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A Jurassic mammaliaform swimmer and transformation of the mammalian pharynx

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Data availability

Life Science Identifiers by the Zoobank for Systematic Palaeontology are registered for the new family Megacaudidae (urn:lsid:zoobank.org:act:20A95F1F-F06A-49B3-8DD8-6701704AD5D5), for the new genus Megacauda (urn:lsid:zoobank.org:act:858D78B8-9973-4C95-8286-CA05B77EFBBC) and for new species M. sungei (urn:lsid:zoobank.org:act:E1D7A965-E547-4705-8696-E63F672564ED). The Holotype PMOL-AM00026A/B is curated by Paleontological Museum of Liaoning, Shenyang, China.

Code availability

Phylogenetic character list, data matrix (in Nexus) are presented in the Supplementary Information, as are the maximum parsimony (PAUP) analysis logs and Bayesian (MrBayes) analysis logs. Branch support values from TNT are provided in the Supplementary Information.

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Acknowledgements

We thank G. Sun and N. Tian for proposing the research collaboration on this fossil; Q. Fang and the staff at the YingHua (GE) Inspection Center (Shanghai) for CT scans of the fossil; and Q. Li for assistance with X-ray fluorescence imaging of the soft tissue preserved in fossil; B. Patterson, L. Heaney and A. Ferguson for access to comparative collection of extant mammals at the Field Museum (Chicago); and D. Y. Huang, J. Jia and E. Panciroli for their discussion during this research.

Funding

During this research, C.-F.Z. was supported by NNSF-China (42161134003) and Taishan Scholar Program of Shandong (tstp20240514), and T.M. was supported by Deutsche Forschungsgemeinschaft (DFG). CT scanning for the Luo laboratory was supported by NSF (BRS-TCN 1702421). P.L. was supported by Research Assistantship from NSF (IOS 2315501) and Ohio University Heritage College of Osteopathic Medicine (Athens, Ohio).

Author information

Authors and Affiliations

  1. College of Paleontology, Shenyang Normal University, Shenyang, China

    Y. Li

  2. Department of Biomedical Sciences, the Heritage College of Osteopathic Medicine, Ohio University, Athens, OH, USA

    P. Li

  3. Department of Organismal Biology and Anatomy, The University of Chicago, Chicago, IL, USA

    A. I. Neander & Z.-X. Luo

  4. Paleontological Museum of Liaoning, Shenyang Normal University, Shenyang, China

    H. Zhang

  5. College of Earth Science and Engineering, Shandong University of Science and Technology, Qingdao, China

    C.-F. Zhou

  6. Bonner Institut für Organismische Biologie, Abteilung Paläontologie, Universität Bonn, Bonn, Germany

    T. Martin

Authors

  1. Y. Li
  2. P. Li
  3. A. I. Neander
  4. H. Zhang
  5. C.-F. Zhou
  6. T. Martin
  7. Z.-X. Luo

Contributions

C.-F.Z., T.M. and Z.X.L. initiated and designed this study. Y.L., H.Z. and C.-F.Z. curated the specimen, investigated the field geology at the type locality and established stratigraphical correlation in the Daohugou region. C.-F.Z. acquired the CT scans. A.I.N. performed the CT visualization. P.L., A.I.N. and Z.X.L. interpreted skull anatomy and P.L. provided primary comparative data. Z.X.L. and A.I.N. interpreted skeletal anatomy. Z.X.L., T.M. and C.-F.Z. carried out dentomandibular interpretation and collated the character list. Y.L., T.M. and Z.X.L. scored the matrix and carried out phylogenetic analysis. A.I.N. and Z.X.L. composed figures. A.I.N. composed the videos. Z.X.L. composed the Supplementary Information with major input from P.L., C.-F.Z. and T.M.; Z.X.L. wrote manuscript with input and edits from all of the authors.

Corresponding authors

Correspondence to C.-F. Zhou, T. Martin or Z.-X. Luo.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature thanks Rebecca German, Suzanne Hand, Elsa Panciroli and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data figures and tables

Extended Data Fig. 1 Holotype main-part of Megacauda (Docodonta, Mammaliaformes) (PMOL–AM00026A).

a, camera photograph: purple triangles indicate the margins of the widened tail. b, XRF photo (from X-ray fluorescence spectroscopy high-light elements Si [silicon] and Y [yttrium]): arrows indicate the distal haemal arches. The calcified costal rib plates (widened and imbricated) are preserved on both left and right aspects of the thorax. c, identification of osteological features on skeleton (a composite outline of elements by complementary preservations of both the main-part A and the counter-part B). Large triangles on indented outline at the base of the tail. Small triangles on the margin of the wide tail. Small arrows on the posterior-most haemal arches on the tail in b. Abbreviations: Ca - caudal vertebra; DV- dorsal vertebrae.

Extended Data Fig. 2 Holotype counter-part of Megacauda (Docodonta, Mammaliaformes) (PMOL–AM00026B).

a, camera photograph: purple triangles indicate the margins of the broad tail. b, XRF photo (from X-ray fluorescence spectroscopy) highlighting (Si) silicon and (Y) yttrium: small arrows indicate the distal extent of haemal arches. The ossified costal rib (broad and imbricated) are visible on both left and right sides of the thorax. c, Identified features on the skeleton (composite outline from complementary preservations of elements and impressions, from both main-part A and counter-part B). Large triangles on indented outline at the base of the tail. Small triangles on the margin of the wide tail. Small arrows on posterior-most haemal arches on the tail (in b and c). Abbreviations: Ca – caudal vertebra.

Extended Data Fig. 3 Upper teeth of Megacauda in paired stereo images from CT rendering (best preserved in holotype main-part PMOL–AM00026A).

a, left upper teeth in oblique lingual view. b, left upper teeth in occlusal view of the molars, oblique lingual view of the anterior teeth. c, left upper teeth in labial view of the molars, in oblique labial view of the anterior teeth. d, right upper teeth in labial view. e, right upper teeth in occlusal view of the molars, in oblique lingual view of the anterior teeth. f, right upper teeth in lingual view. Abbreviation: C – canine, I – incisor, P-premolar, M-molar.

Extended Data Fig. 4 Megacauda (PMOL–AM00026) lower teeth, mandible, mandibular middle ear, and hyoids as preserved with the cranium and the mandibles.

a-c, left mandible and lower teeth in stereo pair of CT images and in labial (a), occlusal (b) and lingual (c) views. The coronoid process is mirrored from the counter-part. d, right mandible and lower teeth in lingual view. e-f, Megacauda main-part (PMOL-AM00026A) hyoids preserved with the cranium, as seen in dorsal view of the cranium (e), as seen in ventral view of the cranium (f). g-h, hyoids and middle ear associated with mandibles with the left mandible in medial view and the right mandible in lateral view (g). The hyoids and middle ear associated with the mandibles, as seen with the left mandible in lateral view and the right mandible in internal view (h). i, the right middle ear elements as preserved, in medial view (i1), dorsal view (i2) and lateral view (i3). j, restoration of the malleus, Meckel’s element, the surangular and the ectotympanic in medial view (j1), dorsal view (j2) and lateral view (j3). Abbreviation: c – canine, i – incisor, p-premolar, m-molar, v3-notch – the notch for inferior alveolar nerve to enter the mandibular canal, “-r” - right side; “-l” - left side.

Extended Data Fig. 5 Pseudocarnassial sectorial cheek teeth of Megacauda sungei.

a, outline reconstruction in occlusal view of the upper posterior premolars (P3-P4) and molars (M1-M4), and their cusp identification. b, occlusal view reconstruction of the lower molars (m1-m5), with identification of m1-m2 interlock and lower molar cusps. c, left: interpretation of occlusion (early stage or phase 1) of the upper molars (P4-M4) with the lower molars (m1-m5), with transparent upper molars on top of the occlusal surfaces of the lower molars; right: wear facets (red) of phase 1 occlusion on the upper and lower molars, the wear facets along upper cusps A and C of the upper molars corresponding to the wear facets along lower cusps b and a are plesiomorphic and also known from mammaliaforms with triconodont-like molars (thus called triconodont facets here). d, left: late occlusion (Phase 2) of the upper and lower molars, with movement of the lower molars in posterior and lingual direction, and away from maximum intercuspation; right: wear facets for shearing between the serrated posterior crest of lower cusps a-c-d (postvallid crest) and the prevallum wear facet (on the precingular surface). These are apomorphic features related to the development of sectorial premolars and anterior molars with compressed crown and recurved primary cusp. e (box), left (docodonts): the prevallum shearing surfaces on the precingulum of the upper premolars (P3-P4) and molars (M1) marked by blue brackets correspond to the lower postvallid crests of the recurved cusps a-c-d (marked by blue brackets) on the sectorialized lower molars (m1-m3), and anterior premolars (not shown) The prevallum-postvallid shearing on the sectorial teeth of Megacauda is geometrically opposite to the carnassial teeth of carnivores and faunivores of crown therians. e (box), right (therians): typical therian carnassial teeth with postvallum/prevallid shearing (schematic based on a eutherian hyaenodontid carnivore).

Extended Data Fig. 6 Dental differences of Megacauda and Castorocauda and their respective diagnostic characters.

a, identification key for molar cusps in lingual view (left) and occlusal view (right). b, differences of p5 – the tallest and longest of premolars (teeth of the two taxa arbitrarily scaled to the same length for comparison): Megacauda (grey shade) is characterized by a single cusp c, by contrast to Castorocauda (black outline) that exhibits two cusps (c1 and c2) posterior to the primary cusp a and above cingulid cusp d. Megacauda shows a taller and more reclined primary cusp a than Castorocauda. c, differences of m1 - a bilaterally compressed sectorial molar (teeth of the two taxa arbitrarily scaled to same length for comparison): Megacauda is characterized by a single cusp posterior to the primary cusp a, in contrast to Castorocauda (black outline) with two cusps (c1 and c2) posterior to cusp. Megacauda shows taller and more reclined cusps than Castorocauda. d, Differences of molars in lingual view between Castorocauda (line drawing) and Megacauda (grey shading): m2: cusp g large and distinct in Castorocauda but absent in Megacauda; m3: cusps b and g are large and distinct in Castorocauda, but are reduced in Megacauda; m4: entoflexus is narrow and distinct in Castorocauda but wider and shallower in Megacauda; cusp c smaller in Megacauda than in Castorocauda. e, Differences of molars in occlusal view between Castorocauda (line drawing) and Megacauda (grey shading): m2 is bilaterally compressed more at the mesial and distal ends in Megacauda, in contrast to the more rectangular mesial and distal ends, and a fully formed distal basin in Castorocauda. m3-m4: cusps b and g are larger in Castorocauda than in Megacauda. Development of distal occlusal basin on m2-m3-m4: present in Castorocauda (plesiomorphic for docodontans) but absent in Megacauda (absence of distal basin is related to bilaterally compression, an apomorphy, as compared to most other docodontans). Castorocauda line drawings in a and e are originals from Luo, reproduced with permission from ref. 13. See Supplementary Table S1 for the full list of check character differences between M. sungei and C. lutrasimilis.

Extended Data Fig. 7 Megacauda interpretative muscle reconstruction of the pterygoid and hyoid region, with comparison to mammaliaform Morganucodon and extant therian Didelphis.

a, Plesiomorphic features of the mammaliaform Morganucodon in which the pterygoid transverse process (plesiomorphic equivalent to hamulus) is more anteriorly placed, closer to the edge of the palatine and the upper tooth row, and also laterally aligned with the mandible. This plesiomorphic configuration lacks any required space for the muscles of the medial pterygoid group (Morganucodon: original graphics by Luo, modifed from ref. 1, previously re-interpreted from SI Ref 138). b, apomorphic configuration of Megacauda, in which the pterygoid hamulus is already therian-like: presence of a hook, placed more posteriorly and away from the palatine border, and also shifted medially from the corner of the palatine pyramidal process, such that the pterygoid hamulus is widely separated from the mandible. This apomorphic configuration has a saddle (concave area) of palatine for the passage of the M. tensor veli palatini to the soft secondary palate. The concave area lateral to the hamulus and posterior to the palatine pyramidal process is the space to accommodate the medial pterygoid muscle in extant therians, and for the M. tensor veli palatini and the levator veli palatini that are developmentally associated with the medial pterygoid muscle. The hook-like shape also indicates the pterygopharyngeal muscle that originates anteriorly from the hamulus in extant therians and is linked to the superior pharyngeal constrictors. c, interpretive reconstruction of soft secondary palate, with M. tensor veli palatini and M. levator veli palatini curving around the pterygoid hamulus, and with M. pterygopharyngeus originating medial to the pterygoid hamulus to connect with the superior pharyngeal constrictor. d and e, marsupial Didelphis virginiana with the derived musculoskeletal characters of the palatine, the pterygoid hamulus, and the hyoid apparatus with the ptergygopharyngeus muscle and the tensor veli palatini. f, Didelphis – visualization of diceCT scanning of the musculoskeletal structure of the palate and pharynx in the region of the pterygoid hamulus and the hyoid apparatus, to illustrate the muscle attachment of the tensor veli palatini and the ptergyopharyngeus muscle (=the superior pharyngeal constrictor muscle). These palatopharyngeal musculoskeletal features of Didelphis provide the comparative evidence for reconstructing the soft secondary palate, the tensor veli palatine and the levator veli palatine for the docodontan Megacauda.

Extended Data Fig. 8 Apomorphic hindlimb features of docodontans Megacauda and Castorocauda for semi-aquatic locomotor function.

a, Megacauda pelvis (in anterolateral view with strong pectineal (pubic) tubercle and large notch for the M. iliopsoas as part of the hypertrophied pelvic-femoral muscles for rotation and adduction of the femur. b, Megacauda: disarticulated hindlimb and pes: femur in anterior view, tibia and fibula in anterolateral view, pes in ventral (plantar) view. c, Castorocauda: disarticulated hindlimb and pes: both Megacauda and Castorocauda show a specialized ankle joint capable of hyper-abduction and eversion. These semi-aquatic docodontans also show an elongation of the lateral pedal digit rays (4 and 5), more robust medial metatarsals, a strong lateral curvature of the phalanges resulting in asymmetrical feet, for paddling function of the hindlimb and pes in swimming, also with abducted foot posture as seen in Tachyglossus. d, land-living Agilodocodon hindlimb in anterolateral view and pes in dorsal view, showing far less elongation of the lower hindlimb and relatively shorter pes, and more symmetrical pedal digit rays10, all of which are plesiomorphic by contrast to specialized limbs of semi-aquatic docodontans.

Extended Data Fig. 9 Megacauda astragalus and calcaneus in paired stereo images from CT rendering, in comparison to those of other docodontans and Morganucodon.

Megacauda astragalus: dorsal view (a), with its contacting side to the calcaneus, seen in ventral view (b), oblique medial view (c), and posterior view (d). e, docodontan Docofossor astragalus in dorsal view for comparison12. Megacauda calcaneus: ventral view (f), dorsal view (g), contacting side to astragalus in medial view (h), and oblique medial view (i). For comparison, docodontan Borealestes calcaneus (j) in dorsal (top), ventral (middle), and medial (bottom) views (redrawn by Luo, from CT rendering of calcaneus by SI-Ref 174). For comparison, mammaliaform Morganucodon calcaneus (k) in dorsal (top), ventral (middle), and medial (bottom) views (original drawings by Luo, re-labelled from SI-Ref 205; previously redrawn from Ref. SI-Ref 197; SI-Ref 229).

Extended Data Fig. 10 Apomorphic rib cage structure of semi-aquatic docodontan Megacauda, in comparison to Ornithorhynchus.

a, Main-part (PMOL-AM00026A) XRF photograph, inset (b) of the posterior region of the rib cage. b, magnified area of the posterior thorax to show the broad and overlapping ossified costal rib plates, in both the left side of the ribcage (b-2), and the right side of ribcage (b-3) on either side of the sternal series (b-1). c, rib 6 shows a flattened and expanded distal plate, in stereo pair (c1 and c2). d, rib 7 shows a flattened and expanded distal plate, in stereo pair (d1 and d2). e, Monotreme Ornithorhynchus: ribcage with calcified costal rib plates that are so broad as to overlap each other, forming an imbricated series of costal plates in thoracic rib 6 through rib 13. Monotremes (both Ornithorhynchus and Tachyglossus) are also unique in exhibiting a fully calcified sternal segment of the thoracic ribs and forming a distinct joint with the intermediate costal rib segment (SI-Ref Gasc 1967). Outline of Ornithorhynchus skeleton in ventral view (e1) (new drawing, differing from previous illustrations of SI-ref 133, SI-Ref 241), and the ribcage in detail (e2) based on new observations of museum specimens). f, schematic reconstruction of thorax of semi-aquatic docodontans, to show the estimated positions of the broad and calcified costal plates in imbricated series (grey). Both Megacauda (Extended Data Fig. 10a–c) and Castorocauda13 show imbricated costal plates and plate-like distal thoracic ribs, as seen in monotremes. g, Land-living docodontan Microdocodon8 (original drawing by Luo, re-labeld from Ref-8 SOM Fig. S10) is characterized by thin, rod-like costal cartilages linking the thoracic ribs to the sternal series, a similar plesiomorphic pattern observed in other docodontans preserved with ribcage such as Agilodocodon10 and Docofossor12. All docodontans lack the ossified and jointed sternal rib segments of rib 1 to rib 5 as seen monotremes.

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Li, Y., Li, P., Neander, A.I. et al. A Jurassic mammaliaform swimmer and transformation of the mammalian pharynx. Nature (2026). https://doi.org/10.1038/s41586-026-11107-0

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