LETTERS. Convergent dental adaptations in pseudo-tribosphenic and tribosphenic mammals. Zhe-Xi Luo 1, Qiang Ji 2 & Chong-Xi Yuan 2

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1 Vol Noveer 2007 doi: /nture06221 onvergent dentl dpttions in pseudo-triosphen nd triosphen mmmls Zhe-Xi Luo 1, Qing Ji 2 & hong-xi Yun 2 Triosphen molrs of sl mrsupils nd plentls re mjor dpttion, with the protoone (pestle) of the upper molr rushing nd grinding in the tlonid sin (mortr) on the lower molr 1 4. The extint pseudo-triosphen mmmls hve reversed triosphen molr in whh pseudo-tlonid is nterior to the trigonid, to reeive the pseudo-protoone of the upper molr. The pseudo-protoone is nlogous to the protoone, ut the nteriorly pled pseudo-tlonid is opposite to the posterior tlonid sin of true triosphen mmmls 5 7. Here we desrie mmml of the Middle Jurss period with highly derived pseudo-triosphen molrs ut predominntly primitive mndiulr nd skeletl fetures, nd ple it in sl position in mmml phylogeny. Its shoulder girdle nd lis show fossoril fetures similr to those of mmmliforms nd monotremes, ut different ompred with those of the erliest-known Lursin triosphen (oreosphenid) mmmls. The find revels muh greter rnge of dentl evolution in Mesozo mmmls thn in their extnt desendnts, nd strengthens the hypothesis of homoplsy of triosphen-like molrs mong mmmls. lss Mmmli lde Yinotheri how nd Rh, 1982 (emended y ref. 3) Fmily Shuotheriide how nd Rh, 1982 (ref. 5) Pseudotrios roustus gen. et sp. nov. Etymology. Pseudo, flse, for superfil reselne; trios, grinding, for the grinding nd rushing funtion of the pseudo-triosphen molr; roustus, strong, for the stout li ones of the new mmml. Holotype. hinese Ademy of Geologl Sienes (AGS)040811A nd AGS040811B re the prt nd ounterprt, respetively, of prtil skeleton with impression nd ronized residues of furs (Fig. 1). Lolity nd ge. Dohugou lolity (41u N, 119u E), Ningheng ounty, Inner Mongoli Region, hin, in the Middle Jurss Jiulongshn Formtion. Mmmliforms from this formtion inude the doodont storoud 8 nd the sl mmml Voltotherium 9 (see lso Supplementry Informtion). Dignosis. Dentl formul: I P 5 -M 3 /I 4-1 -P 5 -M 3 (Fig. 2). Upper molrs hve tringulr pttern of three usps: tller prone, shorter metone nd n elevted pseudo-protoone. Lower molrs hve tringulr usp pttern of the trigonid with lil protoonid (tllest), n nterior pronid nd posterior metonid (lowest). to the trigonid is the pseudo-tlonid sin with n elevted lil rim (pseudo-hypoonid) tht is onneted to the pseudo-hypolophid rest. The lingul rim of pseudo-tlonid sin of Pseudotrios lks the distintive usp pseudo-entoonid tht is present in the osely relted Shuotherium. Pseudotrios is distinguishle from Shuotherium in hving more extensive posterior ingulid extending to the lil side of the trigonid nd in hving less developed pseudo-hypoonid (Fig. 3). It is distinguishle from Shuotherium shilongi nd Shuotherium kermki in hving teeth out 30% smller in size thn omprle teeth, ut lrger thn those of Shuotherium dongi y 20% 5 7. Pseudotrios differs from S. kermki in hving more inflted pseudo-protoone. It is distinguishle from other mmmliforms in the pomorphies of well developed pseudo-tlonid sin positioned nterior to the trigonid on the lower molr in full ousion with the upper pseudo-protoone (Fig. 2e). It is distinguishle from ll theriimorph mmmls (from eutronodontns, multituerultes nd splotheroids to living therins) 10 in the following plesiomorphies: lrge intervle twe s long s the sternl mnurium, with two lterl proesses nd n expnded posterior proess. It is distinguishle from multituerultes to living therins (exept Akidolestes) 11 in retining moile lur ris, in lking distintive hed nek offset from the femorl shft, nd in the presene of n enlrged lesser trohnter nd lterlly projeting greter trohnter. Although similr to Fruitfossor in generlized nd roust li ones, Pseudotrios lks the tuulr postnines nd xenrthrn-like lurs of the ltter 12. (For full dignosis, see Supplementry Informtion.) The most importnt molr feture of the reversed triosphen pttern is the pseudo-tlonid with pestle-to-mortr rushing funtion 5 7 onvergene to the true triosphen molr tht is orrelted with the gret diversiftion of sl plentls nd mrsupils 1 4, nd with sl ustrlosphenid mmmls 3,13 16 (see Fig. 2). Previous ssignment of isolted upper molrs to the lower teeth of Shuotherium 6,7 is now vlidted y the in situ ousion of upper nd lower teeth of the new Pseudotrios (Fig. 2e): pseudoprotoones of left M 1 nd M 2 re preserved in ousion with the nterior pseudo-tlonid sins of lower M 2 nd M 3, respetively. The pseudo-protoone of M 3 is smller nd symmetrl; its prevllum surfe shers ginst the postvllid surfe of the trigonid of M 3 ; nd the post-ingulid of lower M 3 seems to e too smll to hve ny rushing funtion (Fig. 2e), s previously predted 5. The preserved postvllum of upper P 5 orresponds to the nterior side (prevllid) of the trigonid on lower M 1. This mthes the tringulr lower ultimte premolr (P 5 ), s erlier interpreted for Shuotherium nd some ustrlosphenid mmmls 3,13,14, nd ligns ll upper nd lower molrs. The nine is inisiform, nd the nte-molr series shows posterior grdient of inresingly developed preingulids tht further expnds into the pseudo-tlonid sin on M 1. onspuous spes re present etween teeth from the posterior inisors to the lower premolr P 4. The repling teeth t the ultimte lower inisor lous nd the upper nine lous indte typl diphyodont replement s in most mmmliforms 3. The mndile of Pseudotrios is poorly preserved, ut revels severl strutures previously unknown in shuotheriids (Fig. 2g). The dentry ondyle is roust. The mndiulr rmus is grile nd shllow in the symphysel region, ut more roust nd deeper 1 rnegie Museum of Nturl History, Pittsurgh, Pennsylvni 15213, USA. 2 hinese Ademy of Geologl Sienes, Beijing , hin. 93

2 NATURE Vol Noveer 2007 (1.6 mm) elow M 2 in Pseudotrios thn in Shuotherium (1.3 mm). The mndiulr ngle in Pseudotrios is etter developed thn in Shuotherium, nd hs posteriorly fing fet for the etotympn (ngulr) one s in doodontns 9,17. A rugose re is tenttively interpreted s the oronoid sr (Fig. 2g). Pseudotrios is similr to Shuotherium in hving onvex ventrl order of the mndile under M 2. The postdentry trough extends nteriorly to the posterior opening of the mndiulr nl. The shllow Mekel s sulus extends nteriorly to elow M 1, nd is prllel to the ventrl order of the mndile, not interseting the ventrl order s in Shuotherium. Judging from the mndiulr nd molr differenes within shuotheriids, there seems to e wide rnge of morphologl divergene etween Pseudotrios nd Shuotherium s well s etween known speies of Shuotherium, suggesting greter diversity thn shown y the urrently modest reord of this group. Pseudotrios hs 22 thoro-lur vertere (Fig. 1), inuding 13 thor vertere hving the ris onneted y mens of ostl rtilge to the sternere. Three dditionl thoro-lur vertere (Fig. 1, 4 16) hve floting ris without onnetion to the sternl series, nd re tenttively designted thor vertere. The lst six thoro-lur vertere (17 22) hve more vertl pre-zygpophyses nd more roust entr; these re designted lur vertere. The lst three thor vertere nd the nterior two lur vertere with moile ris form grdtion of suessively shorter ris in more posterior vertere, s in modern monotremes, the sl mmml Fruitfossor nd the eutronodont Ynoonodon 12,18. The ostl rtilges of posterior ris re grile nd individulized, unlike the rod nd overlpping (irting) ostl rtilge pltes of extnt monotremes 19. The ervl vertere re osured y the roken elements of the shoulder girdle, ut I 1 4 mx P Mm I 1 4 os? p d ul 10 mm (in) r st2 7 t Ir1 3 lr1 3 I4 6 s1 3 il ep It fe fi gt ti Figure 1 Skeleton of the new Mesozo mmml Pseudotrios roustus., Holotype AGS040811A (min prt)., AGS040811B (ounter-prt). An outline for identiftion of min skeletl fetures is shown to the left of nd., Restortion of Pseudotrios s fossoril mmml with sprwling lis (grey indtes the ones preserved in the holotype). p, ngulr proess of mndile;, nine; (in), vle (inomplete); os?, puttive oronoid sr (dentry); d, dentry ondyle; e, etepondyle (humerus); ep, epipuis; fe, femur; fi, fiul; fr, frontl; gt, greter trohnter (femur); 94 hu, humerus; I 1 4 nd I 1 4, upper nd lower inisors 1 4 (or lveoli), respetively;, intervle; il, ilium; l4 6, lur vertere 4 to 6; lr1 3, lur ris 1 to 3; lt, lesser trohnter (femur);, mnurium (sterner 1); Mm, upper nd lower molrs (in prtil ousion); mx, mxill; o, oipitl ones (outline only); P, upper premolrs 5 or their loi indted y roots; p, prietl (outline only); r, rdius; s1 3, srl vertere 1 3; s(in), spul (inomplete); st2 7, sternere 2 7; t16, thor entrum 16; ti, tii;, thor ri 1; 4 16, thor ris 14 16; ul, uln.

3 NATURE Vol Noveer 2007 three srl vertere re reognizle y the iliosrl ontting surfes. The preserved prts of pelv nd hindli ones re roust. The ilium, the femur nd the preserved proximl prts of the tii nd fiul re very rod nd re thk for their length. An impression overlpping the ilium is interpreted to represent the epipuis, mmmliform plesiomorphy 9. The femorl hed is lrge nd without distintive nek. The greter trohnter is lterlly oriented nd lrger thn the medilly direted lesser trohnter elow the femorl hed. The lterl ondyle is lrger thn the medil ondyle. The proximl end of the fiul is rod; the prfiulr proess is present, ut not strongly developed. The tii hs strong nterior rest, rod trough for the tiilis nterior muse, nd proximo-lterl proess. These fetures re plesiomorph for mmmliforms nd tritylodontids 9, The roust shoulder girdle of Pseudotrios shows mny fossoril fetures tht re lso plesiomorphies for rown Mmmli. This supports sl plement of shuotheriids in mmmliform phylogeny. Pseudotrios reseles Sinoonodon nd modern monotremes (Fig. 4) in tht the intervle is twe the length of the mnurium (ynodont plesiomorphy), nd hs onstrted wist, u-foot posterior expnsion, nd elongte rtultion for the vle (ll mmmliform pomorphies). Tritylodontids hve n elgonte ut trnsversely nrrow intervle with tpering posterior end 22,23 (Fig. 4) sl ynodont ondition 24,25 (Fig. 4). In ontrst, the posterior end of the intervle is expnded nd juxtposed with the expnded nterior side of the T-shped or tringulr mnurium (the first ri-ering element in the sternl struture) in mmmliforms. The intervle nd mnurium together form mssive re for the petorlis muses. The lterl proesses of intervle nd vles form the expnded re of origin for the sternoeidomstoid muse. The expnded musulr tthments suggest powerful foreli with sprwling posture, s in monotremes with urrowing dpttion 19, onsistent with suh fossoril fetures s the expnded deltopetorl rest, the teres mjor tuere, nd n expnded distl end of the humerus. The olernon proess is long, nd its length is 62% tht of the ulnr length nterior to the semilunr noth, similr to Fruitfossor nd extnt fossoril mmmls 12. In ll mmmliforms for whh the petorl girdle is suffiently known 12,20,21, the oroid does not rtulte with the intervle. Extnt monotremes, however, hve hypertrophied (met)oroid Metone Prone 1 mm d Protoonid e Metonid Pronid p-hypoonid p-hlpd Metone M 3 protoonid Prone M 2 pseudo-protoone (symmetrl) M 2 metonid g f M3 pseudoprotoone (symmetrl) Pseudotlonid M 3 M 2 M 1 P 5 r P 3 I 4? P1 d os? 2 mm M 3 M 2 M 1 P5 P 3 P 1 I 4? I 1 i? pdt etf p pfm m-sulus Figure 2 Dentl nd mndiulr fetures of Pseudotrios., Upper molr M 2, rown view, omposite restortion, the ingulr outline onjeturl, sed on AGS040811A, AGS040811B nd ref. 7., Lingul view (with ventrl tilt, sed on mer luid drwing of AGS040811A)., Lil view (AGS040811A). d, Lower molr M 2, lingul view, omposite restortion from AGS040811A nd AGS040811B. e, Upper molrs (M 2,M 3 )in ousion with lower M 2 nd M 3 in lingul view (intt in AGS040811B). f, omposite restortion of the upper teeth, lterl view, only two posterior sym inisors known, sed on AGS040811A nd AGS040811B. g, Mndile nd dentition, medil view, omposite restortion of AGS040811A nd AGS040811B. r, repling (permnent) nine; etf, etotympn (ngulr) fet on the mndiulr ngle; I 1 i, inisor I 1 lveolus; I 4 i?, possile inisor I 4 lveolus; I 4 r erupting (replement) I 4 ; m-sulus, Mekel s groove (dentry); pdt, postdentry trough; pfm, posterior formen of mndiulr nl; p-hlpd, pseudo-hypolophid; p-hypoonid, pseudo-hypoonid; sym, mndiulr symphysis. I 4 r 95

4 NATURE Vol Noveer 2007 (sphen) (trio) d e Medil Dorsl Pseudotlonid Pseudo-tlonid (pseudo-trio) (sphen) Figure 3 omprison of triosphen nd pseudo-triosphen molrs., Skull of the mrsupil Didelphis virginin with its true triosphen molrs., Shemt illustrtion of the upper lower ousion of triosphen molrs (trigonid nterior to tlonid)., Boreosphenidn Kielntherium s exmple of the nestrl triosphen pttern 4. extending to rtulte with the expnded posterior end of the intervle (Fig. 4e, f) 19,26. This provides extr support for the powerful shoulder nd foreli for urrowing in modern monotremes. The posterior u-foot of the intervle nd the expnded nterior end of the mnurium re lso present in Pseudotrios. This primitive morphotype of girdle nd sternl struture, s seen in Sinoonodon, d, Shemt illustrtion of the upper lower ousion of pseudotriosphen molrs (trigonid posterior to pseudo-tlonid). e, Shuotheriids s n exmple of pseudo-triosphen molrs (sed on Pseudotrios). Dshed line shows the protoone-tlonid ousion nd the pseudoprotoone-pseudotlonid ousion. Pseudotrios nd monotremes, differs from the pivotl vulo intervle joint nd moile shoulder girdle of theriimorphs (eutronodontns nd rown therins) (Fig. 4) 11,18, The reversed triosphen molr struture is signifnt y its opposite rrngement of the two funtionl prts (the primitive trigonid versus the derived tlonid) of the triosphen molr of sl lvle ontt Intervle Proelesodon ontt 2 m Bienotheroides s m p d Pseudotrios Sinoonodon e Ornithorhynhus s p m ontt f Thyglossus s g Jeholodens st2 h Didelphis Figure 4 Evolution of intervle nd nterior sternl struture in the ynodont mmml trnsition. (Plement of Pseudotrios in mmml phylogeny is shown in Supplementry Informtion.), ynodont Proelesodon 25., Mmmlimorph Bienotheroides 22., Mmmliform Sinoonodon (restortion from severl speimens). d, Pseudotrios (omposite from AGS040811A nd AGS040811B). e, Monotreme Ornithorhynhus. f, Thyglossus. g, Eutronodont Jeholodens 27. h, Mrsupil Didelphis. Apomorphies of (1) mmmliforms: widening nterior end of intervle; posterior u-foot of intervle to the sme width of mnuium for extensive tthment of petorlis muses. (2) Yinotheri (sensu, ref. 3): T-shped mnurium. (3) Monotremt: grile nd elongte lterl proess of intervle overlpping two-thirds the length of vle; enlrgement of (met)oroid to rtulte diretly with the intervle; presene of prooroid for strengthening the girdle 19,26,30. (4) Theriimorph 10 : shortening of intervle to equl mnurium; redution of the lterl proess of intervle; moile vulo intervle rtultion 18,27. (5) rown Theri: inorportion of eryon intervle into the mnurium 30, the first sternl segment in rtultion of vle nd thor ri 1., vle (green); ontt, rtultion or overlp of vle nd intervle (yellow);, intervle (red);, mnurium (sterner 1) (lue); m, metoroid; p, prooroid 19 ; s, spul (grey); st2, seond sterner; nd, thor ri ostl rtilge 1 nd 2, respetively. 96

5 NATURE Vol Noveer 2007 mrsupils nd plentls (Fig. 3) nd ustrlosphenid mmmls 15,16. Not present in ny modern mmmls, this evolutionry phenomenon suggests muh greter rnge of dentl diversity in mmmls from the Mesozo er thn in modern mmmls. Despite their importne to mmml dentl evolution, shuotheriids were represented previously y only 11 teeth nd n inomplete mndile. Beuse of the onflting hrters of the derived pseudo-triosphen molrs nd primitive mndile represented y its limited fossils, hypotheses out phylogenet ffinities of Shuotherium hve differed widely. Previously, Shuotherium ws onsidered to e intermedite etween the primitive mmmliform Kuehneotherium with molr usps in n otuse tringle, nd the symmetrodonts with molr usps in n ute tringle 5, to e relted to doodont mmmliforms 28, to e etween the ute-tringled symmetrodonts nd the derived pre-triosphen mmml Permus 7, or to e sl mmml nd sister-group to ustrlosphenidns 3, Our nlysis on n improved dt set from the new Pseudotrios exudes n ffinity of shuotheriids to symmetrodonts, or to the de of Permus nd more derived lineges. Shuotheriids re more osely relted to the ustrlosphenidn de thn other Mesozo mmml groups 3, Before the disovery of the geometrlly reversed design of pseudo-triosphen molrs, it ws widely ssumed for more thn entury tht the upper molr protoone, the lower molr tlonid nd their ousl orrespondene evolved together in single origin in erly mmmlin history. The disovery of Shuotherium hnged this ssumption. It is now generlly epted tht protoone-like struture of the upper molr n oude either tlonid in the posterior prt of lower molr or pseudo-tlonid in the nterior prt of the lower molr in different des; the funtionlly dptive upper molr protoone evolved t lest twe in mmmlin history 4 7. Reently it ws lso proposed tht the tlonid sin in the lower triosphen molr hs evolved more thn one in erly mmmls 3,4, This oservtion reeived support from newly disovered ustrlosphenidn mmmls 15,16 tht lk the typl wer fets of the oreosphenid mmmls (s defined y studies for the Lursin triosphen molrs) 1,2. Their detiled ousl reltionship to the struture of upper molrs my differ etween the Lursin nd the Gondwnn Mesozo mmmls with triosphen teeth 15,16. The new fossil of Pseudotrios dds to the inferene tht oth the upper molr protoone nd the lower molr tlonid ould hve undergone onvergent evolution, nd tht there is more thn one pthwy to oine sling nd rushing funtions in single jw motion for more effetive funivory nd omnivory in erly mmmlin history 29. The wider morphologl rnge of tooth form nd funtion in Mesozo mmmls suggests tht their dentl evolution is muh more lile thn n e inferred from enozo mmmls. Reeived 3 April; epted 3 Septeer rompton, A. W. in Erly Mmmls (eds Kermk, D. M. & Kermk, K. A.) (Zool. J. Linn. So., Suppl. 1, London, 1971). 2. MKenn, M.. in Phylogeny of the Primtes (eds Lukett, W. P. & Szly, F. S.) (Plenum, New York, 1975). 3. Kieln-Jworowsk, Z., ifelli, R. L. & Luo, Z.-X. Mmmls from the Age of Dinosurs Origins, Evolution, nd Struture (olui Univ. Press, New York, 2004). 4. Loptin, A. V. & Averinov, A. O. An egilodontid upper molr nd the evolution of mmml dentition. Siene 313, 1092 (2006). 5. how, M. & Rh, T. H. Shuotherium dongi, n. gen. nd sp., therin with pseudotriosphen molrs from the Jurss of Shun, hin. Austr. Mm. 5, (1982). 6. Sigogneu-Russell, D. Disovery of Lte Jurss hinese mmml in the upper Bthonin of Englnd..R. Ad. Si. II 327, (1998). 7. Wng, Y.-Q., lemens, W. A., Hu, Y.-M. & Li,.-K. A prole pseudotriosphen upper molr from the Lte Jurss of hin nd the erly rdition of the Holotheri. J. Vert. Pleontol. 18, (1998). 8. Ji, Q. et l. A swimming mmmliform from the Middle Jurss nd eomorphologl diversiftion of erly mmmls. Siene 311, (2006). 9. Meng, J. Hu, Y.-M., Wng, Y.-Q., Wng, X.-L. & Li,.-K. A Mesozo gliding mmml from northestern hin. Nture 444, (2006). 10. Rowe, T. B. Definition, dignosis, nd origin of Mmmli. J. Vert. Pleontol. 8, (1988). 11. Li, G. & Luo, Z.-X. A reteous symmetrodont therin with some monotremelike postrnil fetures. Nture 439, (2006). 12. Luo, Z.-X. & Wile, J. R. A new Lte Jurss digging mmml nd erly mmmlin diversiftion. Siene 308, (2005). 13. Luo, Z.-X. et l. Dul origin of triosphen mmmls. Nture 409, (2001). 14. Luo, Z.-X. et l. In quest for phylogeny of Mesozo mmmls. At Pleont. Polon 47, 1 78 (2002). 15. Mrtin, T. & Ruhut, O. W. M. Mndile nd dentition of Asfltomylos ptgonus (Austrlosphenid, Mmmli) nd the evolution of triosphen teeth. J. Vert. Pleontol. 25, (2005). 16. Rougier, G. W. et l. New Jurss mmmls from Ptgoni, Argentin: repprisl of ustrlosphenidn morphology nd interreltionship. Am. Mus. Novit. 3566, 1 54 (2007). 17. Lillegrven, J. A. & Krust, G. rnio-mndiulr ntomy of Hldnodon exspettus (Doodont; Mmmli) from the Lte Jurss of Portugl nd its impltions to the evolution of mmmlin hrters. ontri. Geol. Univ. Wyom. 28, (1991). 18. Luo, Z.-X. et l. A new eutronodont mmml nd evolutionry development in erly mmmls. Nture 446, (2007). 19. Augee, M. L. et l. Ehidn Extrordinry Egg-lying Mmml (SIRO Pulishing, ollinswood, 2006). 20. Mrtin, T. Postrnil ntomy of Hldnodon exspettus (Mmmli, Doodont) from the Lte Jursss (Kimmeridgin) of Portugl nd its ering for mmmlin evolution. Zool. J. Linn. So. 145, (2005). 21. Jenkins, F. A. Jr & Prrington, F. R. The postrnil skeletons of the Triss mmmls Eozostrodon, Megzostrodon nd Erythrotherium. Phil. Trns. R. So. Lond. B 273, (1976). 22. Sun, A.-L. & Li, Y.-H. The postrnil skeleton of Jurss tritylodonts from Shun Provine. Vert. PlAsit 23, (1985). 23. Sues, H.-D. & Jenkins, F. A. Jr. in Amniote Pleoiology: Perspetives on the Evolution of Mmmls, Birds, nd Reptiles (eds rrno, M. T. et l.) (Univ. hgo Press, hgo, 2006). 24. Jenkins, F. A. Jr. The postrnil skeleton of Afrn ynodonts. Peody Mus. Nt. Hist. Bull. 36, (1971). 25. Romer, A. S. & Lewis, A. D. The hñres (Argentin) Triss reptile fun. XIX. Post rnil mterils of the ynodonts Proelesodon nd Proinognthus. Brevior 407, 1 26 (1973). 26. Sereno, P.. in Amniote Pleoiology: Perspetives on the Evolution of Mmmls, Birds, nd Reptiles (eds rrno, M. T. et l.) (Univ. hgo Press, hgo, 2006). 27. Ji, Q. et l. A hinese tronodont mmml nd mos evolution of the mmmlin skeleton. Nture 398, (1999). 28. Kermk, K. A. et l. A new doodont from the Forest Mrle. Zool. J. Linn. So. 89, 1 39 (1987). 29. Evns, A. R. & Snson, G. D. The tooth of perfetion: funtionl nd sptil onstrints on mmmlin tooth shpe. Biol. J. Linn. So. 78, (2003). 30. Klim, M. Erly development of the shoulder girdle nd sternum in mrsupils (Mmmli: Mettheri). Adv. Ant. Er. ell Biol. 109, 1 91 (1987). Supplementry Informtion is linked to the online version of the pper t Aknowledgements We thnk A. R. Trum for fossil preprtion, H.-L. You for his ssistne, M. R. Dwson for improving the mnusript, M. A. Klingler for help with grphs, nd J. R. Wile, G.-H. ui nd K.-Q. Go for ess to omprtive mterils. This work ws supported y the 973 Projet y Ministry of Siene nd Tehnology, hinese Ademy of Geologl Sienes (Beijing) (Q.J.), the Ntionl Siene Foundtion (USA), the Ntionl Nturl Siene Foundtion (hin) nd the Ntionl Geogrph Soiety (Z.-X.L.). Author Informtion Reprints nd permissions informtion is ville t orrespondene nd requests for mterils should e ddressed to Z.-X.L. (luoz@rnegiemnh.org). 97