PUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORY CENTRAL PARK WEST AT 79TH STREET, NEW YORK, NY 10024 Number 3398, 15 pp., 4 ®gures March 27, 2003
A New Specimen of Eurylambda aequicrurius and Considerations on ``Symmetrodont'' Dentition and Relationships
GUILLERMO W. ROUGIER,1 BARTON K. SPURLIN,2 AND PETER K. KIK3
ABSTRACT A new specimen of the tinodontid ``symmetrodont'' Eurylambda (Simpson, 1925a, 1929) from the Late Jurassic Como Bluff Quarry, Morrison Formation, is described. The specimen, a complete upper left molariform, is probably an M1. The major crown cusps of Eurylambda show similarities to those of triconodontids on the one hand and to spalacotherioids on the other. Cusp B of basal mammaliaforms is tentatively proposed as homologous with the cusp traditionally described as a stylocone in Eurylambda and with cusp BЈ of Peralestes. These homologies imply that the stylocone is ancestrally a small cusp in the lineage leading to Theria and that the development of a parastylar lobe or ``hook'' is a derived feature of post-tinodontid mammals. If accepted, this scenario results in a more complex origin for the therian upper molar than previously recognized. Wear facet 1 (Crompton, 1971) of holotherians would not be homologous between Kuehneotherium±Eu- rylambda±Zhangheotherium, on the one hand, and the therians, on the other.
INTRODUCTION in a bias favoring the preservation of lower teeth and jaws. Therefore, our knowledge of Information on the upper molars of early dental evolution during the Mesozoic relies Mesozoic mammals is sparse, due largely to heavily on mandibular and lower molar mor- the mechanical fragility of the maxilla when phology (Crompton, 1971; Prothero, 1981). compared with the mandible, which results Discovery of upper molariforms is always a
1 Research Associate, Division of Paleontology, American Museum of Natural History. Assistant Professor, Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louisville, KY 40292. e-mail: [email protected] 2 Department of Neurology, School of Medicine, University of Louisville, Louisville, KY 40292. e-mail: [email protected] 3 Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louis- ville, KY 40292. e-mail: pete[email protected]
Copyright ᭧ American Museum of Natural History 2003 ISSN 0003-0082 2 AMERICAN MUSEUM NOVITATES NO. 3398 welcome addition because it allows the eval- sessing the bases of both roots. The crown uation of models of evolutionary transfor- was broken into halves at the base of the mation that have been based primarily on paracone and has been previously repaired. lower teeth, with occluding upper teeth being The crown of the new specimen is more buc- based on hypothetical reconstructions (Mills, colingually compressed than the type speci- 1964; Kermack et al., 1964; Crompton, men and is slightly longer mesiodistally, with 1971). the major cusps forming an obtuse angle of We report here on an isolated upper left about 171Њ (with the type specimen forming molariform of Eurylambda aequicrurius an angle of about 168Њ). These features sug- (Simpson, 1925a, 1929). This is only the sec- gest that the new specimen is a more anterior ond specimen known for this taxon originally tooth than is the type. If tooth variance in described as Amphidon aequicrurius (Simp- other symmetrodonts is used as a model (Ci- son, 1925a) from the Late Jurassic Morrison felli, 1999; Cifelli and Gordon, 1999; Cifelli Formation in Wyoming. The new specimen and Madsen, 1999; Hu et al., 1997, 1998; was collected at Como Bluff Quarry in 1883 undescribed specimen in prep.), the new by members of the O.C. Marsh expedition tooth probably represents the position direct- and catalogued at the National Museum of ly mesial to the type, which was originally Natural History as an undetermined Meso- regarded as M1 (Simpson, 1925a, 1929) but zoic mammal tooth with the number USNM is here interpreted as a probable M2. How- 2846. Como Bluff is also the locality where ever, USNM 2846 has a slightly more pro- the Yale University party collected the type nounced ecto¯exus than does the type. This specimen of Eurylambda. becomes more pronounced in distal elements Discovery of a second specimen of Eury- of other ``symmetrodonts'', that is zhang- lambda is important because the type, a frag- heotheriids (Hu et al., 1997, 1998; unde- mentary maxilla with one tooth, has previ- scribed specimen in prep.), spalacotheriids ously been the only model for the upper den- (Cifelli and Gordon, 1999; Cifelli and Mad- tition of basal or obtuse angled ``symmetro- sen, 1999) and Peralestes (Owen, 1871; donts'' (Crompton and Jenkins, 1967, 1968; Simpson, 1928; Clemens, 1963). This latter Crompton, 1971; Ensom and Sigogneau- consideration introduces some doubts about Russell, 2000). Furthermore, the incomplete the more mesial position of USNM 2846 type is missing the parastylar region of the proposed here. tooth. Eurylambda has been suggested to be The crown has the shape of an elongated a junior synonym of Tinodon (Prothero, ``D'', with the straight side oriented buccally 1981; Fox, 1985; McKenna and Bell, 1997; and slightly excavated by the shallow ecto- Sigogneau-Russell and Ensom, 1998; Ensom ¯exus. Six cusps are fairly regularly placed and Sigogneau-Russell, 2000) and we tenta- on the outline of the ``D''. The largest of tively follow this interpretation here (see be- them is the centrally placed paracone (cusp low), although for convenience we continue A in terminology of Crompton and Jenkins, to use the name Eurylambda for the upper 1968). Three cusps are distal to the paracone: molars. the metacone (cusp C), cusp D, and a small metastyle (following the terminology of SYSTEMATIC PALEONTOLOGY Crompton and Jenkins (1967) and subse- quent modi®cations) forming the extreme TINODONTIDAE FOX, 1985 distolabial corner of the crown. Mesial to the paracone is the fairly lingual cusp BЈ (as in- Tinodon bellus Marsh, 1879 (Amphidon aequi- terpreted by Hu et al. (1997, 1998) or ``cusp crurius Simpson, 1925a; Eurylambda Simpson, 1929). on anterior crest'' of Patterson (1956), and a small stylocone in the mesiobuccal corner of the crown (see discussion below). The styl- DESCRIPTION ocone is joined to cusp BЈ by a sharp crest (Figures 1, 2) that descends abruptly from the mesial slope MATERIAL: The specimen (USNM 2846) is of BЈ. The small stylocone extends distally an isolated complete left upper molar pos- as a fairly robust crest that reaches the mid- 2003 ROUGIER ET AL.: EURYLAMBDA AEQUICRURIUS 3 dle of the buccal surface of BЈ. The area buc- gard to the main axis of the tooth, and sub- comesial to cusp BЈ is raised and forms a equal in size. They differ markedly from the small platform which sets off a small thick- condition in Spalacotheroides (Cifelli and ening of the mesial slope of the stylocone. Madsen, 1999), whose roots are mesiodistal- Cusp BЈ is a main cusp of the crown and ly compressed and parallel to each other. is not connected to a cingulum of any kind. It has a concave mesial face and a teardrop- DISCUSSION shaped wear facet that truncates its apex. Dis- tal to its apex, cusp BЈ is connected to the The importance of Eurylambda in our un- massive face of the paracone by a short, near- derstanding of early ``symmetrodont'' rela- ly horizontal ridge. A shallow notch is formed tionships has recently been stressed by En- at the intersection of the distal crest of cusp som and Sigogneau-Russell (2000). Eury- BЈ and the mesial edge of the paracone. lambda, putatively a junior synonym of Tin- The paracone is the largest and tallest odon, represents one of the few examples of cusp, being twice as high as the next highest upper tooth morphology among ``symmetro- cusp in the crown, the metacone. The para- donts'' and it has played a crucial role in the cone is slightly asymmetrical mesiodistally, study of dental occlusion (Crompton and with a blunter, more vertical mesial edge and Jenkins, 1967, 1968; Crompton, 1971). Eur- a sharper, longer, and more gently sloping ylambda represents the archetype for basal distal crest. The lingual side of the paracone nontribosphenic holotheres (Hopson, 1994; is almost ¯at while the buccal side is strongly Wible et al., 1995). convex. These differences give the paracone Ensom and Sigogneau-Russell (2000) re- a conspicuous hooklike shape. cently described a new species of Tinodon, The metacone (cusp C) is separated by Tinodon micron, from the Purbeck Lime- deep notches from both the paracone and stone Group, Lower Cretaceous of England. cusp D. It is a fairly symmetrical cusp, al- The type specimen is a lower molar that though the distal ridge is somewhat longer agrees quite closely with the American spe- and less vertical. Its lingual face is grooved cies of Tinodon. In addition to lower molars, by a distinctive wear facet, which covers not Ensom and Sigogneau-Russell (2000) attri- only cusp D, but extends to the metastyle. buted an isolated upper molar to the same Cusp D (which may be interpreted as the species and compared it to Eurylambda.We larger of two metastylar cusps) is separated concur with their analysis in noting the close by a relatively shallow embrasure from the similarities between the two North American metacone (C) and by a deep notch from the specimens of Eurylambda and the English metastyle. The wear facet mentioned above specimen, DORCM GS 694, referred to T. ¯attens its lingual surface while its labial as- micron. The main difference between the pect is strongly convex. The distal metastyle American specimens of Eurylambda and that is a poorly differentiated small cuspule that from the Purbeck is the presence of a well- forms a small, sharp posterior edge for inter- developed cusp BЈ in the former, which is locking with the following molar. All of vestigial in the latter. The mesial lobe of the these cusps, with the exception of those in crown is also broader and proportionally the parastylar area (this portion is missing in larger in the English specimen than in the YPM 13639, the type specimen of Eury- American specimens. The latter also lack the lambda aequicrurius), can be seen in YPM small labial cingulum (or have it extremely 13639 with similar proportions. A shallow, reduced) that is seen on the upper molar re- buccally-concave stylar shelf extends be- ferred to T. micron (Ensom and Sigogneau- tween the stylocone and the metastylar por- Russell, 2000: ®g. 9C). Ensom and Sigog- tion of the tooth. A low crest, the buccal cin- neau-Russell (2000) wondered if the differ- gulum, demarcates the outer margin of the ences they noted between their specimen and stylar shelf. the American Eurylambda specimens were There is no clear demarcation between the due to different positions on the dental series crown and the roots. Both roots are bucco- or to individual variation. The two American lingually compressed, set obliquely with re- specimens agree with each other very close- 4 AMERICAN MUSEUM NOVITATES NO. 3398
Fig. 1. USNM 2846, Eurylambda aequicrurius, left upper molariform (M1?), in lingual (A), labial (B) and occlusal (C) views. 2003 ROUGIER ET AL.: EURYLAMBDA AEQUICRURIUS 5
Fig. 2. USNM 2846, Eurylambda aequicrurius, drawings of the left upper molariform, in lingual (A), labial (B) and occlusal (C) views. Cusp nomenclature follows Hu et al. (1997, 1998) which in turn is a modi®cation of that employed by Hopson and Crompton (1969) and Crompton (1971). Arrows refer to B and C. 6 AMERICAN MUSEUM NOVITATES NO. 3398
Fig. 3. Traditional interpretation of the homologies of the main cusps among mammaliaformes (oc- clusal views): (A) upper and lower molariforms of Morganucodon; (B) upper and lower molariforms of Kuehneotherium; (C) upper and lower molariforms of Tinodon (Eurylambda); (D) upper and lower molariforms of an undescribed zhangheotherid mammal; (E) upper and lower molariforms of Peramus; (F) upper and lower molariforms of Pappotherium. Drawings are not at the same scale. (A, lower), (B), (E), and (F) taken with modi®cations from Hopson (1994). Represented in black are the main cusps of upper and lower molariforms. Abbreviations: entdϭ entoconid, hydϭ hypoconid, proϭ protocone. ly, which suggests that the very reduced cusp Hopson and Crompton (1969), as well as the BЈ (see below) of the English specimen re- latter modi®cations by Crompton (1971) and ¯ects a taxonomic difference more than var- somewhat revised ideas by Prothero (1981) iation within a single species of Tinodon/ (®g. 3). Ensom and Sigogneau-Russell Eurylambda. (2000) doubted the identi®cation by Cromp- Ideas on cusp homologies among basal ton and Jenkins (1967) of the large cusp me- holotheres, including Kuehneotherium, Tin- sial to cusp A in Eurylambda as the stylo- odon/Eurylambda, and spalacotherioids, go cone (ϭ cusp B of basal mammaliaforms back to the original proposals by Patterson such as Morganucodon). Ensom and Sigog- (1956), Crompton and Jenkins (1967, 1968), neau-Russell (2000) thought it possible that 2003 ROUGIER ET AL.: EURYLAMBDA AEQUICRURIUS 7 this cusp was in fact the homologue of cusp a paraphyletic series leading to Theria. In all BЈ present in Peralestes (Patterson, 1956; of these studies, groups traditionally included Sigogneau-Russell and Ensom, 1998; Cifelli in ``Triconodonta'' (Jenkins and Crompton, and Madsen, 1999) and zhangheotheriids 1979), such as Triconodontidae and amphi- (®g. 3; Hu et al., 1997, 1998), a possibility lestids, form a succession of paraphyletic also considered by Simpson (1929) when groups leading to some of the groups tradi- noting similarities between Eurylambda and tionally included in Symmetrodonta. Upper Peralestes (although he used a different cusp teeth of traditional amphilestids, such as Am- nomenclature). philestes (Owen, 1871; Simpson, 1928) and Resolution of this controversy is important Phascolotherium (Owen, 1838, 1871; Simp- because if the above proposal by Crompton son, 1928) are not known at present, al- and Jenkins is accepted, we would have es- though the occlusal pattern as deduced from sentially a structural continuity within holo- the morphology and wear facets of the lower theres for all the major cusps and wear facets dentition suggest that three main cusps were (excluding the protocone) on the upper tooth present on the upper teeth and that they were crown. On the other hand, if Ensom and Sig- arranged to form a broadly obtuse-angled tri- ogneau-Russell are correct and the very lin- angle (Mills, 1964, 1971; Crompton, 1974; gual, large anterior cusp of Eurylambda is Kielan-Jaworowska and Dashzeveg, 1998). homologous with cusp BЈ of the spalacoth- Gobiconodon from the Early Cretaceous erioids, then either the parastyle or the styl- of Mongolia (Tro®mov, 1978; Kielan-Jawo- ocone has disappeared in Tinodon/Eurylamb- rowska and Dashzeveg, 1998) has been var- da and obtuse-angled symmetrodonts (Fox, iously interpreted as an amphilestid (Tro®- 1972, 1984; Cifelli and Madsen, 1999; Sig- mov, 1978; Kielan-Jaworowska and Dash- ogneau-Russell and Ensom, 1998) or the zeveg, 1998) or as a member of a monotypic stylocone and the traditional cusp B is not family Gobiconodontidae (Chow and Rich, homologous with that of cladotheres. Basing 1984; Jenkins and Schaff, 1988). Recent this debate on cusp and crown morphology analyses weakly link Gobiconodon with tri- is problematic because without relative po- conodontids (Luo et al., 2001; Rougier et al., sition (and its concomitant functional impli- 2001; Wang et al., 2001). Regardless of the cations), we are left with no other criteria on ultimate af®nity of gobiconodontids (either which to base homology (Simpson, 1961). with amphilestids or triconodontids), they Relative position is generally accepted as show enough similarity to amphilestids to highly variable among cusps thought to be use their upper molariform morphology as homologous, and is therefore not the best cri- the general pattern expected for the upper terion to be used as the sole determinant of molars of amphilestids which represent a cusp homology. Although the phylogenetic grade instead of a clade (Rougier et al., relationships of nontribosphenic mammals 2001). are currently in a state of ¯ux, the general As noted by Kielan-Jaworowska and framework, as presented by Wible et al. Dashzeveg (1998), Gobiconodon shows an (1995); Hu et al. (1997), and Luo et al. incipient triangular arrangement of the cusps (2001, 2002) can be used to address this of the upper molariforms. In Gobiconodon, problem. the degree of triangulation of the main cusps Symmetrodonta, as considered by Simp- is 140 degrees, averaged for the M3±M5, son (1928, 1929) and Cassiliano and Clem- whereas it is 168Њ for the type specimen of ens (1979), is not supported as a monophy- Eurylambda aequicrurius from Como Bluff. letic group in any of the recent analyses of Note that this is somewhat different from the mammaliaform relationships (Hu et al., crown outline angles measured by Crompton 1997; Ji et al., 1999; Luo et al., 2001, 2002; and Jenkins (1967) and Prothero (1981) for Rougier et al., 1999, 2001). In spite of nu- the type specimen of Eurylambda. The angle merous small disagreements among these is 154Њ in the upper molariform attributed to analyses, the taxa traditionally included in Tinodon micron (from Ensom and Sigog- Symmetrodonta (i.e., kuehneotheriids, tino- neau-Russell, 2000). Therefore, the some- dontids, and spalacotherioids) generally form what triangular arrangement of the main 8 AMERICAN MUSEUM NOVITATES NO. 3398 cusps is likely to be a primitive feature erian Nanolestes (Martin, 2002) from the Ju- shared by symmetrodonts and at least some rassic of Portugal. of their ``triconodont'' outgroups. If cusp B of triconodonts and other basal Gobiconodontids and the more basal mammaliaforms is homologous with BЈ of mammaliaforms, including Triconodontidae spalacotheriids, which also have a stylocone, and Dinnetherium (Jenkins et al., 1983; it follows that B and the stylocone are not Crompton and Luo, 1993), have three main the same cusp (®gs. 3, 4). What, then, is the cusps on the crown. These are a central cusp homologue of the stylocone among basal A (paracone), a distal cusp C (generally ac- mammaliaforms and later cladotheres? In cepted as the metacone, but see Crompton Gobiconodon, Jeholodens, and triconodonts (1971) and comments in Prothero (1981) for there is a small cusp occupying the mesio- opposing views), and a mesial cusp B (gen- buccal corner of the crown. All of these erally thought to be homologous with the forms also lack a distinct parastylar lobe, a stylocone). feature also absent in Eurylambda. There- By comparing the upper teeth of Eury- fore, we deduce the mesiobuccal cusp of tri- lambda with those of Gobiconodon, tricon- conodonts, Gobiconodon, and other basal odontids, Dinnetherium, and Jeholodens mammaliaforms to be the homologue of the (personal obs.), a fairly simple case of posi- stylocone of later mammals. If this proposal tional homology can be defended for a cor- is accepted, the small stylocone of symme- respondence of cusp B of triconodonts trodonts, Peramus (Clemens and Mills, (Simpson 1925b, 1925c; Patterson, 1956; 1971), basal tribosphenic mammals, and their Crompton, 1974) with the stylocone of Eury- relatives such as Vincelestes (Bonaparte and lambda as identi®ed by Crompton and Jen- Rougier, 1987; Rougier, 1993) and Coman- kins (1967) or cusp BЈ above (®g. 4). In all chea (Jacobs et al., 1989) would be a re- of these forms, the three main cusps are tained primitive feature. placed lingually, forming a broad triangle As a corollary of the homologies proposed with a dominant cusp A, and ¯anking cusps above, the parastylar hook would be a de- B and C of similar size. From this, it also rived feature diagnosing all post-tinodontid follows that the large cusp on the paracrista mammals, because it is lacking in basal in spalacotherioids (such as Peralestes)isin mammaliaforms. A parastyle as an individual fact cusp B of basal mammaliaforms; there- cusp seems also to be absent in all, or most, fore, the use of BЈ to designate it is unnec- of these basal forms. A distinctive parastylar essary. We should refer to it simply as cusp hook and parastyle are likely associated with B, re¯ecting the homology between the cusps the increased transverse development of mo- of upper molariforms across precladothere lariforms in more derived mammals by pro- mammaliaforms. In our description of the viding an interlocking mechanism between new specimen of Eurylambda, we used BЈ to successive teeth. describe this cusp so as not to introduce the A possible and noteworthy exception to unfamiliar B without justi®cation. However, the proposals above is Kuehneotherium, if the above proposal is accepted, the names known from isolated teeth and jaws from the should be interchangeable. This proposal Early Jurassic Welsh ®ssure ®llings (KuÈhne, supports Ensom and Sigogneau-Russell's 1950; Kermack et al., 1968; Parrington, (2000) interpretation of the homologies of 1971; Gill, 1974). The upper molariforms of the cusps in Tinodon micron and Zhang- Kuehneotherium have three main cusps in heotherium±Peralestes. Cusp B would be the crown traditionally interpreted as a some- lost among dryolestoids and later mammals. what centrally positioned paracone, an ante- Patterson (1956) illustrated the occurrence of rior stylocone, and a posterior metacone. De- a small cuspule occupying a similar position spite some obvious similarities between to cusp B in the Jurassic dryolestoid Melan- Eurylambda and Kuehneotherium (Crompton odon, but a similar cusp is not known in oth- and Jenkins, 1967), there are differences, no- er dryolestoids (Ensom and Sigogneau-Rus- tably the very lingual position of the cusp sell, 1998; Martin, 1999). A well developed here identi®ed as B (or BЈ)inEurylambda cusp B is however present in the stem zath- and the far more buccally located cusp tra- 2003 ROUGIER ET AL.: EURYLAMBDA AEQUICRURIUS 9
Fig. 4. Interpretation of the homologies of the main cusps among mammaliaformes postulated in this paper: (A) upper and lower molariforms of Morganucodon; (B) upper and lower molariforms of Kuehneotherium; (C) upper and lower molariforms of Tinodon (Eurylambda); (D) upper and lower molariforms of an undescribed zhangheotherid mammal; (E) upper and lower molariforms of Peramus; (F) upper and lower molariforms of Pappotherium. Drawings are not at the same scale. (A, lower), (B), (E), and (F) taken with modi®cations from Hopson (1994). Represented in black are the main cusps of upper and lower molariforms. Abbreviations: entdϭ entoconid, hydϭ hypoconid, proϭ protocone, styϭ stylocone. ditionally identi®ed as the stylocone in Kueh- gier et al., 2001). This basal position for neotherium. As noted by Ensom and Sigog- Kuehneotheium and its lack of af®nities with neau-Russell (2000), cusp B of Eurylambda later cladotheres has been proposed early on is more lingual than in any other symmetro- by Rougier et al. (1996) as an expression of dont and even in Kuehneotherium. Recent the contradiction posed by the primitive phylogenetic analyses place Kuehneotherium mandibular morphology and the supposed as a basal mammaliaform outside the com- derived triangular dentition. In numeric phy- mon ancestor of triconodontids, ``symmetro- logenetic analyses the basal position for donts'', and therians (Luo et al., 2001; Rou- Kuehneotherium is determined mostly by the 10 AMERICAN MUSEUM NOVITATES NO. 3398 plesiomorphic dentary, which retains a broad functional features (such as wear facets) as postdentary groove with a medial ridge, a independent characters in phylogenetic anal- mandibular angle located very far forward, yses seems best approached with caution. In- and a small, backward-sloping coronoid pro- ferring the presence of cusps based on the cess. Considering that several groups with facets present in an opposite element (e.g., nontriangular dentitions are intercalated be- upper molar cusps predicted by wear facets tween Kuehneotherium and Tinodon (includ- on a lower molar) seems risky and subject to ing triconodontids, amphilestids, and possi- too many idiosyncrasies (re¯ecting mostly bly multituberculates), it is likely that the tri- the favored morphology of the person recon- angular arrangement of the cusps in Kueh- structing the missing element). neotherium and the stem group Theria is Along the posterior crest of the tooth of convergent. Kuehneotherium may represent Eurylambda there are two major and one mi- an early and independent acquisition of the nor cusp, referred to here as paracone, meta- reversed triangles occlusal pattern (Rougier cone, and cusp D, respectively. We are fairly et al., 1996, 2001; Luo et al., 2002). The con®dent that the cusp immediately posterior cusp identi®ed as the stylocone in Kueh- to the paracone is homologous with cusp C neotherium is arguably homologous with the of triconodonts and Zhangheotherium;we mesialmost large cusp in the crown of tri- are less certain, however, that the same cusp conodonts, Gobiconodon, Eurylambda, and is homologous with the metacone of therians. spalacotherioids, where it is called B (or BЈ). Crompton (1971) argued that the metacone The minute cusp identi®ed as the parastyle of therians was a neomorph absent in prim- in Kuehneotherium can be either homolo- itive Mesozoic mammals (citing as support- gous with the stylocone of Eurylambda or ing evidence an accessory cusp CЈ that in his simply a neomorph. We think that these is- view would be homologous with cusp C of sues are irresolvable at present. more basal forms). Prothero (1981) and Hop- The possibility that cusp B of a basal son (1997) refuted the presence of accessory mammaliaforme (e.g., Morganucodon) and cusps CЈ in Peramus (Clemens and Mills, the stylocone of a therian are not homolo- 1971), and Hopson (1997) also denied its gous also raises the issue about the homology presence in Amphitherium although no upper of the wear facets purported to connect these teeth are known. In Eurylambda there are cusps. Facet 1 (Crompton, 1971; Crompton clearly three cusps distal to the paracone, two and Kielan-Jaworowska, 1978) constitutes of them large. The two more distal (cusp D one of the most primitive features of the tri- and the metastyle here) can be viewed as a angular molariforms thought to be precursors bi®d metastyle, and the more mesial cusp (C) of the therian tribosphenic molar. However, as the metacone. It is equally plausible, in facet 1 as identi®ed by Crompton (1971) our opinion, that these three cusps represent would not be homologous between Kueh- the metacone (C), cusp CЈ of Crompton neotheriu±Eurylambda±Peralestes and the (1971), and the metastyle. Cusp CЈ is wide- nontriangular mammaliamorphs, on the one spread among cladotheres, being present in hand, and the cladotheres, on the other. In the taxa basal to Theria, including Vincelestes ®rst group it would extend between the stylo- (Rougier, 1993), Comanchea (Jacobs et al., cone and B (BЈ) and the paracone in the sec- 1989), and some basal tribosphenic forms, ond. Changes in occlusal relationships including Pappotherium (Patterson, 1956; among major cusps are, of course, possible. Butler, 1978). We believe that it is dif®cult Furthermore, it is feasible to imagine that the to resolve if the metacone is indeed the ho- functional difference might not be very sub- mologue of cusp C in triconodonts and more stantial between a facet that stops at a poorly basal mammals as traditionally defended developed cusp B or one that continues (after (Patterson, 1956; Prothero, 1981; Hopson, the disappearance of the cusp B) to the para- 1997); however, we also view the evidence cone. This change, however, is of the same marshaled by Crompton (1971) to challenge magnitude as those used to argue in favor of this as being insuf®cient. Until new speci- or in opposition to therian af®nities for am- mens bearing on this problem are discovered philestids. In light of the above, the use of and studied, we choose to follow the tradi- 2003 ROUGIER ET AL.: EURYLAMBDA AEQUICRURIUS 11 tional interpretation that equates cusp C and other mammals have been included in Am- metacone, thus preserving a very general phidontidae (Yabe and Shikama, 1938; Tro- statement of homology spanning from basal ®mov, 1980; Yadagiri, 1985; Krusat, 1989), nonmammalian cynodonts to modern theri- with Manchurodon (Yabe and Shikama, ans. 1938) and Gobiotheriodon (Tro®mov, 1980, 1997) based on the most complete speci- AMPHIDON AND ``SYMMETRODONTS'' mens. These materials, however, do not shed additional light on the morphology of the Simpson (1929) considered the possibility Amphidontidae, if there is such a group at that the type of Eurylambda (YPM 13639) all. Manchurodon has been lost since the was, in fact, an upper molar of Amphidon, brief original description. It is unclear if the instead of Tinodon, and was his original at- dental morphology described really corre- tribution. These caveats were his main rea- sponds to a full buccal view or simply to the sons for erecting a new genus for YPM exposed labial surface of the molariforms. In 13639. Crompton and Jenkins (1967), how- the published source, there is very little to ever, determined that the wear facets of Eury- link Manchurodon with Amphilestes. The af- lambda and Amphidon did not match, ren- ®nities of Manchurodon are at present better dering Simpson's 1929 attribution unlikely. left unresolved. The type and only known specimen of Gobiotheriodon (Tro®mov, 1980, 1997) is Amphidon superstes seems to be better inter- known by a fairly complete lower jaw with preted as an amphilestid with worn-down three molariforms and an attributed upper molars rather than a typical ``symmetrodont'' tooth in a fragment of maxilla. We had the (Rougier et al., 2001). There are ®ve molar- opportunity to study this specimen at the Pa- iforms preserved in the type and only spec- leontological Institute, Moscow. Gobiother- imen (YPM 13638) traditionally interpreted iodon is indeed a symmetrodont, but it re- as p±last m1±4. The ®rst preserved tooth sembles more closely the recently discovered shows little wear in contrast with the heavily Zhangheotherium from Liaoning (Hu et al., worn posterior molariforms, indicating that 1997, 1998) than Amphidon.AsinZhang- this element had a deciduous predecessor and heotherium, the lower jaw is very slender that it erupted later than the more distal teeth. with multiple mental foramina (four or A deciduous predecessor would make this more), and the molar cusps have their bases tooth, by de®nition, a premolar (Clemens and separated, forming individual cones. The Lillegraven, 1986; Luckett, 1993). The mor- presence of individualized cusps contrasts phology of this tooth, however, agrees close- with the condition in spalacotheriids where ly with that of the ®rst molariform of Am- the main molariform cusps are united by philestes and Phascolotherium in showing crests at the time of eruption. In zhangheoth- ®ve cusps that are fairly symmetrically ar- eriids, and presumably also in Gobiotherio- ranged, as well as a faint basal cingulum (the don, wear removes substantial parts of the smaller cusp b is damaged). Replacement of crown to obtain matching surfaces between molariforms is known in the putative amphi- upper and lower molariforms. We think, lestid gobiconodonts Gobiconodon and therefore, that Amphidontidae as a distinct Hangjininia (Jenkins and Schaff, 1988; God- family of Mesozoic mammals is probably efroit and Guo, 1999); additionally, the tooth unwarranted, and that it is based on an arti- identi®ed as m1 in zhangheotheriids has a ®cial grouping of badly preserved and poorly deciduous predecessor (personal obs). There- known fossils. fore, the dentition preserved in Amphidon In addition to Amphidon, there are two can be interpreted as m1±m5, which agrees other amphilestids described from the Mor- well with the almost universal presence of rison Formation, Phascolodon (Simpson, ®ve molariforms among amphilestids. The 1925a) and Aploconodon (Simpson, 1925a). somewhat triangular aspect of the crown of Both of these are known by the type speci- Amphidon is, in our opinion, caused by the mens only, in both cases partial lower den- extreme wear of cusps b and c, and by the titions. We cannot rule out that Eurylambda labial bulging of the base of cusp a. A few represents the uppers of an amphilestid, al- 12 AMERICAN MUSEUM NOVITATES NO. 3398 though the poor matching of wear facets be- stead, facet 1 stretching from stylocone to the tween Amphidon and Eurylambda makes it paracone (A). unlikely (Crompton and Jenkins, 1967). Based on the orientation of the wear facets ACKNOWLEDGMENTS on these amphilestids and their crown outline we would expect the upper molariforms to We thank Robert J. Emry, from the Na- be similar to those of Gobiconodon, with the tional Museum of Natural History, Smithson- main cusps forming a broad triangle, the ian Institution, for allowing us to study the teeth somewhat broad buccolingually, and specimen under his care. the parastylar hook absent. Thus, we support For access to comparative material and for the traditional view that Eurylambda is likely providing important additional information the upper dentition of Tinodon. we thank JoseÂ F. Bonaparte, Museo Argen- tino de Ciencias Naturales, Buenos Aires; CONCLUSIONS James A. Hopson, University of Chicago; Demberilyin Dashzeveg, the Geological In- Eurylambda (Simpson, 1925a, 1929) is stitute of the Mongolian Academy of Scienc- likely to be a junior synonym of Tinodon es; Zo®a Kielan-Jaworowska, Institute of Pa- (Marsh, 1879) as previously argued by leobiology, Polish Academy of Sciences, Crompton and Jenkins (1967). The molar de- Warsaw; Farish A. Jenkins, Jr., Museum of scribed here seems to correspond to a more Comparative Zoology, Harvard, and Zhe-Xi mesial position than the corresponding one Luo, Carnegie Museum of Natural History, in the type. The type of Eurylambda is likely Pittsburgh. We thank Angela Klaus and Kev- an M2, which would make the new specimen in Frischmann from the American Museum a probable M1. of Natural History for extensive help with It is likely that the most mesial of the three the SEM photography. large cusps of the crown in Eurylambda is Earlier versions of the manuscript bene®t- homologous with cusp B of triconodonts, in- ed from comments by Zhe-Xi Luo, S. Net- cluding triconodontids and amphilestids; in tleton, and John Wible. This research has turn, this cusp is also likely to be homolo- been supported by NSF grants DEB 94-0799, gous with cusp BЈ of Peralestes (Simpson, DEB 95-27811, DEB 96-25431, DEB 99- 1928; Clemens, 1963; Cifelli and Madsen, 96051, DEB 99-96172, DEB 01-29061, and 1999; Cifelli and Gordon, 1999) and by a Ralph. E. Powe Junior Faculty Enhance- Zhangheotherium (Hu et al., 1997, 1998). If ment Award from The Oak Ridge Founda- these homologies are accepted, cusp B would tion and a Research Incentive grant from be lost in most, if not all, cladotheres, while University of Louisville to G.W.R. A grant the parastylar hook and likely the parastyle from the Antorchas Foundation to the would be a derived feature of post-tinodontid AMNH and to G.W.R has also provided sup- mammals. If the homologies outlined above port for study trips and presentations at meet- were followed, the history of the upper mo- ings. lariforms would be somewhat more complex than previously assumed in the therian stem REFERENCES lineage. This would imply the loss of a major crown cusp (B) and the derived acquisition Bonaparte, J.F., and G.W. Rougier. 1987. MamõÂ- of the parastylar area, instead of a somewhat feros del CretaÂcico Inferior de Patagonia. IV continuous elaboration of the primitive holo- Congreso Latinoamericano de PaleontologõÂa de therian pattern of triangulated cusps. 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