A new early-diverging probainognathian cynodont and a revision of the occurrence of cf. Aleodon from the Chañares Formation, northwestern Argentina: New clues on the faunistic composition of the latest Middle-?earliest Late Triassic Tarjadia Assemblage Zone.

The Chañares Formation (Ischigualasto-Villa Unión Basin) is worldwide known by its exquisitely preserved fossil record of latest Middle-to-early Late Triassic tetrapods, including erpetosuchids, "rauisuchians," proterochampsids, gracilisuchids, dinosauromorphs, pterosauromorphs, kannemeyeriiform dicynodonts, and traversodontid, chiniquodontid and probainognathid cynodonts, coming from the Tarjadia (bottom) and Massetognathus-Chanaresuchus (top) Assemblage Zones of its lower member. Regarding cynodonts, most of its profuse knowledge comes from the traditional layers discovered by Alfred Romer and his team in the 1960s that are now enclosed in the Massetognathus-Chanaresuchus Assemblage Zone (AZ). In this contribution we focus our study on the probainognathian cynodonts discovered in levels of the Tarjadia Assemblage Zone. We describe a new chiniquodontid cynodont with transversely broad postcanine teeth (Riojanodon nenoi gen. et sp. nov.) which is related to the genus Aleodon. In addition, the specimen CRILAR-Pv 567 previously referred to cf. Aleodon is here described, compared, and included in a phylogenetic analysis. It is considered as an indeterminate Aleodontinae nov., a clade here proposed to included chiniquodontids with transversely broad upper and lower postcanines, by having a cuspidated sectorial labial margin and a lingual platform that is twice broader than a lingual cingulum. Cromptodon mamiferoides, from the Cerro de Las Cabras Formation (Cuyo Basin), was also included in the phylogenetic analysis and recovered as an Aleodontinae. The new cynodont and the record of Aleodontinae indet. reinforce the faunal differentiation between the Tarjadia and Massetognathus-Chanaresuchus Assemblage Zones, in the lower member of the Chañares Formation, and inform on the diverse chiniquodontid clade with both sectorial and transversely broad postcanine teeth.

Endocranial anatomy of the early prozostrodonts (Eucynodontia: Probainognathia) and the neurosensory evolution in mammal forerunners.

Prozostrodon brasiliensis and Therioherpeton cargnini are non-mammaliaform cynodonts that lived ~233 million years ago (late Carnian, Late Triassic) in western Gondwana. They represent some of the earliest divergent members of the clade Prozostrodontia, which includes "tritheledontids", tritylodontids, "brasilodontids", and mammaliaforms (including Mammalia as crown group). Here, we studied the endocranial anatomy (cranial endocast, nerves, vessels, ducts, ear region, and nasal cavity) of these two species. Our findings suggest that during the Carnian, early prozostrodonts had a brain with well-developed olfactory bulbs, expanded cerebral hemispheres divided by the interhemispheric sulcus, and absence of an unossified zone and pineal body. The morphology of the maxillary canal represents the necessary condition for the presence of facial vibrissae. A slight decrease in encephalization is observed at the origin of the clade Prozostrodontia. This new anatomical information provides evidence for the evolution of endocranial traits of the first prozotrodonts, a Late Triassic lineage that culminated in the origin of mammals.

New rhadinosuchine proterochampsids from the late Middle-early Late Triassic of southern Brazil enhance the diversity of archosauriforms.

Proterochampsidae is a clade of non-archosaurian archosauriforms restricted to the Middle to the Late Triassic of the Ischigualasto-Villa Unión Basin of Argentina and the Santa Maria Supersequence of Brazil. A reappraisal of proterochampsid specimens from the Brazilian Dinodontosaurus Assemblage Zone (AZ) of the Pinheiros-Chiniquá Sequence (late Ladinian-early Carnian) is presented here. One of the specimens was preliminary assigned to Chanaresuchus sp., whose type species comes from the Massetognathus-Chanaresuchus AZ of the Chañares Formation of Argentina. However, our revision indicates that it differs from Chanaresuchus, being more closely related to the middle-late Carnian Rhadinosuchus gracilis. We therefore propose the new taxon, Pinheirochampsa rodriguesi, to reallocate this specimen. Additionally, we present a revision of other putative Chanaresuchus occurrences in Brazil, including the only known specimen described for the Santacruzodon AZ (Santa Cruz do Sul Sequence; early Carnian), also proposing it as a new taxon: Kuruxuchampsa dornellesi. Both new species are characterized, among other features, by transverse expansion of the anterior end of the rostrum, similar to the condition present in Rhadinosuchus, but absent in Chanaresuchus, Gualosuchus, Pseudochampsa, and non-rhadinosuchine proterochampsids. These two new species expand the growing knowledge of the non-archosaurian archosauriform diversity during the Middle-Late Triassic in South America and enhance faunal and chronological comparisons between approximately coeval geological units between Argentina and Brazil.

New specimens of Baurutitan britoi and a taxonomic reassessment of the titanosaur dinosaur fauna (Sauropoda) from the Serra da Galga Formation (Late Cretaceous) of Brazil.

The description of new titanosaur specimens unearthed from deposits of the Serra da Galga Formation (Bauru Group, Late Cretaceous) at the BR-262 site, near Peirópolis (Uberaba, Minas Gerais State, Brazil), sheds light on the taxonomy of two taxa previously known from the same area and geological unit: Baurutitan britoi and Trigonosaurus pricei. A comparative revision indicates that T. pricei represents a junior synonym of Ba. britoi, and that the BR-262 specimens belong to that latter species. The information provided by the new specimens also revealed that the paratype of T. pricei (MCT 1719-R), a caudal vertebral series, actually represents a new taxon, named here as Caieiria allocaudata gen. et sp. nov.

Fossil basicranium clarifies the origin of the avian central nervous system and inner ear.

Among terrestrial vertebrates, only crown birds (Neornithes) rival mammals in terms of relative brain size and behavioural complexity. Relatedly, the anatomy of the avian central nervous system and associated sensory structures, such as the vestibular system of the inner ear, are highly modified with respect to those of other extant reptile lineages. However, a dearth of three-dimensional Mesozoic fossils has limited our knowledge of the origins of the distinctive endocranial structures of crown birds. Traits such as an expanded, flexed brain, a ventral connection between the brain and spinal column, and a modified vestibular system have been regarded as exclusive to Neornithes. Here, we demonstrate all of these 'advanced' traits in an undistorted braincase from an Upper Cretaceous enantiornithine bonebed in southeastern Brazil. Our discovery suggests that these crown bird-like endocranial traits may have originated prior to the split between Enantiornithes and the more crownward portion of avian phylogeny over 140 Ma, while coexisting with a remarkably plesiomorphic cranial base and posterior palate region. Altogether, our results support the interpretation that the distinctive endocranial morphologies of crown birds and their Mesozoic relatives are affected by complex trade-offs between spatial constraints during development.

Inner ear biomechanics reveals a Late Triassic origin for mammalian endothermy.

Endothermy underpins the ecological dominance of mammals and birds in diverse environmental settings1,2. However, it is unclear when this crucial feature emerged during mammalian evolutionary history, as most of the fossil evidence is ambiguous3-17. Here we show that this key evolutionary transition can be investigated using the morphology of the endolymph-filled semicircular ducts of the inner ear, which monitor head rotations and are essential for motor coordination, navigation and spatial awareness18-22. Increased body temperatures during the ectotherm-endotherm transition of mammal ancestors would decrease endolymph viscosity, negatively affecting semicircular duct biomechanics23,24, while simultaneously increasing behavioural activity25,26 probably required improved performance27. Morphological changes to the membranous ducts and enclosing bony canals would have been necessary to maintain optimal functionality during this transition. To track these morphofunctional changes in 56 extinct synapsid species, we developed the thermo-motility index, a proxy based on bony canal morphology. The results suggest that endothermy evolved abruptly during the Late Triassic period in Mammaliamorpha, correlated with a sharp increase in body temperature (5-9 °C) and an expansion of aerobic and anaerobic capacities. Contrary to previous suggestions3-14, all stem mammaliamorphs were most probably ectotherms. Endothermy, as a crucial physiological characteristic, joins other distinctive mammalian features that arose during this period of climatic instability28.

First titanosaur dinosaur nesting site from the Late Cretaceous of Brazil.

Titanosaurs were successful herbivorous dinosaurs widely distributed in all continents during the Cretaceous, with the major diversity in South America. The success of titanosaurs was probably due to several physiological and ecological factors, in addition to a series of morphological traits they achieved during their evolutionary history. However, the generalist nesting behaviour using different palaeoenvironments and strategies was key to accomplish that success. Titanosaur nesting sites have been found extensively around the world, with notable records in Spain, France, Romania, India, and, especially, Argentina. Here, we describe the first titanosaur nesting site from the Late Cretaceous of Brazil that represents the most boreal nesting site for South America. Several egg-clutches, partially preserved, isolated eggs and many eggshell fragments were discovered in an Inceptisol palaeosol profile of the mining Lafarge Quarry, at the Ponte Alta District (Uberaba Municipality, Minas Gerais State), corresponding to the Serra da Galga Formation (Bauru Group, Bauru Basin). Although classical mechanical preparation and CT scans have not revealed embryonic remains in ovo, the eggs and eggshell features match those eggs containing titanosaurian embryos found worldwide. The morphology of the egg-clutches and observations of the sedimentary characteristics bolster the hypothesis that these sauropods were burrow-nester dinosaurs, as was already suggested for the group based on other nesting sites. The egg-clutches distributed in two levels along the Lafarge outcrops, together with the geopalaeontological data collected, provide clear evidence for the first colonial nesting and breeding area of titanosaur dinosaurs in Brazil.

A new specimen provides insights into the anatomy of Irajatherium hernandezi, a poorly known probainognathian cynodont from the Late Triassic of southern Brazil.

Irajatherium hernandezi is a poorly known non-mammaliaform cynodont from the Late Triassic of southern Brazil. A new specimen of this cynodont was found in recent fieldwork to the type-locality, Sesmaria do Pinhal (Candelária), providing new insights into the anatomy of this mammalian forerunner. This specimen comprises a partial skull preserving the left canine, two left and three right postcanines, and an isolated exoccipital; the left dentary with the canine and postcanines; a fragment of the right dentary; the proximal portion of the left partial humerus; the right scapula; and indeterminate fragments. Based on new material, it is here suggested that I. hernandezi presents: a rostrum broad and short, possibly long as the temporal region; three foramina on the lateral surface of the maxilla, that could correspond to the external openings of the rostral alveolar, infraorbital, and zygomaticofacial canals; a slender zygomatic arch and an absent postorbital bar; a posteriorly wide temporal fossa; a long secondary palate, slightly surpassing the level of the last postcanine tooth; the cerebral hemispheres of the cranial endocast divided by a median sulcus; the scapular blade long and straight, and the postscapular fossa absent in lateral aspect. Finally, I. hernandezi and other tritheledontids were included in a phylogenetic analysis of Eucynodontia. The analysis recovered unresolved relationships for ictidosaurs/tritheledontids, nested within a polytomy with Tritylodontidae and a clade composed by Pseudotherium argentinus, Botucaraitherium belarminoi, Brasilodon quadrangularis, and Mammaliaformes.

Dental replacement in Mesozoic birds: evidence from newly discovered Brazilian enantiornithines.

Polyphyodonty-multiple tooth generations-in Mesozoic birds has been confirmed since the nineteenth century. Their dental cycle had been assessed through sparse data from tooth roots revealed through broken jawbones and disattached teeth. However, detailed descriptions of their tooth cycling are lacking, and the specifics of their replacement patterns remain largely unknown. Here we present unprecedented µCT data from three enantiornithine specimens from the Upper Cretaceous of southeastern Brazil. The high resolution µCT data show an alternating dental replacement pattern in the premaxillae, consistent with the widespread pattern amongst extinct and extant reptiles. The dentary also reveals dental replacement at different stages. These results strongly suggest that an alternating pattern was typical of enantiornithine birds. µCT data show that new teeth start lingually within the alveoli, resorb roots of functional teeth and migrate labially into their pulp cavities at an early stage, similar to modern crocodilians. Our results imply that the control mechanism for tooth cycling is conserved during the transition between non-avian reptiles and birds. These first 3D reconstructions of enantiornithine dental replacement demonstrate that 3D data are essential to understand the evolution and deep homology of archosaurian tooth cycling.

New cladotherian mammal from southern Chile and the evolution of mesungulatid meridiolestidans at the dusk of the Mesozoic era.

In the last decades, several discoveries have uncovered the complexity of mammalian evolution during the Mesozoic Era, including important Gondwanan lineages: the australosphenidans, gondwanatherians, and meridiolestidans (Dryolestoidea). Most often, their presence and diversity is documented by isolated teeth and jaws. Here, we describe a new meridiolestidan mammal, Orretherium tzen gen. et sp. nov., from the Late Cretaceous of southern Chile, based on a partial jaw with five cheek teeth in locis and an isolated upper premolar. Phylogenetic analysis places Orretherium as the earliest divergence within Mesungulatidae, before other forms such as the Late Cretaceous Mesungulatum and Coloniatherium, and the early Paleocene Peligrotherium. The in loco tooth sequence (last two premolars and three molars) is the first recovered for a Cretaceous taxon in this family and suggests that reconstructed tooth sequences for other Mesozoic mesungulatids may include more than one species. Tooth eruption and replacement show that molar eruption in mesungulatids is heterochronically delayed with regard to basal dryolestoids, with therian-like simultaneous eruption of the last premolar and last molar. Meridiolestidans seem endemic to Patagonia, but given their diversity and abundance, and the similarity of vertebrate faunas in other regions of Gondwana, they may yet be discovered in other continents.

An additional brain endocast of the ictidosaur Riograndia guaibensis (Eucynodontia: Probainognathia): intraspecific variation of endocranial traits.

Recently, the morphology and encephalization of the brain endocast of the Triassic non-mammaliaform probainognathian cynodont Riograndia guaibensis were studied. Here, we analyzed the brain endocast of an additional specimen of this species. The new endocast shows well-defined olfactory bulbs and a median sulcus dividing the hemispheres, traits that were not clearly observed in the first studied specimen. Encephalization quotients were also calculated, revealing similar values to other non-mammaliaform cynodonts and lower than those of the first analyzed specimen. The analyzed cranium is slightly larger than the first studied one and may represent an advanced ontogenetic stage. Hence, these differences may be related to the intraspecific variation of this cynodont or alternatively, to the preservation of each specimen.

Enigmatic dinosaur precursors bridge the gap to the origin of Pterosauria.

Pterosaurs were the first vertebrates to evolve powered flight1 and comprised one of the main evolutionary radiations in terrestrial ecosystems of the Mesozoic era (approximately 252-66 million years ago), but their origin has remained an unresolved enigma in palaeontology since the nineteenth century2-4. These flying reptiles have been hypothesized to be the close relatives of a wide variety of reptilian clades, including dinosaur relatives2-8, and there is still a major morphological gap between those forms and the oldest, unambiguous pterosaurs from the Upper Triassic series. Here, using recent discoveries of well-preserved cranial remains, microcomputed tomography scans of fragile skull bones (jaws, skull roofs and braincases) and reliably associated postcrania, we demonstrate that lagerpetids-a group of cursorial, non-volant dinosaur precursors-are the sister group of pterosaurs, sharing numerous synapomorphies across the entire skeleton. This finding substantially shortens the temporal and morphological gap between the oldest pterosaurs and their closest relatives and simultaneously strengthens the evidence that pterosaurs belong to the avian line of archosaurs. Neuroanatomical features related to the enhanced sensory abilities of pterosaurs9 are already present in lagerpetids, which indicates that these features evolved before flight. Our evidence illuminates the first steps of the assembly of the pterosaur body plan, whose conquest of aerial space represents a remarkable morphofunctional innovation in vertebrate evolution.

Two Late Cretaceous sauropods reveal titanosaurian dispersal across South America.

South American titanosaurians have been central to the study of the evolution of Cretaceous sauropod dinosaurs. Despite their remarkable diversity, the fragmentary condition of several taxa and the scarcity of records outside Patagonia and southwestern Brazil have hindered the study of continental-scale paleobiogeographic relationships. We describe two new Late Cretaceous titanosaurians from Quebrada de Santo Domingo (La Rioja, Argentina), which help to fill a gap between these main areas of the continent. Our phylogenetic analysis recovers both new species, and several Brazilian taxa, within Rinconsauria. The data suggest that, towards the end of the Cretaceous, this clade spread throughout southern South America. At the same locality, we discovered numerous accumulations of titanosaurian eggs, likely related to the new taxa. With eggs distributed in three levels along three kilometres, the new site is one of the largest ever found and provides further evidence of nesting site philopatry among Titanosauria.

The Late Triassic Ischigualasto Formation at Cerro Las Lajas (La Rioja, Argentina): fossil tetrapods, high-resolution chronostratigraphy, and faunal correlations.

Present knowledge of Late Triassic tetrapod evolution, including the rise of dinosaurs, relies heavily on the fossil-rich continental deposits of South America, their precise depositional histories and correlations. We report on an extended succession of the Ischigualasto Formation exposed in the Hoyada del Cerro Las Lajas (La Rioja, Argentina), where more than 100 tetrapod fossils were newly collected, augmented by historical finds such as the ornithosuchid Venaticosuchus rusconii and the putative ornithischian Pisanosaurus mertii. Detailed lithostratigraphy combined with high-precision U-Pb geochronology from three intercalated tuffs are used to construct a robust Bayesian age model for the formation, constraining its deposition between 230.2 ± 1.9 Ma and 221.4 ± 1.2 Ma, and its fossil-bearing interval to 229.20 + 0.11/- 0.15-226.85 + 1.45/- 2.01 Ma. The latter is divided into a lower Hyperodapedon and an upper Teyumbaita biozones, based on the ranges of the eponymous rhynchosaurs, allowing biostratigraphic correlations to elsewhere in the Ischigualasto-Villa Unión Basin, as well as to the Paraná Basin in Brazil. The temporally calibrated Ischigualasto biostratigraphy suggests the persistence of rhynchosaur-dominated faunas into the earliest Norian. Our ca. 229 Ma age assignment to Pi. mertii partially fills the ghost lineage between younger ornithischian records and the oldest known saurischians at ca. 233 Ma.

Osteology and systematics of Uberabatitan ribeiroi (Dinosauria; Sauropoda): a Late Cretaceous titanosaur from Minas Gerais, Brazil.

Uberabatitan ribeiroi is a Late Cretaceous titanosaur (Dinosauria, Sauropoda) from southeastern Brazil. Here we provide a detailed revision of all its available specimens, including new elements from the type-locality. One new autopomorphy is added to diagnosis of the taxon: astragalus with a well-developed anteroposterior crest that mediodistally delimits the tibial articulation. Linear regressions were conducted in an attempt to circumscribe specimens within the type-series, revealing that it is composed of several individuals, with inferred total body lengths varying from 7 to 26 meters. Phylogenetic analyses including U. ribeiroi show that the Brazilian taxon corresponds to a non-saltasaurid lithostrotian titanosaur.

A New Clevosaurid from the Triassic (Carnian) of Brazil and the Rise of Sphenodontians in Gondwana.

The early evolution of lepidosaurs is marked by an extremely scarce fossil record during the Triassic. Importantly, most Triassic lepidosaur specimens are represented by disarticulated individuals from high energy accretion deposits in Laurasia, thus greatly hampering our understanding of the initial stages of lepidosaur evolution. Here, we describe the fragmentary remains of an associated skull and mandible of Clevosaurus hadroprodon sp. nov., a new taxon of sphenodontian lepidosaur from the Late Triassic (Carnian; 237-228 Mya) of Brazil. Referral to Sphenodontia is supported by the combined presence of a marginal dentition ankylosed to the apex of the dentary, maxilla, and premaxilla; the presence of 'secondary bone' at the bases of the marginal dentition; and a ventrally directed mental process at the symphysis of the dentary. Our phylogenetic analyses recover Clevosaurus hadroprodon as a clevosaurid, either in a polytomy with the Late Triassic to Early Jurassic Clevosaurus and Brachyrhinodon (under Bayesian inference), or nested among different species of Clevosaurus (under maximum parsimony). Clevosaurus hadroprodon represents the oldest known sphenodontian from Gondwana, and its clevosaurid relationships indicates that these sphenodontians achieved a widespread biogeographic distribution much earlier than previously thought.

Early evidence of molariform hypsodonty in a Triassic stem-mammal.

Hypsodonty, the occurrence of high-crowned teeth, is widespread among mammals with diets rich in abrasive material, such as plants or soil, because it increases the durability of dentitions against wear. Hypsodont postcanine teeth evolved independently in multiple mammalian lineages and in the closely related mammaliaforms since the Jurassic period. Here, we report the oldest record, to our knowledge, of hypsodont postcanines in the non-mammaliaform stem-mammal, Menadon besairiei, from the early Late Triassic. The postcanines are long and columnar, with open roots. They were not replaced in older individuals and remained functional after the total wear of the crown enamel. Dental histology suggests that, convergently to hypsodont mammals, wear was compensated by the prolonged growth of each postcanine, resulting in dentine hypsodont teeth most similar to extant xenarthran mammals. These findings highlight the constraints imposed by limited tooth replacement and tooth wear in the evolutionary trajectories of herbivorous mammals and stem-mammals.