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Convergent evolution of diverse jaw joints in mammaliamorphs

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Abstract

The evolution of a single-dentary-boned lower jaw and its secondary craniomandibular articulation between the dentary condyle and the squamosal glenoid has been regarded as a pivotal vertebrate innovation and defining mammalian trait1,2,3,4,5,6,7. Here we report two mammaliamorphs with novel shapes of secondary jaw joint, offering insight into the evolution of the mammalian jaw. The first, Polistodon8, a Middle Jurassic herbivorous tritylodontid with a relatively large body size and a lifestyle that is likely to have been fossorial, uniquely evolved a dentary–jugal articulation. The second, an Early Jurassic morganucodontan, exhibits a dentary–squamosal joint that lacks a bulbous condyle, supporting the hypothesis that the mammalian dentary condyle was formed by expansion of the lateral ridge of the dentary9. These diverse joints reflect repeated evolutionary experimentation in advanced cynodonts, in which secondary jaw joints arose independently7,10, and in which the load-bearing dentary–squamosal joint is a synapomorphy of mammaliaforms. Although body miniaturization might have driven this transformation11, our findings indicate that other factors were involved, such as jaw-muscle reorganization, feeding ecology and masticatory behaviour7,12,13,14,15,16,17. The ecomorphological diversity of these taxa suggest that phenotypic plasticity and environmentally induced morphological changes18,19,20 could have shaped jaw-joint evolution, emphasizing how ecological pressures and developmental flexibility guided the diversification of jaw structures in mammalian ancestors.

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Fig. 1: Skull and dentition of Polistodon chuannanensis (holotype, ZDM8601).
Fig. 2: Partial skull and teeth of Camurocondylus (IVPP V8685, holotype).
Fig. 3: Jaw joints of Polistodon chuannanensis and dentary condyle of Camurocondylus lufengensis gen et sp. nov. in comparison with Didelphys virginiana.
Fig. 4: Summary of types of primary and secondary jaw joints in selected advanced cynodonts, including mammals.

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Data availability

All material related to the data for phylogenetic analysis is presented in this Article and its Supplementary Information. Life science identifiers for the new genus and species have been registered at ZooBank: article: 278B9AE4-94E2-4003-ACFE-5C6497C7514E; Polistodon: 6B85D440-D84E-4A14-B952-28A1D60B016B; Polistodon chuannanensis: 2176D79D-3A6C-4293-94F1-3684488EA7B7; Camurocondylus: 454DF3EF-A9E7-4C62-9987-BB551CE54F64; Camurocondylus lufengensis: FC798AB8-CF78-498D-8B34-745EBC496CAD. The character list and data matrix for the phylogenetic analysis are presented in the Supplementary Information and have been deposited in MorphoBank (http://morphobank.org/permalink/?P5985). The 3D stl files for the skulls of Polistodon and Camurocondylus have been uploaded to the Archives of Digital Morphology, Key Laboratory of Vertebrate Evolutionary Systems Science of the Chinese Academy of Sciences (http://www.admorph.ivpp.ac.cn): Polistodon: https://doi.org/10.12112/M.36.Fossil; Camurocondylus: https://doi.org/10.12112/M.38.Fossil.

Code availability

The PAUP commands for parsimony-based analysis are in the Supplementary Information. The character list and data matrix for the phylogenetic analysis are in the Supplementary Information and have also been deposited in Zenodo (https://doi.org/10.5281/zenodo.16193266; ref. 72).

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Acknowledgements

We thank G. Peng and G. Wei for access to the specimens; Y. Hou, P. Yin and J. Wang for CT scanning of the specimens; A. Shi for help with drawings; J. Choiniere and J. Benoid for access to collections; and S. Lautenschlager and DigiMorph.org for digital data. F.M. was supported by the National Natural Science Foundation of China (42122010 and 42288201) and by the Youth Innovation Promotion Association of the Chinese Academy of Sciences (Y2023017). S.J., Y.Y., Y.L. and X.S. were supported by the Project Foundation of the Sichuan Provincial Cultural Heritage Administration (SCWW2023A02). P.W. and J.C. were supported by the Chongqing Municipal Planning and Natural Resource Bureau (ZC-2021018).

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  1. These authors contributed equally: Fangyuan Mao, Shan Jiang

Authors and Affiliations

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

    Fangyuan Mao, Jun Liu & Jicheng Ren

  2. Division of Paleontology, American Museum of Natural History, New York, NY, USA

    Fangyuan Mao & Jin Meng

  3. Research Center of the Jurassic Stratigraphy and Paleontology, Zigong Dinosaur Museum, Zigong, China

    Shan Jiang, Yong Ye, Yu Liu & Xin Shen

  4. Key Laboratory of Dinosaur Research and Protection, Sichuan Provincial Department of Culture and Tourism, Zigong, China

    Shan Jiang, Yong Ye, Yu Liu & Xin Shen

  5. Bureau of Land and Resources of Lufeng County, Lufeng, China

    Tao Wang

  6. Fossil Research Center of Chuxiong Prefecture, Chuxiong, China

    Guofu Wang

  7. Chongqing Key Laboratory of Paleontology and Paleoenvironment Co-evolution (Sichuan-Chongqing Joint Construction), Chongqing Institute of Paleontology, Chongqing, China

    Ping Wang & Juan Chen

  8. Earth and Environmental Sciences, Graduate Center, City University of New York, New York, NY, USA

    Jin Meng

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Contributions

F.M. and J.M. conceived the study and wrote the paper. F.M. and J.R. performed the CT scanning and rendering work. S.J., Y.Y., Y.L., X.S., T.W., G.W., P.W. and J.C. participated in the fieldwork and provided stratigraphic data. J.L. helped with refining the data matrix, and provided discussions and manuscript edits. All authors edited and approved the manuscript.

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Correspondence to Fangyuan Mao or Jin Meng.

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Extended data figures and tables

Extended Data Fig. 1 Holotype of Polistodon chuannanensis (ZDM8601).

a, Dorsal view with the lower jaws in occlusion; b, Lateral (left); c, ventral; d, lateral (right); e, anterior; f, posterior; g, ventrolateral (right). Abbreviations: anp, angular process; apn, anterior process of the nasal; cop, coronoid process; dec, dentary condyle; dja, dentary–jugal articulation; glj, glenoid of the jugal; iav, incisor alveolus; i, incisor; jug, jugal; lac, lacrimal; nab, nasal boss; occ, occipital condyle; pdt, postdentary trough; sac, sagittal crest; sjs, squamosal-jugal suture; sma, septomaxilla; squ, squamosal; sym, symphysis; vgz, ventral groove of the zygma (of jugal).

Extended Data Fig. 2 Digital rendered holotype of Polistodon chuannanensis (ZDM8601).

a, Ventral view of the skull with both lower jaws in occlusion. b, Lateral (right) view of the cranium; c, lateral (right) view of the squamosal, jugal, and the mandible; d, lateral (right) view of the mandible (same position in c); e, posterior view of the squamosal and jugal; f, posterior view of the squamosal, jugal, and the mandible from both sides; g, posteromedial view of the right squamosal, jugal and mandible with the coronoid attached to the dentary; h, posterior view of the skull with the left mandible digitally removed. i–k, Ventral (i), dorsal (j), and posterior (k) views of the two lower jaws. Abbreviations: anp, angular process; cop, coronoid process; cor, coronoid; dec, dentary condyle; dja, dentary–jugal articulation; dpg, dorsal process of the glenoid; glj, glenoid of the jugal; i, incisor; jug, jugal; lac, lacrimal; loj, lower jaw(s); maf, masseteric fossa; mef, mental foramen; nas, nasal; occ, occipital condyle; ocp, occipital plate (bones fused in the occiput); par, parietal; prm, premaxilla; sjs, squamosal-jugal suture; sma, septomaxilla; squ, squamosal; sym, symphysis; vgz, ventral groove of the zygma (of jugal); vpg, ventral process of the glenoid.

Extended Data Fig. 3 Digital rendered postcanines of Polistodon chuannanensis (holotype, ZDM8601).

a–c, Lateral (right, but medial for left dentition) (a), occlusal (b), and lateral (left, right medial for right dentition) (c) views of the two lower dentitions as preserved. d–g, Lingual (d), occlusal (e), dorsal (f), and lateral (g) views of the right upper postcanines. h, Occlusal view of the left upper postcanines. i,j, Lateral (i) and lingual (j) views the left dentary (digitally rendered as semitransparent) and teeth. Abbreviations: -r, right side; -l, left side; anr, anterior root; arp, articular process; cop, coronoid process; dec, dentary condyle; i, incisor; pc, lower postcanine; PC, upper postcanine; por, posterior root.

Extended Data Fig. 4 Comparison of the jaw system of P. chuannanensis (holotype, ZDM8601) with the typical tritylodontid pattern represented by Bienotheroides.

a–c, Ventral view of the cranium of P. chuannanensis (a) and Bienotheroides (b, 16YPDC-01; c, 19YP-67). d–f, Lateral views of the same specimens in a–c. g–i, Medial view of the zygomatic arch formed by the jugal and squamosal, corresponding to a–c, respectively. j–l, Medial (j), posteromedial (k), and lateral (l) views of the dentary of P. chuannanensis. m–o, Medial (m), posteromedial (n), and lateral (o) views of the dentary of Bienotheroides (20YX-1#-0510-1)45. Abbreviations: ang, angular process; arp, articular process; bwp, bulged wall of the postdentary trough; cop, coronoid process; dec, dentary condyle; dpg, dorsal process of the glenoid; glj, glenoid of the jugal; i, incisor; jug, jugal; maf, masseteric fossa; mcg, Mecklian groove; mef, mental foramen; mer, medial ridge; pdt, postdentary trough; sjs, squamosal-jugal suture; squ, squamosal; vgz, ventral groove of the zygoma (of jugal); vpg, ventral process of the glenoid.

Extended Data Fig. 5 Holotype specimen of Camurocondylus lufengensis gen. et sp. nov. (IVPP V8685).

a–c, Lateral (right and left) (a,b) and ventral (c) views of the skull as preserved (a,b) and after the mandibles were removed (c). d, Computed tomographic section showing the tooth root condition in upper and lower jaws. e,f, Ventral (e) and dorsal (f) views of the skull with the bones segmented (in colour). g,h, Lateral (g) and lateroventral (h) views of the right upper and lower dentitions in preserved occlusal relation.

Extended Data Fig. 6 Dentitions of Camurocondylus lufengensis gen. et sp. nov. (holotype, IVPP V8685).

a–c, Occlusal (a), labial (b) and lingual (c) views of the left ultimate upper premolar and M1-3. d–f, Occlusal (d), lingual (e) and labial (f) views of the right ultimate premolar and M1-3. g–i, Occlusal (g), labial (h) and lingual (i) views of left m1-3. j–l, Occlusal (j), lingual (k), and labial (l) views of the right ultimate premolar and m1-3. Tooth measurements (length/width in mm; asterisk denoting estimate): left pc (px 1.1*/0.69*; m1 2.29/0.84; m2 2.45/1.84; m3 1.95/1.04); left PC (P1 0.44*/0.23*; P2 0.63*/0.39; P3 1.89/0.72; M1 2.04/1.85; M2 1.98/0.96; M3 1.13/0.69); right pc (px 1.8*/0.82; m1 2.33/0.97; m2 2.41/1.21; m3 1.93/0.93); right PC (P1 p.45*/0.24*; P2 0.55*/0.32*; P3 1.79/0.78; M1 2.05/0.91; M2 1.95/1.0; M3 1.1/0.77).

Extended Data Fig. 7 Comparison of load-bearing dentary condyles in mammaliaforms.

a–j, The dentary condyles of various extinct groups (a-c, f-j, corresponding to the taxon names at the bottom of the panels) in comparison with those of extant mammals (d,e). In each panel, the dorsal, lateral, and posterior views of the condyle are arranged from the top to the bottom. To facilitate comparison some images have been photographically reversed so that the views are consistent throughout. All images are original except for Morganucodon, which is from Kermack et al.13 used with permission. The images are not at the same scale. Abbreviations: ang, angular process; ann, angular notch; cop, coronoid process; dec, dentary condyle; lpc, lateral projection of the condyle; ltr, lateral ridge; maf, masseteric fossa; mpc, medial projection of the condyle.

Extended Data Fig. 8 Images of jaw joints rendered from CT scans.

Images show jaw joints that are used as the basis for Fig. 4. a, Thrinaxodon (AMNH 5630), exclusive articular–quadrate joint. b, Probainognathus11, articular–quadrate joint with surangular–squamosal contact. Interpretation of the jaw joint of Thrinaxodon and Probainognathus in Fig. 4 also refers to Allin and Hopson25 (Fig. 28.8) and Luo73 (Fig. 3). c, Riograndia7 articular–quadrate joint with the dentary–squamosal contact. Interpretation of its jaw joint also refers to Soares et al.74, Rodrigues et al.75, and Kerber et al.76. d, Polistodon (ZDM8601), dentary–jugal joint; its primary joint in Fig. 4 is reconstructed based on tritylodontid specimen (IVPP V14232). e, Dianoconodon28 (IVPP V4257), partial but load-bearing dentary–squamosal jaw joint (PDSJ) with a bulbous dentary condyle and coexisting with a reduced but functional articular–quadrate joint. f, Camurocondylus (IVPP V8685), PDSJ with no expanded dentary condyle; its primary joint in Fig. 4 is reconstructed based on other morganucodontans, including Dianoconodon. g, Feredocodon71 (IMMNH-PV01925), exclusive dentary–squamosal joint (EDSJ) with postdentary bones remaining long and attached to the dentary but the articular–quadrate joint has completely lost its jaw suspension function. h, Qishou77 (JZT-D061), EDSJ for palinal jaw movement. i, Liaoconodon78,79,80 (IVPP V16051), EDSJ with the ossified Mechel’s cartilage (ONC) bridging the dentary and the detached postdentary bones (middle ear ossicles) in which the anterior processes of the malleus and ectotympanic are long and have substantial overlap with the OMC. j, Origolestes79,80 (JZD-DB0064), EDSJ with a long OMC that is decoupled from the middle ear ossicles in which the anterior processes of the malleus and ectotympanic are reduced. k, Tachyglossus aculeatus (AMNH teaching specimens, no catalogue number), EDSJ highly specialized and teeth are lost. l, Didelphis (AMNH 137900), EDSJ with medial extension of the dentary condyle, the middle ear ossicles fully suspended at the basicranial region, and a horseshoe shaped ectotympanic. m, Erinaceus (AMNH 57218), EDSJ with the ectotympanic expanded to from the tympanic bulla. All images are original except for b (3D rendering courtesy of Stephan Lautenschlager and DigiMorph.org; https://www.digimorph.org/index.phtml) and c (open-access publication).

Extended Data Fig. 9 Strict consensus tree of 211 best trees.

Tree length = 3,026; consistency index (CI) = 0.3275; homoplasy index (HI) = 0.6725; retention index (RI) = 0.8079; rescaled consistency index (RC) = 0.2646. See Supplementary Information for additional information. The blue polygon represents the load-bearing dentary–squamosal jaw joint that diagnoses Mammaliaformes (in blue shape), including mammals. Red arrows point to the phylogenetic positions of the two taxa reported in the study. Red dots indicate taxa that are used in Fig. 4 in the main text that possess the secondary jaw joint except for Thrinaxodon that has only the exclusive articular–quadrate joint. See Supplementary Information for the result of the bootstrap analysis.

Extended Data Table 1 Measurements of the Polistodon postcanine teeth (occlusal length and width)
Full size table

Supplementary information

Supplementary Information

Supplementary Information Part I provides a category summarizing different types of the primary and secondary jaw joints in known mammaliamorphs and their close relatives. In the Systematic Palaeontology, we provide the differentiate diagnoses (the amended diagnosis is in the main text) for Polistodon channanensis He and Cai, 1984 and a redescription of the holotype specimen (ZDM8601). Illustrations of ZDM8601 are in the main text and Extended Data figures. We also provide the differentiate diagnoses for the new morganucodontan genus and species Camurocondylus lufengensis. gen. et sp. nov. (the diagnosis is in the main text) and a detailed description of the holotype specimen (IVPP V8685). Supplementary Information Part I also has the character list that includes specific coding for three relevant genera, Polistodon, Camurocondylus and Diarthrognathus, in addition to other taxa used in a previous study (Mao et al., 2024a). The character list contains brief explanations about the characters and character coding that are modified based on the new data recognized in this study. Supplementary Information Part II includes the data matrix, PAUP phylogenetic analyses and the resulting logs of the analysis about the strict consensus tree. The related character list, dataset, settings and logs of these analyses are presented deposited at MorphoBank (http://www.morphobank.org) and Zenodo (https://doi.org/10.5281/zenodo.10597270) (See also Methods in the main text). Supplementary Information Part III is the online link to the 3D stl files of reconstructed skulls of the holotype of Polistodon channanensis (ZDM8601) and the holotype of Camurocondylus lufengensis. gen. et sp. nov. (IVPP V8685).

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Mao, F., Jiang, S., Liu, J. et al. Convergent evolution of diverse jaw joints in mammaliamorphs. Nature (2025). https://doi.org/10.1038/s41586-025-09572-0

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