Osteology of Pseudochampsa ischigualastensis gen. et comb. nov. (Archosauriformes: Proterochampsidae) from the early late triassic ischigualasto formation of Northwestern Argentina

Osteohistological insight into the growth dynamics of early dinosaurs and their contemporaries

Dinosauria debuted on Earth's stage in the aftermath of the Permo-Triassic Mass Extinction Event, and survived two other Triassic extinction intervals to eventually dominate terrestrial ecosystems. More than 231 million years ago, in the Upper Triassic Ischigualasto Formation of west-central Argentina, dinosaurs were just getting warmed up. At this time, dinosaurs represented a minor fraction of ecosystem diversity. Members of other tetrapod clades, including synapsids and pseudosuchians, shared convergently evolved features related to locomotion, feeding, respiration, and metabolism and could have risen to later dominance. However, it was Dinosauria that radiated in the later Mesozoic most significantly in terms of body size, diversity, and global distribution. Elevated growth rates are one of the adaptations that set later Mesozoic dinosaurs apart, particularly from their contemporary crocodilian and mammalian compatriots. When did the elevated growth rates of dinosaurs first evolve? How did the growth strategies of the earliest known dinosaurs compare with those of other tetrapods in their ecosystems? We studied femoral bone histology of an array of early dinosaurs alongside that of non-dinosaurian contemporaries from the Ischigualasto Formation in order to test whether the oldest known dinosaurs exhibited novel growth strategies. Our results indicate that the Ischigualasto vertebrate fauna collectively exhibits relatively high growth rates. Dinosaurs are among the fastest growing taxa in the sample, but they occupied this niche alongside crocodylomorphs, archosauriformes, and large-bodied pseudosuchians. Interestingly, these dinosaurs grew at least as quickly, but more continuously than sauropodomorph and theropod dinosaurs of the later Mesozoic. These data suggest that, while elevated growth rates were ancestral for Dinosauria and likely played a significant role in dinosaurs' ascent within Mesozoic ecosystems, they did not set them apart from their contemporaries.

Cranial osteology and paleoneurology of Tarjadia ruthae: An erpetosuchid pseudosuchian from the Triassic Chañares Formation (late Ladinian-?early Carnian) of Argentina

Tarjadia ruthae is a quadrupedal terrestrial pseudosuchian from the Middle-early Upper Triassic of the Chañares Formation, La Rioja Province, Argentina. Originally, this species was identified as an indeterminate archosaur and later as a doswelliid archosauriform based on very fragmentary specimens characterized by the ornamentation of the skull roof and osteoderms. Additional specimens (including skulls and postcrania) recovered in the last decade show that Tarjadia is an erpetosuchid, an enigmatic pseudosuchian group composed of six species registered in Middle-Upper Triassic continental units of Tanzania, Germany, Scotland, North America, Brazil, and Argentina. Tarjadia ruthae from Argentina and Parringtonia gracilis from Tanzania are the best preserved and more abundant species. Although the monophyly of Erpetosuchidae is well supported, alternative high-level positions within Archosauria have been suggested, such as sister taxon to Crocodylomorpha, Aetosauria, or Ornithosuchidae. In order to improve the knowledge about the erpetosuchids, we performed a detailed description and paleoneurological reconstruction of the skull of Tarjadia ruthae, based on two articulated partial skulls (CRILAR-Pv 478 and CRILAR-Pv 495) and other fragmentary specimens. We analyzed the stratigraphic and geographic occurrence of historical and new specimens of Tarjadia and provided a new emended diagnosis (the same for the genus as for the species, due to monotypy) along with a comparative description of the cranial endocast. The skull of Tarjadia is robust, with a thick and strongly ornamented skull roof, triangular in dorsal view, with concave lateral margins at mid-length that form an abrupt widened posterior region. The external nares are the smallest openings of the skull. The antorbital fossa is deeply excavated and has a small heart-shaped fenestra with both lobes pointing anteriorly. The supratemporal fenestrae are as large and rounded as the orbits, and the infratemporal fenestrae are L-shaped with an extensive excavation along the jugal, quadratojugal and quadrate. The hemimandibles are low, slightly concave on the dentigerous region and strongly convex on the posterior region, conferring them a S-shaped profile in dorsal view. The external mandibular fenestra is small and elliptic, being twice longer than high. The maxillary dentition is restricted to the anterior to mid region of the rostrum. Since the braincase of both specimens is partially damaged, the dorsal surface of the brain could not be entirely reconstructed. As a result, the endocast is anteroposteriorly elongated and seemingly flat, and the cephalic flexure seems to be lower than expected for a suchian. The labyrinth is twice wider than high, the semicircular canals are remarkably straight, and the anterior canal is longer than the posterior one.

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.

A late Permian archosauriform from Xinjiang shows evidence of parasagittal posture

Archosaurs diversified and became dominant during the Mesozoic Era, but their earliest relatives (non-archosaurian archosauromorphs) were already scarcely present in the late Permian. Here we describe a new species of non-archosaurian archosauriform from the upper Permian of Xinjiang, China. Preserved as a partial hindlimb, it possesses a few derived features shared with other archosauriforms, including a much stouter tibia than fibula, a longer metatarsal III than metatarsal IV, and a hooked metatarsal V. Phylogenetic analysis confirmed the new taxon to be a non-archosaurian archosauriform. The morphology of the knee, crus, and pes shows traits that are commonly related with a parasagittal posture, including an entirely proximo-distal articulation of the femur and fibula, the slender and closely spaced tibia and fibula, and a mesaxonic foot with a reduced fifth toe. The new taxon shows that the parasagittal posture evolved before the end-Permian Mass Extinction.

Redescription, taxonomic revaluation and phylogenetic affinities of Proterochampsa nodosa (Archosauriformes: Proterochampsidae) from the early Late Triassic of the Candelaria Sequence (Santa Maria Supersequence)

Proterochampsidae are a group of predatory, putatively semi-aquatic, archosauriforms endemic to the Middle and Upper Triassic strata of Argentina and Brazil (Ischigualasto Formation and Santa Maria Supersequence). Here, the Brazilian species Proterochampsa nodosa from the Candelária Sequence (Santa Maria Supersequence of the Paraná Basin) is revisited with a detailed description, phylogenetic analysis and taxonomic revaluation. Through the application of computed tomographic scans, we were able to reveal part of the previously hidden morphology of its mandible and palate. We also discuss further differences between P. nodosa and P. barrionuevoi. The new phylogenetic analysis bolsters the monophyly of Proterochampsidae, the genus Proterochampsa and its position as the basalmost proterochampsid taxon in the clade.

A new phylogenetic hypothesis of Tanystropheidae (Diapsida, Archosauromorpha) and other "protorosaurs", and its implications for the early evolution of stem archosaurs

The historical clade "Protorosauria" represents an important group of archosauromorph reptiles that had a wide geographic distribution between the Late Permian and Late Triassic. "Protorosaurs" are characterized by their long necks, which are epitomized in the genus Tanystropheus and in Dinocephalosaurus orientalis. Recent phylogenetic analyses have indicated that "Protorosauria" is a polyphyletic clade, but the exact relationships of the various "protorosaur" taxa within the archosauromorph lineage is currently uncertain. Several taxa, although represented by relatively complete material, have previously not been assessed phylogenetically. We present a new phylogenetic hypothesis that comprises a wide range of archosauromorphs, including the most exhaustive sample of "protorosaurs" to date and several "protorosaur" taxa from the eastern Tethys margin that have not been included in any previous analysis. The polyphyly of "Protorosauria" is confirmed and therefore we suggest the usage of this term should be abandoned. Tanystropheidae is recovered as a monophyletic group and the Chinese taxa Dinocephalosaurus orientalis and Pectodens zhenyuensis form a new archosauromorph clade, Dinocephalosauridae, which is closely related to Tanystropheidae. The well-known crocopod and former "protorosaur" Prolacerta broomi is considerably less closely related to Archosauriformes than was previously considered.

Deep evolutionary diversification of semicircular canals in archosaurs

Living archosaurs (birds and crocodylians) have disparate locomotor strategies that evolved since their divergence ∼250 mya. Little is known about the early evolution of the sensory structures that are coupled with these changes, mostly due to limited sampling of early fossils on key stem lineages. In particular, the morphology of the semicircular canals (SCCs) of the endosseous labyrinth has a long-hypothesized relationship with locomotion. Here, we analyze SCC shapes and sizes of living and extinct archosaurs encompassing diverse locomotor habits, including bipedal, semi-aquatic, and flying taxa. We test form-function hypotheses of the SCCs and chronicle their evolution during deep archosaurian divergences. We find that SCC shape is statistically associated with both flight and bipedalism. However, this shape variation is small and is more likely explained by changes in braincase geometry than by locomotor changes. We demonstrate high disparity of both shape and size among stem-archosaurs and a deep divergence of SCC morphologies at the bird-crocodylian split. Stem-crocodylians exhibit diverse morphologies, including aspects also present in birds and distinct from other reptiles. Therefore, extant crocodylian SCC morphologies do not reflect retention of a "primitive" reptilian condition. Key aspects of bird SCC morphology that hitherto were interpreted as flight related, including large SCC size and enhanced sensitivity, appeared early on the bird stem-lineage in non-flying dinosaur precursors. Taken together, our results indicate a deep divergence of SCC traits at the bird-crocodylian split and that living archosaurs evolved from an early radiation with high sensory diversity. VIDEO ABSTRACT.

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.

The phylogenetic relationships of basal archosauromorphs, with an emphasis on the systematics of proterosuchian archosauriforms

The early evolution of archosauromorphs during the Permo-Triassic constitutes an excellent empirical case study to shed light on evolutionary radiations in deep time and the timing and processes of recovery of terrestrial faunas after a mass extinction. However, macroevolutionary studies of early archosauromorphs are currently limited by poor knowledge of their phylogenetic relationships. In particular, one of the main early archosauromorph groups that need an exhaustive phylogenetic study is "Proterosuchia," which as historically conceived includes members of both Proterosuchidae and Erythrosuchidae. A new data matrix composed of 96 separate taxa (several of them not included in a quantitative phylogenetic analysis before) and 600 osteological characters was assembled and analysed to generate a comprehensive higher-level phylogenetic hypothesis of basal archosauromorphs and shed light on the species-level interrelationships of taxa historically identified as proterosuchian archosauriforms. The results of the analysis using maximum parsimony include a polyphyletic "Prolacertiformes" and "Protorosauria," in which the Permian Aenigmastropheus and Protorosaurus are the most basal archosauromorphs. The enigmatic choristoderans are either found as the sister-taxa of all other lepidosauromorphs or archosauromorphs, but consistently placed within Sauria. Prolacertids, rhynchosaurs, allokotosaurians and tanystropheids are the major successive sister clades of Archosauriformes. The Early Triassic Tasmaniosaurus is recovered as the sister-taxon of Archosauriformes. Proterosuchidae is unambiguosly restricted to five species that occur immediately after and before the Permo-Triassic boundary, thus implying that they are a short-lived "disaster" clade. Erythrosuchidae is composed of eight nominal species that occur during the Early and Middle Triassic. "Proterosuchia" is polyphyletic, in which erythrosuchids are more closely related to Euparkeria and more crownward archosauriforms than to proterosuchids, and several species are found widespread along the archosauromorph tree, some being nested within Archosauria (e.g., "Chasmatosaurus ultimus," Youngosuchus). Doswelliids and proterochampsids are recovered as more closely related to each other than to other archosauromorphs, forming a large clade (Proterochampsia) of semi-aquatic to aquatic forms that includes the bizarre genus Vancleavea. Euparkeria is one of the sister-taxa of the clade composed of proterochampsians and archosaurs. The putative Indian archosaur Yarasuchus is recovered in a polytomy with Euparkeria and more crownward archosauriforms, and as more closely related to the Russian Dongusuchus than to other species. Phytosaurs are recovered as the sister-taxa of all other pseudosuchians, thus being nested within Archosauria.

Cranial Anatomy and Palaeoneurology of the Archosaur Riojasuchus tenuisceps from the Los Colorados Formation, La Rioja, Argentina

Riojasuchus tenuisceps Bonaparte 1967 is currently known from four specimens, including two complete skulls, collected in the late 1960s from the upper levels of the Los Colorados Formation (Late Triassic), La Rioja, Argentina. Computed tomography (CT) scans of the skulls of the holotype and a referred specimen of Riojasuchus tenuisceps and the repreparation of the latter allows recognition of new features for a detailed analysis of its cranial anatomy and its comparison with a wide variety of other archosauriform taxa. The diagnosis of Riojasuchus tenuisceps is emended and two autapomorphies are identified on the skull: (1) a deep antorbital fossa with its anterior and ventral edges almost coinciding with the same edges of the maxilla itself and (2) a suborbital fenestra equal in size to the palatine-pterygoid fenestra. Also, the first digital 3D reconstruction of the encephalon of Riojasuchus tenuisceps was carried out to study its neuroanatomy, showing a shape and cranial nerve disposition consistent to that of other pseudosuchians.