Marina Bento Soares

New stem-sauropodomorph (Dinosauria, Saurischia) from the Triassic of Brazil

Post-Triassic theropod, sauropodomorph, and ornithischian dinosaurs are readily recognized based on the set of traits that typically characterize each of these groups. On the contrary, most of the early members of those lineages lack such specializations, but share a range of generalized traits also seen in more basal dinosauromorphs. Here, we report on a new Late Triassic dinosaur from the Santa Maria Formation of Rio Grande do Sul, southern Brazil. The specimen comprises the disarticulated partial skeleton of a single individual, including most of the skull bones. Based on four phylogenetic analyses, the new dinosaur fits consistently on the sauropodomorph stem, but lacks several typical features of sauropodomorphs, showing dinosaur plesiomorphies together with some neotheropod traits. This is not an exception among basal dinosaurs, the early radiation of which is characterized by a mosaic pattern of character acquisition, resulting in the uncertain phylogenetic placement of various early members of the group.

A taphonomic model for the Mesosauride assemblage of the Irati Formation (Paraná Basin, Brazil)

The Irati Formation (Permian, Parana Basin, Brazil) is made up of a succession of black bituminous and nonbituminous shales and mudstones with interbedded carbonate layers, which record deposition in marine environments. Mesosaur remains are abundant at several sites in this formation, where they occur preserved in centimetre- thick beds and displaying various degrees of skeletal disarticulation. This paper seeks to establish a model to explain the processes that generated the mesosaurian taphocoenosis in the Irati Formation. Three types o successions containing mesosaur remains were analysed: 1) Carbonate turbidite facies in the State of Sao Paulo; 2) Carbonate tempestite facies in Rio Grande do Sul; 3) Black shale facies in the State of Parana. Three taphonomic classes are established: Class I) articulated skeletons; Class II) partially articulated skeletons; Class III) isolated bones (including Class IIIA with complete isolated bones and Class IIIB with fragmented isolated bones). Based upon the sedimentological and taphonomic evidence, a taphofacies model is proposed. The hypothesis here advanced is that all accumulations of mesosaur remains resulted from catastrophic mortality caused by storm events. The basic assumption is that these reptiles died and were buried through the action of successive storms that occurred in the Irati Sea. The taphonomic model points to repeated episodic events of burial, erosion, reworking and redeposition of skeletons and suggests significant time averaging.

Two New Cynodonts (Therapsida) from the Middle-Early Late Triassic of Brazil and Comments on South American Probainognathians

We describe two new cynodonts from the early Late Triassic of southern Brazil. One taxon, Bonacynodon schultzi gen. et sp. nov., comes from the lower Carnian Dinodontosaurus AZ, being correlated with the faunal association at the upper half of the lower member of the Chañares Formation (Ischigualasto-Villa Unión Basin, Argentina). Phylogenetically, Bonacynodon is a closer relative to Probainognathus jenseni than to any other probainognathian, bearing conspicuous canines with a denticulate distal margin. The other new taxon is Santacruzgnathus abdalai gen. et sp. nov. from the Carnian Santacruzodon AZ. Although based exclusively on a partial lower jaw, it represents a probainognathian close to Prozostrodon from the Hyperodapedon AZ and to Brasilodon, Brasilitherium and Botucaraitherium from the Riograndia AZ. The two new cynodonts and the phylogenetic hypothesis presented herein indicate the degree to which our knowledge on probainognathian cynodonts is incomplete and also the relevance of the South American fossil record for understanding their evolutionary significance. The taxonomic diversity and abundance of probainognathians from Brazil and Argentina will form the basis of deep and complex studies to address the evolutionary transformations of cynodonts leading to mammals.

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.

The African cynodont Aleodon (Cynodontia, Probainognathia) in the Triassic of southern Brazil and its biostratigraphic significance

In this contribution we report the first occurrence of the enigmatic African probainognathian genus Aleodon in the Middle-early Late Triassic of several localities from the state of Rio Grande do Sul in southern Brazil. Aleodon is unusual among early probainognathians in having transversely-expanded postcanine teeth, similar to those of gomphodont cynognathians. This genus was previously known from the Manda Beds of Tanzania and the upper Omingonde Formation of Namibia. The Brazilian record of this genus is based upon multiple specimens representing different ontogenetic stages, including three that were previously referred to the sectorial-toothed probainognathian Chiniquodon theotonicus. We propose a new species of Aleodon (A. cromptoni sp. nov.) based on the specimens from Brazil. Additionally, we tentatively refer one specimen from the upper Omingonde Formation of Namibia to this new taxon, strengthening biostratigraphic correlations between these strata. Inclusion of A. cromptoni in a phylogenetic analysis of eucynodonts recovers it as the sister-taxon of A. brachyrhamphus within the family Chiniquodontidae. The discovery of numerous specimens of Aleodon among the supposedly monospecific Chiniquodon samples of Brazil raises concerns about chiniquodontid alpha taxonomy, particularly given the extremely broad geographic distribution of Chiniquodon. The discovery of Brazilian Aleodon and new records of the traversodontid Luangwa supports the hypothesis that at least two subzones can be recognized in the Dinodontosaurus Assemblage Zone.

A new prozostrodontian cynodont (Therapsida) from the Late Triassic Riograndia Assemblage Zone (Santa Maria Supersequence) of Southern Brazil

We report here on a new prozostrodontian cynodont, Botucaraitherium belarminoi gen. et sp. nov., from the Late Triassic Riograndia Assemblage Zone (AZ) of the Candelária Sequence (Santa Maria Supersequence), collected in the Botucaraí Hill Site, Candelária Municipality, state of Rio Grande do Sul, Brazil. The new taxon is based on a single specimen (holotype MMACR-PV-003-T) which includes the left lower jaw, without postdentary bones, bearing the root of the last incisor, canine and four postcanines plus one partial crown inside the dentary, not erupted, and two maxillary fragments, one with a broken canine and another with one postcanine. The features of the lower jaw and lower/upper postcanines resemble those of the prozostrodontians Prozostrodon brasiliensis from the older Hyperodapedon AZ and Brasilodon quadrangularis and Brasilitherium riograndensis from the same Riograndia AZ. The inclusion of Botucaraitherium within a broad phylogenetic analysis, positioned it as a more derived taxon than tritylodontids, being the sister-taxon of Brasilodon, Brasilitherium plus Mammaliaformes. Although the new taxon is based on few cranial elements, it represents a additional faunal component of the Triassic Riograndia AZ of southern Brazil, in which small-sized derived non-mammaliaform cynodonts, closely related to the origin of mammaliaforms, were ecologically well succeed and taxonomically diverse.

The lower jaw and dentition of the traversodontid Exaeretodon riograndensis Abdala, Barberena & Dornelles, from the Brazilian Triassic (Santa Maria 2 Sequence, Hyperodapedon Assemblage Zone)

Exaeretodon riograndensis Abdala, Barberena, & Dornelles, 2002 is the most abundant traversodontid preserved in the basal Santa Maria 2 Sequence (Hyperodapedon Assemblage Zone), southern Brazil and is closely related to Exaeretodon argentinus Cabrera, 1943 from the Ischigualasto Formation, Argentina. Previous anatomical studies of E. riograndensis have focused mainly on cranial material and little is known about the morphology of its lower dentition or postdentary bones. We describe the first fairly complete postdentary series of the mandible of E. riograndensis and provide additional information on its lower dentition. The postdentary bones of E. riograndensis include a complex coronoid, an angular with a delicate reflected lamina and a stout retroarticular process of the articular, contrasting with the morphology reconstructed for Argentinean specimens, which possess a small retroarticular process. Apart from that, the postdentary bones do not differ significantly from those known for E. argentinus, a fact expected due to the great similarity between other skeletal features of these species. Furthermore, the lower postcanines of E. riograndensis have virtually the same structure as those of E. argentinus, with an approximate quadrangular shape in occlusal view. Moreover, the transverse cusp row is placed anteriorly and comprises a lingual and a buccal cusp, and the occlusal basin delimited by the four main cusps is relatively deep. The new material does not add any taxonomically diagnostic features to E. riograndensis. However, the fossils greatly improve our understanding of the anatomy of the Brazilian species.

A New Dicynodont (Therapsida: Anomodontia) from the Permian of Southern Brazil and Its Implications for Bidentalian Origins

Dicynodonts were a highly successful group of herbivorous therapsids that inhabited terrestrial ecosystems from the Middle Permian through the end of the Triassic periods. Permian dicynodonts are extremely abundant in African deposits, but are comparatively poorly known from the other regions of Gondwana. Here we describe a new South American dicynodont, Rastodon procurvidens gen. et sp. nov., from the Boqueirão farm site of the Rio do Rasto Formation, Paraná Basin, Guadalupian/Lopingian of Brazil. Diagnostic features of R. procurvidens include uniquely anteriorly-curved maxillary tusks, well-developed ridges extending from the crista oesophagea anteriorly along the pterygoid rami, strong posterior angulation of the posterior pterygoid rami, and a bulbous, well-developed retroarticular process of the articular. Phylogenetic analysis indicates that R. procurvidens is the earliest and most basal member of Bidentalia, a cosmopolitan clade that includes Permian and Triassic dicynodonts whose dentition is usually reduced to a pair of maxillary tusks.

Pterosauria from the Late Triassic of Southern Brazil

A few postcranial remains of a Late Triassic pterosaur from the early Coloradian Caturrita Formation of Rio Grande do Sul are communicated. The general morphology of the coracoid, proximal portion of the humerus, femur, tibia and fibula suggests that it is more primitive than the pterosaurs from the Norian of northern Italy. The morphology and proportions of the different bones support their assignment to a primitive pterosaur. An almost complete maxilla with three teeth is tentatively referred to the same taxon because it was collected at some distance from the postcrania cited above.

The oldest archosauromorph from South America: postcranial remains from the Guadalupian (mid-Permian) Rio do Rasto Formation (Paraná Basin), southern Brazil

Abstract In this contribution, we report a distal portion of a left humerus that likely belongs to an indeterminate basal archosauromorph from the Guadalupian (mid-Permian) Rio do Rastro Formation (Paraná Basin) of southern Brazil. A precise taxonomy of the fragmented and isolated humerus UFRGS-PV-0546-P is not warranted at generic nor familiar level but, likely, this specimen belongs to an Archosauromorpha due to the lack of both the entepicondylar and the ectepicondylar foramina. The narrow distal end of the humerus, the rounded radial and ulnar condyles, and the moderately developed supinator process with a shallow ectepicondylar groove (not notched) are features reminiscent of tanystropheids rather than that of other archosauromorphs. This material likely represents the first and oldest Permian archosauromorph from South America and indicates the presence of this lineage before the P/T boundary.