jugal; light blue, pterygoid; dark blue, quadrate; yellow, basicranium.

Extended Data Fig. 3 CT data of the proatlas and atlas in the Chicago Archaeopteryx and the comparative anatomy of the proatlas and atlas (atlas intercentrum and neurapophyses) in paravians.

Elements, all figured in right lateral view, are to scale relative to each other but not between taxa. The proatlas is colored olive green; the atlas neurapophysis is pink; the atlas intercentrum is light green. Sinovenator is based on PMOL (Paleontology Museum of Liaoning) -AD00102; Tsaagan is based on IGM, Institute of Geology, Mongolian Academy of Sciences, 100/105; and Deinonychus is based on YPM, Yale Peabody Museum, 5210. Dashed lines indicate poor preservation. Fused atlas (only right neurapophysis could be reconstructed) in Archaeopteryx FMNH PA 830 in caudal (a) and cranial (b) view; right proatlas in lateral (c), medial (d), caudal (e), and cranial (f) view.

Extended Data Fig. 4 Close up photograph (a) and interpretative line drawing (b) of the left wrist in Archaeopteryx FMNH PA 830.

Scale bar equals 2 mm. Anatomical abbreviations not listed in Fig. 1 caption: sl, semilunate carpal. Grey indicates damaged bone. The carpals were identified based on 2D observations; the smallest carpals (scapholunare and distal carpal 3) could not be reconstructed from the CT data.

Extended Data Fig. 5 The plantar foot pads of terrestrial, arboreal, and raptorial birds in right lateral and plantar aspects.

Extant arboreal taxa Hylocichla mustelina (a) and Cyanocitta cristata (b); terrestrial taxon Coturnix coturnix (c-d); raptorial taxon Buteo jamaicensis (e-f). Fossil taxa represented by (g-h) enantiornithine indet. HPG-15-1 (Xing et al., 2017); (i-j) Archaeopteryx FMNH PA 830; and (k) Microraptor STM5-109 (Pittman et al. 40). All extant specimens represent recently deceased individuals being prepared for the skeletonization at the Field Museum and thus lack catalog numbers. All extant bird photos by A. D. Clark.

Extended Data Fig. 6 Preserved feather tracts in the Chicago Archaeopteryx FMNH PA 830.

Scale bar equals five centimeters.

Extended Data Fig. 7 Plumage details in Archaeopteryx FMNH PA 830.

a, scapulars and left humeral and ulnar tertials under UVABC; b, c, close up of the left tertials showing the preservation of barbs, rachises, and vane symmetry; d, open pennaceous body feathers preserved dorsal to the proximal caudal vertebrae and projecting off the left femur; e, close up of the open pennaceous feathers preserved in (d). Scale bar in (a) equals 2 cm, 5 mm in (b-c, e), and 1 cm in (d). Anatomical abbreviations not listed in Fig. 1 caption: ba, barbs; cv, wing coverts; op, open pennaceous body feathers; rh, rachis.

Extended Data Fig. 8 The function of tertial feathers.

In many modern birds, the tertial feathers (black arrows; red feathers, inset) are morphologically distinct. These feathers function as protection for the remiges from abrasion when the bird is not flying and are particularly developed in species that feed on the ground, such as (a) wagtails (Motacillidae; pictured, Eastern Yellow Wagtail Motacilla tschutschensis; photo by Y. Kiat), and (b) many shorebirds (Charadriiformes; pictured, Buff-breasted Sandpiper Calidris subruficollis; photo by I. Davies, ML110171881, adapted with permission from Cornell Lab of Ornithology | Macaulay Library). These feathers may also help close the gap between the secondary feathers and the bird’s body, thereby improving aerodynamic performance. Additionally, the tertials may be used for display and visual communication, as observed among some cranes (Gruiformes), such as the elongated tertials in the (c) Blue Crane (Anthropoides paradiseus; photo by M. McCloy, ML136331631, adapted with permission from Cornell Lab of Ornithology | Macaulay Library), or the prominent white tertials contrasting with the dark wing and body plumage in the (d) Pale-winged Trumpeter (Psophia leucoptera; photo by T. Palliser, ML62921801, adapted with permission from Cornell Lab of Ornithology | Macaulay Library).

Extended Data Fig. 9 Reconstruction of the plumage of Archaeopteryx by Michael Rothman.

The black and white pattern of the feathers is based on previous research that indicates the isolated wing covert was black and white (Carney et al., 2012). This Chicago specimen reveals the presence of the enlarged tertial tract.

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O’Connor, J., Clark, A., Kuo, PC. et al. Chicago Archaeopteryx informs on the early evolution of the avian bauplan. Nature 641, 1201–1207 (2025). https://doi.org/10.1038/s41586-025-08912-4

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