A sectorial toothed cynodont (Therapsida) from the Triassic Santa Cruz do Sul fauna, Santa Maria Formation, Southern Brazil

ABSTRACT A sectorial toothed cynodont from the Triassic Santa Cruz do Sul fauna, Santa Maria Formation, Parana Basin, southern Brazil, is described. The taxon is represented by a tiny portion of a right lower jaw which preserves partially the last postcanine. A comparative analysis of the postcanine morphology of the Santa Cruz do Sul specimen with South American Triassic cynodonts is made. The crown morphology of the Santa Cruz do Sul cynodont is closer to that of the juvenile single specimen of cf. Probainognathus from the Carnian Ischigualasto Formation and of juveniles of Probainognathus jenseni Romer, 1970 from the Ladinian Chañares Formation in Argentina. There are, however, some important differences between the tooth of the new specimen and those of P. jenseni juveniles, and therefore we provisionally assign the new Santa Cruz do Sul material to cf. Probainognathus. The fauna of Santa Cruz do Sul, dominated by traversodontid cynodonts, is now composed of a proterochampsid archosauriform, three traversodontids and two sectorial toothed cynodonts and we refer to it as Santacruzodon Assemblage Zone. We also propose the name of Riograndia Assemblage Zone for the faunas from the Upper Triassic Caturrita Formation, on the basis of the abundance yet restricted record of this taxon in these faunas. A brief summary of the Brazilian Middle and Upper Triassic biostratigraphy is presented within the framework of two different time scales.


INTRODUCTION
Triassic vertebrates are a landmark of the southern Brazilian fossil record, with the outcrops from the Santa Maria Formation recording a fauna interpreted as ranging from the Middle Triassic (?Anisian, Ladinian) to the Carnian (Schultz et al. 2000;Abdala & Teixeira 2004;Langer et al. 2007, but see discussion below), corresponding to the Sequence Santa Maria 2 of Zerfass et al. (2003). Several vertebrate groups including fi sh, parareptiles, archosaurs and therapsids are represented in beds of this unit (Perez & Malabarba 2002;Langer et al. 2007;Richter & Toledo 2008). Th e Santa Cruz do Sul fauna represents a recent addition to the knowledge of the Brazilian Triassic Bertoni-Machado & Holz 2006). Th is fauna is represented by a restricted outcrop know as "Schoenstatt Sanctuary" located on the SW outskirts of the city of Santa Cruz do Sul. A remarkable dominance of cynodonts is a highlight of this fauna where traversodontids are abundant and remains of carnivorous chiniquodontids and archosauromorph proterochampsids are also represented Abdala & Ribeiro 2002, 2003Machado & Kischlat 2003). Four diff erent traversodontids were originally recognized for this fauna Abdala & Ribeiro 2002): Santacruzodon hopsoni Abdala & Ribeiro, 2003; a second taxon, the largest form of this fauna, that was identifi ed by Melo et al. (2009) as the Malagasy traversodontid Menadon (Flynn, Parrish, Rakotosamimanana, Ranivoharimanana, Simpson & Wyss, 2000) (Flynn et al. 2000;Kammerer et al. 2008); a third form represented by a tiny fragment of maxilla including four postcanines

RÉSUMÉ
Un cynodonte à dentition sectorielle de la faune triassique de Santa Cruz do Sul, Formation Santa Maria, sud du Brésil. Un cynodonte à dentition sectorielle de la faune de Santa Cruz do Sul, de la Formation Santa Maria, Bassin du Paraná, sud du Brésil, est décrit. Le taxon est représenté par une petite portion de la branche de la mandibule droite, avec la dernière post-canine partiellement préservée. Une analyse comparative de la morphologie de la post-canine du spécimen de Santa Cruz do Sul avec celle de cynodontes triassiques d'Amérique du Sud est réalisée. La morphologie de la couronne dentaire du cynodonte de Santa Cruz do Sul est semblable à celle présentée par l'individu jeune rattaché à cf.

MATERIAL
Th e studied material, UFRGS-PV1121T, is a fragment of a right lower jaw with the last postcanine and three alveoli preserved anteriorly some of them with remains of dental roots. Sectorial toothed cynodonts are, however, rare in the Schoenstatt assemblage, with chiniquodontid cynodonts being represented by at least three specimens. We describe in this contribution a new record of a sectorial toothed cynodont in the Santa Cruz do Sul fauna. Th e specimen is represented by a fragment of the lower jaw of a tiny animal, preserving the last postcanine. Th e morphology of the tooth resembles the postcanine pattern observed in juvenile specimens of Probainognathus Romer, 1970 from the Chañares fauna of Argentina (Abdala 1996) and to the    (Fig. 5A). In medial view the meckelian canal is placed near the ventral margin of the dentary, at approximately one quarter of the height of the bone (Fig. 2B). Most of the meckelian canal is horizontal, except for the posterior portion, at the level of the preserved postcanine, which is directed postero-dorsally (Fig. 2B).

UFRGS-PV 1121T PVL 4445 PVL 4447
Th e preserved postcanine is 1.8 mm in anteroposterior length (Table 2) and shows a large main cusp, which is aligned with the anterior and posterior accessory cusps (Fig. 3). Th e top of the anterior accessory cusp is broken, yet, this cusp seems to be smaller than the posterior accessory one. An additional posterior accessory cusp is somewhat displaced towards the lingual side of the postcanine, but clearly visible in the labial margin (Fig. 3A). An also broken anterior lingual cusp is positioned at the base of the anterior accessory cusp, and probably is totally hidden from the labial view of the sectorial margin. Th ere are two bulbous cingular cusps ventrally to the second posterior accessory cusp and the anterior lingual cusp respectively, and a posterior cingular crest (Fig. 3B). Th e root of the penultimate postcanine is single, as evidenced by the removal of bone in the lingual side of the mandibular ramus, with no evidence of a central furrow (Fig. 2).

DISCUSSION
Th e tiny fragment of lower jaw with tooth described is more likely that of a juvenile individual. Th e tooth is composed of an aligned series of cusps oriented dorsally, with the most anterior and posterior cusps displaced lingually and with a lingual cingulum formed by bulbous cusps and a posterior crest. A comparison between the crown of UFRGS-PV 1121T and those of other South American sectorial toothed cynodonts follows. We restrict this comparison to the posterior lower postcanines, wherever possible (Fig. 4).
Chiniquodontid  THERAPSIDA Broom, 1905CYNODONTIA Owen, 1861EUCYNODONTIA Kemp, 1982PROBAINOGNATHIA Hopson, 1990 cf. Probaino gnathus sp. (Figs 2; 3) DESCRIPTION UFRGS-PV-1121T is represented by a fragment of the right mandibular ramus of approximately 7 mm in length (Fig. 2). Th e horizontal portion is remarkably low and presents one ellipsoid empty alveolus, two alveoli preserving the base of the crowns and the fourth alveolus with a partial postcanine. Th e coronoid process is rising immediately behind the last postcanine, whereas the ventral margin of the horizontal ramus is straight GEODIVERSITAS • 2011 • 33 (2) and are therefore quite diff erent from the new specimen from Santa Cruz do Sul. Protheriodon estudianti, represented by a tiny, poorly preserved specimen, was recently described from the Dinodontosaurus AZ of the Santa Maria Formation (Bonaparte et al. 2006). Its posterior lower postcanines, visible only labially on the right side, show tiny accessory cusps in relation to the main one, being the overall morphology of the crown diff erent from that of UFRGS-PV1121T. Th e small cynodonts Charruodon tetracuspidatus and Th erioherpeton cargnini, from the Brazilian Hyperodapedon AZ are represented only by their holotypes. Th e posterior upper postcanine of T. cargnini (Fig. 4B) and the anterior lower one of C. tetracuspidatus (Fig. 4C), the only tooth crown known in each specimen, lack cingulum (Bonaparte & Barberena 1975, 2001Abdala & Ribeiro 2000;Oliveira 2006).
Lower postcanines of the tritheledontids Riograndia guaibensis (Fig. 4D), Chaliminia musteloides (Fig. 4E) and Irajatherium hernandezi (Fig. 4F) show a decreasing height of the aligned cusps posteriorly ; Martinelli et al. anterior accessory cusp is displaced lingually (Fig. 4I). Th e lingual cingulum in the lower postcanines of juveniles is mostly formed by cingular crests originating from the anterior and posterior cusps (Abdala 1996). In some cases it is possible to recognize the presence of a few tiny isolated cuspules contributing to the cingulum. Th ere is no evidence of a furrow in the postcanine roots. Several characters observed in the only preserved postcanine crown of UFRGS-PV1121T, including the lingual location on the crown of the anterior lingual cusp and the second posterior accessory cusp, and the presence of cingular cusps and a lingual cingular crest, are similar to those of P. jenseni juveniles. UFRGS-PV 1121T is remarkably smaller than the juvenile specimens of P. jenseni currently known (see Table 2 for comparative measurements) and presents some important diff erences that should be mentioned. Th e posterior lower postcanines of P. jenseni show a comparatively taller and anteroposteriorly shorter crown, and the horizontal ramus of the lower jaw is comparatively taller than that of the specimen of Santa Cruz do Sul. A tiny (approximately 40 mm of skull length) juvenile specimen from the Ischigualasto Formation was also described as cf. Probaino gnathus sp. (Bonaparte & Crompton 1994). Th e lower postcanines are not visible due to jaw occlusion, whereas the posterior uppers show four aligned cusps in labial view, the second being the largest. Th e morphology of these upper postcanines is similar to the labial morphology of the lower postcanine of UFRGS-PV1121T, both in the number and degree of development of the marginal cusps as well as in the general proportion between the height and anteroposterior length of the crown. Unfortunately it is not possible to observe the lingual side of the postcanines from the Ischigualasto Formation specimen, hampering a complete knowledge of its tooth morphology.
Th e postcanine preserved in the new specimen from Santa Cruz do Sul is therefore very similar, in labial view, to the posterior upper postcanines of cf. Probaino gnathus sp. from the Ischigualasto Formation. Th ere is also a general similarity between the lower postcanine of UFRGS-2005;Martinelli & Rougier 2007) besides the lack of cingular cusps, being therefore diff erent from the Santa Cruz do Sul specimen. Diff erences between UFRGS-PV1121T and the Late Triassic brasilodontids (Fig. 4H) are also clear because the lower postcanines in this group present a tiny anterior accessory cusp very low in the crown in relation to the main cusp (Bonaparte et al. 2003. Th e posterior accessory cusp is also more developed and high in the specimen from Santa Cruz do Sul.
Sectorial toothed cynodonts more similar to the new record of Santa Cruz do Sul are Prozostrodon brasiliensis from the Santa Maria Formation, Brazil, Probainognathus jenseni from the Middle Triassic Chañares Formation in Argentina and cf. Probaino gnathus sp. from the Carnian Ischigualasto Formation also in Argentina.
Prozostrodon brasiliensis posterior lower postcanine sectorial margin is also tetracuspidated in labial view but, diff erent from UFRGS-PV1121T, the posteriormost marginal cusp is not displaced lingually and there is a well-developed lingual cingulum formed by a series of clearly diff erentiated isolated cusps (Barberena et al. 1987: fi g. 4G). In addition, the postcanine roots have a furrow, especially well developed in the most posterior postcanines (Bonaparte & Barberena 2001).
Several adult specimens of Probainognathus jenseni show a worn out postcanine series with the teeth showing chisel-like edges (Romer 1970). Postcanines of juveniles of this species present a main cusp with an aligned anterior and two posterior accessory cusps, whereas an additional PV1121T with those of juvenile P. jenseni from the Chañares Formation. Taking into account the fragmentary nature of UFRGS-PV1121T, the diff erences mentioned above to juveniles of P. jenseni and the overall similarity to the specimen from the Ischigualasto Formation, we provisionally identify this specimen from Santa Cruz do Sul as cf. Probaino gnathus. Additional material is necessary to provide a more precise taxonomic identifi cation of this taxon in Santa Cruz do Sul as well as in Ischigualasto.

THE FAUNA OF SANTA CRUZ DO SUL AND THE BRAZILIAN TRIASSIC BIOSTRATIGRAPHY
Two groups, cynodonts and archosauriforms, and six diff erent taxa are now recognized in Santa Cruz do Sul (Table 3)  Besides the Santacruzodon AZ, at least three other faunal associations can be recognized in the Triassic Santa Maria and Caturrita formations (Fig. 5). Th e oldest one, dominated by dicynodonts, is the Dinodontosaurus AZ that, considering the stratigraphic scale of Gradstein & Ogg (2004: fi g. 5A), would span from probably the Upper Anisian to the Lower Ladinian. Th e Anisian age for the lower bound of this AZ is based on the record in outcrops from the Santa Maria Formation of Luangwa, a traversodontid cynodont known from African Anisian beds (Abdala & Teixeira 2004). In addition, recent phylogenetic analyses support a monophyletic clade including the Mariante rhynchosaur, recorded in association with Dinodontosaurus, and Anisian rhynchosaurs (Montefeltro 2008). Above the Santacruzodon AZ is recognized the rhynchosaur-dominated Hyperodapedon AZ representing a Carnian fauna. Finally, in levels of the Caturrita Formation, there are faunas mostly represented by small animals and referred to as Ictidosauria Cenozone (Rubert & Schultz 2004) or Ictidosaur Assemblage Zone . Considering that the tritheledontid (= ictidosaur) Riograndia guaibensis is one of the most common representatives and unique to this fauna, we propose the name Riograndia Assemblage Zone to refer to this fauna (Fig. 5). Th is younger Triassic fauna, representing a very important new addition to the knowledge of the Brazilian Triassic, is part of the Middle Norian in Gradstein & Ogg (2004) time scale (Fig. 5A).
Several diff erences with the Geologic Time Scale 2004 (Gradstein & Ogg 2004), concerning the Triassic time scale have been raised recently (see among others, Muttoni et al. 2004;Furin et al. 2006;Lehrman et al. 2006). Th e modifi ed time scale (Fig. 5B) is based mainly on magnetostratigra-taxon in the Brazilian fauna, whereas Massetognathus, which is also recorded in Santa Cruz do Sul , is the most common representative from the Chañares Formation. In Santa Cruz do Sul there is also the traversodontid Menadon (Melo et al. 2009), that occurs in Madagascar (Flynn et al. 1999 Th e more remarkable similarities of the Santacruzodon AZ are with the upper Ladinian/lower Carnian "Isalo II" fauna from Madagascar, which yielded chiniquodontids, Menadon and another traversodontid that resemble forms from the Santa Cruz do Sul fauna (Flynn et al. 1999(Flynn et al. , 2000Flynn & Wyss 2002;Abdala & Ribeiro 2003;Langer et al. 2007). Dicynodonts, a fairly common group in Middle and Late Triassic faunas from Gondwana, are absent from the Santacruzodon AZ and poorly recorded in the Malagasy fauna (Flynn et al. 1999 (2) phy, conodont biostratigraphy and high precision dates of marine sediments and their correlation with the continental Newark astrochronological polarity time scale (Kent & Olsen 1999). Th e main diff erences between this proposal and the Geologic Time Scale 2004 are the temporal extension of the Norian to 228 Ma and of the Carnian to 235 Ma (Fig. 5B), implying a temporal range of the Upper Triassic that is approximately two-thirds of the complete Triassic (Gallet et al. 2003). In the context of the Muttoni et al. (2004) time scale, almost all the southern Brazilian Triassic record, historically regarded as Middle-Late Triassic, would be restricted to the Upper Triassic (Fig. 5B). Th e Dinodontosaurus AZ, as well as the Chañares Formation, would extend from the upper Ladinian to the lower Carnian, the Santacruzodon AZ would be upper Carnian, the Hyperodapedon AZ and also the Ischigualasto Formation would be lower to middle Norian, and the Riograndia AZ would be upper Norian.
Th e use of diff erent time scales will indeed infl uence our understanding of the timing of the origin and fi rst diversifi cation pulse of important Mesozoic clades, including dinosauriforms, dinosaurs, mammaliamorphs and mammaliaforms.