Adriana Cecilia Mancuso

The precise temporal calibration of dinosaur origins.

Significance Many hypotheses have been put forth to explain the origin and early radiation of dinosaurs, but poor age constraints for constituent fossil assemblages make these scenarios difficult to test. Using precise radioisotopic ages, we demonstrate that the temporal gap between assemblages containing only dinosaur precursors and those with the first dinosaurs was 5–10 million years shorter than previously thought. Thus, these data suggest that the origin of dinosaurs was a relatively rapid evolutionary event. Combined with our synthesis of paleoecological data, we demonstrate there was little compositional difference between the dinosaur precursor assemblages and the earliest dinosaur assemblages, and thus, the initial appearance of dinosaurs was not associated with a fundamental shift in ecosystem composition, as classically stated. Dinosaurs have been major components of ecosystems for over 200 million years. Although different macroevolutionary scenarios exist to explain the Triassic origin and subsequent rise to dominance of dinosaurs and their closest relatives (dinosauromorphs), all lack critical support from a precise biostratigraphically independent temporal framework. The absence of robust geochronologic age control for comparing alternative scenarios makes it impossible to determine if observed faunal differences vary across time, space, or a combination of both. To better constrain the origin of dinosaurs, we produced radioisotopic ages for the Argentinian Chañares Formation, which preserves a quintessential assemblage of dinosaurian precursors (early dinosauromorphs) just before the first dinosaurs. Our new high-precision chemical abrasion thermal ionization mass spectrometry (CA-TIMS) U–Pb zircon ages reveal that the assemblage is early Carnian (early Late Triassic), 5- to 10-Ma younger than previously thought. Combined with other geochronologic data from the same basin, we constrain the rate of dinosaur origins, demonstrating their relatively rapid origin in a less than 5-Ma interval, thus halving the temporal gap between assemblages containing only dinosaur precursors and those with early dinosaurs. After their origin, dinosaurs only gradually dominated mid- to high-latitude terrestrial ecosystems millions of years later, closer to the Triassic–Jurassic boundary.

Dinosaur origins: evidence from the footprint record

Triassic tracks and trackways assigned to dinosaur trackmakers or closest relatives have been mentioned from several Middle to the latest Triassic successions from both northern and southern Pangea. At present, the earliest gondwanan records are those from the Middle Triassic Los Rastros Formation in west-central Argentina. A reanalysis of Los Rastros ichnites at the Ischichuca area, including new material, has revealed the presence of a more diverse ichnofauna than previously suspected. The ichnocoenosis includes several tracks and trackways of bipeds with functionally tridactyl digitigrade pes, well developed claws, and a parasagittal posture of the hindlimbs. Previously, some large tridactyl footprints from the Ischichuca area were allied to theropod dinosaurs, although no synapomophies are preserved in the three-toed footprints that might discriminate among theropods, basal saurischians and basal ornithischian groups as their possible trackmakers. If the Ischichuca trackmakers are referred to a dinosaur taxon and/or to a close dinosaur sister-taxon, their presence in the Los Rastros levels suggests that derived dinosauriforms (including dinosaurs) had diverged and acquired their characteristic functionally tridactyl pes by at least the Middle Triassic, something that the body-fossil record has failed to document to date.

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

Abstract The Triassic Ischigualasto-Villa Unión 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.

Fluvial and eolian ichnofaunas from the Lower Permian of South America (Patquía Formation, Paganzo Basin)

The Lower Permian Patquia Formation is the youngest unit of the Paganzo Basin, western Argentina. The lower section consists of red mudstones, and fine- and coarse grained sandstones deposited in fluvial systems with extensive and thick floodplain deposits. These rocks contain a low-diversity and relatively abundant association of trace fossils suggesting the activity of a sub-superficial to superficial fauna. The association is characterized by Rusophycus carbonarius (Cubichnia), Cruziana problematica (Repichnia), and Palaeophycus tubularis (Domichnia) of the Scoyenia ichnofacies. Disarticulated fish remains are also present. The upper section is dominated by red, cross-bedded, medium- to fine-grained sandstones deposited in eolian systems that host a lowdiversity and low-abundance association of trace fossils that indicates the activity of a mainly superficial fauna. Tetrapod footprints (Chelichnus duncani, oval digit imprints, short parallel grooves, and sinusoidal grooves), horizontal to vertical burrows (Palaeophycus tubularis and Skolithos isp. respectively), and arthropod trackways are the typical components of these deposits. The association shows elements of the three presently proposed eolian ichnofacies (Chelichnus, Octopodichnus and Entradichnus ichnofacies) suggesting the necessity of revision and the possible integration of these separate ichnofacies into a single model. The record of fossil vertebrate tracks is uncommon in Lower Permian strata of South America. Therefore, the ichnologic record of the Patquia Formation is a significant contribution to the understanding of Lower Permian South American ichnofaunas.

Tetrapod and invertebrate trace fossils from aeolian deposits of the lower Permian of central-western Argentina

Abundant tetrapod footprints are described from the Early Permian Yacimiento Los Reyunos Formation including both collected and in situ specimens. The slabs come from several quarries at the Sierra Pintada and Sierra de las Peñas area, south-west of Mendoza, Argentina. The trace fossil assemblage, which constitutes one of the oldest known from Gondwana, comprises excellent-preserved tetrapod tracks (Chelichnusduncani, Chelichnusgigas and ‘pear-like’ footprints) and invertebrate simple sub-horizontal (Palaeophycustubularis) and vertical (Skolithos isp.) burrows formed in a aeolian dune field. The analysis of the tetrapod track producers indicates the presence of at least three different taxa of sprawling to semi-erect therapsids, thus suggesting the presence of members of this clade, or closest relatives, in the Early Permian of southern Gondwana. Moreover, a series of measurements and simple indexes were developed to estimate body proportions and locomotion styles of the putative trackmakers. The new assemblage, analysed in the context of other known Permian assemblages from Pangea, is one the few known in Gondwana to be present in an aeolian environment. The evaluation of the assemblage, in the light of aeolian ichnofacies (Chelichnus, Octopodichnus and Entradichnus), shows that it has common elements with the Chelichnus and Entradichnus ichnofacies.

PRELIMINARY CHEMOMETRIC STUDY OF BONE DIAGENESIS IN EARLY TRIASSIC CYNODONTS FROM MENDOZA, ARGENTINA

Abstract. The non-mammalian therapsids dominated the terrestrial ecosystems during the Late Paleozoic—Early Mesozoic. The cynodonts have been studied from a taxonomic, osteologic and morphological perspective. However, taphonomy using chemometrics has been barely explored. This report includes a rib and an appendicular bone of cynodonts from the Puesto Viejo Group (Mendoza, Argentina). These fossils are studied for the first time using SEM-EDX. Semi-quantitative data derived from SEM-EDX spectra is evaluated by principal component analysis to gain new insights regarding the different diagenetic pathways of bone microstructure. The multivariate model supports the distinction of different sampled areas (bone, transition zone and rock matrix), in terms of chemical parameters. Differentiation is based mainly on varying contents of Ca, P, F, Si, Al, K, O, Mn and Fe. Variable concentrations of Fe and Mn could be related to different facies (floodplain and crevasse splay). These results along with thin section petrographical analysis confirm —in one of the cases— the substitution of hydroxyapatite by fluorapatite in the bone microstructure. Fossil-diagenetic processes observed herein include substitution, fracturing, brittle deformation and different permineralization events. Permineralization stages during burial history include infilling of vascular canals, trabeculae and fractures with hematite, manganite and calcite. The presence of calcite and iron enrichment indicates local reducing conditions below water-table during precipitation. This chemometric approach to the study of Triassic cynodont remains proved useful for assessing the chemical changes in bone microstructure.

Chlorococcalean algae as palaeoenvironmental indicators: new data from the Los Rastros Formation, Triassic of central-western Argentina

Chlorococcalean algae as palaeoenvironmental indicators: new data from the Los Rastros Formation, Triassic of central-western Argentina. Lacustrine strata of the Los Rastros Formation (Triassic) at the Ischigualasto Provincial Park section, San Juan province, yield a distinctive palynological assemblage of miospores and Chlorococcalean algae. The miospore association is characterized by a relative abundance of corystosperm pollen grains with subordinate spores and inaperturates, diploxylonoid disaccates, monocolpates, monosaccates and striate pollen grains. The phytoplankton is represented by the Chlorococcalean Botryococcus and Plaesiodictyon. Geological data and variations in phytoplankton content indicate that the lacustrine system should be moderately eutrophic towards the lower third of the section, evolving into a body with oligotrophic conditions through the middle and upper part of the unit. Comparisons with the Rio Gualo section suggest palaeoenvironmental similarities between the two areas. One new disaccate pollen grain species, Platysaccus rhombicus, is proposed.

CHEMOMETRIC APPROACH TO CHAROPHYTE PRESERVATION (TRIASSIC CERRO PUNTUDO FORMATION, ARGENTINA): PALEOLIMNOLOGIC IMPLICATIONS

ABSTRACT A first-time chemometric study of energy-dispersive X-ray (EDX) data of Charophyta gyrogonites is presented. Specimen provenance is a microbialitic carbonate lacustrine succession from the Triassic (Anisian, 243.8 ± 1.9 Ma) of the Cerro Puntudo Formation, San Juan, Argentina. Gyrogonites from three different strata of the succession are studied. Data obtained by EDX include major and minor components, which are analyzed by principal component analysis (PCA). The aim of this study is twofold: first to determine the preservation features of gyrogonites by way of a chemometric approach (i.e., EDX followed by PCA) and then to infer the likely, original chemical composition of the paleolake inhabited by charophytes. EDX spectra show the presence of O, Ca, and minor elements (e.g., Si and Mg), indicating a predominantly calcium carbonate (CaCO3) composition. Principal component analysis supports differences obtained between central and peripheral areas of the gyrogonites, indicating a higher CaCO3 content in their central part. On the other hand, in their outer part, the CaCO3 diminishes and the presence of Si compounds is recorded. No significant differences among gyrogonites from the three different strata are found, implying a similar preservation mode. This suggests a differential diagenetic pattern for the external cells of the gyrogonites than their centers. These results have implications regarding the chemical composition of the paleolake water (Si and Ca availability) and the provenance and catchment areas. Results are encouraging regarding the usefulness of a chemometric approach for studies of fossil remains in lacustrine environments when other techniques of chemical analysis are not available.

A new lungfish (Dipnoi) from the Late Triassic of South America

http://zoobank.org/urn:lsid:zoobank.org:pub:69F156B4-1A84-4C98-86C3-4FE4626024AC Citation for this article: Agnolín, F. L., S. Bogan, F. Brissón Egli, F. E. Novas, M. P. Isasi, C. Marsicano, A. Zavattieri, and A. Mancuso. 2016. A new lungfish (Dipnoi) from the Late Triassic of South America. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1245665.

TETRAPOD TRACKS TAPHONOMY IN EOLIAN FACIES FROM THE PERMIAN OF ARGENTINA

Abstract: Ancient desert deposits preserve a copious ichnofossil record, particularly Permian-age deposits where the record of tetrapod footprints is present and abundant in almost all desert settings. We propose to analyze, from a taphonomic perspective, Permian footprints preserved in eolian deposits from Argentina with a detailed sedimentological study of the trackway-bearing levels, in order to find evidence of processes that may have enhanced their preservation. We defined four taphonomic modes based on preservation quality, and the morphological and extra-morphological features of the footprints. Mode 1 includes footprints with detailed impressions of the palm, digits and claws. Mode 2 includes tracks with palm and digit impressions associated with small bulbous-shape marginal rims. Mode 3 includes tracks characterized by large, bulbous, marginal rims and randomly preserved palm impressions. Mode 4 includes footprints with shallow digit and palm impressions associated with sand-crescent marginal rims. The Los Reyunos footprints suggest preservation in: (1) dry sand, evidenced by sediment slipping down-slope structures and (2) subsurface damp sand, evidenced by digit impressions and claw drag traces. Also, we found vertical water content variations along the dune foresets, evidenced by a varying amount of sediment slipping down-slope in the same trackway. Moreover, differences in the time of entombment are suggested by the morphology of rims (bulbous-shape or sand-crescent). The stratigraphic genetic framework resulting from the Los Reyunos taphonomic analysis supports changes in the interstitial subsurface water and rapid entombment of the tracking surface due to a high rate of sediment supply as the main factor for footprint preservation.