New information on Riograndia guaibensis Bonaparte , Ferigolo & Ribeiro , 2001 ( Eucynodontia , Tritheledontidae ) from the Late Triassic of southern Brazil : anatomical and biostratigraphic implications

The tritheledontid Riograndia guaibensis was the first cynodont described for the “Caturrita Formation” fauna from the Late Triassic of southern Brazil (Santa Maria 2 Sequence). The type materials did not preserve anatomical information regarding braincase, occiput, basicranium, zygomatic arch, postdentary bones and craniomandibular joint. Here new materials are described and supply the missing information. Riograndia shows a suite of important anatomical features quite derived among the non-mammaliaform eucynodonts, such as the partial closure of the medial orbital wall and braincase, extensive secondary osseous palate, wide primary palate, basicranium with jugular foramen separated from the periphery of fenestra rotunda, narrow zygomatic arch and much reduced postdentary bones. Many of these features constitute synapomorphies shared only with the other members of mammaliamorpha. Thus, the almost complete cranial, mandibular and dental information from the new fossils of Riograndia can bring a significant improve in the understanding of the anatomy and phylogenetic relationships of the tritheledontids and help to elucidate the transformational steps involved in the cynodont-mammal transition. Additionally, Riograndia is a key taxon in refining the “Caturrita Formation” biostratigraphy, enabling the connection of several fossiliferous outcrops that have a rich tetrapod fauna that can be correlated with other Triassic faunas from Gondwana and Laurasia.


INTRODUCTION
The upper Triassic beds of southern Brazil assigned as "Caturrita Formation" (e.g.Andreis et al. 1980) or the "upper portion of the Santa Maria 2 Sequence" (Zerfass et al. 2003) became renowed for their primitive dinosaur containning, represented by the prosauropod Unaysaurus tolentinoi Leal, Azevedo, Kellner & Da Rosa, 2003, and the theropod (sensu Langer et al. 2009) Guaibasaurus candelariensis Bonaparte, Ferigolo & Ribeiro, 1999.Other than these dinosaurs, these beds also produce the dinosauriform Sacisaurus agudoensis Ferigolo iensis Araújo &Gonzaga, 1980, an indetermina tosaur (Lucas andKischlat 2003), isolated teeth chosaurs (Dornelles 1990) and a stereospondyl am ian (Dias-da-Silva et al. 2009).However, the mo prising findings from these layers are related with and extremely rich association of small tetrapods, are less than 15 cm length.This fauna has yield procolophonid Soturnia caliodon Cisneros  We judge important to emphasize here that, unlike Liu and Olsen (2010), we follow the view of Bonaparte et al. (2003Bonaparte et al. ( , 2005Bonaparte et al. ( , 2010) ) who consider Brasilodon and Brasilitherium different from each other and, therefore, as valid taxa.This whole fauna indicates a Norian age for the layers where it occurs (see Geological Setting and Biostratigraphy section).
Previous knowledge about Riograndia guaibensis is based on the description of the holotype MCN-PV-2264, represented by an incomplete skull with dentition, and its related materials, which consist of a left mandible with dentition, but lacking post dentary bones (MCN-PV2265), and a fragmented right mandible with three postcanines teeth (MCN-PV2271) (Bonaparte et al. 2001) (Fig. 1).Bonaparte et al. (2001) recognized several synapomorphies shared by Riograndia and the "ictidosaurs" (or Tritheledontidae sensu Hopson and Kitching 1972), but proposed the monotypic family Riograndidae to include the only species Riograndia guaibensis.
The type series of Riograndia (Bonaparte et al. 2001) did not provide anatomical information on the orbital wall, braincase, basicranium, primary palate, occiput, zygomatic arch, postdentary bones and craniomandibular joint.Fortunately, new and more complete materials of Riograndia were collected, supplying the missing information concerning these anatomical regions.The new materials include four well-preserved skulls with teeth and several mandibular specimens with teeth, two of them with the postdentary bones preserved.It is noteworthy that although many postcranial elements have been found in field works, none of them can be confidently assigned to Riograndia.These new cranial, mandibular and dental materials show important de- ap, articular process; cp, coronoid process; d, dentary; f, frontal; gld, groove for dental lamina; if, infraorbital foramen; i1, lower incisor 1; j, jugal; l, lacrimal; lf, lacrimal foramina; m, maxilla; mc, Meckel canal; n, nasal; p, parietal; pa, palatine; pm, premaxilla; sm, septomaxilla; sy, symphysis.Modified from Bonaparte et al. (2001).
Although considered by many authors (Hopson andBarghusen 1986, Hopson 1991, Crompton and Luo and incomplete specimens.Thus, the cranial, mandibular and dental information from the new specimens of Riograndia should greatly improve our understanding of the anatomy of the Tritheledontidae, should help to elucidate the intermediate transformational steps in the cynodont-mammal transition, and provides new data to unravel the interrelationships within Cynodontia.Martinelli et al. (2005) showed Riograndia to be the most basal taxon within Tritheledontidae (Fig. 2a), while Martinelli and Rougier (2007) placed Riograndia as the most basal taxon of the monophyletic Ictidosauria Clade, and as sister taxon of the most inclusive Tritheledontidae, composed by Irajatherium, Chaliminia, Elliotherium, Pachygenelus, Diarthrognathus, and Tritheledon (Fig. 2b).Recently, Liu and Olsen (2010) used, among the tritheledontids, only Riograndia and Pachygenelus in their analysis and did not recognize Tritheledonta as a monophyletic group (Fig. 2c).Nevertheless, the authors support the close relationship between Riograndia and Pachygenelus.They emphasize that the possibility of a monophyletic Ictidosauria (sensu Martinelli and Rougier 2007) group cannot be excluded and needs additional tests.In any case, Riograndia is closely related to other tritheledontids (or tritheledontians) than other non-mammaliaform cynodonts; the new materials presented here reinforce this phylogenetic hypothesis.In this paper we adopted the view of Martinelli et al. (2005), who place Riograndia as the most basal Tritheledontidae.
Riograndia also plays an important role in the enlightenment of the biostratigraphic context of the Upper Triassic sequence from southern Brazil, because this non-mammaliaform cynodont is commonly represented in the faunal association of the "Caturrita Formation", both in number of individuals and in wide geographic distribution.

SYSTEMATIC PALAEONTOLOGY
Hipodigm: MCN-PV2265, an almost complete jaw with complete dentition lacking the postd bones and MCN-PV2271, fragment of the midd tion of right lower jaw, with three postcanines.

Locality and horizon:
The specimens MCN-64, MCN-PV2265, MCN-PV2271, UFRGS-PV-0 and UFRGS-PV-0601-T are from Sesmaria do 1 outcrop (Candelária municipality); UFRGS-PV T, UFRGS-PV-0833-T, UFRGS-PV-0842-T and SINOS-4881 are from Linha São Luiz outcrop nal do Soturno municipality).The specimens U Emended diagnosis: small tritheledontid (sensu Martinelli et al. 2005) with the combination of the following derived characters: three upper and three lower incisors; reduced upper incisor 1 and hypertrophied lower incisor 1; upper canines larger than the lower ones; postcanines blade-like with 5-9 small, sharp and subequal cuspules evenly distributed on the almost semicircular border of the crown in the upper postcanine crown and in the posterodorsal border of the lower ones; postcanine teeth forming an angle with the long axis of the palate; elongated septomaxilla bordering the posterior margin of the external nares; frontal with an anterolateral projection contacting the nasal medially; absence of prefrontal; postorbital bar absent; dorsoventrally deep lacrimal; contact of the ventral process of frontal with the ascending process of palatine; sphenopalatine foramen bordered by the ascending process of palatine; ossified orbitosphenoid; weak anterior portion of the zygomatic arch; dorsoventrally narrow zygomatic arch; wider amplitude of the zygomatic arch at the half of the arch; quadrate suspended by the squamosal; premaxilla bordering the posterior margin of the incisive foramen; maxilla participating of the anterior margin of the subtem-tine foramina in the secondary osseous palate; wide primary palate; intermediate pterygopalatine ridges reaching the basisphenoid; broad interpterygoid vacuities; basisphenoid wing excluded from the margin of the fenestra ovalis; jugular foramen separated from the fenestra rotunda; two hypoglossal foramina outside the margin of the jugular foramen; prootic and opisthotic unfused; ascending process of the alisphenoid moderately expanded; open pterygoparoccipital foramen; postdentary bones reduced in a rod-like bar; dentary symphysis unfused; the articular process of the dentary in contact with the squamosal.

GEOLOGICAL SETTING AND BIOSTRATIGRAPHY
Several specimens of Riograndia were collected from four outcrops (Fig. 3) of Rio Grande do Sul State, southern Brazil, making possible to attempt the temporal correlation among them.
This taxon occurs predominantly in the Linha São Luiz outcrop (29   with red mudstones interbedded with small-scale trough cross-bedded sandstone lenses.Rhytmites and sigmoidal massive to climbing cross-laminated sandstone bodies are also present.This facies association is interpreted as a lacustrine-deltaic depositional system.This basal portion of the Santa Maria 2 Sequence encompasses the Hyperodapedon Assemblage Zone (sensu Abdala et al. 2001), in which the most abundant components are the rhynchosaur Hyperodapedon and the traversodontid cynodont Exaeretodon.They permit to correlate these levels with those of the Ischigualasto Formation from Argentina, whose basal layer was dated as 230.3-231.4± 0.3My (Rogers et al. 1993, Furin et al. 2006, Martinez et al. 2011).Upwards, the sandstone content of the Santa Maria 2 Sequence increases.The arenous layers occur as narrow, massive or stratified (horizontal and trough cross bedding) lenses interpreted as amalgamated sandstone bodies related to high width/depth ratio channels.This succession is interpreted as the progressive replacement of a lacustrine basin by a fluvial system.
H. Zerfass (personal communication) does not exclude the hypothesis that the upper layers of the Santa Maria 2 Sequence could in fact constitute another depositional sequence, but more detailed stratigraphic work in the area is needed to test this hypothesis, once the currently available evidences do not suggest any significant depositional gap inside this package.Nowadays, seven fossiliferous outcrops are assigned to the upper portion of the Santa Maria 2 Sequence in the Rio Grande do Sul State, as shown in the Table I.
The individualization of a biostratigraphic unit for the upper portion of the Santa Maria 2 Sequence was first proposed by C.M.S. Scherer (unpublished data) who have recognized that the Botucaraí outcrop, from which were recovered the dicynodont Jachaleria candelariensis and isolated archosaur's teeth, had distinct sedimentary faces and represented a younger horizon than the Rhynchosauria Cenozone (= Hyperodapedon AZ) (Schultz et al. 2000).In such a stratigraphic framework, the Botucaraí outcrop should be correlated with the basal layers of the Los Colorados Formation of Ar-chosaurs).This correlation indicated (at that time) a Norian age for the Botucaraí fauna and allowed the recognition of an informal biostratigraphic unit to the Brazilian beds containing that dicynodont genus, named "Jachaleria Interval" (C.M.S. Scherer unpublished data).Some years later, other outcrops were found near the Botucaraí Outcrop, and in Agudo and Faxinal do Soturno cities (ca. 100 km west from the Botucaraí region).These outcrops correspond to the upper levels of the Santa Maria 2 Sequence (sensu Zerfass et al. 2003), and have produced the dinosaur Guaibasaurus candelariensis (Bonaparte et al. 1999(Bonaparte et al. , 2006) ) and small vertebrates.Based on these new discoveries, especially on the abundant materials of the "ictidosaur" Riograndia (Bonaparte et al. 2001), Rubert andSchultz (2004) proposed a formal biostratigraphic name -Ictidosauria Assemblage Zone -to replace the "Jachaleria Interval".Further, this denomination was changed to Mammaliamorpha Assemblage Zone by Schultz and Soares (2006) based on the argument that this last name better reflects the phylogenetic status of the small non-mammaliaform cynodonts (e.g.Riograndia, Irajatherium, Brasilodon, Brasilitherium) of the upper levels of the Santa Maria 2 Sequence.
Langer et al. ( 2007) discussed the possible correlations of this Assemblage Zone, claiming that "the whole fauna might be intermediate between those sampled at localities of La Chilca and La Esquina, both from Los Colorados Formation, Argentina (Bonaparte 1982, Abdala et al. 2001), including the earliest records of certain clevosaurid, tritheledontian and leptopleurine clades.Alternatively, it might congregate temporally separate assemblages.Given their separated occurrences and based on their phylogenetic affinities, Jachaleria candelariensis and Sacisaurus agudoensis would be assigned to an older fauna, while Irajatherium hernandezi and Clevosaurus brasiliensis would characterize a younger one.In this case, forms that occur together with most of these taxa, such as Riograndia and brasilodontid cynodonts would have longer temporal ranges, occurring along that entire time interval".Moreover, the presence of the prosauropod Unaysaurus does not allow "main" -2011/2/12 -13:39 -page 335 -#7 NEW INFORMATION ON Riograndia guaibensis FROM THE LATE TRIASSIC OF SOUTHERN BRAZIL Fig. 4 -Comparative chart showing six stratigraphic proposes for the Brazilian Triassic.Riograndia guaibensis occurs in the lower porti Caturrita Formation (sensu Andreis et al. 1980) or in the upper portion of the Santa Maria 2 Sequence (sensu Zerfass et al. 2003).Fm., Fo JUR, Jurassic; Mb, member; My, millions of years; PER, Permian; s.s., sensu stricto.Modified from Scherer et al. (2000).Geological Tim based on Gradstein and Ogg (2004). is no strong stratigraphic evidence for a significant discordance between this sequence and the underlying sequences that include the Hyperodapedon AZ.Besides, if a stratigraphic gap inside the Santa Maria 2 Sequence occurs, it is likely to be positioned below Jachaleria and Sacisaurus, but not between these layers and the Riograndia AZ (sensu Abdala and Ribeiro, 2010).Moreover, if the levels containing Jachaleria (as well as the phytosaur and the stereospondyl amphibian) and Sacisaurus, whose outcrops revealed also teeth of Riograndia and Brasilitherium (Ferigolo and Langer 2006), represent a fauna with a distinct age from that of the Riograndia AZ, the choice of this taxon as a guide to name this biozone is not so suitable.
In this paper we preferred to assume that the whole fauna of the upper layers of the Santa Maria 2 Sequence constitutes a different faunal association from that occurring in the lower part of the sequence (Hyperodapedon AZ) and that of the most representative taxon of this fauna, which is Riograndia guaibensis.A synthesis of our biostratigraphic opinion regarding the biostratigraphy of the South Brazilian Triassic is presented in Figure 5.

DESCRIPTION AND COMPARATIVE ANATOMY
For purposes of description, only the anatomical regions that were not mentioned by Bonaparte et al. (2001) will be presented.Also, the reinterpretation of some elements not so clear in the holotype was possible thanks to the analysis of the new materials.

MEDIAL ORBITAL WALL
The medial orbital wall of Riograndia was described by Bonaparte et al. (2001).However, some new information on the ascending process of palatine and orbitosphenoid was provided by the specimens UFRGS-PV-0596-T and UFRGS-PV-0601-T.
Palatine (Fig. 7a, b, c, d).In the medial orbital wall, the ascending process of the palatine contacts the lacrimal anteriorly and the frontal dorsally.This is very similar to that of Diarthrognathus (Crompton 1958), Elliothe-Although this pattern is a derived condition among nonmammaliaform cynodonts, the palatine of Riograndia does not reach the development degree of tritylodontids, in which this bone is wider and contributes for the total closure of the medial orbital wall.The sphenopalatine foramen (not reported by Bonaparte et al. 2001) is enclosed by the ascending process of palatine, at the level of sixth postcanine (UFRGS-PV-0596-T).This foramen is for the passage of the major palatine nerves and vessels to the palate and the caudal nasal nerves and vessels to the nasal cavity (Wible et al. 2004).Among the non-mammaliaform cynodonts, mentions about the sphenopalatine foramen are scarce.Bonaparte and Barberena (2001) described that this opening is completely enclosed by the ascending process of palatine in the medial orbital wall of Prozostrodon, and the same pattern is observed in the tritylodontids (e.g.Bienotheroides, Sun 1984;Tritylodon, Rowe 1988;Kayentatherium, Luo 1994) and Sinoconodon (Wible 1991, Crompton andLuo 1993).In another way, Bonaparte et al. (2003) described the sphenopalatine foramen in Brasilodon, which was placed between the ascending process of palatine and orbitosphenoid as in Morganucodon.
Orbitosphenoid.No reference on the orbitosphenoid was made by Bonaparte et al. (2001) because this structure was not preserved in the holotype.None of the new materials shows evidence of an ossified orbitosphenoid, but Rodrigues et al. (2006) have confirmed the presence of this element in two specimens of Riograndia (UFRGS-PV-0596T and UFRGS-0601-T) through a comparative investigation using C.T. Scanning method.According to Rodrigues et al. (2006), the orbitosphenoid of Riograndia is less developed than that of Prozostrodon (UFRGS-PV-0248-T) and Brasilitherium (UFRGS-PV-0760-T), but none of these taxa reaches the ossification degree of the tritylodontids and mammaliaforms, in which this element forms the floor of the anterior portion of the braincase (Kielan-Jaworowska et al. 2004).
Parietal (Fig. 6a, b, e).The sutural relationships of the ventral border of the parietal with the elements of the braincase can be better observed in the specimen UFR-GS-PV-0601-T.In lateral view, posterior to the contact with the frontal, the parietal contacts the alisphenoid and the prootic ventrally.Despite the sutures are not clear, it can be inferred that, from the frontal-parietal contact, the parietal delineates a descending bend that covers the dorsal margin of the alisphenoid and the prootic until it reaches the squamosal.Along the ventral margin of the parietal, in contact with the prootic and the alisphenoid, a narrow and shallow open groove is placed.The groove is not well defined in its origin, but its poste-bitalis of the stapedial artery.Riograndia shares t siomorphic condition, an opened orbitotemporal with other non-mammaliaform cynodonts (Wib Hopson 1993) excepting tritylodontids (Hopson 1 Squamosal (Fig. 6a, b, e).The cranial area com by the squamosal is very restricted.In the pos most portion of the temporal region, the squamos tacts the anterior lamina of the prootic anterior the parietal dorsally.Its anterodorsal limit is s at the level of the anterior opening of the post ral canal, which is placed in the prootic-squamo ture.Posteriorly, the squamosal borders dorsally "main" -2011/2/12 -13:39 -page 338 -#10
Quadrate (Fig. 6a, b).In the holotype of Riograndia (Bonaparte et al. 2001) the quadrate is not preserved.The information of this element comes from a fragmented right quadrate, which is displaced from its natural position, of UFRGS-PV-0596-T.When observed in anterior or posterior view, the quadrate shows a triangular shape, resembling that of Probainognathus (Romer 1970), but being slender.In Riograndia, the trochlear area is less expanded than that of Probainognathus, a fact that gives an elongated aspect for the quadrate that is unusual among non-mammaliaform cynodonts.The trochlea presents a cylindrical shape and its convex articular facet contacts the concave area of the articular bone.In its ventral surface, the trochlea bears a pair of elongated condyles, the medial trochlear condyle, and the lateral one, which is wider than the former as in Procynosuchus (Kemp 1979), Thrinaxodon (Fourie 1974), Probainognathus (Romer 1970) it is similar to other tritheledontids (Luo and Crompton 1994), but different from Brasilodon and Brasilitherium (Bonaparte et al. 2005, Luo 2007).
Alisphenoid (Fig. 6e).The alisphenoid is identified in two specimens (UFRGS-PV-0596-T and UFRGS-PV-0601-T).In lateral view, the ascending process of the alisphenoid is a spatulated plate.In its dorsal border, the ascending process shows an anterior expansion that meets the ventroposterior margin of the frontal.It seems that the alisphenoid-frontal contact in Riograndia occupies a wider area than that of Pachygenelus (Wible and Hopson 1993), Brasilodon and Brasilitherium (Bonaparte et al. 2003(Bonaparte et al. , 2005)), similarly to the patterns observed in Diartrognathus (Crompton 1958), Sinoconodon (Crompton and Luo 1993) and Morganucodon (Kermack et al. 1981).The major part of the anterior margin of the alisphenoid delimitates the posterior border of the sphenorbital fissure, as in most non-mammaliaform cynodonts, excepting in tritylodontids (Luo 1994).Below the contact with the frontal, the ascending process of alisphenoid becomes narrower near its base as in Morganucodon (Kermack et al. 1981).Throughout its anterior margin, a well-defined thin vertical ridge is delimited, resembling that of Diarthrognathus (Crompton 1958).The suture between the alisphenoid and the parietal has been already mentioned.The specimen UFRGS-PV-0601-T shows that Riograndia exhibits the general feature of non-mammaliaform cynodonts that is the ascending process of the alisphenoid being anteroposteriorly broad and widely in contact with the anterior lamina of the prootic.The alisphenoid quadrate ramus is preserved in the specimens UFRGS-PV0601-T and UFRGS-PV-0833-T, and will be described together with other basicranial elements.
Prootic (Fig. 6e).The prootic of Riograndia is laterally projected, forming a lateral flange that is supported by the quadrate ramus of the alisphenoid.This lateral flange is well developed as in Pachygenelus (Wible and Hopson 1993), but does not exhibit a vertical component as in tritylodontids (Hopson 1964).The anterior lamina  terior opening of the posttemporal canal communicated through this groove, which corresponds to the open channel for the passage of the superior ramus of the stapedial artery and other vessels that exit through the pterygoparoccipital foramen (Rougier et al. 1992).Thus, in Riograndia it seems that these blood vessels should run in an open channel in the lateral flange of the prootic until they join the sinus canal system (orbitotemporal canal) and posttemporal veins (Wible and Hopson 1993).This is the same configuration as in Pachygenelus (Wible and Hopson 1993).However, the open ascending channel for the superior ramus of the stapedial artery is different and more primitive than the enclosed channel of this vessel in mammaliaforms, such as Morganucodon (Wible and Hopson 1993) and Hadrocodium (Luo et al. 2001).
ZYGOMATIC ARCH (FIG.6C, D) At least in one of the new specimens, UFRGS-PV-0833-T, the zygomatic arches are preserved in all of their length.Some information, related to the root of the zygomatic arch, was also provided by the specimen UFRGS-PV-0596-T.The specimen UFRGS-PV-0833-T confirms the suggestion of Bonaparte et al. (2001) that the zygomatic arch of Riograndia is slender.In a general way, the zygomatich arch resembles that of Pachygenelus, whose arch delineates, in lateral view, a straight line.The prominent zygomatic process of the maxilla, at the expense of the anterior process of the jugal, constitutes an important element of the suborbital region of the skull of Riograndia.This is a condition also presented by other tritheledontids (Hopson and Barghusen 1986), Brasilodon (Bonaparte et al. 2003) and mammals (Luo 1994).In the root of the zygomatic arch, the maxilla contacts posteriorly the jugal and medially the lacrimal Palatine (Fig. 7a, b).In its posteriormost portion, the palatal plate becomes broader, following the general trend of the whole skull.The contribution, in extension, of the palatine in the secondary palate is greater than that of the maxilla.The former makes up to 50% of the length of the postcanine row, the same derived condition of other tritheledontids, tritylodontids (Kielan-Jaworowska et al. 2004), Brasilodon (Bonaparte et al. 2003(Bonaparte et al. , 2005) ) and Morganucodon (Kermack et al. 1981).
the palate, the greater palatine foramen is symmetrically positioned, very close to the suture between the maxilla and the palatine (but completely enclosed by the palatine).These structures, considered apomorphic for epicynodonts (Hopson andBarghusen 1986, Kielan-Jaworowska et al. 2004), served as a passage of the greater palatine branch of the sphenopalatine nerve (a branch of the maxillary ramus of the trigeminal nerve) and the greater palatine artery (Kermack et al. 1981, Rougier et al. 1992).In front of the anterior border of these foramina, there is a shallow groove that surpasses the anterior limit of the palatine and reaches the maxilla, where the nerve and the artery should run forward together.Posteriorly to the greater palatine foramina, in the same symmetric position, there are two smaller foramina, which should correspond to the lesser palatine foramina, only reported in tritylodontids among non-mammaliaform cynodonts (Kemp 1983, Kielan-Jaworowska et al. 2004).In this group, the foramina are placed in the same position of those from Sinoconodon (Crompton and Luo 1993) and Morganucodon (Kermack et al. 1981), that is, near the posterolateral limit of the palatine, behind the last postcanine level.In Riograndia this second pair of palatine foramina is placed more medially behind the great palatine foramina at the level of the last postcanine.Both pairs of foramina are clearly present in the secondary palate of the two specimens of Riograndia (UFRGS-PV-0596-T and UNISINOS-4881) and the holotype, which was confirmed in the reexamination done by the senior author.In this sense, Riograndia shares with Tritylodontidae and Mammaliaformes the derived condition of the presence of a lesser palatine foramen (Luo 1994, Kielan-Jaworowska et al. 2004).The posterior palatal plate of the palatine forms the floor and the lateral walls of the posterior portion of the nasopharyngeal cavity (Kermack et al. 1981, Hopson andBarghusen 1986).This region is well preserved in the specimen UNISINOS-4881, but some information was also provided by UFRGS-PV-0596-T.In the medial region of the primary palate of the last specimen, the palatine projects posteriorly, contacting medially the vomer.The posteriormost portion of the palatine is enclosed medial and laterally by the pterygoids.
ridges of Riograndia are parts of a system of ridg will be discussed in the description of the pterygo Pterygoid (Figs. 6e;7a,b).The pterygoids ar tively well preserved in three specimens, UFRG 0596-T, UFRGS-PV-0601-T and UNISINOS-488 specimen shows the contact of pterygoid with th illa or the vomer.Laterally, in the anterior por the pterygoids, the pterygoid flanges are placed pterygoid flanges of Riograndia are smaller than of Pachygenelus (Bonaparte et al. 2003) and hav angular shape, with the apex directed ventropost which allows a lateral contact with the mandible.specimen UFRGS-PV-0596-T the pterygoid flan in close contact with the region of the supposed noid bone of the lower jaw.In the medial region anterior portion the pterygoids are a little concav meeting of the two pterygoids in the sagittal pla fines a medial ridge.This ridge reaches anterio internal nares area.At the level of the interpte vacuity the medial ridge suffer an interruption, b reconstituted posteriorly until it reaches the ba noid.Symmetrically placed in each side of th dial ridge, the pterygopalatine (or intermediate) run parallel to it without keeping contact.These tures correspond to the site of the attachment anterior pterygoid muscle, which promotes a st union between palate and neurocranium (Kemp The pterygopalatine ridges border laterally the pterygoid vacuities and reach the anterior mar the basisphenoid as in Pachygenelus and Morganu (Luo 1994).Kermack et al. (1981) Maier et al. 1996) infer that this system would be covered by soft tissue, extending the length of the secondary palate and dividing the nasopharyngeal cavity into two air passages: lateral and medial.The interpterygoid vacuities of Riograndia correspond to a well-defined structure that was crossed by the cultriform process of the basisphenoid.Some very thin and small bone fragments dispersed in the interpterygoid vacuity area of UFRGS-PV-0596-T could indicate that the aperture was covered by an incipient sheet of bone.Among epicynodonts, the interpterygoid vacuities were observed in juveniles specimens of Thrinaxodon (Martinelli and Rougier 2007), and among eucynodonts this feature is present in juveniles of Probelesodon and Lumkuia, in adult specimens of tritheledontids, as Diarthrognathus (Crompton 1958), Chaliminia (Bonaparte 1980, Martinelli andRougier 2007) and Pachygenelus (Hopson and Barghusen 1986), and in Brasilodon and Brasilitherium (Bonaparte et al. 2005).The presence of the interpterygoid vacuities in epicynodonts has been interpreted as a retention of a primitive character or a reversion to a plesiomorphic therapsid condition, which was still conserved in the pre-epicynodonts Dvinia and Procynosuchus (Hopson and Barghusen 1986).According to Kemp (1979), the vacuities are a remnant structure related to some skull kinetism of basal therapsids still present in these Permian cynodonts.Martinelli and Rougier (2007) have considered that the re-acquisition of the interpterygoid vacuities in the derived nonmammaliaform cynodonts, which bear a high resemblance with the pattern present in most basal forms (e.g.Procynosuchus) and in immature individuals of more derived forms (e.g.Thrinaxodon, Lumkuia, Probelesodon, Kayentatherium), could be the result of heterochronic processes like paedomorphosis.We consider that, in the case of Riograndia and other tritheledontids, it is plausible to accept that the presence of the interpterygoid vacuity is a consequence of the new remodeling of the primary palate, in connection with the increase of the nasopharyngeal system and brain, then to assume that the vacuity is related with skull kinetism as in primitive cynodonts.So, instead of being interpreted as a homoplastic feature shared with pre-epicynodonts, transversely enlarged pterygoids, is similar to that of other tritheledontids and Morganucodon.The broadening of the primary palate is related with an evolutionary trend observed in the advanced non-mammaliaform eucynodonts, which includes an increase in the cerebral volume and an inflation of the nasopharyngeal system, promoting the posterior displacement of the choanas and the separation of the pterygopalatine ridges (Rowe 1993).Regarding the pterygoid quadrate ramus, it is possible to observe in the specimens UFRGS-PV-0601-T and UNISINOS-4881 that each pterygoid shows a lateral inflexion, becoming more convex posterioly.At this point, the pterygoids seem to be fused to the alisphenoid and, from there, a posterolateral projection can be observed.As the suture between these two bones is indistinguishable, it is probable that the mentioned projection corresponds to the alisphenoid quadrate ramus.This condition is in agreement with the eucynodontia general pattern, which is the reduction or lack of the pterygoid quadrate ramus (Hopson and Barghusen 1986).No sign of an ectopterygoid was observed in the specimens of Riograndia, as in other thitheledontids, tritylodontids and basal mammaliaforms (e.g.Sinoconodon and Megazostrodon).
In a general way these elements follow the basic nonmammaliaform cynodont pattern, with a triangular outline, displaying a greater transverse development in the posterior sector.The well-developed basipterygoid process is defined in the left side of the skulls UFRGS-PV-0596-T and UNISINOS-4881; it forms the anteromedial margin of the ventral opening of the cavum epiptericum and contacts anteriorly the pterygoids.The parasphenoid overlaps the basisphenoid, covering this bone ventrally in all of its extension.The limit between the basisphenoid and basioccipital can be only described based on the basioccipital anterior articular facet.The was not completely preserved in any of the specimens, we can infer that it was reduced because the area situated in both sides of the basisphenoid is occupied by the prootic in the specimens UFRGS-PV-0596-T and UFRGS-PV-0833-T.Riograndia shares with Probainognathus (Romer 1970, Allin 1986), chiniquodontids (Romer 1969, Teixeira 1982), Diarthrognathus (Crompron 1958), Pachygenelus (Luo 1994) and tritylodontids (Sues 1986) the derived condition that is a reduced basisphenoid wing excluded from the margin of the fenestra ovalis.The basisphenoid wing of Riograndia is more reduced than that of Diarthrognathus (Crompton 1958), resembling more that of Pachygenelus (Bonaparte et al. 2003).However, it is not as reduced as in mammaliaforms, such as Adelobasileus (Lucas and Luo 1993, Luo et al. 1995), and Hadrocodium (Luo et al. 2001).The cultriform process of the parasphenoid was not conserved in any of the specimens, although signs of ossification of the basisphenoid-parasphenoid complex, medial to the cavum epiptericum, reveal a short ridge.Probably the ossification that forms this ridge corresponds to the basisphenoid, which was originally fused to the parasphenoid and projects anteriorly through its cultriform process.Based on the medial pterygoid ridges, we can infer that the cultriform process of Riograndia would extend anteriorly between the pterygoids, lying over the referred ridge and crossing the interpterygoid vacuities until joining the vomer.In the area of origin of the medial pterygoid ridge of UFRGS-PV-0596-T it is possible to observe remnants of bone adhered in both sides, which may correspond to pterygoid fragments that were originally fused to the parasphenoid and basisphenoid.The fusion of these elements is characteristic of advanced non-mammaliaform eucynodonts and Morganucodon (Kermack et al. 1981).This feature is also related to the cranial anatomical remodeling observed along the evolutionary history of the Cynodontia, in function of the increase of the cerebral and nasopharyngeal system volume as discussed in the case of the interpterygoid vacuities.The transversal enlargement of the pterygoids and its fusion to the cultriform process of the parasphenoid in Riograndia illustrates this evolutionary trend, which is fully established in Morganucodon Alisphenoid (Fig. 7d).The anterior margin of t sphenoid cannot be observed because it is broken only specimen that bears this bone partially pre (UFRGS-PV-0601-T).The quadrate ramus of t sphenoid projects posteriorly and covers the pte and the prootic laterally.In ventral view, the qu ramus of the alisphenoid participates on the for of the lateral border of the cavum epiptericum an jects posteriorly until the level of the anterior of the basioccipital, but without surpassing it.Ri dia shares this feature with other tritheledonti non-mammaliaform cynodonts, excepting trity tids (Rowe 1988), which present the plesiom condition.
Basioccipital (Fig. 7e).About the basioccipit previous information was provided by the holot Riograndia (Bonaparte et al. 2001).The basioc preserved in the specimens UFRGS-PV-0596-UFRGS-PV-0601-T, presents a typical hexagonal but wider anteriorly.In its median portion the short prominent and thickened ridge that runs alo bone surface.In consequence, the basiocipital margins are depressed.In the central portion of th there are two small nutritional foramina.Based anterior contact facet of the basioccipital, we c that this bone meets the basisphenoid-parasp complex through an anteriorly concave suture lin erally, the basioccipital meets the prootic throug agonal suture that converges anteriorly to the s plane.The posteriormost sector of the basioccipi ticipates on the ventral border of the foramen ma The sutures between basioccipital and exoccipit wide and run anterioly diagonal to the sagittal pla Exoccipital (Fig. 7c, e).The exoccipitals of Ri dia are preserved in the specimens UFRGS-PV-0 UFRGS-PV-0601-T, UFRGS-PV-0833-T.In th cranium, the contact between the exoccipital a paroccipital process of the opisthotic is located la to each occipital condyle, below half of the ext of the posterior margin of the jugular foramen.N "main" -2011/2/12 -13:39 -page 344 -#16 344 MARINA B. SOARES, CESAR L. SCHULTZ and BRUNO L.D. HORN form cynodonts is the hypoglossal foramina in confluence with the jugular foramen.The derived condition presented by Riograndia is also observed in Oligokyphus (Kühne 1956), Pachygenelus (Bonaparte et al. 2003) and Brasilitherium (Bonaparte et al. 2005).However, one can verify that the ossified area between the jugular foramen and the hypoglossal foramina ("jugular process" of Kermack et al. 1981) in Pachygenelus, Brasilitherium and mammaliaforms is wider than that in Riograndia (Lucas and Luo 1993, Luo et al. 2001).
Prootic and opisthotic (Fig. 7d, e).The prootic and the opisthotic are partially represented in the basicranium of UFRGS-PV-0596-T, UFRGS-PV-0601-T and UFRGS-PV-0833-T.Riograndia presents the primitive condition of unfused prootic and opisthotic, a feature evidenced by the specimen UFRGS-PV-0833-T, in which both elements are preserved around the pterygoparoccipital foramen.Perpendicular to the posterior margin of this foramen, we can observe a vertical straight line that should correspond to the suture between the prootic and the opisthotic.The supposed suture is situated in a similar position to the suture between the prootic and the opisthotic of Probainognathus and Chiniquodon (Abdala 1996).Regarding tritheledontids, Wible (1991) has made reference to a visible suture between these two bones in a juvenile specimen of Pachygenelus, but no conclusion about the character state of adult individuals was possible to establish.In Diarthrognathus, Crompton (1958) affirms that there is no sign of suture dividing the prootic and the opisthotic.In Brasilodon (Bonaparte et al. 2005), the prootic and the opisthotic are separated.The tritylodontids Oligokyphus (Crompton 1964) and Kayentatherium (Sues 1986) show these two elements fused, as in Brasilitherium (Bonaparte et al. 2005) and mammaliaforms (e.g.Morganucodon, Sinoconodon and Hadrocodium), whose fused prootic and opisthotic form the petrosal bone (Luo et al. 1995).The suture between the paroccipital process and the exoccipital is placed laterally to the occipital condyle (UFRGS-PV-0833T).The paroccipital process does not meet the quadrate because the latter bone is in contact exclusively with the squamosal.This is the same plesiomorphic paroccipital foramen of Riograndia (UFRGS-PV-0833-T) is anteriorly bordered by the prootic and posteriorly by the paroccipital process, as in Pachygenelus (Wible and Hopson 1993).Running laterally to the prootic, the alisphenoid quadrate ramus posteriormost limit reaches the anterior margin of the pterygoparocipital foramen, but without covering it.Riograndia follows the same pattern of Pachygenelus (Wible and Hopson 1993), tritylodontds (Lucas and Luo 1993, Luo 1994), morganucodontids and Sinoconodon (Wible and Hopson 1993).
In Riograndia, the jugular foramen is separated from the fenestra rotunda (or perilymphatic foramen) by a narrow bone bar.This separation is a derived condition among non-mammaliaform cynodonts, being shared with tritylodontids (e.g.Tritylodon, Oligokyphus) (Rowe 1988), Pachygenelus (Wible 1991) and Brasilitherium (Bonaparte et al. 2005).However, in the last two taxa, the distance between the jugular foramen and the fenestra rotunda is wider than in Riograndia.The posterior and lateral margins of the jugular foramen are dominated by the paroccipital process, while the anterior and medial ones are formed by the prootic.The fenestra ovalis is a large opening (greater than the jugular foramen and the fenestra rotunda), totally enclosed by the prootic and positioned lateroanteriorly to the jugular foramen and the fenestra rotunda (without contacting the basisphenoid wing), a derived state shared with Probainognathus (Luo 1994), chiniquodontids (Romer 1969), Diarthrognathus (Crompton 1958), Pachygenelus (Allin 1986) and mammaliaforms (Luo 1994).Despite this derived condition, the fenestra ovalis of Riograndia still has an osseous ring, which is a primitive condition of non-mammaliaform cynodonts (Luo et al. 1995).This ring, which contacts the stapes, is a common feature among non-mammaliaform cynodonts (e.g.Probainognathus, Luangwa, Massetognathus, Kayentatherium, Diarthrognathus).The cavum epiptericum is posteromedially bordered by the prootic, laterally by the alisphenoid quadrate ramus and anteriorly by the basipterygoid process of the basisphenoid.Only a very narrow ossified portion separates the anterior margin of the fenestra ovalis from the cavum epiptericum.Thus, as in most part of the non-mammaliaform cynodonts [ex-

Squamosal.
In UFRGS-PV-0596-T the squamosal notch, to lodge the quadrate, is placed laterally to the contact between the squamosal and the paroccipital process, and medially to the glenoid cavity.The facet on the paroccipital process for the squamosal is parallel to the sagittal plane.Its cranial moiety does not extend anteriorly to the level of the posterior margin of the pterygoparoccipital foramen, which is not encircled by the squamosal.

OCCIPITAL REGION
Comparisons with other tritheledontids are difficult because the occipital region is poorly preserved in Diarthrognathus (Crompton 1958), Chaliminia (Bonaparte 1980) and Elliotherium (Sidor and Hancox 2006); no published data about Pachygenelus is available.The specimens UFRGS-PV-0596-T and UFRGS-PV-0601-T have conserved the whole occipital region.In lateral view, the ventral margin of the occipital plate is more posteriorly projected than the dorsal one.In posterior view, the occipital plate shows a triangular shape, with the apex delimited by the union of the sagittal and lambdoidal crests.
Interparietal.Due to the high level of fusion of the occipital plate, the presence of an interparietal cannot be observed in UFRGS-PV-0596T, which has the best preserved skull roof and occipital region.Riograndia seems to follow the pattern showed by other advanced non-mammaliaform cynodonts as Diarthrognathus (Crompton 1958), Pachygenelus (Luo 1994), Therioherpeton (Bonaparte andBarberena 1975, 2001), tritylodontids (Rowe 1988) and mammaliaforms (Lucas and Luo 1993), which lack an independently ossified interparietal.
Supraoccipital (Fig. 7c).The supraoccipital is strongly fused to the other occipital plate elements.The sagittal and lambdoidal crests project over the supraoccipital; throughout the medial portion of the bone a broad and short ridge is delineated, supporting the sagittal crest and promoting a greater robustness of this portion of the occipital plate.In each side of the medial ridge there are two shallow depressed areas that correspond to sites tabulars is not clear, but the former contacts laterally the latter.
Basioccipital (Fig. 7e).The posteriormost portion of the basioccipital participates on the ventral border of the foramen magnum.In occipital view, the basioccipital separates the two occipital condyles through a shallow notch.This feature represents a derived condition shared with Chiniquodon (Abdala 1996), Lumkuia (Hopson and Kitching 2001), Pachygenelus (Bonaparte et al. 2003), Brasilodon and Brasilitherium (Bonaparte et al. 2003(Bonaparte et al. , 2005)), among others.However, in Riograndia and in these mentioned taxa, a well-developed notch for the odontoid process of the axis, like that of Adelobasileus (Lucas and Luo 1993), Sinoconodon (Crompton and Luo 1993) and Morganucodon (Kermack et al. 1981), has not been formed yet.
Tabular (Fig. 7c).The tabular of Riograndia occupies the whole area lateral to the supraoccipital, being limited dorsolaterally by the descending lambdoidal crests of parietals and laterally by the squamosal.Ventrolaterally each tabular makes contact with the paroccipital process, and ventromedially contacts the exoccipitals.Both posttemporal canals, through which the arteria and the vena diploetica magna pass (Wible and Hopson 1993), are dorsally encircled by the tabulars very close to the squamosal margin.The ventral margin of each posttemporal canal touches the suture between the tabular and the paroccipital process, being, thus, bordered by the exoccipitals.This condition, shared with Probainognathus (Romer 1970), Diarthrognathus (Crompton 1958), Pachygenelus (Lucas and Luo 1993), Brasilodon (Bonaparte et al. 2003), and Oligokyphus (Crompton 1964), among others, is considered by Luo (1994) as intermediate between the plesiomorphic condition, characterized by a posttemporal canal totally encircled by the tabular; and the derived condition exhibited by mammaliaforms, in which the posttemporal canal is dorsally bordered by the squamosal.
Exoccipital (Fig. 7c, e).The suture between the exoccipitals and the supraoccipital is located at the level of the dorsal border of the foramen magnum, perpendicular to the sagittal plane, but it is almost indistinguishable "main" -2011/2/12 -13:39 -page 347 -#19 NEW INFORMATION ON Riograndia guaibensis FROM THE LATE TRIASSIC OF SOUTHERN BRAZIL the occipital condyles of Riograndia is oval, similar to the pattern observed in Pachygenelus (Bonaparte et al. 2003).The condyles do not reach the top of the dorsal border of the foramen magnum, being restricted to its ventral one-third, which is a primitive condition (Rowe 1988).As mentioned above, the occipital condyles are well separated by the posterior margin of the basioccipital.Following the general pattern of the whole occipital plate, we can observe that the ventral portion of the condyles is more posteriorly projected than the dorsal one.This condition is also observed in the occipital plate of Adelobasileus (Lucas and Luo 1993).The nonbifurcated paroccipital process occupies the base of the occipital plate, lateral to each occipital condyle.Dorsally, the paroccipital process makes an irregular contact with the tabular and borders the ventral margin of the posttemporal canal, as mentioned before.
LOWER JAW Bonaparte et al. (2001) have described the almost complete lower jaw of the specimen MCN-PV2265 in the medial view, but nothing was reported about the postdentary bones of Riograndia.All the new specimens (UFRGS-PV-0596-T, UFRGS-PV-0624T, UFRGS-PV-0833T, UFRGS-PV-0623T) confirm the characteristics observed in the type series (Bonaparte et al. 2001): a mandible with rather robust construction with a high and thick unfused symphysis, a high coronoid process and a distinct angular process.The postdentary bones complex of Riograndia is described for the first time in this contribution.
Postdentary bones (Fig. 8e).The information about the postdentary bones of Riograndia comes from the specimens UFRGS-PV-0596-T, UFRGS-PV-0624-T and UFRGS-PV-0833-T.The postdentary trough is dorsally limited by the medial ridge of the dentary that runs anteriorly until the level of pc6.Posteriorly, the medial ridge becomes wider and projects above the articular process, which represents the posteriormost edge of the dentary.However, the articular process of the dentary does not form a condyle as seen in mammaliaforms.
The relative position of the articular process of the den-and Hopson 1992), Diarthrognathus (Crompton Chaliminia (Bonaparte 1980, Martinelli andR 2007) and Morganucodon (Kermack et al. 1981 the three specimens that have preserved the postd bones show some degree of fragmentation in the ments, but in general we can observe that the su lar, angular, articular and prearticular compose ile unity, lodge in the postdentary trough, as in advanced non-mammaliaform eucynodonts and maliaforms (Crompton and Luo 1993).The ar prearticular and angular are fused, as in other tr dontids (Allin and Hopson 1992).Supposedly, t angular runs parallel over the rod formed by the ular, prearticular and angular, and finishes at the riormost level of the retroarticular process.The ular seems to form most part of the retroarticul cess, which is concave and exhibits the ventral posteriorly projected.The reflected lamina of t gular was not preserved, and there is no eviden contact between the surangular and the squam any of the analyzed specimens.Regardind the mandibular joint, it seems that Riograndia p the same condition of other tritheledontids (Lu Crompton 1994) and Brasilitherium (Bonaparte 2003), that is the articular process of the den contact with the squamosal, instead of the sura as in most eucynodonts (Crompton 1958(Crompton , 1972)).the dentary-squamosal articulation (or contact) sents a synapomorphy of Tritheledontidae, Bra rium and mammaliaforms (Luo 1994, Luo 2007 splenial, which should cover medially the Mec groove, was not preserved in any of the specimens in the UFRGS-PV-0596-T was possible to disting most medially pronunciated area in the medial the mandible that can suggest the presence of th noid bone.jatherium.Some dental series (e.g.UFRGS-PV-0622-T, 788-T) show that, in upper and lower postcanines, the cuspules are slightly displaced posteriorly and curved backwards.This curvature is better defined in the central cusp and in the distal accessory cuspules.Tritheledontids as Pachygenelus (Gow 1980) and Irajatherium (Martinelli et al. 2005) also show the postcanines cusps recurved backwards.The dentition of the newly studied specimens confirms the description of Bonaparte et al. (2001), excepting for the upper canine size.Despite the fact that the canines are small in UFRGS-PV-0596-T, these elements are well developed in the specimens, UNISINOS-4881 and UFRGS-PV-0788-T, showing that these teeth are much more robust than the incisors in the upper dentition.The same was observed by Sidor and Hancox (2006)  Riograndia shows important anatomical cha that are quite derived among the eucynodonts, those of the medial orbital wall, lateral wall of the case, basicranium, secondary osseous palate, p palate and craniomandibular joint.Many of thes acters constitute synapomorphies shared only wit tritheledontids and their closely related taxa, wh cludes tritylodontids, Brasilodon, Brasilitheriu Mammaliaformes.
In a recent cladistic analysis, Riograndia sidered the most basal tritheledontid (Martinell 2005).In regard with the phylogenetic posit Riograndia in the Tritheledontidae Clade, Sid Hancox (2006) noted that the highly distinctiv tal features were responsible for the basal posi this taxon because its postcanines lack a more b shape and cingulum, features present in other ledontids.We recognize that the dental charact have an important phylogenetic significance, in ment with Sidor and Hancox (2006), and we co the dental characteristics of Riograndia to be d instead of generalized, as pointed out by Bonap al. (2001).We follow the view of Cabreira (200 the spatulated (leaf-like) postcanine teeth with cant wear and inferior first incisor with an ope are products of the adaptative features related herbivore diet.It is likely that Riograndia is mo vanced than previously thought.Thus, we presum the use of the new cranial and mandibular anat information about Riograndia, decribed in the p study, in future cladistic analysis, will make pos more accurate understanding of its phylogeneti tion and provide a more solid representation of th acters of the Tritheledontidae.(1993) made comments about the anatomical transformations that ocurred in the group comprising the tritheledontids, tritylodontids and mammaliaforms.According to the author, this group bears one of the most distinctive and strongly diagnosed taxa within Cynodontia.Rowe (1993) postulates that virtually all parts of the skeleton were remodeled in association with a dramatic reduction in body size.In this sense, the cranial reorganization involved inflation of the nasopharyngeal cavity, remodeling of the orbit, further enclosure of the brain, and further modifications of the acoustic and masticatory systems.Similar to other tritheledontids (and tritylodontids), Riograndia shows a wider skull roof as compared to other non-mammaliamorpha eucynodonts.This derived feature is also reflected in the long osseous secondary palate and in the broad and long primary palate with wide interpterygoid vacuities.The development of a more extensive medial orbital wall formed by the contact between the frontal and the palatine, an ossified orbitosphenoid and the development of the prootic anterior lamina provide a better enclosure of the brain.
A more sophisticated basicranium with complete separation of the jugular foramen and the fenestra rotunda and a possible contact between the dentary and the squamosal associated with the reduction of the postdentary bones to a thin rod and an unfused symphysis, revelead advances in both auditory and masticatory systems.
In fact, the cerebral volume of Riograndia (UFRGS-PV-596-T and 601-T) was inferred by Rodrigues et al. (2006) through C.T. Scanning method using nasal cavities and braincases.The obtained data, concerning morphological aspects, showed that this taxon had high respiratory rates and significant encephalization, which suggests their elevated metabolic status.The Triassic members of the Mammaliamorpha (sensu Kielan-Jaworowska et al. 2004) clade have a wide distribution in the South American continent (Rio Grande do Sul and Argentina).Riograndia, Irajatherium and Chaliminia demonstrate that many derived characters, considered synapomorphies of Mammaliamorpha, were already established before the end of the Triassic, when the tritheledontids and mammaliaforms just started their diversification.This geographic area during approach, a second contribution of the record of Riograndia in South American Upper Triassic is to allow the biostratigraphic relationships of the whole fauna of the "Caturrita Formation" or upper portion of the Santa Maria 2 Sequence through the association of its taxa in the Riograndia AZ.This biozone reflects an important moment in the beginning of the evolutionary history of both dinosaurs and mammaliamorpha, just before the two taxa spread out across the Pangea.The anchorage of the Riograndia AZ enhances the more accurate temporal correlations with other Triassic faunas of Gondwana and Laurasia.
. The lacrimal of Riograndia occupies a large area in the root of the zygomatic arch as in Chaliminia(Bonaparte 1980, Martinelli andRougier 2007), Diarthrognathus(Crompton 1958) and Pachygenelus(Bonaparte et al. 2003).The lacrimal-jugal contact in the root of the zygomatic arch is much reduced in lateral view 2003), Morganucodon(Kermack et al. 1981), Sinoconodon(Crompton and Luo 1993) and the docodont Haldanodon(Lillegraven and Krusat 1991), in which this bone and the maxilla rather than the jugal are the dominant elements.The weak representation of the jugal in the root of the zygomatic arch showed by Riograndia contrasts with the general non-mammaliaform cynodont pattern whose jugal is the dominant element.Regarding the dorsal extension of the zygomatic arch, Riograndia presents the same condition of most non-mammaliaform cynodonts (excepting traversodontids and tritylodontids) and mammaliaforms, in which the maximal dorsal height of the arch is situated in a plane below half of the orbit(Hopson and Kitching 2001).The squamosal zygomatic portion is positioned in the middle of the the arch, overlying the jugal, and configures a dorso-ventrally compressed bar exhibiting the same depth of the jugal.Due to the lateral curvature of the squamosal and the jugal, the zygomatic arch of Riograndia shows, in dorsal view, a round shape, with its maximal lateral projection placed in the middle of the arch, as in Thrinaxodon(Fourie 1974), Probainognathus(Romer 1970), Pachygenelus (Allin and Hopson 1992), and Brasilodon(Bonaparte et al. 2003), among others.PALATEThe secondary osseous palate was described byBonaparte et al. (2001); the specimens UNISINOS-4881 and UFRGS-PV-0596-T bring additional information.The primary palate has received little description by previous authors and is well represented in UFRGS-PV-0596-T, UFRGS-PV-0833-T and UNISINOS-4881.

TABLE I Fossil record from the Santa Maria 2 Sequence, Upper Triassic of Rio Grande do Sul, Brazil.
Riograndia AZ could be correlated with the lowe of the Los Colorados Formation (e.g.Jachaleri rata), thus been interpreted as Norian, Late No Rhaetic in age.
, among others.A lateral notch is situated above the lateral trochlear condyle, which separates the trochlea of the lateral margin of the articular facet of the dorsal plate without forming a neck, as in Thrinaxodon and Probainognathus.The dorsal plate of Riograndia's quadrate shows a sharper dorsal angle than in Pachygenelus and Morganucodon (whose quadrate exhibits a rounded dorsal margin), resembling more the pattern of Probainognathus.The articular facet of the dorsal plate is concave as in Probain- (Crompton 1958ton 1994)s, tritylodontids, , Allin and Hopson 1992, Luo 1994, Luo and Crompton 1994fering from Probainognathus, in which the quadrate articulates with the alisphenoid, the quadrate of Riograndia is exclusively suspended by the squamosal as in the tritheledontids Diarthrognathus and Pachygenelus(Crompton 1958, Crompton and Hylander 1986, Allin and Hopson 1992, Luo 1994, Luo and Crompton 1994).Moreover, the quadrate of Riograndia was not covered by the squamosal in lateral view, as in Diarthrognathus, (Martinelli and Rougier 2007)a new specimen of minia(Martinelli and Rougier 2007)and the st new materials of Irajatherium (Oliveira et al. have increased the knowledge about the Trithel dae.Now, the detailed description of several new imens of Riograndia from this contribution mak taxon one of the most complete, known for nea anatomical regions of the skull and dentition, of ledontids. CONCLUSIONA complete diagnosis of the Tritheledontidae clade, as well as a fully understanding of the relationships among its members, was still not possible due to the incomplete state of most taxa in the group (e.g.Chaliminia, Diarthrognathus, Tritheledon).In this sense, many phy-