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A heterodontosaurian ornithischian in the UpperTriassic of southern Patagonia?A. M. BAEZ and C. A. MARSICANODepartamento de Clenclas Geol6glcas, FCEN,Unlversldad de Buenos Aires, Ciudad Universitariol, Pab.II. 1428 Buenos Aires, ArgentinaHeterodontosauria (= Heterodontosaundae sensuWelshampel and Witmer, 1990) IS a clade of euornlthopodornithlschians supported by several derived dental andcranial features, In addition to some postcranlal characters

(Sereno, 1986). The dental and cranial features may beassOCiated With the development of a powerful masticatorymechanism (Crompton and Attndge, 1986) Mostdescnbed taxa of these small-bodied dinosaurs are fromthe Elliot and Clarens Formations of South Afnca(Haughton, 1924; Crompton and Chang, 1962; Thulborn,1974; Gow, 1975; Santa Luca, 1980). However, theoccurrence of heterodontosaunan remains In the KayentaFormation of western United States (Attndge et al .. 1985)and Lower Lufeng FormatIOn of southwest China (Luo andWu, 1994), has also been reported.In general. the lower part of the Upper Karoo Supergroupof South Afnca (Molteno and lower part of the ElliotFormations) IS considered to represent the Late 1nasslc,whereas deposition of the upper part of the succession(Clarens Formation and Drakensberg VolcaniCS) e~tendedInto the Early and Middle Jurassic (e.g. Tankard et al.,1982; Cole, 1992; Veevers et al., 1994). However, onthe baSIS of the faunal content, these units were COl-relatedWith others In the Northern Hemisphere, the ages of which

are determined more accurately. The companson of thetetrapod assemblages recorded In the Elliot Formation Withthose of the Newark and Glenn Canyon Supergroups, andBntlsh Fissures led to the establishment of the TriasslcJurassIcboundary within thiS formation (Olsen and Galton,---------------14 Journal of African Earth SCiences1984). a conclusion Widely accepted by vertebratepalaeontologists. Thus, all Karoo heterosaurian beanng bedswere redated as Early Jurassic; known North Amencanand Chinese records of these dinosaurs are also from theLower Jurassic.In a recent field trip to the continental outcrops of the EITranquiloGroup, in Santa Cruz Province (69W; 48S),fragmentary remains of heterodontosaurians werediscovered In concretIOns (Baez and MarSicano, 1997).The fossils were collected from the Laguna ColoradaFormation, which consists of flUVial and flood-plain depOSits(Jalfin and Herbst, 1995). These deposits have also yieldedfossil plants (e.g. Bonetti, 1964; Jalfin and Herbst, 1995)and plateosaurid prosauropods (Casamlquela, 1980;

Bonaparte and Vince, 1979); the Identity of the latter isunder Investigation. The EI Tranquilo Group is generallyrecognised to be of Triassic age. Late Ladinian-Early Carnianmacro and microfloras occur in the Canadon LargoFormation, which unconformably underlies the vertebratebearing unit. RadiometriC dating of intrUSive granitOidcomplexes in the Laguna Colorada Formation gave 202 2Ma (Pankhurst et al., 1993), thereby placing a minimumage near the Hettangian-Slnemunan limit (Ogg, 1995) ThiseVidence, together With the palaeoflorlstlc content(Dicroidium flora), suggests a Late Triassic (probably Nonan)age for the heterodontosaunan beanng beds.The matenal consists of a weathered, left posterior maXillaryfragment With dentition, and an isolated caniniform Withmesial and lateral serrations. The three maxillary teethpreserved bear flat wear facets, and are columnar andclosely packed. The mesial and distal surfaces of the crownare in broad contact, a feature that has been considered adenved character that supports the sister-group relatIOnshipof Heterodontosaurus and Lycorhinus from Elliot Formation.

Like the teeth of Heterodontosaurus, no Cingulum orconstnctlon separates crown and root, and the wear facetsof adjoining teeth form a Single, continuous surface. Thus,a relatively advanced heterodontosauflan,Heterodontosaurus or a closely related taxon, IS representedIn the Upper TriaSSIC of southern South America. Theextension of the temporal range of the most derivedheterodontosaurlans, and consideration of the phylogenyof ornlthischians as now understood (Weishampel andWitmer, 1990; Brett-Surman, 1997), Indicate that a moreextensive phyletiC diversification of these dinosaurs wastaking place in the Late TriaSSIC than that documented byactual records.ReferencesAttridge. J . Crompton, A Wand Jenkins. F A 1985 The Southern

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African LiasSiC prosauropod Massospondylus discovered In NorthArnenca Journal Vertebrate Palaeontology 5, 128-132.Baez. A M and MarSicano, C A 1997 First record of the orn,th,,,ch,andinosaur Heterodontosau",s from South Amenca Ameghmlana 34. 531Bonaparte. J F. and Vince, M 1979 EI hallazgo del prImp, nldo dedinosaUrios tnaslcos (Saurl~qula) Prosauropodal, TnasJco Superior dePatagonia. Ameghmiana 16, 173-182Brett-Surman. M K 1997 Ornithopod a In The Complete Dmosaur (Editedby Farlow. J 0 and Brett-Surman, M K I pp330-346 Indiana UniverSityPress, Blcomlngton & IndianapoliSBonetti, M. R. I. 1964. Breve nollcla sobre los conOClmlentos actuales dela flora Tnaslca de EI TranqUllo, prOVinCia de Santa Cruz Ameghmlana3, 182Casamlquela. R M 1980. La presencia del genera Plateosaurus(Prosauropoda) en el T"8SICO superior de la Formacl6n EI Tranqullo,Patagonia Actas Segundo Congreso Arg