Ischigualasto-Villa Union Basin Research Articles

New Specimen Sheds Light on the Anatomy and Taxonomy of the Early Late Triassic Dinosauriforms from the Chañares Formation, NW Argentina

The lower Carnian levels of the Chañares Formation (Ischigualasto-Villa Unión Basin, La Rioja Province) have yielded some of the most informative dinosaur precursor species known so far. However, these species are based on partial skeletons that in several cases hamper the comparison between them because of the absence of overlapping bones. This has generated a contentious debate during the last 20 years about the synonymy between two of these species, Lewisuchus admixtus and Pseudolagosuchus major. Here, we describe a new dinosauriform partial skeleton (CRILAR-Pv 552) recently collected in the Chañares Formation that preserves previously unknown anatomical regions for the dinosaur precursors of this unit (e.g., premaxilla, inner ear, anterior zeugopodium) and allows comparisons with other dinosauriform specimens. CRILAR-Pv 552 is referred to Lewisuchus admixtus because it possesses a proportionally large skull, a laterally projected, shelf-like ridge on the jugal, and recurved, finely serrated middle-posterior maxillary and dentary teeth ankylosed to the bone, and the absence of a coracoid foramen. The new specimen preserves a dorsally bowed dentary with a lateroventral shelf that is identical to a dentary associated with the holotype of Lewisuchus admixtus. Additionally, the morphology of the new specimen is completely congruent with that of specimens of Pseudolagosuchus major, bolstering the hypothesis that the latter species is a subjective junior synonym of Lewisuchus admixtus. A preliminary phylogenetic analysis with updated scorings for Lewisuchus admixtus found this species at the base of Silesauridae. Anat Rec, 303:1393-1438, 2020. © 2019 American Association for Anatomy.

THE VALIDITY OF LAGOSUCHUS TALAMPAYENSIS ROMER, 1971 (ARCHOSAURIA, DINOSAURIFORMES), FROM THE LATE TRIASSIC OF ARGENTINA

The Chanares Formation (latest Middle–earliest Late Triassic) of northwestern Argentina is part of the Ischigualasto–Villa Union Basin and preserves a rich fossil vertebrate record discovered by a crew of the Museum of Comparative Zoology (Harvard University) led by Alfred Romer. A diverse non-dinosaurian dinosauromorph assemblage has been described from this formation and includes the iconic, small-sized “proto-dinosaur” Lagosuchus talampayensis Romer, 1971. This species has been crucial to understand the body plan of dinosaur precursors and the higher-level phylogenetic relationships of dinosaurs. More recently, authors considered this genus and species as nomina dubia. The aim of this contribution is to reassess the taxonomic validity of L. talampayensis. New observations indicate that the holotype of L. talampayensis differs from other avemetatarsalians, but not from the hypodigm of Marasuchus lilloensis, in the presence of a globose femoral head (strongly convex proximal and medial surfaces) with a distinctly dorsal orientation and a sub-oval outline in proximal view as a result of incipient posterior and posteromedial tuberosities. Thus, Lagosuchus talampayensis is here considered a senior synonym of “Marasuchus lilloensis” (Romer, 1972). Our conclusions have implications in the knowledge about early dinosauromorph diversity and the taxonomic richness of the group in the Chanares Formation.

U[sbnd]Pb and Sm[sbnd]Nd constraints on miocene units in the Ischigualasto-Villa Unión foreland basin, Sierras Pampeanas, Argentina: Sedimentary provenance, landscape evolution coupling flat-slab subduction

Foreland basin systems are intimately linked to thrust belts and activation and reactivation of structures in association with convergent plate dynamics, capable of controlling regional uplifts, sedimentation, as well as drainage patterns among other processes. The Ischigualasto-Villa Unión basin (IVB) is a Miocene broken foreland, occurring in-between the Famatina-Sañogasta, Valle Fértil and Maz Ranges in the Western Sierras Pampeanas, Argentina, Central Andes. We conducted U–Pb geochronology by SHRIMP in igneous zircons and LA-ICPMS in detrital zircons as well as whole rock Sm–Nd analyses in the Quebrada del Médano and Desencuentro Formations – the uppermost units of the IVB. The new and compiled data provide new insights on the influence of flat-slab on the uplift of the Sierras Pampeanas and Precordillera and consequent shift of the drainage patterns in the basin infill. The SHRIMP age for the youngest zircons from a volcanic tuff of the middle portion of the Desencuentro Formation yielded a mean 206Pb/238U age of 8.49 ± 0.33 Ma whereas the other grains gave concordant ages from 10 to 1433 Ma. The detrital zircon ages from both Formations indicated that different sourced materials contributed to the basin infill, through time and space. The major age modes in the detrital populations were: 1) 1250-950 Ma, (2) 850-600 Ma, (3) 540-500 Ma, (4) 490-460 Ma, (5) 350-320 Ma, (6) 300-250, (7) 40 Ma and (8) 16-8 Ma. From a statistical point of view, we defined two similar aged groups with distinct major contribution provenance: a first cluster with main Cambrian-Ordovician ages and a second one with main Permian and Mesoproterozoic ages. The bulk picture suggests a critical change in the IVB provenance history since the mid-late Miocene, roughly consistent with the Pampean flat-slab subduction model that controls uplifts of the Western and Eastern Sierras Pampeanas and associated regional drainage shifts in Central Andes, as like geodynamic processes in Western North America. We interpret the statistic change in the distribution of the IVB zircons as product of: (1) the presence of axial rivers flowing parallel to the basement arches in the more distal foreland; (2) a disconnection between the IVB and the Bermejo basin related to the uplift of the IVB western margin; (3) the uprising of the IVB eastern ranges. Our findings are also consistent with the acquired TDM ages (1.2–0.9 Ga) and the coupled εNd(t) constraints (−6 to −5) for samples collected along the stratigraphy of both IVB Formations that indicated recycled Grenville-aged sources were likely overload by sourced rocks with high lithologic erodibility.

A New Early Coelophysoid Neotheropod from the Late Triassic of Northwestern Argentina

Neotheropoda includes the vast majority of the predatory dinosaurs and their oldest members are Late Triassic in age. The Triassic neotheropod record is restricted to North America and Europe with the exception of a few specimens from South America, which includes Zupaysaurus rougieri and Lucianovenator bonoi. Here, the South American record of the group is enriched with the description of the new genus and species Powellvenator podocitus from the middle Norian Los Colorados Formation (Ischigualasto-Villa Union Basin) of northwestern Argentina. The new taxa are represented by previously undescribed partial hindlimbs collected by J. F. Bonaparte and associated to the hypodigm of the pseudosuchian Riojasuchus tenuisceps. In addition, a specimen originally interpreted by Bonaparte in 1972 as an indeterminate coelurosaur is here referred to the new species. Powellvenator podocitus differs from other basal dinosaurs in character-states that include an astragalus with a distinctly sigmoid posterodorsal margin and a rounded dorsal expansion on the anteromedial portion of the astragalar body in anterior view, calcaneum with a laterally projected flange, and strongly reduced shaft of metatarsal II. The phylogenetic relationships of the new species were tested in a comprehensive analyses focused on early neotheropods, which recovered Powellvenator podocitus within Coelophysoidea and as the sister-taxon of Procompsognathus triassicus and Coelophysinae. Powellvenator podocitus represents the first coelophysoid from South America together with Lucianovenator bonoi from the Marayes-El Carrizal Basin.

Archosauriform remains from the Late Triassic of San Luis province, Argentina, Quebrada del Barro Formation, Marayes–El Carrizal Basin

Here we present archosauriform remains from ‘Abra de los Colorados’, a fossiliferous locality at Sierra de Guayaguas, NW San Luis Province. Two fossiliferous levels were identified in outcrops of the Quebrada del Barro Formation (Norian), which represent the southernmost outcrops of the Marayes–El Carrizal Basin. These levels are composed by massive muddy lithofacies, interpreted as floodplain deposits. The specimens consist of one incomplete maxilla (MIC-V718), one caudal vertebra (MIC-V719), one metatarsal (MIC-V720) and one indeterminate appendicular bone (MIC-V721). The materials can be assigned to Archosauriformes but the fragmentary nature and lack of unambiguous synapomorphies preclude a more precise taxomic assignment. The maxilla is remarkably large and robust and represents the posterior process. It preserved one partially erupted tooth with ziphodont morphology. This bone shows some anatomical traits and size match with ‘rauisuchians’ and theropods. MIC-V719 corresponds to a proximal caudal vertebra. It has a high centrum, a ventral longitudinal furrow, expanded articular processes for the chevrons, a posteriorly displaced diapophysis located below the level of the prezygapophyses, and short prezygapophyses. This vertebra would be from an indeterminate archosauriform. MIC-V720 presents a cylindrical diaphysis, with a well-developed distal trochlea, which present resemblances with metatarsals of theropods, pseudosuchians, and silesaurids, although the size matches better with theropods. MIC-V721 has a slender diaphysis and a convex triangular articular surface, and corresponds to an indeterminate archosauriform. Despite being fragmentary, these materials indicate the presence of a diverse archosauriforms association from Late Triassic beds of San Luis. Thus, they add to the faunal assemblage recently reported from this basin at San Juan Province, which is much rich and diverse than the coeval paleofauna well known from Los Colorados Formation in the Ischigualasto–Villa Union Basin.

Archosauromorph Remains from the Tarjados Formation (Early—Middle Triassic, NW Argentina)

Abstract. The Agua de la Pena Group of the Ischigualasto-Villa Union Basin (northwestern Argentina) documents the evolution of archosauromorph assemblages in western Gondwana during the late Middle and Late Triassic. However, the South American archosauromorph record in the aftermath of the Permo/Triassic mass extinction (Early—early Middle Triassic) is remarkably scarce and restricted to isolated bones. Here, we describe a recently collected isolated second sacral vertebra and rib that represents one of the few fossils known from the Lower—Middle Triassic Tarjados Formation, the unit that underlies the Agua de la Pena Group. This specimen is identified as an archosauromorph because of the presence of a non-notochordal vertebra and a bifurcated distal end of the second sacral rib. A quantitative phylogenetic analysis places the new specimen as an archosauromorph more derived than protorosaurs. In particular, this specimen resembles Pamelaria, Prolacerta and early rhynchosaurs in the presence of a squared ...

Paleoenvironments and taphonomy of a Triassic lacustrine system (Los Rastros Formation, central-western Argentina)

The Triassic Ischigualasto-Villa Union Basin is an extensional basin located in central-western Argentina. It includes the Los Rastros Formation, a lacustrine-deltaic sequence comprising several coarsening-upward cycles of black shale, siltstone, and sandstone. We performed a taphonomic analysis of the floral and faunal fossils of the Los Rastros succession and have defined five plant taphofacies, four invertebrate taphofacies, and four vertebrate taphofacies. Our taphonomic model characterizes four subenvironments within the lacustrine-deltaic environment of Los Rastros Formation. These include 1) offshore lacustrine, 2) prodelta, 3) deltaic mouth bar, and 4) deltaic plain subenvironment. Our analysis of fossil assemblages allows us to reconstruct the structure of the original ecosystem. The lake margins were vegetated with small ginkgophytes, corystosperms, and sphenophytes. River margins were characterized by riparian thickets of sphenophytes, while the proximal floodplains supported closed woodlands of corystosperms, cycadophytes, and pteridophytes. More distal floodplains were covered with open conifer forests. The invertebrate fauna included insects (Blattoptera, Hemiptera, Coleoptera) associated with lakeshore vegetation and conchostracans that inhabited both the lake shoreline and smaller ponds in the floodplains. Fish and temnospondyl amphibians probably inhabited the delta plain and incoming fluvial systems. The activity of nonmammalian therapsids, crurotarsal archosaurs, and putative dinosaurs is recorded by trackway surfaces in the lake shoreline subenvironment.