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Jurassic shuotheriids show earliest dental diversification of mammaliaforms

Nature volume 628, pages 569–575 (2024)Cite this article

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Abstract

Shuotheriids are Jurassic mammaliaforms that possess pseudotribosphenic teeth in which a pseudotalonid is anterior to the trigonid in the lower molar, contrasting with the tribosphenic pattern of therian mammals (placentals, marsupials and kin) in which the talonid is posterior to the trigonid1,2,3,4. The origin of the pseudotribosphenic teeth remains unclear, obscuring our perception of shuotheriid affinities and the early evolution of mammaliaforms1,5,6,7,8,9. Here we report a new Jurassic shuotheriid represented by two skeletal specimens. Their complete pseudotribosphenic dentitions allow reidentification of dental structures using serial homology and the tooth occlusal relationship. Contrary to the conventional view1,2,6,10,11, our findings show that dental structures of shuotheriids can be homologized to those of docodontans and partly support homologous statements for some dental structures between docodontans and other mammaliaforms6,12. The phylogenetic analysis based on new evidence removes shuotheriids from the tribosphenic ausktribosphenids (including monotremes) and clusters them with docodontans to form a new clade, Docodontiformes, that is characterized by pseudotribosphenic features. In the phylogeny, docodontiforms and ‘holotherians’ (Kuehneotherium, monotremes and therians)13 evolve independently from a Morganucodon-like ancestor with triconodont molars by labio-lingual widening their posterior teeth for more efficient food processing. The pseudotribosphenic pattern passed a cusp semitriangulation stage9, whereas the tribosphenic pattern and its precursor went through a stage of cusp triangulation. The two different processes resulted in complex tooth structures and occlusal patterns that elucidate the earliest diversification of mammaliaforms.

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Fig. 1: Type specimens of F. chowi.
Fig. 2: Right dentition and dental morphology of F. chowi (holotype, IMMNH-009166).
Fig. 3: Tooth occlusion and competing hypotheses for tooth cusp homology of Feredocodon.
Fig. 4: Primary tooth patterns of mammaliaforms in the phylogenetic frame.

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Data availability

All material related to the data for phylogenetic analyses is presented in the article, the Extended Data and the Supplementary Information (see also ref. 64). Life Science Identifiers for the new genera and species have been registered at ZooBank as Feredocodon (http://zoobank.org; LSID urn:lsid:zoobank.org:act:AFAD135D-9191-4016-BD38-345F630E0245) and F. chowi (LSID urn:lsid:zoobank.org:act:2EE7FBE9-80E1-42E4-B8F1-E2A985808BCA). The character list and data matrix for the phylogenetic analysis have been deposited in MorphoBank (http://www.morphobank.org; project number 5075).

Code availability

The PAUP commands for parsimony-based analyses, the MrBayes commands for Bayesian analyses, the results and logs of phylogenetic analyses (including the complete apomorphic list for the consensus tree from the PAUP analysis) and ancestral state reconstruction (from the Bayesian analyses) have been deposited in Zenodo (https://doi.org/10.5281/zenodo.10597270)64.

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Acknowledgements

We thank S. Xie for specimen preparation; T. Jiang for access to comparative specimens housed in their collections; G. Peng, S. Jiang, B. Hao, G. Guang and H. Hu for discussions of localities and stratigraphies; Y. Hou, P. Yin and J. Wang for CT scanning of the specimens; and A. Shi and Y. Xu for helping with drawings. F.M. was supported by the National Natural Science Foundation of China (42288201; 42122010; 42072002), and the Youth Innovation Promotion Association CAS (2019076).

Author information

Authors and Affiliations

  1. Key Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, Beijing, China

    Fangyuan Mao & Chi Zhang

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

    Fangyuan Mao & Jin Meng

  3. Inner Mongolia Museum of Natural History, Hohhot, China

    Zhiyu Li & Zhili Wang

  4. Museums Victoria, Melbourne, Victoria, Australia

    Thomas Rich & Patricia Vickers-Rich

  5. School of Earth, Atmosphere and Environment, Monash University, Melbourne, Victoria, Australia

    Thomas Rich & Patricia Vickers-Rich

  6. School of Earth and Planetary Sciences, Curtin University, Perth, Western Australia, Australia

    Patricia Vickers-Rich

  7. Earth and Environmental Sciences, City University of New York, New York, NY, USA

    Jin Meng

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Contributions

F.M. and J.M. designed the study and wrote the paper; F.M. conducted the CT scan and rendering work; Z.L. and Z.W. collected and curated specimens and provided related stratigraphic data; C.Z. ran the Bayesian analyses; T.R. and P.V.-R. 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 Nomenclature, cusp homology and tooth occlusal relationships used in this study.

Tooth drawings are based on Crompton and Jenkins32 for morganucodontans and Kuehneotherium, Luo and Martin10 for Haldanodon, Davis49 for Kokolellia; some figures are reversed from the original ones for convenience of comparison. Based on the new evidence, we assume that the primary cusps of shuotheriids can be homologized with those of docodontidans, other non-allotherian mammaliaforms such as morganucodontans (perhaps further to non-mammaliamorph cynodonts, such as Thrinaxodon, as suggested by Crompton and Jenkins32), and the tribosphenic molar. The dot-line diagrams in the lower box show the cusp relationship. Dark dots represent main lower cusps; gray dots are main upper primary cusps. Abbreviations: A, cusp A (paracone, pa); a, cusp a (protoconid, prd); art, anterior root; b, cusp b (paraconid, pad); B, cusp B (parastyle?, pas, or stylocone?, stc); C, cusp C (metacone, me); c, cusp c (metaconid, med); d, cusp d (hypoconid, hyd); df, docodont cuspule f10,24; end, entoconid; g, cusp g (kühnecone31); hyd, hypoconid (cusp d); hyld, hypoconulid; lrt, lingual root; me, metacone (cusp C); med, metaconid (cusp c); pa, paracone (cusp A); pad, paraconid (cusp b); pas, parastyle (cusp B?); ppr, pseudoprotocone; pr, protocone; prd, protoconid (cusp a); prt, posteiror root; ptad, pseudotalonid; ptrb, pseudotrigon basin; ptrd, pseudotrigonid; stc, stylecone (cusp B?); tal, talonid; trb(s), trigon basin (secondary); trd, trigonid. X, cusp X (pseudoprotocone, ppr). The terminologies are based on Crompton and Jenkins32, Butler6, Luo and Martin10, Davis49, Averianov9, and this study.

Extended Data Fig. 2 The holotype of Feredocodon chowi (IMMNH-PV01035).

A. Slab B of the holotype, which contains most of the postcranial elements. B. Slab A of the holotype, containing the skull and mostly impressions of the postcranium. The green arrows in A and B mark two matching points of the two slabs. C-D. Close-up views of the red-boxed areas in A. E. CT-rendered skull and forepart of the postcranium. The red boxes A and B correspond to the close-up views in Extended Data Fig. 2. Abbreviations: ang, angular process; cal, calcaneus; cl, clavicle; cor, coronoid process of the dentary; cv-(1-7), cervical vertebra 1 to 7; d-(I-V), digit I to V of the manus; dcf, distal condyles of the femur; dco, dentary condyle; del, deltopectoral crest; den, dentary; dppm, dorsal process of the premaxilla; ect, ectepicondyle; ent, entepicondyle; fe, femur; feh, femoral head; fi, fibula; glf, glenoid fossa; gtr, greater trochanter; gtu, greater tubercle; hu, humera; huh, humeral head; icl, interclavicle; il, ilium; ltr, lesser trochanter; lv (1-5), lumbar vertebra 1 to 5; ltu, lesser tubercle; mas, manubrium of the sternum; mtc, metacarpals; prm, promontorium; r(1-16), rib 1 to 16; ra, radia; rac, radial condyle of the humerus; sc, scapula; sen, semilunar notch; sv-1-2, sacral vertebra 1 to 2; thv-1, first thoracic vertebra; ti, tibia; ul, ulna; ulc, ulnar condyle of the humerus; zya, zygomatic arch. The left and right side structure is denoted with l- and r-, respectively.

Extended Data Fig. 3 The paratype of Feredocodon chowi (IMMNH-PV01925).

A. Line drawing based on slab A of IMMNH-PV01925. B-C. Slabs A and B of IMMNH-PV01925. The labelled red boxes correspond to the close-up views of C-H in Extended Data Fig. 4. D-E. Reconstructed dorsal and ventral views of the partial cranium and neck region from slab B of IMMNH-PV01925. Abbreviations: ata, atlantal arch; atb, atlas body; axb, axis body; cav, caudal vertebra (with number); cvb-3, the third cervical vertebra; dec, dentary condyle; den, dens of the atlas; efan, efflected angular (mandibular) process; fe, femur; fi, fibula; iam, internal auditory meatus; il, ilium; inc, intercentrum; jf, jugular (posterior lacerate) foramen; ju, jugal; lv-1-5, lumbar vertebra 1 to 5; M(1-3), upper molar 1 to 3; mrp, medial ridge protuberance; occ, occipital condyle; pdb, postdentary bones; pfi, parafibular process; prm, promontorium; pub, pubic bone; qre, quadrate ramus of epipterygoid; r(1-16), rib 1 to 16; rar, retroarticular process; sc, scapula; sq, squamosal; sv-1-2, sacral vertebra 1 to 2; ti, tibia; ul, ulna; vlt, ventral limb of the ectotympanic (reflected lamina of the angular); zma, zygomatic arch; zpma, zygomatic process of the maxilla. The left and right side structure is denoted with l- and r-, respectively. The red line in D and E indicates the crack separating the skull that is digitally rejoined from two separate CT-scans.

Extended Data Fig. 4 The paratype of Feredocodon chowi (IMMNH-PV01925).

A-B. Dorsal and ventral views of CT-data reconstructed skull and anterior postcranium preserved on slab A of IMMNH-PV01925. C. Left scapula and forelimb. D. Right forelimb, posterior cervical and anterior thorasic vertebrae. E.Pelvic girdle and anterior caudal vertebrae in ventral view and proximal hind limbs. F. Right scapula and forelimb. G, Right pes and three caudal vertebrae in ventral view. H. Left pes (disarticulated). C-H correspond to the red-boxed areas in Extended Data Fig. 3b and c. Abbreviations: ac, acromion; ast, astragalus; atl, atlantal lamina; cal, calcaneus; cav-#, caudal vertebra (with number); cor, coronoid process of the dentary; cv-#, cervical vertebra with number; dch, distal condyle(s) of the humerus; del, deltopectoral crest; feh, femoral head; fi, fibula; glf, glenoid fossa of the squamosal; gls, glenoid fossa of the scapula; gtr, greater trochanter; gtu, greater tubercle; haa, haemal arch; hm, hamate; huh, humeral head; icl, interclavicle; il, ilium; isc, ischium; ju, jugal; ltu, lesser tubercle; lv-1-5, lumbar vertebra 1 to 5; mc-1-5, metacarpal 1 to 5; mt-1-5, metatarsal 1 to 5; nav, navicular; pi, pisiform; r-#, rib with number; ra, radius; sc, scapula; sca, scaphoid; scs, scapular spine; spax, spinous process of the axis; spth, spinous process of the thoracic vertebra; sv-1-2, sacral vertebra 1 to 2; tdm, tibial distal malleolus; tpca, transverse process of caudal vertebra; trm, trapezium; trq, triquetrum; ul, ulna. The left and right side structure is denoted with l- and r-, respectively.

Extended Data Fig. 5 Dentitions of Feredocodon chowi (holotype [IMMNH-PV01035] and paratype [IMMNH-PV01925]).

A-C. Right upper dentition of the holotype in lingual, occlusal, and labial views. D-F. Right lower dentition of the holotype in lingual, occlusal, and labial views. G-I. Upper teeth of the paratype in occlusal, lingual, and labial views. J-L, Lower teeth of the paratype in lingual, occlusal, and labial views. See Supplementary Information for description.

Extended Data Fig. 6 Cheek teeth of Feredocodon in occlusal relationship and comparison with other forms.

A. Lingual view of left P2-M3 and p2-m3 in occlusion (in preserved condition). B. The same teeth tilted lingually. C. Dorsal view in which the roots of the upper and the crowns of the lower are exposed. D. Labial view. E. Crown view of the upper teeth and ventrolabial views of the lower teeth. F. Close-up lingual view (slightly tilted) of the right P6-M3 and p6-m3. G. Lingual view of the left P6-M3 and p6-m3. H. Occlusal relationship of tribosphenic molars. I. Molar occlusal relationship of the docodont Haldanodon (adapted from figure 10.20 of ref. 23). J. Molar occlusal relationship of the shuotheriid Pseudotribos (adapted from figure 2e of ref. 4). Abbreviations: dr, distal root; lr, lingual root; met, metacone ( = cusp c); mr, mesial root; ppr, pseudoprotocone; pr, protocone; prd, protoconid ( = cusp a); pspr, pseudoprotocone (original lablelling); pstal, pseudotalonid; talb, talonid basin. Note 1. The occlusal pattern of Feredocodon is similar to that of Haldanodon; however, in Feredocodon the pseudoprotocone of M2 occludes in the pseudotalonid of m2, whereas in Haldanodon the pseudoprotocone of M2 was interpreted to occlude in the pseudotalonid of m3; the latter was interpreted for Pseudotribos. In both Feredocodon and Haldanodon the pseudoprotocone does not occlude into the pseudotalonid basin, differing from the tribosphenic condition.

Extended Data Fig. 7 Cheek teeth comparison of shuotheriids and docodontans.

Identifications of the ultimate lower premolar and molars are from the original studies. The shaded teeth are our interpretation of the ultimate lower premolars in compared taxa. The lower molars are shown in lingual and occlusal views, whereas the upper ones are in occlusal view. The figures are modified and derived from the following resources: Shuotherium lower molars (A1-A2, new images from cast of IVPP V6448); Shuotherium upper molar (A3) (new image from cast of IVPP V7467); Agilodocodon (C1-3) (adapted from figures 2C–E of ref. 24); Haldanodon (D1-3) (adapted from figures 5D and 8D-E of ref. 10); Simpsonodon (E1-3) (adapted from figures 5A-B, 32A of ref. 5); Borealestes (F1-3) (adapted from figures 10C and 15B of ref. 60); Sibirotherium (G1-2) (adapted from figures 1b and 3 of ref. 79). Figures are not on scale and may be reversed photographically for convenience of comparison.

Extended Data Fig. 8 Serial homology and morphological gradient as illustrated by the dentitions of Feredocodon.

The most common structures in the dentitions are the paracone (cusp A) in the upper dentition and the protoconid (cusp a) in the lower; they are the earliest formed in development and most primitive in evolution. The least common structures are the pseudoprotocone and cusp g, which are the latest formed cusps in development and most derived in evolution. Yellow dots indicate initial condition of the feature in teeth. Green dots indicate occurences of structures in corresponding teeth. Dark green indicates the parastyle, homologous with cusp B30. The empty circle indicates lack of cusp B (present in the left P4 of IMMNH-009166), probably a “development noise”80.

Extended Data Fig. 9 Comparison of hypotheses on lower molar cusp homologies of shuotheriids and docodontans.

A. Conventional interpretation of the lower molar structures of Shuotherium; the neomorphic pseudotalonid of the molars is marked in green. B. A hypothetical dental occlusal relationship in Shuotherium based on the conventional interpretation. In addition to what has been explained in the text and Fig. 3, an additional issue is that the ultimate upper premolar of Shuotherium would not have a pseudoprotocone because there is no area in px to receive it. This contradicts the fact that px is already submolariform (enlarged and widened). C. Dental structurers reinterpreted under the new hypothesis (see Fig. 3 and text). D. A hypothetical dental occlusal relationship in Shuotherium under the new hypothesis. The ultimate upper premolar (Px) and even the penultimate premolar may have the pseudoprotocone, as in Feredocodon. E-H. Comparison of cusp homology hypotheses in the lower molar of shuotheriids and docodontans. Abbreviations: c-g, cusp g; trd, trigonid; pad(b), paraconid (cusp b); phy, pseudohypoconid; ptad, pseudotalonid; ptrd, pseudotrigonid.

Extended Data Table 1 Measurements (Length/Width in mm)
Full size table

Supplementary information

Supplementary Information

The Supplementary Information provides the definitions of dental structures recognized and used in the study. It contains a detailed description about systematic paleontology, including the definition and emended diagnosis for higher level taxa (clades) as well as the new genus and species reported in the study. It also provides the data and methods for phylogenetic analyses. In particular, the character list includes specific coding for two new species and six known shuotheriid and docodontan taxa. The character list also contains brief explanations about the characters and character coding that are modified based on the new data recognized in this study. The phylogenetic analyses include PAUP analysis and Bayesian tip-dating analysis; only the essential results, including the consensus trees, are presented. The related character list and dataset are presented in MorphoBank (http://www.morphobank.org; project number 5075), and the related detailed settings and logs of these analyses are deposited in Zenodo (https://doi.org/10.5281/zenodo.10597270, ref. 64). See also Methods in the main text.

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Mao, F., Li, Z., Wang, Z. et al. Jurassic shuotheriids show earliest dental diversification of mammaliaforms. Nature 628, 569–575 (2024). https://doi.org/10.1038/s41586-024-07258-7

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