Asociación Paleontológica Argentina. Publicación Especial 7 ISSN 0328-347\ VII International Symposium on Qesozoic Terrestrial Ecosystems= 157-162. Buenos Aires, 30-6-2001

Non-tribosphenic Gondwanan mammalsA and the alternative development of molars with a reversed triangle cusp pattern

Rosendo PASCUAU and Francisco J. GOIN1

Abstract. Several lineages of non-holotherian Gondwanan mammals developed complex, tribosphenic- like rnolars, but not tribosphenic occlusion. The reversed triangle occlusal pattern, characteristic of holotherian lineages leading to tribosphenic occlusion, could ha ve been acquired without a cusp rotation process. A review of the molar structure in primitive monotreme prototherians and gondwanathere al- lotherians suggests that a tricuspid pattern could ha ve been accomplished by expansion of cingula, ele- vation of their cusps, and connection by ridges of two of the latter to one of the original ones, leading to a triangular pattern in some, or all, their molars. This cusp and ridge arrangement is already outlined in the lower cheekteeth of Steropodoú galmaú° Archer et al64the oldest (Early Cretaceous- and most primitive Platypoda so far known. Similarly, one isolated lower molariform of the primitive gondwanathere Ferugí°other°um uúúdhauseú° Bonaparte (Late Cretaceous- shows its anterolabial cusp connected by two ridges to two lingual cusps, forming a V-shaped, trigonid-like structure.

Key words. Qammals. Qonotremata. Gondwanatheria. Gondwana. Qolar pattern. Evolution.

Introduction al641<<<-,Upper Jurassic of Africa (Heinrich, 1<<8, 1<<<-,Early Cretaceous of Australia (Archer et al64 In relation with the Laurasian record, the 1<85?Kielan-Jaworowska et al[, 1<87?Flannery et al[, Gondwanan Qesozoic record of land mammals is ex- 1<<5?Rich et al64 1<<<-, Africa 0e6g64Sigogneau- tremely poor, both chronologically and geographi- Russell, 1<<5-, and South America (Patagonia? cally, although it is currently increasing and becom- Bonaparte, 1<86,1<88,1<<6?Rougier, 1<<0?Rougier ing richer, mainly because of new discoveries in et al[, 1<<2-.The next and younger record comes Cretaceous beds of southern South America from Late Cretaceous beds of southern South (Patagonia-. Although the marnmalian record in the America (Patagonia? Bonaparte, 1<<6and references remaining Gondwanan continents is even less com- therein? Pascual et al642000-, Qadagascar and India plete, its comparison with that of South America sug- (Krause et al641<<7?Prasad and Sahni, 1<88?Pascual gests that there was a markedly regional differentia- et al64 1<<<-. Except for a few Early Cretaceous tion, e.g., between South America and Australia, and records from Australia (Rich et al6 1<<7,1<<<-,and even within the present Gondwanan continents. For one from the Early Cretaceous of Patagonia example, the Qesozoic record from Patagonia and (Bonaparte, 1<86,1<88?Rougier, 1<<0-,all remaining that from the present intertropical regions of the known Cretaceous mammals of Gondwana are from South American continent indicate that, by the Late the Late Cretaceous, and pertain to non-tri- Cretaceous, both represent distinct biogeographic re- bosphenic groups. Both the Early and Late gions, the latter more closely allied to Africa, while Cretaceous records attest to a distinct Gondwanan the former more closely allied to the Eastern history (Pascual, 1<<6?Pascual, 1<<8?Vizcaíno et al64 Gondwanan continents (Crisci et al[, 1<<3-,as it is at 1<<8-,a partially hidden evolutionary experiment present (Crisci et al641991)6 because, with the exception of Qonotremata and The earliest stages of mammalian evolution in probably the \enarthra, the Qesozoic Gondwanan Gondwana are unknown, since the oldest remains radiation left no representatives in modern commu- come from Qiddle Jurassic of Qadagascar (Flynn et nities. So distinct was this faunal radiation that we called it "the other history" (Pascual, 1<<7-,not on- ly because none of the ancestors of the peculiar 'Facultad de Ciencias Naturales y Quseo, Departamento Cenozoic South American Tribosphenida 0seúsu Científico Paleontología Vertebrados, Paseo del Bosque, 1<00La QcKenna and Bell, 1<<7-were recorded, including Plata, Argentina. [email protected](R.Pascual-? the primordial, presumably native xenarthrans [email protected](F.Goin- (Pascual, 1<88-, but because primitive non-tri- ©Asociación Paleontológica Argentina 0328-347\/01$00.00+50 158 R. Pascual and F.J.Goin bospheníc mammals such as the Prototheria, Molar morphology in Qonotremata and Allotheria (Gondwanatheria-, Triconodonta, and Gondwanatheria Docodonta evolved endemic taxa, including some peculiar ecological specializations (compared to Moúotremata their Laurasian counterparts-. Furthermore, some holotherians 0seúsu QcKenna and Bell, 1<<7-,proba- Archer et al6 (1<85-described a fragment of a Iow- bly the Dryolestida, differentiated taxa with a mosa- er jaw with three molar s of the Early Cretaceous ic of therian-like features, which makes their phylo- monotreme 5teropodoú galmaú° Archer et al6 (figure genetic relationships quite problematic when com- 1.C-, by then dubiously assigned to the Ornitho- pared with that of Laurasian mammals. Some exam- rhynchidae. 5teropodoú molars were interpreted as ples are the Lower Cretaceous. Patagonian tribosphenic, and thus supposedly derived from a V°úcelestes (Bonaparte, 1<86, 1<88? Rougier, 1<<0? relatively advanced stage in therian evolution. Two Rougier et al64 1<<2-, and the Australian Ausktr°- years later, Kielan-jaworowska et al6 (1<87-examined bospheúos (Rich et al641<<7,1<<<-.Even some non-tri- a cast of the holotype, and on the basis of wear facets bosphenic mammals, such as the Docodonta, per- (figure 1.C- and other morphological features they sisted longer than in Laurasia, and gave origin to concluded that its teeth are not tribosphenic, but per- more advanced types than their Laurasian relatives tain to a Theria (sic- "...derived from therians before (Pascual et al64 2000-. Others, like the Gondwa- the development of tribosphenic teeth, possibly du- natheria, developed quite multituberculate-Iike ring the Jurassic period". forms (Pascual et al641<<<-?remarkably, the last and most advanced gondwanatheres (early Paleocene- became equally rodentiform as the most advanced rodents presently known (Pascual et al641<<3,1<<

hyd Raising the issue hyld

Admitting, e /= _.._ (1- Crompton and Jenkins' (1<68-hypothesis that the common ancestor of both docodonts and therians (i.e.,holotherians seúsu QcKenna and Bell, 1<<7-may ~~~LGAP have been an early morganucodontid with a molar pattern like that of Morgaúucodoú -the tricuspid cusp-on-line pattern? see figure 16A-4and (2- that one of the basic modifications leading to EQBRASURES the docodont molar pattern included the lingual ex- pansion of both upper and lower molar's cingula (Jenkins, 1<6

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Figure 2. Qolar morphology in non-tribosphenic Gondwanan mammals. A, first lower left molariform of Sudamer°ca amegh°úo° Scillato- Yané and Pascual (Sudamericidae, Gondwanatheria- in occlusal view? cAemonotremes (the Recent Orú°thorhyúchus aúat°úus low ridges to two lingual cusps, shaping a V-shaped, (Shaw-, the Paleocene South American trigonid-like structure, followed by two lingual and Moúotrematum sudamer°caúum Pascual et al64and the two labial cusps, mutually connected by incomplete Qiocene species of the Australian Obdurodoú transverse ridges. The first lower molariform of [Pascual et al641<<2a, 1<<2b, fig. 1?Archer et al6419924 Sudamer°ca amegh°úo° (figure 2.A-, the most advanced fig. 1]-, it can be observed that the number of cusps in gondwanathere so far known (Pascual et al641999)4 the lingual row of lower molars is larger than on the has also four cusps in the lingual row, and three in buccal row (figures 2.B, C). Although upper molars the buccal one, the latter higher than the lingual ones. of Steropodoú are unknown, those of the remaining This different height of lingual and buccal cusps is genera show that the number of lingual and buccal the reverse of the condition in lower molar s of multi- cusps is reverse with respect to the lower molars. In tuberculates. The first upper molariform of such a manner, one cusp of one row (labial on the Sudamer°ca amegh°úo°4 contrary to the first lower mo- lower molars and lingual on the uppers- becomes lariform, has more cusps on the buccal accessory row, connected to two of the opposite row, giving a V- and the first lingual cusp is connected by ridges to shaped aspect, opened lingually on the lower cheek- the first and second buccal cusps, forming a áA teeth and buccally on the uppers, shaping the two V- shaped structure, reversed to that of the first lower shaped lobes characteristic of the ornithorhynchid molariform, i.e., buccally opened. cheek-teeth. This cusp and ridge arrangement is al- ready outlined in the known lower cheekteeth of bonclusions Steropodoú galmaú° (figure l.C? Archer et al641<85, fig. 1?Jenkins, 1<<0, fig. 2?Kielan-Jaworowska et al6419874 Recognition of the fact that therian molars were fig. 1-, and shapes an apparent "pre-tribosphenic" originally reversed triangles led Patterson (1<56=43- state. Their shearing surfaces and other features sug- to ask if this condition was inherited or if it evolved gest that, like in the more advanced or- within the order Symmetrodonta. To him, the "great nithorhynchids (Ienkins, 1<<0, fig. 2-, the molar oc- age" (sic- of the oldest known symmetrodont clusion was also performed by a reversed-triangle ("Duchy 33", from the European latest Triassic or ear- pattern of cusps and ridges, which explains the non- liest Jurassic- suggests that this type was inherited tribosphenic shearing surfaces remarked by Kielan- from the Therapsida. However, the reversed triangles Jaworowska et al6 (1<87-. In such a manner, one of the occlusal pattern of the non-therian Docodoúta branches of the two V-shaped lobes in upper molars (jenkins, 1<6

A.P.A. Publicación Especial 7, 2001 Qolar pattern in non-tribosphenic Gondwanan rnarnrnals 161 original ones, thus shaping a triangular pattem with- [enkins, EA. 1<6<.Occ1usion in Docodoú (Qarnmalía, Docodonta-. out passing through the cusp rotation of the ancestral Post°lla 13<=1-24. [enkins, EA. 1<<0. Qonotremes and the biology of Qesozoic triangular stage leading to the tribosphenic molar mammals. Netherlaúds Ioumal ox Zoology 40=5-3I. (Crompton and [enkins, 1<68-. Further, these features Kielan-Jaworowska, Z., Crompton, A.W. and [enkins, EA. 1<87. also suggest that the reversed triangle occlusal pat- The origin of egg-lying mammals. Na°ure 326=871-873. tern, characteristic of the lineages leading to the tri- Koenigswald, W. von, Goin, F-J.and Pascual, R 1<<<.Hypsodonty and enamel microstructure in the Paleocene gondwanatherian bosphenic occlusion, could have been acquired with- mammal Sudamerica. Acta Paleoútolog°ca Poloú°ca 44=263-300. out such reversed cusp rotation in upper and lower Krause, D.W. 1<<3. Vucet°ch°a (Gondwanatheria- is a junior syn- molars? in some cases 0e6g64Steropodoú galmaú°)4 de- onym of Ferugl°other°um (Qultituberculata-. [ourúal ox veloping a complex, "tribosphenic-like" pattern, but Paleoútology 67=321-324. Krause, D.W., Kielan-Jaworowska, Z., and Bonaparte, J.E 1<<2. not tribosphenic occlusion. Ferugl°other°um Bonaparte, the first known multituberculate from South America. [ourúal ox Vertebrate Paleoútology 12=351- 376. Acknow ledgements Krause, D.W., Prasad, G.V.R, Koenigswald, W. von, Sahni, A. and Grine, EE. 1<<7. Cosmopolitanism among Gondwanan Late We thank Richard Cifelli, Guillermo Rougier, an anonymous Cretaceous mammals. Na°ure 3<0=504-507. reviewer, and the editors of the Asociación Paleontológica QcKenna, Q.e. and Bell S.K. 1<<7. Class°x°cat°oú ox mammals above Argentina for their useful comments and suggestions. Special the spec°es leve/6 Columbia University Press, New York, 631 p. thanks to Qrs. Laura H. Zampatti, Qrs. LuCÍa Balarino, Qr. Pascual, R 1<<6. Late Cretaceous-Recent land mammals. An ap- Agustín Viñas, and Qr. Qarco D. Pabón, whose effective assistance proach to South American geobiotic evolution. Mastozoología permitted the composition of the figures that illustrate this work. Neotrop°cal 3=133-152. Pascual, R 1<<7. The Gondwanan history of mammals, the other history. r Iúterúat°oúal Ther°olog°cal Coúgress (Acapulco-, References Abstracts: f78Sf79[ Pascual, R 1<<8. The history of South American land mammals= Archer, Q., Flannery, T.E, Ritchie, A. and Qolnar, RE. 1<85. First the seminal Cretaceous-Paleocene transition. Asoc°ac°óú Qesozoic mammal from Australia-an Early Cretaceous Paleoútológ°ca Argeút°úa6 Publ°cac°óú Espec°al 5, Paleógeúo de monotreme. Nature 318 (6040-=363-366. Amér°ca del Sur y de la Peúíúsula Aútárt°co4 Buenos Aires, pp. <- Archer, Q., [enkins, EA., Hand, S.J., Qurray, P. and Godthelp, H. 18. 1<<2. Description of the skull and non-vestigial dentition of a Pascual, R, Archer, Q., Ortiz-Jaureguizar, E., Prado, J.L., Qiocene platypus 0Obdurodoú d°cksoú° n. sp.- from Riversleigh, Godthelp, H. and Hand, S. 1<<2a. First discovery of Australia, and the problem of monotreme origins. In=Augee, monotremes in South America. Nature 356=704-705. Q (ed.-, Platypus aúd Ech°dúas6 The Royal Zoological Society of Pascual, R, Archer, Q., Ortiz-Jaureguizar, E., Prado, J.L., New South Wales, Australia. 1.Evolution, pp.15-27. Godthelp, H. and Hand, S., 1<<2b. The first non-Australian Bonaparte, J.E 1<86. Sobre Mesuúgulatum houssay° y nuevos monotreme= an early Paleocene South American platypus mamíferos cretácicos de Patagonia, Argentina. In=4° Coúgreso (Qonotremata, Omithorhynchidae-. En= Q. Augee (ed.-= Argeút°úo de Paleoútología y B°oestrat°graxía6 Actas 2=63-<5. Platypus aúd Ech°dúas6 The Royal Zoological Society of New Bonaparte, J.E 1<88. Specialized dentitions in two Cretaceous South Wales, Australia, 1.Evolution= 1-14. mammals from Patagonia. In=Teeth Revisited= Proceedings of Pascual, R, Goin, EJ., Ortiz-Jaureguizar, E., Carliní, A.A. and the 7° International Symposium on Dental Qorphology. Prado, J.L. 1<<3. Perugí°other°um and Sudamer°ca4 Mémo°res du Museum úat°oúal d'U°sto°re úature°le4 (série C- 53= Qultituberculata and Gondwanatheria. One more evolution- 207-215. ary process occurred in isolation. Amegh°ú°aúa 30=110-11I. Bonaparte, J.E 1<<6. Cretaceous Tetrapods of Argentina. In= Pascual, R, Ortiz-Jaureguizar, E. and Prado, J.L. 1<<6.Land mam- Arratia, G. (ed.-, Contributions of Southem South America to mals= paradigm for Cenozoic South American geobiotic evo- Vertebrate Paleontology, Müúchúer Geow°sseúschaxtl°che lution. In= G. Arratia (ed.-, Contributions of Southem South Abhaú°uúgeú4 (A- 30=73-130. America to Vertebrate Paleontology. M°°úchúer Crisci, j.v

A.P.A. Publicación Especial 7, 2001