LETTERS

Convergent dental adaptations inpseudo-tribosphenic and tribosphenic mammalsZhe-Xi Luo1, Qiang Ji2 & Chong-Xi Yuan2

Tribosphenic molars of basal marsupials and placentals are amajor adaptation, with the protocone (pestle) of the upper molarcrushing and grinding in the talonid basin (mortar) on the lowermolar1–4. The extinct pseudo-tribosphenic mammals have areversed tribosphenic molar in which a pseudo-talonid is anteriorto the trigonid, to receive the pseudo-protocone of the uppermolar. The pseudo-protocone is analogous to the protocone, butthe anteriorly placed pseudo-talonid is opposite to the posteriortalonid basin of true tribosphenic mammals5–7. Here we describea mammal of the Middle Jurassic period with highly derivedpseudo-tribosphenic molars but predominantly primitive man-dibular and skeletal features, and place it in a basal position inmammal phylogeny. Its shoulder girdle and limbs show fossorialfeatures similar to those of mammaliaforms and monotremes, butdifferent compared with those of the earliest-known Laurasiantribosphenic (boreosphenid) mammals. The find reveals a muchgreater range of dental evolution in Mesozoic mammals than intheir extant descendants, and strengthens the hypothesis of homo-plasy of ‘tribosphenic-like’ molars among mammals.

Class MammaliaClade Yinotheria Chow and Rich, 1982 (emended by ref. 3)

Family Shuotheriidae Chow and Rich, 1982 (ref. 5)Pseudotribos robustus gen. et sp. nov.

Etymology. Pseudo, false, for superficial resemblance; tribos, grinding,for the grinding and crushing function of the pseudo-tribosphenicmolar; robustus, strong, for the stout limb bones of the new mammal.Holotype. Chinese Academy of Geological Sciences (CAGS)040811Aand CAGS040811B are the part and counterpart, respectively, of apartial skeleton with impression and carbonized residues of furs(Fig. 1).Locality and age. Daohugou locality (41u 18.9799 N, 119u 14.3189 E),Ningcheng County, Inner Mongolia Region, China, in the MiddleJurassic Jiulongshan Formation. Mammaliaforms from this forma-tion include the docodont Castorocauda8 and the basal mammalVolaticotherium9 (see also Supplementary Information).Diagnosis. Dental formula: I21-C1-P5-M3/I4-C1-P5-M3 (Fig. 2).Upper molars have a triangular pattern of three cusps: a taller para-cone, a shorter metacone and an elevated ‘pseudo-protocone’. Lowermolars have a triangular cusp pattern of the trigonid with a labialprotoconid (tallest), an anterior paraconid and a posterior metaco-nid (lowest). Anterior to the trigonid is the pseudo-talonid basin withan elevated labial rim (pseudo-hypoconid) that is connected to thepseudo-hypolophid crest. The lingual rim of pseudo-talonid basin ofPseudotribos lacks the distinctive cusp ‘pseudo-entoconid’ that ispresent in the closely related Shuotherium. Pseudotribos is distin-guishable from Shuotherium in having a more extensive posteriorcingulid extending to the labial side of the trigonid and in having aless developed pseudo-hypoconid (Fig. 3). It is distinguishable from

Shuotherium shilongi and Shuotherium kermacki in having teethabout 30% smaller in size than comparable teeth, but larger thanthose of Shuotherium dongi by 20%5–7. Pseudotribos differs fromS. kermacki in having a more inflated pseudo-protocone. It isdistinguishable from other mammaliaforms in the apomorphies ofa well developed pseudo-talonid basin positioned anterior to thetrigonid on the lower molar in full occlusion with the upperpseudo-protocone (Fig. 2e). It is distinguishable from all therii-morph mammals (from eutriconodontans, multituberculates andspalacotheroids to living therians)10 in the following plesiomorphies:a large interclavicle twice as long as the sternal manubrium, with twolateral processes and an expanded posterior process. It is distinguish-able from multituberculates to living therians (except Akidolestes)11

in retaining mobile lumbar ribs, in lacking a distinctive head–neckoffset from the femoral shaft, and in the presence of an enlarged lessertrochanter and a laterally projecting greater trochanter. Althoughsimilar to Fruitafossor in generalized and robust limb bones,Pseudotribos lacks the tubular postcanines and xenarthran-like lum-bars of the latter12. (For a full diagnosis, see SupplementaryInformation.)

The most important molar feature of the reversed tribosphenicpattern is the pseudo-talonid with a pestle-to-mortar crushingfunction5–7—a convergence to the true tribosphenic molar that iscorrelated with the great diversification of basal placentals andmarsupials1–4, and with basal australosphenid mammals3,13–16 (seeFig. 2). Previous assignment of isolated upper molars to the lowerteeth of Shuotherium6,7 is now validated by the in situ occlusion ofupper and lower teeth of the new Pseudotribos (Fig. 2e): pseudo-protocones of left M1 and M2 are preserved in occlusion with theanterior pseudo-talonid basins of lower M2 and M3, respectively. Thepseudo-protocone of M3 is smaller and asymmetrical; its prevallumsurface shears against the postvallid surface of the trigonid of M3; andthe post-cingulid of lower M3 seems to be too small to have anycrushing function (Fig. 2e), as previously predicted5. The preservedpostvallum of upper P5 corresponds to the anterior side (prevallid) ofthe trigonid on lower M1. This matches the triangular lower ultimatepremolar (P5), as earlier interpreted for Shuotherium and some aus-tralosphenid mammals3,13,14, and aligns all upper and lower molars.The canine is incisiform, and the ante-molar series shows a posteriorgradient of increasingly developed precingulids that further expandsinto the pseudo-talonid basin on M1. Conspicuous spaces are presentbetween teeth from the posterior incisors to the lower premolar P4.The replacing teeth at the ultimate lower incisor locus and the uppercanine locus indicate a typical diphyodont replacement as in mostmammaliaforms3.

The mandible of Pseudotribos is poorly preserved, but revealsseveral structures previously unknown in shuotheriids (Fig. 2g).The dentary condyle is robust. The mandibular ramus is gracileand shallow in the symphyseal region, but more robust and deeper

1Carnegie Museum of Natural History, Pittsburgh, Pennsylvania 15213, USA. 2Chinese Academy of Geological Sciences, Beijing 100037, China.

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(1.6 mm) below M2 in Pseudotribos than in Shuotherium (1.3 mm).The mandibular angle in Pseudotribos is better developed than inShuotherium, and has a posteriorly facing facet for the ectotympanic(angular) bone as in docodontans9,17. A rugose area is tentativelyinterpreted as the coronoid scar (Fig. 2g). Pseudotribos is similar toShuotherium in having a convex ventral border of the mandible underM2. The postdentary trough extends anteriorly to the posterior open-ing of the mandibular canal. The shallow Meckel’s sulcus extendsanteriorly to below M1, and is parallel to the ventral border of themandible, not intersecting the ventral border as in Shuotherium.Judging from the mandibular and molar differences within shuother-iids, there seems to be a wide range of morphological divergencebetween Pseudotribos and Shuotherium as well as between knownspecies of Shuotherium, suggesting a greater diversity than shownby the currently modest record of this group.

Pseudotribos has 22 thoraco-lumbar vertebrae (Fig. 1), including13 thoracic vertebrae having the ribs connected by means of costalcartilage to the sternebrae. Three additional thoraco-lumbarvertebrae (Fig. 1, tr14–16) have floating ribs without connection tothe sternal series, and are tentatively designated thoracic vertebrae.The last six thoraco-lumbar vertebrae (17–22) have more verticalpre-zygapophyses and more robust centra; these are designatedlumbar vertebrae. The last three thoracic vertebrae and the anteriortwo lumbar vertebrae with mobile ribs form a gradation of succes-sively shorter ribs in more posterior vertebrae, as in modern mono-tremes, the basal mammal Fruitafossor and the eutriconodontYanoconodon12,18. The costal cartilages of posterior ribs are gracileand individualized, unlike the broad and overlapping (imbricating)costal cartilage plates of extant monotremes19. The cervical vertebraeare obscured by the broken elements of the shoulder girdle, but

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Figure 1 | Skeleton of the new Mesozoic mammal Pseudotribos robustus.a, Holotype CAGS040811A (main part). b, CAGS040811B (counter-part).An outline for identification of main skeletal features is shown to the left ofa and b. c, Restoration of Pseudotribos as a fossorial mammal with sprawlinglimbs (grey indicates the bones preserved in the holotype). ap, angularprocess of mandible; C, canine; cl(in), clavicle (incomplete); cos?, putativecoronoid scar (dentary); dc, dentary condyle; ec, ectepicondyle (humerus);ep, epipubis; fe, femur; fi, fibula; fr, frontal; gt, greater trochanter (femur);

hu, humerus; I1–4 and I1–4, upper and lower incisors 1–4 (or alveoli),respectively; ic, interclavicle; il, ilium; l4–6, lumbar vertebrae 4 to 6; lr1–3,lumbar ribs 1 to 3; lt, lesser trochanter (femur); mb, manubrium (sternebra1); Mm, upper and lower molars (in partial occlusion); mx, maxilla; oc,occipital bones (outline only); P, upper premolars5 or their loci indicated byroots; pa, parietal (outline only); ra, radius; s1–3, sacral vertebrae 1–3; sc(in),scapula (incomplete); stb2–7, sternebrae 2–7; t16, thoracic centrum 16; ti,tibia; tr1, thoracic rib 1; tr14–16, thoracic ribs 14–16; ul, ulna.

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three sacral vertebrae are recognizable by the iliosacral contactingsurfaces.

The preserved parts of pelvic and hindlimb bones are robust. Theilium, the femur and the preserved proximal parts of the tibia andfibula are very broad and are thick for their length. An impressionoverlapping the ilium is interpreted to represent the epipubis, amammaliaform plesiomorphy9. The femoral head is large and with-out a distinctive neck. The greater trochanter is laterally oriented andlarger than the medially directed lesser trochanter below the femoralhead. The lateral condyle is larger than the medial condyle. Theproximal end of the fibula is broad; the parafibular process is present,but not strongly developed. The tibia has a strong anterior crest, abroad trough for the tibialis anterior muscle, and a proximo-lateralprocess. These features are plesiomorphic for mammaliaforms andtritylodontids9,20–23.

The robust shoulder girdle of Pseudotribos shows many fossorialfeatures that are also plesiomorphies for crown Mammalia. Thissupports a basal placement of shuotheriids in mammaliaform phylo-geny. Pseudotribos resembles Sinoconodon and modern monotremes(Fig. 4) in that the interclavicle is twice the length of the manubrium(cynodont plesiomorphy), and has a constricted waist, a club-foot

posterior expansion, and elongate articulation for the clavicle (allmammaliaform apomorphies). Tritylodontids have an elgonate buttransversely narrow interclavicle with a tapering posterior end22,23

(Fig. 4b)—a basal cynodont condition24,25 (Fig. 4). In contrast,the posterior end of the interclavicle is expanded and juxtaposed withthe expanded anterior side of the ‘T-shaped’ or triangular manu-brium (the first rib-bearing element in the sternal structure) in mam-maliaforms. The interclavicle and manubrium together form amassive area for the pectoralis muscles. The lateral processes ofinterclavicle and clavicles form the expanded area of origin for thesternocleidomastoid muscle. The expanded muscular attachmentssuggest a powerful forelimb with a sprawling posture, as in mono-tremes with burrowing adaptation19, consistent with such fossorialfeatures as the expanded deltopectoral crest, the teres major tubercle,and an expanded distal end of the humerus. The olecranon processis long, and its length is 62% that of the ulnar length anterior tothe semilunar notch, similar to Fruitafossor and extant fossorialmammals12.

In all mammaliaforms for which the pectoral girdle is sufficientlyknown12,20,21, the coracoid does not articulate with the interclavicle.Extant monotremes, however, have a hypertrophied (meta)coracoid

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Figure 2 | Dental and mandibular features of Pseudotribos. a, Upper molarM2, crown view, composite restoration, the cingular outline conjectural,based on CAGS040811A, CAGS040811B and ref. 7. b, Lingual view (with aventral tilt, based on camera lucida drawing of CAGS040811A). c, Labial view(CAGS040811A). d, Lower molar M2, lingual view, composite restorationfrom CAGS040811A and CAGS040811B. e, Upper molars (M2, M3) inocclusion with lower M2 and M3 in lingual view (intact in CAGS040811B).f, Composite restoration of the upper teeth, lateral view, only two posterior

incisors known, based on CAGS040811A and CAGS040811B. g, Mandibleand dentition, medial view, composite restoration of CAGS040811A andCAGS040811B. Cr, replacing (permanent) canine; etf, ectotympanic(angular) facet on the mandibular angle; I1i, incisor I1 alveolus; I4i?, possibleincisor I4 alveolus; I4r erupting (replacement) I4; m-sulcus, Meckel’s groove(dentary); pdt, postdentary trough; pfmc, posterior foramen of mandibularcanal; p-hlpd, pseudo-hypolophid; p-hypoconid, pseudo-hypoconid; sym,mandibular symphysis.

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extending to articulate with the expanded posterior end of the inter-clavicle (Fig. 4e, f)19,26. This provides extra support for the powerfulshoulder and forelimb for burrowing in modern monotremes. Theposterior club-foot of the interclavicle and the expanded anterior endof the manubrium are also present in Pseudotribos. This primitivemorphotype of girdle and sternal structure, as seen in Sinoconodon,

Pseudotribos and monotremes, differs from the pivotal claviculo–interclavicle joint and mobile shoulder girdle of theriimorphs (eutri-conodontans and crown therians) (Fig. 4)11,18,26–28.

The reversed tribosphenic molar structure is significant by itsopposite arrangement of the two functional parts (the primitive tri-gonid versus the derived talonid) of the tribosphenic molar of basal

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Figure 3 | Comparison of tribosphenic and pseudo-tribosphenic molars.a, Skull of the marsupial Didelphis virginiana with its ‘true’ tribosphenicmolars. b, Schematic illustration of the upper–lower occlusion oftribosphenic molars (trigonid anterior to talonid). c, BoreosphenidanKielantherium as example of the ancestral tribosphenic pattern4.

d, Schematic illustration of the upper–lower occlusion of pseudo-tribosphenic molars (trigonid posterior to pseudo-talonid). e, Shuotheriidsas an example of pseudo-tribosphenic molars (based on Pseudotribos).Dashed line shows the protocone-talonid occlusion and thepseudoprotocone-pseudotalonid occlusion.

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Figure 4 | Evolution of interclavicle and anterior sternal structure in thecynodont–mammal transition. (Placement of Pseudotribos in mammalphylogeny is shown in Supplementary Information.) a, CynodontProbelesodon25. b, Mammaliamorph Bienotheroides22. c, MammaliaformSinoconodon (restoration from several specimens). d, Pseudotribos(composite from CAGS040811A and CAGS040811B). e, MonotremeOrnithorhynchus. f, Tachyglossus. g, Eutriconodont Jeholodens27.h, Marsupial Didelphis. Apomorphies of (1) mammaliaforms: wideninganterior end of interclavicle; posterior club-foot of interclavicle to the samewidth of manubium for extensive attachment of pectoralis muscles. (2)Yinotheria (sensu, ref. 3): ‘T-shaped’ manubrium. (3) Monotremata: gracileand elongate lateral process of interclavicle overlapping two-thirds the

length of clavicle; enlargement of (meta)coracoid to articulate directly withthe interclavicle; presence of procoracoid for strengthening the girdle19,26,30.(4) Theriimorpha10: shortening of interclavicle to equal manubrium;reduction of the lateral process of interclavicle; mobileclaviculo–interclavicle articulation18,27. (5) Crown Theria: incorporation ofembryonic interclavicle into the manubrium30, the first sternal segment inarticulation of clavicle and thoracic rib 1. cl, clavicle (green); cl–ic contact,articulation or overlap of clavicle and interclavicle (yellow); ic, interclavicle(red); mb, manubrium (sternebra 1) (blue); mc, metacoracoid; pc,procoracoid19; sc, scapula (grey); stb2, second sternebra; tr1 and tr2, thoracicrib costal cartilage 1 and 2, respectively.

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marsupials and placentals (Fig. 3) and australosphenid mammals15,16.Not present in any modern mammals, this evolutionary phenom-enon suggests a much greater range of dental diversity in mammalsfrom the Mesozoic era than in modern mammals.

Despite their importance to mammal dental evolution, shuother-iids were represented previously by only 11 teeth and an incompletemandible. Because of the conflicting characters of the derivedpseudo-tribosphenic molars and primitive mandible representedby its limited fossils, hypotheses about phylogenetic affinities ofShuotherium have differed widely. Previously, Shuotherium was con-sidered to be intermediate between the primitive mammaliaformKuehneotherium with molar cusps in an obtuse triangle, and thesymmetrodonts with molar cusps in an acute triangle5, to be relatedto docodont mammaliaforms28, to be between the acute-triangledsymmetrodonts and the derived pre-tribosphenic mammalPeramus7, or to be a basal mammal and sister-group to australosphe-nidans3,13–15. Our analysis on an improved data set from the newPseudotribos excludes an affinity of shuotheriids to symmetrodonts,or to the clade of Peramus and more derived lineages. Shuotheriidsare more closely related to the australosphenidan clade than otherMesozoic mammal groups3,13–15.

Before the discovery of the geometrically reversed design ofpseudo-tribosphenic molars, it was widely assumed for more thana century that the upper molar protocone, the lower molar talonidand their occlusal correspondence evolved together in a single originin early mammalian history. The discovery of Shuotherium changedthis assumption. It is now generally accepted that a protocone-likestructure of the upper molar can occlude either a talonid in theposterior part of lower molar or a pseudo-talonid in the anterior partof the lower molar in different clades; the functionally adaptive uppermolar protocone evolved at least twice in mammalian history4–7.Recently it was also proposed that the talonid basin in the lowertribosphenic molar has evolved more than once in early mam-mals3,4,13–16. This observation received support from newly discov-ered australosphenidan mammals15,16 that lack the typical wear facetsof the boreosphenid mammals (as defined by studies for theLaurasian tribosphenic molars)1,2. Their detailed occlusal relation-ship to the structure of upper molars may differ between theLaurasian and the Gondwanan Mesozoic mammals with tribosphe-nic teeth15,16. The new fossil of Pseudotribos adds to the inference thatboth the upper molar protocone and the lower molar talonid couldhave undergone convergent evolution, and that there is more thanone pathway to combine slicing and crushing functions in a single jawmotion for more effective faunivory and omnivory in early mam-malian history29. The wider morphological range of tooth form andfunction in Mesozoic mammals suggests that their dental evolution ismuch more labile than can be inferred from Cenozoic mammals.

Received 3 April; accepted 3 September 2007.

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Supplementary Information is linked to the online version of the paper atwww.nature.com/nature.

Acknowledgements We thank A. R. Tabrum for fossil preparation, H.-L. You for hisassistance, M. R. Dawson for improving the manuscript, M. A. Klingler for help withgraphics, and J. R. Wible, G.-H. Cui and K.-Q. Gao for access to comparativematerials. This work was supported by the 973 Project by Ministry of Science andTechnology, Chinese Academy of Geological Sciences (Beijing) (Q.J.), the NationalScience Foundation (USA), the National Natural Science Foundation (China) andthe National Geographic Society (Z.-X.L.).

Author Information Reprints and permissions information is available atwww.nature.com/reprints. Correspondence and requests for materials should beaddressed to Z.-X.L. (luoz@carnegiemnh.org).

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