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BMC Ecology and EvolutionPaleontologyPeer-reviewed journal article

Mandibular morphology clarifies phylogenetic relationships near the origin of crown birds

Abi CraneiD, Juan BenitoiD, Albert CheniD, Daniel T. KsepkaiD, Daniel J. FieldiD

PDFDOIPreprint

In-depth summary

Claude Opus 5.5 · xhigh effort · 8 Nov 2026 · from the full text of version 1 (29 Sept 2025) · CC BY 4.0

Mandibular µCT reconstructions favour galliform-like, not palaeognath, affinities for Asteriornis; Vegavis and pelagornithid jaws lack key galloanseran synapomorphies and instead share features with stem bird Ichthyornis.

Where Asteriornis, Vegavis and pelagornithids sit near the base of crown birds is disputed, partly because incomplete fossils supply few characters and adult avian lower-jaw bones can fuse until they cannot be told apart. Using µCT of juvenile extant birds to characterize individual mandibular bones of major crown lineages for the first time, the work re-examines the jaw evidence, including the lone dentary character that had grouped Asteriornis with palaeognaths.

Key results

  1. ShownReassembling the better-preserved right ramus shows that the caudal fork of the Asteriornis dentary is shallow and asymmetric, with a smaller dorsal prong as in galliforms and anhimids. It does not have the deep, near-equal fork of palaeognaths.

    Segmented µCT of holotype NHMM 2013 008; the deep-fork reading came from the broken left side. This fork was the only synapomorphy uniting Asteriornis with Palaeognathae in the Torres et al. (2021) dataset. Other palaeognath-like traits (no splenial-prearticular contact, small splenial, straight surangular margin) also occur in galliforms; a fully unfused splenial also occurs in Megapodiidae. · Dentary; Does Asteriornis have a palaeognath-like…

  2. SuggestedThe Asteriornis mandible is mostly galliform-like. The hooked 'retroarticular process' of the initial description is reinterpreted as a displaced medial mandibular process, and the caudal jaw is too damaged to show whether a true retroarticular process existed.

    Galliform-like: narrow rami, small rod-shaped splenial, straight surangular margin, low elongate lateral mandibular process. Galloanseran-like: a large, dorsoventrally narrow medial process and two diagonally set cotyles (as in megapodes and anhimids, but also Ichthyornis; bicotyly is potentially plesiomorphic for Neornithes). Few anseriform-like traits, some shared with Ichthyornis. · Post-dentary complex; Asteriornis—the case for galloanseran affinities…; Wonderchicken or Wonderduck?

  3. SuggestedThe Vegavis jaw (AMNH FARB 30899) shows no feature that clearly links it to Anseriformes or Galloanserae. Instead it resembles Ichthyornis and pelagornithids, including a probable simple joint within the ramus (intraramal joint).

    No lateral dentary ridge, recessus conicalis, hooked retroarticular process or anseriform-type medial process base; its two cotyles lie almost perpendicular to the ramus, unlike galloanserans. Shared with Ichthyornis and pelagornithids: a lateral groove, a ramus deepest at the dentary/post-dentary contact, and breaks at matching points in AMNH FARB 30899 and MACN-PV 19.748, read as a possible intraramal joint. · Inferences from taphonomy; Post-Dentary Complex; Does Vegavis have an anseriform-like mandible?; Is the Vegavis mandible clearly neornithine?

  4. SuggestedIn the pelagornithid Dasornis toliapica (holotype NHMUK 44096), the dentary and splenial appear fused. The intraramal joint is a ball-and-socket articulation like that of Ichthyornis, not like the intraramal hinges of neoavians.

    A cotyle in the dentary/splenial receives a round angular/prearticular condyle at the joint's ventral edge, as in Ichthyornis. In neoavians (e.g. Morus) the angular is a thin sheet between dentary and splenial, which stay sutured in adults; any condylar joint there is a separate internal ossification. This questions galloanseran and crown-group placement of pelagornithids. · Dentary/Splenial; Intraramal joint; Do pelagornithids have a neornithine mandible?; Implications for pelagornithid phylogenetic…

  5. ClaimedThe authors propose that the ancestral crown-bird mandible had two articular cotyles, an unforked dentary, a straight mandibular profile and a rostrally extensive prearticular. On this view, the forked-dentary suite shared by galliforms and palaeognaths evolved independently.

    Two cotyles in all galloanserans, Asteriornis, Vegavis, Pelagornithidae, some neoavians and Ichthyornis (hesperornithines have three); the tricondylar joints of palaeognaths and Neoaves differ. The unforked-dentary suite occurs in anseriforms, pelagornithids, in part Vegavis, Ichthyornis and Hesperornithes. No formal ancestral-state analysis; the authors say further analyses are needed. · Inferring mandibular morphology of the ancestral…

Methods, data and reproducibility

Mandibles of skeletally immature extant birds and fossil Asteriornis, Dasornis, Ichthyornis and Vegavis were µCT-scanned or taken from published scans and segmented in VGSTUDIO MAX, separating bones where the data allowed (not for the Vegavis post-dentary complex). Fragmented or composite jaws were reassembled in Autodesk Maya and compared qualitatively; no phylogenetic analysis is reported.

  • Asteriornis maastrichtensis holotype mandible (latest Cretaceous, Belgium), µCT-scanned at the Cambridge Biotomography Centre, segmented and digitally reassembled · Asteriornis maastrichtensis (fossil bird) · NHMM 2013 008
  • Vegavis iaai skull-specimen mandible: CT data and meshes from Torres et al. (2025), compared with the referred specimen MACN-PV 19.748 · Vegavis iaai (fossil bird, Vega Island, Antarctica) · AMNH FARB 30899
  • Dasornis toliapica (Pelagornithidae) holotype left mandible fragment around the intraramal joint, µCT-scanned at the Natural History Museum, London · Dasornis toliapica (fossil pelagornithid bird) · NHMUK 44096
  • Ichthyornis mandibles of varying completeness (UTCT and Harvard scans, plus a predentary mesh from Torres et al. 2021), combined into one composite model scaled to BHI 6421 · Ichthyornis (fossil stem bird) · n = four specimens · YPM 1450, BHI 6421, KUVP 119673, AMNH FARB 32773
  • Skeletally immature Palaeognathae (Struthio camelus, Dromaius novaehollandiae, Tinamus solitarius), µCT-scanned and segmented bone by bone · birds (Palaeognathae)
  • Skeletally immature Anseriformes (Chauna chavaria, Thalassornis leuconotus, Anser fabalis) and Galliformes (Megapodius pritchardii, Gallus gallus), µCT-scanned and segmented · birds (Galloanserae)
  • Skeletally immature neoavians with intraramal joints (Morus bassanus, Diomedea exulans), µCT-scanned and segmented · birds (Neoaves)
  • Comparative material: extant bird specimens at the University of Cambridge Museum of Zoology (UMZC) and surface scans of an undescribed Miocene pelagornithid from Peru · birds · MUSM 1677
Methods & tools
High-resolution µCT scanning, VGSTUDIO MAX 3.4 (Volume Graphics), Autodesk Maya 2023
Reagents & environment
VGSTUDIO MAX 3.4 (Volume Graphics) for segmentation and rendering; Autodesk Maya 2023 for 3D reconstruction.
Reproducibility gaps
The CT datasets generated in the study are available only from the corresponding author on reasonable request; no public repository is given. The Vegavis CT data and the Ichthyornis predentary mesh were reused from Torres et al. (2025) and Torres et al. (2021).
Limitations (6)
  • The caudal ends of both Asteriornis mandibles are too poorly preserved to confirm or rule out a retroarticular process. The apparent separation of surangular and angular may reflect taphonomic displacement. · stated by the authors
  • Vegavis CT data (high-radiodensity inclusions, poor contrast) did not allow the post-dentary bones to be separated. Assigning AMNH FARB 30899 to Vegavis iaai warrants further study, given its differences from MACN-PV 19.748. · stated by the authors
  • Pyrite infill in the fragmentary Dasornis holotype hampers CT assessment of internal anatomy, so several described details are hypotheses to be tested if better pelagornithid material is found. · stated by the authors
  • The galliform-like traits of Asteriornis might instead be galloanseran symplesiomorphies. Whether the similarities between pelagornithids and Ichthyornis are convergent or symplesiomorphic is unresolved. · stated by the authors
  • No phylogenetic analysis or coded character matrix is reported, so conclusions about affinities rest on qualitative comparison of figured bones and their support cannot be quantified. · noted in this summary
  • Extant comparisons rely on µCT-segmented juveniles of only a few species per clade (for Neoaves, only Morus bassanus and Diomedea exulans), so mandibular variation across crown birds is thinly sampled. · noted in this summary

Read this if you work on early crown-bird phylogeny or avian skull evolution and want bone-by-bone mandible comparisons bearing on Asteriornis, Vegavis or pelagornithids.

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Abstract

BackgroundThe phylogenetic relationships of fossil birds near the origin of the avian crown group remain debated, in part due to a limited amount of character evidence from incomplete fossils. The avian lower jaw provides a potentially rich source of additional character data, yet fusion of the individual bony elements composing the avian post-dentary complex has impeded efforts to explore its phylogenetic signal. Here, we use high-resolution {micro}CT-scanning to separate the individual bony elements of the mandibles of several immature crown birds and key fossil taxa, and use those data to assess support for alternative phylogenetic hypotheses for fossils near the origin of crown birds. ResultsWe find that evidence from Asteriornis fails to support interpretations of derived mandibular similarities with palaeognaths, and instead strongly favours galloanseran, and specifically galliform, affinities. Our results also illustrate striking similarities in the architecture of the lower jaws between the toothed ornithurine Ichthyornis, Pelagornithidae and Vegavis, which, in addition to the absence of derived features linking them to Galloanserae, highlights questions regarding the phylogenetic position of these perennially controversial taxa. ConclusionsOur data reveal new insight into patterns of morphological evolution near the origin of the avian crown group while raising new phylogenetic questions, emphasising the potential untapped value of detailed comparative investigations of morphological complexes such as the post-dentary complex of the mandible for informing the early evolutionary history of crown birds.

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