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Home , Khasia, La Meseta Formation, Microbiotheria, Microbiotheriidae, Polydolopimorphia

Revista de la Asociación Geológica Argentina 62 (4): 597-603 (2007) 597

NEW (MAMMALIA) FROM THE OF , AND THE ORIGINS AND AFFINITIES OF THE

Francisco J. GOIN1, Natalia ZIMICZ, Marcelo A. REGUERO1, Sergio N. SANTILLANA2, Sergio A. MARENSSI2 y Juan J. MOLY1

¹ División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, (B1900FWA) La Plata. E-mail: [email protected]. 2 Instituto Antártico Argentino, Dirección Nacional del Antártico, Cerrito n° 1248, C1010AAZ Ciudad Autónoma de Buenos Aires.

ABSTRACT: We describe and comment on an isolated upper molar belonging to Woodburnodon casei gen. et sp. nov. (Mammalia, Marsupialia, Microbiotheria, Woodburnodontidae fam. nov.), from the Eocene of the La Meseta Fm (TELM 5 or Cucullaea I Member), Marambio (Seymour) Island, Antarctic Peninsula. With a body mass estimated between 900 to 1,300 g (depending on the type of equation and the possible molar locus of the type specimen), it represents the largest known Microbiotheria, living or extinct. Besides its size, other diag- nostic features include a proportionally large metacone, reduced or absent para- and metaconules, and an unusual labial notch between stylar cusps C and D. Woodburnodon casei is an undoubted Microbiotheria; however, its reference to the is discarded: almost all its morphological characters are plesiomorphic when compared with South American microbiotheriids, even with respect to the oldest representatives of this family. This suggests (a) a quite ancient and southern origin for Woodburnodon and its ancestors, and (b) that the origins and initial radiation of the Microbiotheria may have occurred from a generalized peradectoid. The new taxon, here referred to the new family Woodburnodontidae, constitutes the second microbiotherian known from these Antarctic levels and age; this confirms the association of representatives of this within a common, Andean-Patagonian-Antarctic biogeographic region, already present since the Late . Microbiotherians stand as the plesiomorphic sister-group of Bonapartheriiform , the latter including Glasbius and allied taxa.

Keywords: Antarctica, Eocene, La Meseta Fm., Marsupials, Microbiotheria.

RESUMEN: Nuevo marsupial (Mammalia) del Eoceno de la Antártida, y los orígenes y afinidades de los Microbiotheria. Se describe y comenta un molar superior aislado perteneciente a Woodburnodon casei gen. et sp. nov. (Mammalia, Marsupialia, Microbiotheria, Woodburnodontidae fam. nov.), procedente de niveles eocénicos de la Fm. La Meseta (Miembro TELM 5 o Cucullaea I), Isla Marambio (Seymour), Península Antártica. Con una masa corporal estimada entre 900 y 1.300 g (dependiendo de la ecuación analizada y del locus molar posible), se trata del más grande microbioterio, fósil o viviente, conocido hasta el momento. Además del tamaño, sus rasgos diag- nósticos incluyen un metacono proporcionalmente grande, para- y metacónulo reducidos o ausentes, y una inusual muesca labial entre las cúspides estilares C y D. Woodburnodon casei es un indudable Microbiotheria, si bien su pertenencia a los Microbiotheriidae debe descartar- se. Casi todos sus caracteres son plesiomorfos en comparación con los microbiotéridos sudamericanos, incluso cuando se lo compara con los más tempranos representantes de la familia. Esto sugiere (a) un origen antiguo y austral para Woodburnodon y sus ancestros, y (b) que la evolución inicial de los Microbiotheria pudo haber tenido su origen a partir de un generalizado peradectoideo. El nuevo taxón, aquí asig- nado a la nueva familia Woodburnodontidae, constituye el segundo Microbiotheria conocido para los mismos niveles y edad de la Antártida, lo que confirma la asociación de los representantes de este orden con una región biogeográfica común, Andino-Patagónico- Antártica, ya existente desde por lo menos el Cretácico Tardío. Los Microbiotheria constituyen el grupo hermano plesiomorfo de los mar- supiales Bonapartheriiformes, estos últimos incluyendo a Glasbius y taxones afines.

Palabras clave: Antártida, Eoceno, Fm. La Meseta, Marsupiales, Microbiotheria.

INTRODUCTION vocally related to South American native 2006). Additionally, an enigmatic, insectivo- mammalian lineages: gondwanatherians re-like represented by tiny isolated Extinct Antarctic are exclusively (Goin et al. 2006); polydolopid, microbio- tooth, was lost after a brief description known from several localities at different theriid, and didelphimorphian marsupials (Goin and Reguero 1993). levels of the (Maram- (Woodburne and Zinsmeister 1982, Goin et The first mention of an extinct Antarctic bio/, Antarctic Peninsula; al. 1999); tardigrade xenarthrans, and astra- marsupial (and of extinct mammals at all) Early to Late Eocene; Reguero et al. 2002). potherian and litoptern ungulates (Reguero was that of Woodburne and Zinsmeister's Up to now the recognized taxa are unequi- et al. 2002 and literature cited; Bond et al. (1982) description of a polydolopine poly- 598 F. J. GOIN, N. ZIMICZ, M. A. REGUERO, S. N. SANTILLANA, S. A. MARENSSI Y J. J. MOLY

dolopimorphian. After two decades of field leocene ( age) Mirandatherium ali- morphians". prospecting, it is clear that polydolopines pioi (Paula Couto 1952). The specimen first Abbreviations, methods and termino- comprise the most abundant marsupials of described by Goin and Carlini (1995) can be logy. MLP, División Paleontología Verte- this age in Antarctica. Successive discove- referred to Marambiotherium glacialis. brados, Museo de La Plata; MPM-PV, Mu- ries led to the description of new polydolo- Here we describe a third specimen belon- seo Regional Provincial "Padre Manuel Je- pines, as well as several referable to ging to an Antarctic microbiotherian mar- sús Molina", Río Gallegos, IAA, Instituto microbiotheriids (Microbiotheria), dero- supial. Its large size and plesiomorphic den- Antártico Argentino, Dirección General del rhynchids (Didelphimorphia), and prepido- tal features prevent any possible referral Antártico, Buenos Aires. Molar dimensions lopids (; Goin et al. either to Marambiotherium glacialis or to any were obtained with a digital calliper. L, 1999). The first Antarctic microbiotherian other member of the Microbiotheriidae. length; W, width; all measurements are in to be described was an edentulous dentary The new taxon sheds new light on the mm. Body mass estimations follow Gordon (Goin and Carlini 1995). A few years later, ancient origin (probably, late Cretaceous) of (2003); Y, ln body mass (in grams); X, ln Goin et al. (1999) described Marambiotherium microbiotherians in former Gondwanan molar area (in square millimeters); r, Pear- glacialis on the basis of this and a new den- , on its extreme southern biogeo- son Correlation Coefficient; r2, Determi- tary bearing an m4. This taxon is a small-si- graphic range, and on the variously propo- nation Coefficient (for r and r2, see Gor- zed microbiotheriid with a few derived fea- sed affinities between microbiotherians, don 2003); g, grams. Molar nomenclature tures suggesting affinities with the Late Pa- other australidelphians, and "polydolopi- follows Goin et al. (2003). SYSTEMATICS

Infraclass Huxley, 1880 Supercohort MARSUPIALIA Illiger, 1811 Superorder Szalay, 1982 Order MICROBIOTHERIA Ameghino, 1889

Family WOODBURNODONTIDAE nov.

Etymology. Derived from Woodburnodon gen. nov., only known genus of the family. Included genera. Woodburnodon gen. nov. Distribution and diagnosis. As for the type species.

Woodburnodon casei, gen. et sp. nov. (Figure 1 a-e)

Etymology. Generic and specific names honour Drs. Michael Woodburne and Judd A. Case, respectively, pioneer researchers of Antarctic mammals; the generic suffix -odon comes from the Greek odontos, tooth. Gender is masculine. Type (and only known) species. W. casei sp. nov. Generic diagnosis. As for the type species. Type specimen. MLP 04-III-1-2, an isola- ted, worn upper right molar (M2 or M3). Hypodigm. The type only. Locality, stratigraphy, and age. Locality Figure 1: Woodburnodon casei gen. et sp. nov. (Microbiotheria). a-e, MLP 04-III-1-2 (Type), an iso- IAA 1/95 (Reguero et al. 2002), Seymour lated upper right molar (M2 or M3) in labial (a), anterior (b), lingual (c), posterior (d), and (Marambio) Island, Antarctic Peninsula. La occlusal (e) views. Scale: 1 mm. Meseta Formation (Elliot and Trautman New marsupial (Mammalia) from the Eocene of Antarctica, ... 599

1982); TELM 5 (Sadler 1988) or Cucullaea I of the stylar shelf; three minute wear facets consisting of a simple least squares regres- Member (Marenssi et al. 1998). Following suggest that it was originally higher than the sion analyses. Measurements used for cons- Marenssi (2006) and Dutton et al. (2002) we preserved structure, thus suggesting its tructing all equations were taken from assign an early Middle Eocene age to the homology with a very labio-lingually com- Gordon (2003). A complication is that the mammal-bearing levels at TELM 5. pressed stylar cusp C. The ectoflexus is very molar locus of specimen MLP 04-III-1-2 is Measurements. Length: 4.96 mm, width: shallow; posterior to it there is a peculiar, uncertain (see above). Molar area (L x W) is 5.58 mm. obliquely set notch between stylar cusps C 27.67 mm2. Diagnosis (for the type and only known and D (Fig. 1 a, e). Four equations were constructed to estima- species of the genus). Differs from all other te the body mass of Woodburnodon casei. In microbiotherians in the following combina- MOLAR LOCUS OF THE the first two, the genus Caluromys was exclu- tion of features: very large size; metacone TYPE SPECIMEN ded of the regression; in the last two it was proportionally large; preparacrista compa- included (see Gordon 2003). ratively longer and more perpendicularly The molar locus of specimen MLP 04-III- When Caluromys was excluded, the follo- oriented in relation to the molar axis; a 1-2 cannot be certainly determined. For a wing equations were constructed: labial notch is present between stylar cusps microbiotherian, its postmetacrista is well- a) Assuming the specimen as a M2, then C and D. developed, suggesting that the specimen is Y=1.708 X + 1.3478 r=0.994; r2=0.989. In Description and comments. The type spe- not an M4. In turn, the preparacrista is not this case, body mass result is 1,118.08 g; cimen is quite worn and lacks enamel on extremely short, as is characteristic of most b) Assuming the specimen as a M3, then the paracone, centrocrista and postmeta- first (and many second) upper molars of Y= 1.6356 X + 1.36 r= 0.993; r2 = 0.987, crista; the trigon area is very wide; accor- microbiotherians, and is not obliquely set resulting in a body mass of 889.95 g. dingly, the protocone is stout and robust. but quite transversal to the antero-posterior When Caluromys was included, the following The metacone is larger and higher than the molar axis, thus suggesting the tooth is an equations were constructed: paracone (Fig. 1a). The preparacrista seems M3. As argued below, Woodburnodon casei is a a) Assuming the specimen as a M2, then to end not at the anterolabial corner of the generalized, non-microbiotheriid microbio- Y= 1.6908 X + 1.5432 r= 0.9707 r2= tooth (as in most microbiotheriids) but at therian. Thus, it could be argued that the 0.9424 resulting in a body mass of 1.283.9 g. an intermediate point between stylar cusps preparacrista is that of an M2. In short, we b) Assuming the specimen as a M3, then A and B; as a consequence, it is more per- conclude that specimen MLP 04-III-1-2 Y= 1.557 X + 1.7592 r= 0.9419 r2= 0.8872 pendicularly oriented with respect to the could be either an M2 or an M3. and the body mass estimation is 1.021.85 g. antero-posterior molar axis than in other Taking in account that microbiotheriids ha- microbiotherians. The anterobasal cingu- WEAR FACETS ve a reduced M/m4, it is therefore probable lum is short and shallow. The metacone is that Woodburnodon also had its last upper relatively large, high and has convex slopes Even though specimen MLP 04-III-1-2 is molar reduced in size regarding the M3, a on both its labial and lingual faces. The cen- partially broken and worn, it can be obser- condition also present in Caluromys. For this trocrista is broken but its preserved portion ved that the wear facet of the postmetacris- reason we favor the last set of body mass indicates it was completely straight (Fig. 1 c, ta is well-developed -other cutting crests of estimations, i.e., that one including Caluro- e). Because of its poor preservation at the this molar are not observable due to the mys in the equation. In short, our best esti- lingual slopes of the paracone and metaco- strong wear of the whole tooth. However, mate is that Woodburnodon had a body mass ne (Fig. 1 c, e), neither the paraconule nor the most conspicuous feature of this speci- between 1,000 and 1,300 g. the metaconule are observable; however, men is the important development of the The size of specimen MLP 04-III-1-2, and judging from the preserved structures, even grinding facet of the protocone (at its inner the inferred body mass of Woodburnodon if these cusps were present they must have face). This, together with the proportionally casei, indicates that this species constitutes been quite reduced. The postmetacrista is large protocone and trigon basin, suggests the largest known microbiotherian marsu- not shortened but moderately developed that the grinding of food during mastica- pial, extinct or living. and straight. The stylar shelf is noticeably tion was a predominant feature of Woodbur- The second largest microbiotherians are reduced, as are the stylar cusps, which are nodon casei. The extent of the wear facet at referable to the genus Pachybiotherium. Up to labio-lingually compressed. Notwiths-tan- the inner face of the protocone in this spe- very recently, upper molars for representati- ding, stylar cusps A, B, C and D are re-cog- cies much resembles that of the upper ves of this genus were unknown. Work in nizable. Stylar cusp B is small, low, and is set molars of the living Caluromys derbianus (Ca- progress by F. Goin, M. Tejedor, A. Abello immediately labial to the paracone, while luromyidae), of frugivorous feeding habits. and G. Martin will describe a new species of stylar cusp D is much smaller, labio-lin- Pachybiotherium, for which an upper molar is gually compressed, and located postero- BODY MASS ESTIMATE known (MPM-PV 1806, a right M3). This labially to the metacone. Between these new species is slightly larger than the type cusps, and anterolabial to the metacone, is a Body mass was estimated following proce- species of the genus, P. acclinum Ameghino, thickening of the enamel at the labial face dures stated in Gordon (2003), basically and much larger than P. minor Goin. Our 600 F. J. GOIN, N. ZIMICZ, M. A. REGUERO, S. N. SANTILLANA, S. A. MARENSSI Y J. J. MOLY

estimate of the body mass of this new spe- is oriented not perpendicular to the antero- paracone. Pediomyids, as well as other cies of Pachybiotherium, based on its upper posterior molar axis but oblique to it; in metatherians (e.g., alphadontids), have para- molar, is 255.99 g using an equation lacking Woodburnodon, even though the preparacris- cones and metacones that are subequal in Caluromys, and 312.07 g if Caluromys is in- ta is worn, it is not significantly short or height and size. Another example is the cluded in the equation. Thus, and taking obliquely oriented. In short, Woodburnodon reduction of the stylar shelf: while pediom- into account their body mass estimates, we casei cannot be assigned to the Microbiothe- yids have much reduced the anterior por- can conclude that Woodburnodon casei was no riidae, as it lacks the basic synapomorphies tion of the stylar shelf, in Woodburnodon this less than three, and probably four, times lar- that characterize this family. Therefore, we structure is equally reduced, both anteriorly ger than the largest species of Pachybiothe- recognize the new family Woodburnodon- and posteriorly. A third feature is the rium. tidae for its inclusion. strong, winged conules of pediomyids, which are reduced or absent in Woodbur- DISCUSSION Origins of the Microbiotheria nodon and all other microbiotherians. Fi- nally, the preparacrista in Woodburnodon is A new family of microbiotherian mar- When compared with the oldest known mi- more or less perpendicular to the molar supials crobiotherian, cordillerensis (Muizon axis, while in pediomyids is invariably orien- 1991), from the Early of Tiu- ted anteriorly. In short, these features sug- Woodburnodon casei represents the second pampa, Bolivia, it is striking that Woodbur- gest that some of the more derived micro- Microbiotheria known from the Eocene of nodon casei is more plesiomorphic than this biotherians convergently evolved a pediom- the Antarctic Peninsula. Taking into ac- taxon concerning all of the above mentio- yid-like morphology, i.e., a further reduc- count their poor representation in the fossil ned features. In fact, Woodburnodon appears tion in the stylar shelf and cusps, and a record, the currently known array of Mid- to be more plesiomorphic than all other more oblique orientation and shortening of dle Eocene Antarctic microbiotherians (two known microbiotherians. This suggests that the preparacrista. Thus, Woodburnodon adds species) should be regarded as moderately woodburnodontid's upper molar morpho- empirical evidence of the independence of diverse. logy is close to, or represents the, ancestral pediomyid and microbiotherian lineages. Three microbiotherian apomorphic featu- microbiotherian molar pattern. The fact Two other, more radical, alternative infe- res are present in specimen MLP 04-III-1- that this taxon comes from levels rences can be made on this topic: (1) Wood- 2, type of Woodburnodon casei: wide protoco- of Antarctica is noteworthy, and is an indi- burnodon is indeed a Pediomyidae whose ne, comparatively short postmetacrista, and cation that the very origin of microbiothe- ancestors reached Antarctica from North reduced stylar shelf. Additionally, the per- rians could be tied to the southernmost re- America, via , by the Late sistence of a straight centrocrista is a gene- gions of the Austral (biogeographic) King- Cretaceous; more modern microbiotheriids ralized feature present in all known micro- dom (sensu Morrone 2002), probably du- evolved from this pediomyid lineage. For biotherians. The reduction or absence of ring Early Paleocene or even Late Creta- the reasons stated above, we cannot favor para- and metaconules is also a derived fea- ceous times (see below and Woodburne and this hypothesis. (2) At least some of the ture present in microbiotherians with Case 1996). Late Cretaceous, North American taxa cu- known upper molars. Previously (e.g. Marshall 1987, Marshall et rrently assigned to the Pediomyidae are ac- Besides its microbiotherian affinities, it is al. 1990), microbiotheriids and North A- tually advanced microbiotherians that arri- not possible to refer the new species to the merican pediomyids were grouped in a ved on that from Southern South Microbiotheriidae, the only family pre- common clade, the Microbiotheria. Even America before the end of the Cretaceous. viously known for this order: (1) microbio- though this taxonomic conclusion has been Testing this last hypothesis would need a theriids have upper molars with the stylar abandoned (see, e.g., Kielan Jaworowska et phylogenetic analysis including all known shelf even more labio-lingually reduced al. 2004, Case et al. 2005), it was reasonably microbiotherians together with peradec- than in Woodburnodon. (2) Microbiotheriid based. Some advanced pediomyids (e.g. toids, early didelphoids, as well as more ba- stylar cusps are indistinguishable as such, Protolambda florencae, from the Late Cretace- sal metatherians (e.g., alphadontids and because of their extreme reduction and la- ous; see Fox 1987: fig. 5, and Davis 2007: allies). However, on the basis of Woodbur- bio-lingual compression. Actually, most mi- fig. 11) show derived features in common nodon's upper molar morphology, as well as crobiotheriid upper molars have a reduced, with microbiotheriids (upper molars with on the new evidence on the evolution of ridge-like structure labially bordering the reduced stylar shelf, short and obliquely set the Pediomyidae (Davis 2007), this last hy- stylar shelf, making impossible to make a preparacrista, robust protocone). The pothesis seems even less probable. secure identification of the stylar cusps. upper molar morphology of Woodburnodon Following Johanson (1996, see also Eaton Woodburnodon casei is plesiomorphic in that casei sheds light on this topic, as it is more 1993) Kielan Jaworowska et al. (2004) upda- its stylar cusps are still observable and pla- clearly derivable from a peradectoid-like ted the dental diagnosis of the Peradecti- ced in the plesiomorphic condition. (3) Fi- pattern than from any pediomyid-like mor- dae. Regarding the upper molars, they state nally, especially on M1-2, microbiotheriids phology. For instance, Woodburnodon's upper that these molars have "… relatively smaller have an extremely short preparacrista which molar has a metacone that is larger than the stylar cusp A placed close to stylar cusp B New marsupial (Mammalia) from the Eocene of Antarctica, ... 601

and about on the same level on the stylar Hatcheriiformes, the Bonapartheriiformes, biotheria. Its range is restricted to the tem- shelf; cusp B reduced in size and subequal and the Polydolopiformes (on the origin of perate forests of the Southern Andean Cor- to stylar cusp C; ectoflexus shallow on all the molar pattern of the Polydolopiformes, dillera, from 36° to 43° South latitude. It is upper molars, not becoming deeper poste- see Goin et al. 2003). Main derived features regarded as a strictly arboreal (Szalay riorly; metacone taller than paracone; pos- leading to polydolopimorphians include the 1994), with frugivorous-insectivorous fee- tmetacingulum lost; preparacrista extends enlargement of the metaconule, the pairing ding habits (Mann 1955, Amico and Aizen from paracone to a point anterior to cusp B, of StB and D with the paracone and meta- 2000). An interesting case of mutualism has rather than terminating at that cusp; conu- cone, respectively, and the further reduction been described by Amico and Aizen (2000) les and conular cristae reduced, with pos- of the stylar shelf. In fact, Woodburnodon involving Dromiciops as a seed disperser of tmetaconular crista lost." (Kielan Jawo- casei fits quite well the pattern expected for the parasite epiphyte Tristerix corymbosus rowska et al. 2004: XX). [Note that this an ancestor of Hatcheriiformes as well as (Loranthaceae). Seed dispersal of this plant diagnosis was made as compared to the of Bonapartheriiforms: reduced stylar requires not only effective ingestion and "Alphadontidae", also regarded as basal shelf, large protocone, short preparacrista transport, but also an adequate placement "didelphimorphians". Case et al. (2005), on and postmetacrista -the true nature of on the living branches of its host tree (No- the contrary, included , Woodburnodon's metaconule is unknown be- thofagus dombeyi). Dromiciops discards the exo- and allies within the Alphadelphia, a clade cause of the poorly preserved type speci- carpium of Tristerix fruits and eats the re- outside of, and basal to, the Didelphimor- men of this species. However, a series of maining parts, including the seeds; the latter phia. While we still recognize the Alpha- peculiar apomorphies of the Polydolo- are transported throughout the intestine, dontidae as a basal clade -i.e., Case et al.'s piformes (Roberthoffstetteria, polydolopids and defecated later --up to 98% of the se- (2005) Alphadelphia-we now accept the and allies), are definitely not derivable from eds ingested are deposited on the host tree. exclusion of the Peradectidae from the Al- Woodburnodon; this suggests now to us that Passage through the digestive tract of Dro- phadelphia, and, based on Kielan Jawo- these marsupials may belong to a different miciops is crucial for seed germination. Thus, rowska et al.'s (2004) arguments, we regard clade, with quite different origins than Dromiciops is a highly effective seed disper- it as a ?Didelphimorphia (Goin 2007). Goin those of Hatcheriiformes + Bonaparthe- ser of this plant. To Aizen et al. (2002), the (2003, 2007) argued that peradectids, mayu- riiformes. Particularly, the lingual alignment high dependence of Tristerix on its marsu- lestids, and caroloameghiniids belong to a of the paraconule and the metaconule with pial disperser could be a result of the anti- natural group: the Peradectoidea]. the protocone, the expansion of anterior quity of their co-evolutionary relationships It is clear from the above-mentioned diag- and posterior cingula, and the persistence (Tristerix is the oldest known Lorantaceae, nosis that the Microbiotheria were derived of a StC which is placed lingually with res- being first recorded in the late Cretaceous, from a peradectoid-like ancestor; in turn, pect to other stylar cusps in the upper ca. 70 Ma). the reduction of stylar cusps and shelf, as molars, seem to be contradictory with other The mutualistic relationship between Dro- well as the enlargement of the protocone in previously related groups, as the bonapar- miciops and a plant related to the Nothofagus the upper molars, constitute derived featu- theriiforms. New phylogenetic analyses flora provides an interesting clue on the res diagnostic of the Microbiotheria. should test these contrasting hypotheses, possible trophic relationships of extinct especially taking in account the new infor- Antarctic microbiotheriids. Other micro- Affinities of the Microbiotheria mation provided by Woodburnodon and seve- biotheriid associations, such as one propor- ral other basal metatherians. We wonder, tionally rich assemblage from the early Ne- The new evidence provided by Woodburno- for instance, whether the inclusion in a ogene of Central Patagonia, have also been don casei is relevant for the interpretation of future analysis of the North American, late related to the nearby presence of a Nothof- the affinities of several marsupial groups Cretaceous Albertatherium could reveal an agus flora (Goin et al. 2007). The upper that have been variously referred as derived independent origin of polydolopines and molar morphology of Woodburnodon casei from, or related to, the Microbiotherians. allies, more closely related to a much more includes an enlarged, bulky protocone and a Recent studies of metatherian phylogeny basal metatherian radiation than that of wide trigon basin indicative of frugivorous consistently relate microbiotherians with microbiotherians and bonapartheriiformes. feeding habits. In turn, frugivory suggests part, or all, the Australidelphian marsupial Upper molars of Albertatherium have their an arboreal or semi-arboreal type of loco- clades (see, e.g., Asher et al. 2004, and litera- para- and metaconules set quite lingual with motion. In short, as already suggested for ture cited). Regarding South American mar- respect to the paracone and metacone, res- polydolopine polydolopimorphians (Wood- supials, outstands the problem of polydolo- pectively, as well as a lingual StC. burne and Case 1996), the origin, radiation, pimorphian relationships. Goin (2003), and dispersal of microbiotherians may be Goin and Candela (2004), and Oliveira and Ecology, , and the timing related to the origin, radiation, and dispersal Goin (2006) argued that microbiotherians of the microbiotherian radiation. of the Nothofagus flora. The distribution, have a sister-group relationship with poly- locomotion, and feeding habits of the sin- dolopimorphians. According to Case et al. Dromiciops gliroides Thomas is the only li- gle living species of Microbiotheriidae, (2005) polydolopimorphians include the ving representative of the Order Micro- Dromiciops gliroides, strongly support this last 602 F. J. GOIN, N. ZIMICZ, M. A. REGUERO, S. N. SANTILLANA, S. A. MARENSSI Y J. J. MOLY

hypothesis. Hill and Dettmann (1996) iden- los mutualismos planta-animal del bosque Goin, F.J. 2007. A review of the Caroloameghi- tified several rapid evolutionary radiations templado de Sudamérica austral. Revista Chi- niidae, Paleogene South American "primate- following the appearance of Nothofagus in lena de Historia Natural 75 (1):79-97. like" marsupials (?Didelphimorphia, Peradec- the fossil record. The oldest of these occu- Amico, G. and Aizen, M.A. 2000. Mistletoe seed toidea). In Kalthoff, D., Martin T., and Möors rred by the Late Campanian-Maastrichtian dispersal by a marsupial. Nature 408: 929- T. (eds.) Festband für Herrn Professor of southernmost Patagonia and the An- 930. Wighart v. Koenigswald anlässlich seines 65. tarctic Peninsula. This suggests a possible Asher, R.J., Horovitz, I. and Sánchez-Villagra, M. Geburtstages, Palaeontographica Schweizer- timing for the microbiotherian radiation. 2004. First combined cladistic analysis of bart'sche Verlagsbuchhandlung, Abt. 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