The dawn of dinosaurs is marked by the appearance of the saurischian lineages in the Late Triassic fossil record, around 230 million years ago. This early burst of diversification of the group is majoritarily represented by sauropodomorphs and herrerasaurids in late Carnian to early Norian of Brazil, Argentina, India, and Zimbabwe. However, "silesaurids", an older and enigmatic group of quadrupedal dinosauromorphs, were recently found, in some works, as stem ornithischians. In this scenario, dinosaurs would have originated far earlier than the end of the Ladinian, a time in which "silesaurids" are already spread through Gondwana. Despite being also recorded in more recent dinosaur-bearing beds in Brazil, "silesaurids" are absent in strata from the early Carnian, an important time frame for dinosaur evolution. Here we present a new "silesaurid", Itaguyra occulta gen. et sp. nov., that fills up the remaining gap of occurrence of these dinosauromorphs and provides new clues to the success of these putative early ornithischians.

Bones and teeth incorporate stable isotopes of C and O from an animal's food and water sources that can provide paleoecological and paleoenvironmental information. In this study, thirty-six samples of fossil bones and teeth were collected from two Triassic allostratigraphic units of the Rio Grande do Sul State, Brazil, and analyzed for S13C and S18O values. Specific strata include Pinheiros-Chiniqua Sequence (Late Ladinian/Early Carnian) which includes the Dinodontosaurus Assemblage Zone (15 specimens analyzed) and the basal portion of the Candelaria Sequence (Late Carnian) in which the Hyperodapedon Assemblage Zone occurs (21 specimens analyzed). The latter assemblage zone was deposited during the Carnian Pluvial Episode, generally characterized by warmer and wetter conditions. Specimens of tooth enamel from the Dinodontosaurus Assemblage Zone have S13C values ranging from -10.8 to -7.0 %o (VPDB) and S18O values from 16.3 to 23.5 %o (VSMOW). In contrast, specimens from the Hyperodapedon Assemblage Zone have overlapping but generally higher S13C values ranging from -9.0 to -4.8 %o and overlapping but higher S18O values from 19.7 to 27.4 %o. Evidently, the Carnian Pluvial Episode coincided with higher S13C and S18O values, which could represent warmer (higher S18O) conditions in this region. Although higher S13C values can indicate greater aridity, atmospheric S13C compositions were dramatically (2-3 %o) higher during the Carnian than the earlier Middle Triassic epoch. Within uncertainties, the 2 %o increases to fossil S13C values could be consistent with either constant or increased precipitation. Climate change during the Late Triassic could have helped drive profound faunal turnover during this episode, including dinosaur evolution and diversification.

The Triassic period (251.9 to 201.4 Ma) is the first period of the Mesozoic Era, initiating after the Permian-Triassic mass extinction and ending at the Triassic-Jurassic mass extinction. This period witnessed the origin of modern ecosystems, and also the rise of the dinosaurs. This influence on both geological and ecological history of the Earth has prompted close attention to the paleoclimate and paleoecology of the Triassic period over the last 60 years. This review builds on past work to summarize knowledge of Triassic climate and ecology, especially focusing on data acquired over the last decade for key South American strata. The main climatic events that occurred in the Triassic period can be subdivided in three key intervals, from the base to top: pre-, syn- and post-Carnian Pluvial Episode (CPE). Each of these three intervals corresponds with major climatic events or episodes, positioned relative to the Permian-Triassic mass extinction, CPE, and Triassic-Jurassic mass extinction. The pre-CPE interval of the Early and Middle Triassic represents relatively warm and dry conditions. The CPE of the earliest Late Triassic marks a global increase in effective precipitation (precipitation minus evaporation) for at least 1-2 Myr, causing major floristic and faunal turnovers and possibly triggering the rise of the dinosaurs. The post-CPE interval of the Late Triassic represents a return to warm and dry conditions. Triassic deposits in Brazil and Argentina represent windows for investigating the interrelationship(s) between changes in climate, floras, and faunas. Alternate triggers for dinosaur diversification are also evaluated, including classical considerations of competition, opportunism, and body temperature, in addition to more novel and subtle factors.

Osteohistological evidence is widely used to infer paleobiological traits of fossil vertebrates, such as ontogeny and growth rates. Mesosaurs, an enigmatic group of aquatic reptiles from the early Permian, are the most well-known Paleozoic amniotes from Africa and South America. Their fossils are abundant in South America, ranging from the central-west region of Brazil to the southernmost areas, as well as parts of Paraguay and Uruguay. In this contribution, we examined the bone microstructure of Mesosaurus tenuidens by analyzing thin sections of axial and appendicular elements of several specimens collected from various Brazilian sites. The microstructure of the bones showed minimal histological variability among elements, predominantly composed of parallel-fibered tissues, indicating slow growth rhythm, along with increased bone density attributed to pachyosteosclerosis. The cortical area consists of poorly vascularized parallel-fibered bone tissue, which was interrupted by multiple cyclical growth marks, some of them being supernumerary, suggesting a strong influence of seasonality. Moreover, the organization of growth marks suggests distinct life history trajectories among individuals collected from different outcrops, reflecting environmental heterogeneity throughout the basin. Internally, the endosteal domain exhibits greater vascularization compared to the cortices and frequently contained calcified cartilage. In the ontogenetic series, there was a progressive filling of the medullary region from small to large individuals. The presence of the External Fundamental System (a proxy indicating somatic maturity) was observed in femora and ribs, suggesting that determinate growth was already occurring in Permian mesosaurs and may not be an exclusive specialization of crown amniotes.

During a field expedition led by the Universidade Regional de Blumenau (FURB) in 2003, two blocks of rock containing cranial remains of a short-snouted temnospondyl were collected. The specimen (CPGEO/FURB-PV-0678) consists of an almost complete skull, which lacks the post-orbital region. In addition to the bones exposed in the rock block, it was also possible to describe the complete palate and other internally preserved bones with the help of CT scans got through a medical tomography. It was possible to observe two rows of teeth: one in the maxilla and another in the palatine, in addition to small denticles in the vomer. Dermal ornamentations are visible on the bones of the cranial roof, which resemble those present in taxa of the Rhinesuchidae family. In the character analysis, the results indicated that the specimen in question belongs to the Australerpetinae clade, being a sister group to Broomistega putterilli Shishkin & Rubidge 2003.

Prestosuchus chiniquensis is an iconic non-crocodylomorph loricatan from the Brazilian Triassic beds and the best-known taxon, represented by several specimens. The completeness and preservation of its skeleton make it a valuable taxon for paleobiological studies. We explore the microstructure of bone tissues of appendicular elements and ribs of three specimens of Prestosuchus to access a variety of aspects of its paleobiology, such as histovariability, ontogeny, and growth patterns. Integrating our data and other osteohistologically studied P. chiniquensis specimens, we proposed for the first time an ontogenetic model for non-crocodylomorph loricatans. The model encompasses six distinct age classes (I-VI) that allow us to infer the growth patterns of P. chiniquensis and possibly expand to other phylogenetically close taxa. During early ontogeny (age classes I-II), sustained fast growth was maintained by a fibrolamellar complex. In mid ontogeny (age classes III-IV), highly vascularized parallel-fibered bone predominates, suggesting intermediary growth rates. A change for a poorly vascularized parallel-fibered/lamellar bone would mark the attainment of sexual (age classes IV-V) and skeletal maturity, comprising the age class VI. An external fundamental system (EFS) present in the outermost cortex is the main histological feature that characterize the age class VI. Major histovariability features are present between appendicular bones and ribs of skeletally immature individuals. The most prominent of them is the presence of fibrolamellar complex and highly vascularized parallel-fibered bone in appendicular bones and poorly vascularized parallel-fibered bone in ribs. In advanced ontogenetic stages, the histovariability between appendicular bones and ribs tends to be minor. Our data also support previous hypothesis of the presence of one new taxon among the specimens assigned to P. chiniquensis, increasing the diversity of non-crocodylomorph loricatans. The new taxon, represented by the specimen UFRGS-PV-0152-T, awaits a formal anatomical description. Our study advances the preliminary understand of the ontogeny and growth patterns of non-crocodylomorphs loricatans and Pseudosuchia as a whole.

The nasal cavity of living mammals is a unique structural complex among tetrapods, acquired along a series of major morphological transformations that occurred mainly during the Mesozoic Era, within the Synapsida clade. Particularly, non-mammaliaform cynodonts document several morphological changes in the skull, during the Triassic Period, that represent the first steps of the mammalian bauplan. We here explore the nasal cavity of five cynodont taxa, namely Thrinaxodon, Chiniquodon, Prozostrodon, Riograndia, and Brasilodon, in order to discuss the main changes within this skull region. We did not identify ossified turbinals in the nasal cavity of these taxa and if present, as non-ossified structures, they would not necessarily be associated with temperature control or the development of endothermy. We do, however, notice a complexification of the cartilage anchoring structures that divide the nasal cavity and separate it from the brain region in these forerunners of mammals.

Prestosuchus chiniquensis is the best represented pseudosuchian archosaur from the Pinheiros-Chiniquá Sequence, Middle-Late Triassic (Ladinian/Carnian) of the Santa Maria Supersequence, Southern Brazil. Several incomplete specimens attributed to this species have been described, but the morphology of the postcranial skeleton of P. chiniquensis is poorly known. In this contribution we present the postcranial material of the UFRGS-PV-0629-T specimen, concluding its description, as its skull and endocast have already been described. Additionally, histological data provided new information on the poorly known ontogenetic series of P. chiniquensis , and on its growth patterns suggesting a longer period of slow growth when compared to other basal Loricata species. A phylogenetic analysis placed UFRGS-PV-0629-T in a group composed by the lectotype, paralectotype, and other described P. chiniquensis specimens, further corroborating our taxonomic hypothesis, that specimens of basal Loricata collected in Brazil are closely related to each other. Due to the association of characters found in the phylogenetic analysis, the specimen UFRGS-PV-0629-T is attributed as the most complete material ever found for P. chiniquensis. As such, it is clear that the material presented here provides important new information on P. chiniquensis. Based on the results presented here, we revised the diagnosis for P. chiniquensis. However, it also evidences the need for new discoveries and studies of other specimens seeking to understand this and other closely related species, which were important components of worldwide trophic webs of the Triassic biotas.

AbstractRhynchosauria is a group of extinct, exclusively Triassic, terrestrial, and herbivorous archosauromorphs, characterized by a peculiar maxillary‐mandibular apparatus. They reached global distribution during the Carnian, with the Hyperodapedontinae clade. The rhynchosaurian record from South America is included in the Ladinian‐?earliest Carnian Dinodontosaurus Assemblage Zone (Pinheiros‐Chiniquá Sequence, Brazil) and Tarjadia Assemblage Zone (Chañares Formation, Argentina), and for strictly Carnian Hyperodapedon Assemblage Zone (Lower Candelária Sequence, Brazil) and Ischigualasto Formation (Argentina). Here, we present the first record of Hyperodapedontinae from the Brazilian Early Carnian Santacruzodon Assemblage Zone (Santa Cruz Sequence). The most diagnostic elements belong to a nearly complete left pes, which presents a combination of hyperodapedontine traits. The presence of a second individual of cf. Hyperodapedontinae is based on a fragmentary, isolated left metatarsal IV. An isolated fragment of premaxilla could belong to the second specimen (based on preservation mode) or to a third individual. This new report fills a gap within the South American rhynchosaurian distribution, strengthening biostratigraphic correlation with other regions from Gondwana (i.e., Madagascar), where similar and coeval faunas are known. The inclusion of these specimens in a phylogenetic dataset resulted in low resolution results, due to missing data because postcranial characters for rhynchosaurs are still poorly explored.

Rhynchosauria is a group of extinct, exclusively Triassic, terrestrial, and herbivorous archosauromorphs, characterized by a peculiar maxillary-mandibular apparatus. They reached global distribution during the Carnian, with the Hyperodapedontinae clade. The rhynchosaurian record from South America is included in the Ladinian-?earliest Carnian Dinodontosaurus Assemblage Zone (Pinheiros-Chiniquá Sequence, Brazil) and Tarjadia Assemblage Zone (Chañares Formation, Argentina), and for strictly Carnian Hyperodapedon Assemblage Zone (Lower Candelária Sequence, Brazil) and Ischigualasto Formation (Argentina). Here, we present the first record of Hyperodapedontinae from the Brazilian Early Carnian Santacruzodon Assemblage Zone (Santa Cruz Sequence). The most diagnostic elements belong to a nearly complete left pes, which presents a combination of hyperodapedontine traits. The presence of a second individual of cf. Hyperodapedontinae is based on a fragmentary, isolated left metatarsal IV. An isolated fragment of premaxilla could belong to the second specimen (based on preservation mode) or to a third individual. This new report fills a gap within the South American rhynchosaurian distribution, strengthening biostratigraphic correlation with other regions from Gondwana (i.e., Madagascar), where similar and coeval faunas are known. The inclusion of these specimens in a phylogenetic dataset resulted in low resolution results, due to missing data because postcranial characters for rhynchosaurs are still poorly explored.

The Permian mesosaurs are well known for being the earliest amniotes to exhibit adaptations for living in a marine environment (Irati-Whitehill Sea). In addition to their set of skeletal features associated with aquatic dwelling life, their dentition includes important characteristics related to feeding in this habitat, which is described in this work, based on the analysis of mesosaur specimens from the Lower Permian Irati Formation of Brazil. Mesosaurs have several slender, conical teeth bordered by enamel apicobasal ridges, a feature predominantly found in aquatic amniotes. Internally, the dentine walls are formed by the arrangement of layers of orthodentine and globular dentine. To prevent tooth loss, the basal area is equipped with plicidentine, a particular type of orthodentine, allied with cementum, alveolar bone trabeculae, and periodontal space that reinforces anchorage and provides some flexibility. The teeth are replaced in a labio-vertical path, and the dentition replaces alternately. This feature is regarded as plesiomorphic, and it ensures the oral cavity is supplied with enough teeth. However, these features do not the assessment of whether mesosaurs teeth were capable of piercing prey with resistant tegument. Instead, we interpret this adaptation as a mechanism for catching prey, at least in adults, and we endorse a possible ontogenetic dietary shift from small to large forms.

Body size influences most aspects of an animal's biology, consequently, evolutionary diversification is often accompanied by differentiation of body sizes within a lineage. It is accepted that miniaturization, or the evolution of extremely small body sizes, played a key role in the origin and early evolution of different mammalian characters in non-mammaliaform cynodonts. However, while there are multiple studies on the biomechanical, behavioral, and physiological consequences of smaller sizes, few explore the evolutionary processes that lead to them. Here, we use body mass as a universal size measurement in phylogenetic comparative analyses to explore aspects of body size evolution in Cynodontia, focusing on the cynodont-mammal transition, and test the miniaturization hypothesis for the origin of Mammaliaformes. We estimated the body masses of 29 species, ranging from Theriocephalia to Mammaliaformes, providing the largest collection of Triassic cynodont body mass estimates that we know of, and used these estimates in analyses of disparity through time and RRphylo. Unexpectedly, our results did not support the miniaturization hypothesis. Even though cynodont body size disparity fell during the Late Triassic, and remained lower than expected under a purely Brownian motion model of evolution up until the Early Jurassic, we found that rates of body size evolution were significantly lower in prozostrodontians leading to the first Mammaliaformes than in other lineages. Evolution rates were higher in medium and large-sized taxa, indicating that size was changing more rapidly in those lineages and that small sizes were probably a persistent plesiomorphic character-state in Cynodontia.

Sampling bias and repeated resampling can lead to misinterpretation of the taphonomic history of a fossiliferous deposit. This is the case of the Schoenstatt Sanctuary site (Santa Cruz do Sul, Rio Grande do Sul, Brazil), the typelocality and richest vertebrate fossil occurrence of the early Carnian Santacruzodon Assemblage Zone (Santa Cruz Sequence, Santa Maria Supersequence, Paran ' a Basin). This site is characterized by the abundance of traversodontid non-mammalian cynodonts, especially Santacruzodon hopsoni and Menadon besairiei (plus Massetognathus ochagaviae). The abundance of cranial remains in a 2.5 m-thick massive mudstone led earlier workers to conclude that the accumulation was biogenically controlled (e.g., selective scavenging). Here we propose a new taphonomic interpretation, based on re-investigation of >2700 fossils, collected at the site over the past 27 years. Rhizoliths and early diagenetic features (e.g., micrite crusts) indicate the occurrence of several pedogenically modified levels within the outcrop. This occurrence of multiple calcic vertisols, added to the absolute dating already obtained for the Santa Cruz Sequence, may be related to the global Carnian Pluvial Event. Re-analysis of the historical fossil collections from this site provides the basis for a biostratigraphic division of Massetognathus (lower strata), Menadon (lower and middle strata) and Santacruzodon (upper strata), leading to a three-fold subzonation of the assemblage zone (AZ). The taphonomic mode of some Menadon skeletons suggest they were scattered by floodwaters, while burrowing and co-habitation behaviours are indicated for Santacruzodon. The revision led also to identification of at least 12 new non-cynodont morphotypes, including kannemeyeriiform dicynodont (already known for the AZ), archosauriform, rhynchosaurian and procolophoid taxa, increasing the faunal diversity of outcrop and Santacruzodon AZ.

The Linha São Luiz Geosite is one of the most remarkable outcrops from the central region of Rio Grande do Sul state, southern Brazil. With more than 20 meters of vertical exposure, the locality preserves records of distinct sedimentary episodes, and one of the richest fossil assemblages known in southern Brazil. After a quarter of a century from the first expeditions to the site, the Linha São Luiz still yields new discoveries, registering exquisitely preserved fossils from micro and macrovertebrates, invertebrates, and plants, as well as trace fossils generated by these groups. In this contribution, we assembled representatives from distinct fields of paleontological study to provide a summary of the fossil assemblage from this site. We also briefly discuss the history of research and report geoconservation strategies, which are being implemented at the locality, in order to preserve this important window to the Brazilian Mesozoic.

Investigating the evolutionary trajectory of synapsid sensory and cephalic systems is pivotal for understanding the emergence and diversification of mammals. Recent studies using CT-scanning to analyze the rostral foramina and maxillary canals morphology in fossilized specimens of probainognathian cynodonts have contributed to clarifying the homology and paleobiological interpretations of these structures. In the present work, μCT-scannings of three specimens of Riograndia guaibensis, an early Norian cynodont from southern Brazil, were analyzed and revealed an incomplete separation between the lacrimal and maxillary canals, with points of contact via non-ossified areas. While the maxillary canal exhibits a consistent morphological pattern with other Prozostrodontia, featuring three main branches along the lateral region of the snout, the rostral alveolar canal in Riograndia displays variability in the number of extra branches terminating in foramina on the lateral surface of the maxilla, showing differences among individuals and within the same skull. Additionally, pneumatization is observed in the anterior region of the skull, resembling similar structures found in reptiles and mammals. Through this pneumatization, certain branches originating from the maxillary canal extend to the canine alveolus. Further investigation is warranted to elucidate the functionality of this structure and its occurrence in other cynodont groups.

The acquisition of the load-bearing dentary-squamosal jaw joint was a key step in mammalian evolution1-5. Although this innovation has received decades of study, questions remain over when and how frequently a mammalian-like skull-jaw contact evolved, hindered by a paucity of three-dimensional data spanning the non-mammaliaform cynodont-mammaliaform transition. New discoveries of derived non-mammaliaform probainognathian cynodonts from South America have much to offer to this discussion. Here, to address this issue, we used micro-computed-tomography scanning to reconstruct the jaw joint anatomy of three key probainognathian cynodonts: Brasilodon quadrangularis, the sister taxon to Mammaliaformes6-8, the tritheledontid-related Riograndia guaibensis9 and the tritylodontid Oligokyphus major. We find homoplastic evolution in the jaw joint in the approach to mammaliaforms, with ictidosaurs (Riograndia plus tritheledontids) independently evolving a dentary-squamosal contact approximately 17 million years before this character first appears in mammaliaforms of the Late Triassic period10-12. Brasilodon, contrary to previous descriptions6-8, lacks an incipient dentary condyle and squamosal glenoid and the jaws articulate solely using a plesiomorphic quadrate-articular joint. We postulate that the jaw joint underwent marked evolutionary changes in probainognathian cynodonts. Some probainognathian clades independently acquired 'double' craniomandibular contacts, with mammaliaforms attaining a fully independent dentary-squamosal articulation with a conspicuous dentary condyle and squamosal glenoid in the Late Triassic. The dentary-squamosal contact, which is traditionally considered to be a typical mammalian feature, therefore evolved more than once and is more evolutionary labile than previously considered.

Sampling (or sample) bias is a widespread concern in scientific research, across several disciplines. The concept of sampling bias originated in statistical studies. The consequence of a biased sample is that scientists will conclude about a population different from their target. In paleontology, sampling bias is typically related to fieldwork context. Human factors, known as sullegic (e.g. collection method, historic resampling) and trephic (transport, and curatorial processes) factors can generate bias. Other factor is the ugly fossil syndrome (i.e. choosing based on completeness of the specimens, or according to the researcher interest). Thus, sampling implies information loss. Biased samples add artificial results and can be considered an additional taphonomic filter. Therefore, sampling bias and the collector role and choices are frequently linked and almost indistinguishable. Compared to the treatment of this topic in other research fields, little related discussion has been held in vertebrate paleontology, especially regarding what happens at the interface between the biosphere, lithosphere, and anthroposphere, and during the transition between the anthroposphere and the patrisphere (museums). Numerous questions still arise. As a community, we must pay attention, to minimize the loss of information, from field activities to cataloging.

Traversodontid cynodonts form one of the most abundant tetrapod clades in continental Triassic beds of southern Brazil, with more than eleven species described. Within this clade, Protuberum cabralense is one of its most bizarre members, characterized by a robustly built skull and the presence of rounded protuberances in the dorsal surface of the ribs, ilium and neural spines in presacral vertebrae. The holotype and paratypes were collected in the 1970's but described only in 2009, being positioned in the clade Gomphodontosuchinae. Here we reanalyzed the skull of the holotype specimen and reinterpreted some anatomical traits, such as the number of upper incisors, position of the paracanine fossa, number of postcanines, morphology of the pterygo-paraoccipital foramen, among others. Through CT scan images the endocranial cavities are described and compared with Santagnathus mariensis (whose endocranial cavities are firstly described here) and other traversodontids. Also, two new occurrences of the species are presented, increasing its geographical distribution within the Dinodontosaurus Assemblage Zone (Pinheiros-Chiniqua Sequence, latest Ladinian-earliest Carnian) and its relevance to biostratigraphy. The reinterpretation of several traits resulted in a novel phylogenetic placement for Protuberum cabralense, nested outside the Gomphodontosuchinae clade, even as an earlier divergent than massetognathine forms.

The Schoenstatt Sanctuary site (Santa Cruz do Sul, Rio Grande do Sul, Brazil) is the type-locality and richest fossil deposit of the early Carnian Santacruzodon Assemblage Zone (Santa Cruz Sequence, Santa Maria Supersequence, Paraná Basin). This deposit is characterized by the abundance of traversodontid non-mammalian cynodonts Santacruzodon hopsoni and Menadon besairiei. By high abundance of cranial remains, this bone accumulation was described as biogenically controlled (e.g., selective scavenging), considering the outcrop as a 2,5-m-thick massive mudstone deposit. Here we propose a new taphonomic interpretation, based on the revision of more than 2700 fossil remains, collected at the site over 27 years. Rhizoliths and diagenetic features (like crusts) indicates the existence of different levels composing the outcrop. Being the Santa Cruz Sequence as deposited in arid to semi-arid palaeoenvironment, this abundance of rhizolith levels could be related to the global Carnian Pluvial Event. Reconstruction of the historical sampling provides a possible biostratigraphic division of Menadon (lower and middle strata) and Santacruzodon (upper strata), leading to a sub-zonation of the assemblage zone (AZ). The revision led also to identification of at least 12 new morphotypes for both outcrop and Santacruzodon AZ, increasing the faunal diversity. The presence of tusked dicynodonts is confirmed. Previously identified as generic indeterminate cynodonts, at least nine new archosauriforms are here reported. Rhynchosaurian and procolophonoid remains are reported for the first time. Preservation mode of some Menadon specimens suggest scattering by flooding events, while burrowing and co-habitation behaviour is proposed for Santacruzodon, for which the area may have been a breeding site, given the apparent presence of only juvenile individuals. The problem of sampling bias and historical resampling is highlighted, having led to misinterpretation of the taphonomic history of this emblematic site.