869 Journal of Vertebrate Paleontology 23(4):869–876, December 2003  2003 by the Society of Vertebrate Paleontology THE AFFINITIES OF ROBERTHOFFSTETTERIA NATIONALGEOGRAPHICA (MARSUPIALIA) AND THE ORIGIN OF THE POLYDOLOPINE MOLAR PATTERN FRANCISCO J. GOIN 1 , ADRIANA M. CANDELA 1 , and CHRISTIAN DE MUIZON 2 1 Departamento Paleontologı ´a Vertebrados, Museo de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina, fgoin@museo.fcnym.unlp.edu.ar; 2 UMR 8569 CNRS, Laboratoire de Pale ´ontologie, Museum national d’Histoire naturelle, 8, Rue Buffon, F-75005 Paris, France ABSTRACT—A reanalysis of the upper and lower molar morphology of the early Paleocene marsupial Roberthoff- stetteria nationalgeographica led to an interpretation of the molar homologies of the Polydolopinae (Polydolopimor- phia). Several derived features support the attribution of Roberthoffstetteria to the Polydolopimorphia: thick dentary, molars with thick enamel, upper molars with reduced preparacrista that points to stylar cusp A, open centrocrista (that is, postparacrista not connected to the premetacrista), and metaconule large to very large, forming a ‘‘hypocone.’’ Other derived features support the monophyly of Roberthoffstetteria + polydolopines: alignment of paraconule, protocone, and metaconule in a lingual row; well-expanded anterior and posterior cingula, which are level with the trigon basin; variable occurrence of accessory cuspules at the labial face of the upper molars; and the labially placed paraconid with respect to the metaconid in the lower molars. Both Roberthoffstetteria and the polydolopines are plesiomorphic in having stylar cusp E, which is absent in most other South American marsupials. Several character complexes may be involved in the evolution of the polydolopine molar pattern. INTRODUCTION Roberthoffstetteria nationalgeographica Marshall, Muizon and Sige ´, 1983 (Fig. 1) is one of the most peculiar marsupials recorded at the early Paleocene locality of Tiupampa, Bolivia. Complete upper and lower molar series, as well as the upper premolar series, are known for this taxon. Upper and lower molars are brachydont, bunodont, almost crestless, and sub-qua- drangular in occlusal view, indicating frugivory or omnivory (Thenius, 1989). Marshall and Muizon (1988) and Muizon (1992) suggested affinities between Roberthoffstetteria, Procar- oloameghinia (middle Paleocene of Brazil), and Glasbius intri- catus (latest Cretaceous of North America). Muizon (1992) pointed out that its many dental specializations suggest that Roberthoffstetteria evolved from a quite older (early or middle Maastrichtian?) ancestor. A reanalysis of the type and several referred specimens of R. nationalgeographica led us to new hypotheses of relationships for this taxon. Here we propose that some of the unique derived features of Roberthoffstetteria may provide the key to inter- preting the molar pattern and cusp homologies of the Polydol- opinae (Marsupialia, Polydolopimorphia). The polydolopine molar pattern is one of the most aberrantly specialized among extinct or living metatherians (Fig. 2; Mar- shall, 1982). It certainly inspired Ameghino’s own concept of Allotheria, a clade that also included caenolestoids, rodents, and multituberculates. Polydolopine upper molars consist of two or three series of aligned cusps, resembling those of multituber- culates and some rodents. Other derived dental features of po- lydolopines include reduction of the incisor, premolar, and mo- lar formula (I?/2?, C1/1?, P1–2/1–2, and M3/3), sectorial pre- molars, enlarged and procumbent ?canine, wrinkled molar ba- sins, and very high, blade-like trigonid on m1 (Marshall, 1982). Homology between the individual cusps of upper and lower polydolopine molars and those of the basic mammalian tribos- phenic pattern has never been established. As a consequence, polydolopine origins, affinities, and evolution have been subject of intense debate over the last century (e.g., Marshall, 1987; Goin and Candela, 1995). Abbreviations MLP, Departamento Paleontologı ´a Verte- brados, Museo de La Plata; MACN, Seccio ´n Paleontologı ´a Ver- tebrados, Museo Argentino de Ciencias Naturales ‘‘Bernardino Rivadavia,’’ Buenos Aires; MHNC, Museo de Historia Natural de Cochabamba, Bolivia; MNHN, Laboratoire de Pale ´ontolo- gie, Museum National dHistoire Naturelle, Paris; StA, StB, StC, StD, StE, stylar cusps A, B, C, D, and E, respectively. DENTAL MORPHOLOGY OF ROBERTHOFFSTETTERIA NATIONALGEOGRAPHICA The molar morphology of Roberthoffstetteria nationalgeo- graphica was described in detail by Marshall et al. (1983), Mar- shall and Muizon (1988), and Muizon (1992). Here we restrict our descriptions to the morphological features which are rele- vant to the identification of homologies in the polydolopine molar pattern. A short description of the morphology of the dentary, maxillary, and premolars is also included. The upper premolar and molar series of MNHN 8353 were figured by Muizon (1992:fig. 2a–c; as MNHN Vil 99, holotype of R. na- tionalgeographica). In this paper we figure a better preserved specimen, MNHN 8351 (Fig. 1A). Dentary and Maxilla The preserved mandibular portions reveal that the dentary is a wide, robust bone, especially posteriorly. Neither the alveolar nor the ventral edges of this bone are straight, but instead are slightly convex ventrally. The labial side of the dentary is quite thick and convex, especially below m3, while the lingual side is almost flat. The horizontal ramus of the dentary is deeper below m1 than below m4. The maxilla is stout; the palate is not preserved. The suture between the maxilla and the zygomatic arch is almost parallel to the alveolar edge. MHNC 8353 has a well developed alve- olus just in front and lingual to P1, indicating a large canine. The absence of diastemata in the upper and lower dental series suggest a short rostrum. Upper and Lower Premolars MHNC 8353 preserves the complete P1–P2 and part of the crown of P3. All upper premolars are double-rooted and enlarge