end-Triassic Extinction Research Articles

Reevaluating hydrodynamic performance of Late Triassic–Early Jurassic ammonoid shells with a 1D trajectory model

Biomechanical analyses provide unique insights on state shifts in the ecology of extinct communities. Ammonoids present a compelling case study for coupling biomechanical analysis with ecology given their robust fossil record of external conchs. We present a trajectory model to evaluate hydrodynamical advantages and challenges associated with ammonoid conch form. The model is a one-dimensional calculation estimating the dynamic feedbacks between different components of an ammonite’s motion including thrust, drag, acceleration, and distance traveled. Computational fluid dynamics simulations were performed on eleven different ammonoid conch morphotypes and integrated into a mathematical model to analyze the dynamics of swimming across a combination of conch diameters (5, 10, and 20 cm) and jet rhythms (a single jet or series of three pulses). We compared the efficacy of short-term bursts of motion to that of long-term cruising and found: inflated shapes (i.e., spherocones) offer the fastest short-term motion, but at the greatest costs; heavily streamlined shapes (i.e., platycones) offer long cruise distances, but with ineffective short-term motion; and visibly-coiled shapes (i.e., serpenticones) appear to offer intermediate performance in both locomotion styles. Size is critical in ranking the performance of different conch shapes in both locomotion styles because ranking is determined predominantly by the amount of thrust an animal is capable of generating. With increasing size, Reynolds number increases and the effects of second-order morphological characters become more pronounced and alter the performance ranking of conchs. Finally, we present a visual analysis of the flow regimes and shape details that may drive these hydrodynamic consequences. We speculate that serpenticone morphologies capitalized on these subtleties with a morphology that provided reasonably high-speed swimming at small sizes relevant to juveniles while maintaining relatively efficient coasting locomotion at the larger sizes relevant to adult animals. We highlight the ubiquitous serpenticones of the Early Jurassic as a case study for applying biomechanical data to a paleoecological context. The broad range of morphotypes expressed by ammonoids in the Late Triassic is dramatically pared down during the End Triassic extinction. In the few million years following the extinction, ammonoids diversify into a suite of shapes with a more restricted range of locomotor performance, and only much later is the full range of morphology recovered.

­The End-Triassic Extinction as Recorded in the Deep-Sea Archive of the Budva Basin, Dinarides, Montenegro

­The End-Triassic Extinction as Recorded in the Deep-Sea Archive of the Budva Basin, Dinarides, Montenegro

Recurring photic zone euxinia in the northwest Tethys impinged end-Triassic extinction recovery

The end-Triassic extinction (ETE) is associated with rapid atmospheric CO2-driven warming amplified by positive feedbacks involving weathering, nutrient oversupply, water column anoxia and sul.fate reduction. These conditions culminated in photic zone euxinia (PZE) at least locally in the Western Tethys, but its broader extent across northwest Europe has yet to be identified. Here we present new biomarker and bulk δ13Corg isotopic data that document redox and palaeoecological changes from two end-Triassic sites in the Western Tethys: Felixkirk, in the Cleveland Basin, northeast England, and Lavernock Point, in the Bristol Channel Basin, S Wales. The presence of aryl isoprenoids and Chlorobi-derived isorenieratane indicates shoaling of anoxia and toxic H2S to the photic zone caused by flooding in restricted or semi-enclosed basins. Oscillating redox conditions and severe PZE prevented a swift recovery of at least the benthic ecosystem across several British basins. Additionally, in concert with recent discoveries in the Bristol Channel Basin, in the Cleveland Basin we find that the ‘initial’ negative carbon isotope excursion (CIE) is related to a localized change in organic matter sources. PZE in British and other European basins points towards H2S toxicity as an extinction driver for the Western Tethys, highlighting the need for a global characterization of redox changes across the end-Triassic and other extinction events.

A high-precision U–Pb zircon age constraints the timing of the faunistic and palynofloristic events of the Carnian Ischigualasto Formation, San Juan, Argentina

The Late Triassic represents records many of the most critical biotic crises in life-history (from Carnian to the end-Triassic extinction, ETE). Some hypotheses propose that these life changes were linked with shifts in the environmental setting during the initial stages of the Pangea breakup. These crises were likely not sudden events but a series of step-changes that reached their peak at the end of the Triassic. The Carnian-Norian Ischigualasto Formation is a world-renown terrestrial sequence that records both the Triassic Dinosaur Diversification Event (DDE) and at least one crucial terrestrial environmental and biotic turnover. A notable increase in humidity and volcanism was the setting of critical faunal replacement represented by the passage of the Hyperodapedon-Exaeretodon-Herrerasaurus to Exaeretodon tetrapod biozones. That environmental modification, together with the paleofaunistic turnover, has been accurately constrained by a new high-precision CA-TIMS U/Pb zircon age of 228.91 ± 0.14 Ma obtained from a tuff layer located in the exact stratigraphic position of the major biotic-paleoenvironmental disturbance in the middle part of the Valle de la Luna Member, at ~310 m from the base of the Ischigualasto Formation. Palynological assemblages containing the first records in westernmost Gondwana of European Tethyan and Onslow palynological species are also recognized close to the dated tuff level. Thus, this age is the first isotopic record for the Onslow Microflora diagnostic species in Gondwana. Moreover, the CA-TIMS date promotes a discussion about the accurate chronostratigraphic position for the beginning (after the Carnian Pluvial Event) of a series of environmental-biological disturbances that culminated at the end of the Triassic.

Extreme growth plasticity in the early branching sauropodomorph Massospondylus carinatus.

There is growing evidence of developmental plasticity in early branching dinosaurs and their outgroups. This is reflected in disparate patterns of morphological and histological change during ontogeny. In fossils, only the osteohistological assessment of annual lines of arrested growth (LAGs) can reveal the pace of skeletal growth. Some later branching non-bird dinosaur species appear to have followed an asymptotic growth pattern, with declining growth rates at increasing ontogenetic ages. By contrast, the early branching sauropodomorph Plateosaurus trossingensis appears to have had plastic growth, suggesting that this was the plesiomorphic condition for dinosaurs. The South African sauropodomorph Massospondylus carinatus is an ideal taxon in which to test this because it is known from a comprehensive ontogenetic series, it has recently been stratigraphically and taxonomically revised, and it lived at a time of ecosystem upheaval following the end-Triassic extinction. Here, we report on the results of a femoral osteohistological study of M. carinatus comprising 20 individuals ranging from embryo to skeletally mature. We find major variability in the spacing of the LAGs and infer disparate body masses for M. carinatus individuals at given ontogenetic ages, contradicting previous studies. These findings are consistent with a high degree of growth plasticity in M. carinatus.

A possible serpulid tube worm of the genus Filograna from the upper Permian Cadeby Formation of South Yorkshire, UK

Simple, tubular clusters from a single boundstone hand specimen, collected from loose material in the Permian (Lopingian) Cadeby Formation at Hazel Lane Quarry, Doncaster Metropolitan Borough, are tentatively identified as the serpulid worm, Filograna sp. A. The tubes are calcareous, found in vertical clusters, with some short sections growing horizontally, and with external diameters between 0.6 and 1 mm. Recent work on the Permian Tethyan deposits of Italy have identified occurrences of the genus Filograna from before the Permian–Triassic extinction, but the importance of this example is its provenance in the Zechstein English Shelf biota, where reported occurrences of Serpulidae have been rare.

End-Triassic crisis and “unreefing” led to the demise of the Dachstein carbonate platform: A revised model and evidence from the Transdanubian Range, Hungary

The Dachstein platform was an extensive carbonate platform developed on the westernmost shelf of the Neotethys during the Late Triassic, now preserved in various tectonic units disrupted during the Alpine orogeny. Despite being the focus of a multitude of sedimentological, paleontological and other studies, the demise of this platform remains controversial, with contrasting views on the timing and causes of cessation of its growth, the duration of the gap above, which at many places includes the Triassic-Jurassic boundary (TJB), and the depositional environment of overlying strata. Here we present new carbonate sedimentological, stable isotope and cyclostratigraphic data from sections in the Transdanubian Range (Hungary) which capture the termination of uppermost Triassic Dachstein Limestone and the onset of Hettangian (Early Jurassic) sedimentation following a hiatus. Previously, the TJB in the Transdanubian Range was regarded as a textbook case of a tectonically-driven platform drowning event or, alternatively, cessation of carbonate production due to emergence caused by a significant sea level fall at the TJB. However, recognition of global biotic change and environmental perturbations at the TJB calls for an assessment of their possible role in the demise of the Dachstein platform. Oxygen and carbon isotopic composition of bulk carbonates were measured in sections at Kőris-hegy (Bakony Mts.), Tata (Tata Horst), and Vöröshíd (Gerecse Mts.) Paleogeographically, these three sections represent a proximal to distal platform transect. Other sections at Pisznice and Tölgyhát (Gerecse Mts.) yielded additional sedimentological data. The sharp surface separating the Dachstein Limestone from the overlying Jurassic formations carries no or only minimal relief at outcrop scale. Thin section studies reveal small-scale irregularities, stylolites, microborings with ferruginous filling, or a thin clay-rich layer at the TJB, indicative of a submarine, or perhaps polygenetic, hardground. In the first meters of the lowermost Jurassic beds abundant ooids occur, and crinoids become common. In each of the studied sections, an abrupt negative carbon isotope shift is recorded at the TJB, and a gradual rebound to more positive values characterizes the lowermost Jurassic strata. Chemostratigraphy allows correlation with sections elsewhere. In the Transdanubian Range, the initial carbon isotope excursion and at least the first part of the purported main carbon isotope excursion are not preserved due to the gap at the TJB. Combined bio- and cyclostratigraphy of lowermost Jurassic strata permits an astrochronologic duration estimate of the early Hettangian hiatus that was not longer than a few hundreds of thousand years. Our results highlight the role of submarine erosion, perhaps partly related to acidification, and point to an abrupt change in carbonate production related to the end-Triassic extinction of several groups in the platform system. “Unreefing”, the ecological collapse of reefs, led to a regime shift, the transformation of the rimmed platform to a carbonate ramp, with a significant gap in production and preservation of carbonate sediment. This model is not uniformly applicable to Late Triassic platforms as several of them, unlike the Dachstein platform, survived with unrimmed architecture in the Early Jurassic elsewhere on the Neotethyan shelf. However, the model may explain features of the carbonate platform sedimentary record across other events with reef collapse. • Demise of Dachstein carbonate platform linked to end-Triassic global change. • Change in sedimentary regime caused facies contrast across Triassic-Jurassic boundary. • Collapse of reef ecosystem led to transition of rimmed platform to carbonate ramp. • Top surface of Dachstein Limestone points to submarine erosion rather than emergence. • Carbon isotope and cyclostratigraphy hint at short gap at Triassic-Jurassic boundary.

Fossil seed fern Lepidopteris ottonis from Sweden records increasing CO2 concentration during the end-Triassic extinction event

The end-Triassic event (ETE), a short global interval occurring at the end of the Triassic Period (~201.5 Ma), was characterized by climate change, environmental upheaval, as well as widespread extinctions in both the marine and terrestrial realms. It was associated with extensive perturbations of the carbon cycle, principally caused by the volcanic emplacement of the Central Atlantic Magmatic Province in relation to the break-up of Pangea. The correlated change in atmospheric CO2 concentrations (pCO2) can be reconstructed with the stomatal proxy, which utilizes the inverse relationship between stomatal densities of plant leaves (here stomatal index (SI), which is the percentage of stomata relative to epidermal cells) and pCO2. Fossilized Lepidopteris leaves are common and widespread in Triassic strata, thus offering great potential for high-resolution pCO2 reconstructions. A dataset of leaf cuticle specimens belonging to the seed fern species Lepidopteris ottonis from sedimentary successions in Skåne (Scania), southern Sweden, provided the possibility of pCO2 reconstruction at the onset of the ETE. Here, we tested the intra- and interleaf variability of L. ottonis SI, and estimated the pCO2 during the onset of the ETE. Our findings confirm L. ottonis as a valid proxy for palaeo-pCO2, also when using smaller leaf fragments. Importantly, the statistical analyses showed that the SI values of abaxial and adaxial cuticles are significantly different, providing a tool to distinguish between the two sides and select cuticles for analysis. Reconstructed pCO2 increased from ~1000 pre ETE to ~1300 ppm at the onset of the event, a significant increase of ~30% over a relatively short time period. The pCO2 recorded here is similar to previously published estimates, and strongly supports the observed pattern of elevated pCO2 at the onset of the ETE.

Widespread elevated iridium in Upper Triassic\u2013Lower Jurassic strata of the Newark Supergroup: implications for use as an extinction marker

Anomalous levels of iridium in sedimentary strata are associated with mass extinction events caused by impact events. In the case of the end-Triassic extinction event, the anomalies as well as the extinctions are linked to the eruption of the Central Atlantic Magmatic Province (CAMP) flood basalts. We report new data on concentrations of iridium in continental strata of the Fundy, Deerfield, Hartford and Newark basins, both above and below the oldest CAMP flows in these basins, that demonstrate that these anomalies are more common than previously known. We conclude that the enrichments were at least in some instances likely derived locally by concentration due to leaching directly from the lavas into sediments proximal to the CAMP flows due to post-eruptive hydrothermal activity. In other instances, the enrichments likely record the global fallout of aerosols and/or ash particles during the eruptions of the CAMP basalts. The common association of the highest levels of enrichment with organic matter suggests either redox control or stabilization by formation of organometallic complexes following post-eruptive redistribution. These findings demonstrate the importance of considering the distribution and magnitude of iridium anomalies in considering the source of the iridium and possible extinction mechanisms.

Paleoenvironments and geochemistry across a continuous Permian–Triassic boundary section at Bűkk Mountains, Hungary

The Bálvány North Permian-Triassic boundary sediments were deposited on a carbonate platform in the tropical part of the western PaleoTethys ocean. The overall elemental geochemistry of the detailed two-metre-thick section across the boundary that we studied shows that the clastic content of the sediments came from dominantly silica-rich continental sources though with some more silica-poor inputs in the uppermost Permian and lowest Triassic limestones as shown by Ni/Al and Nb/Ta ratios. These inputs bracket, but do not coincide with, the main extinctions and associated C, O and S changes. Increased aridity at the Permian-Triassic boundary with increased wind abrasion of suitable Ti-bearing heavy minerals accounts for both the high Ti/Al and Ti/Zr ratios. Various geochemical redox proxies suggest mainly oxic depositional conditions, with episodes of anoxia, but with little systematic variation across the Permian–Triassic extinction boundary. The lack of consistent element geochemical changes across the Permian-Triassic boundary occur not only in adjacent shallower-water marine sections, and in other marine sections along the SW Tethys margin such as the Salt Range sections in Pakistan, but also in deeper shelf and oceanic sections, and in non- marine African and European continental sediments. In the absence of significant changes in physical environments, chemical changes in the atmosphere and oceans, reflected in various isotopic changes, drove the Permian–Triassic extinctions.

Phylomitogenomics provides new perspectives on the Euphasmatodea radiation (Insecta: Phasmatodea)

Phasmatodea species diversity lies almost entirely within its suborder Euphasmatodea, which exhibits a pantropical distribution and is considered to derive from a recent and rapid evolutionary radiation. To shed light on Euphasmatodea origins and diversification, we assembled the mitogenomes of 17 species from transcriptomic sequencing data and analysed them along with 22 already available Phasmatodea mitogenomes and 33 mitogenomes representing most of the Polyneoptera lineages. Maximum Likelihood and Bayesian Inference approaches retrieved consistent topologies, both showing the widespread conflict between phylogenetic approaches and traditional systematics. We performed a divergence time analysis leveraging ten fossil specimens representative of most polyneopteran lineages: the time tree obtained supports an older radiation of the clade with respect to previous hypotheses. Euphasmatodea diversification is inferred to have started ~ 187 million years ago, suggesting that the Triassic-Jurassic mass extinction and the breakup of Pangea could have contributed to the process. We also investigated Euphasmatodea mitogenomes patterns of dN, dS and dN/dS ratio throughout our time-tree, trying to characterize the selective regime which may have shaped the clade evolution.

Evaluation of the ontogeny and sexual dimorphism in a new species of Middle Triassic Darwinulocopina (Crustacea, Ostracoda) from Argentina

AbstractDarwinulocopina is an interesting group of ostracods that includes some of the first invaders of freshwater habitats during the late Palaeozoic. The Permian–Triassic extinction greatly reduced their diversity, culminating in a single extant family. The darwinulids are regarded as ‘ancient asexuals’ given that a parthenogenetic mode of reproduction is assumed for all post‐Triassic members of the group. However, the high diversity achieved during the late Palaeozoic is often associated with sexual reproduction. Here we studied a monospecific assemblage of ostracods from the Middle Triassic of the Cuyo Basin, Province of Mendoza, Argentina, recognizing a new species of Darwinulocopina,Prasuchonella?huarpesp. nov. We discuss the traditional length/height and length/width graphical method for identifying ontogeny/sexual dimorphism in fossil ostracod assemblages. A geometric morphometric analysis was performed on both lateral and dorsal views of almost 170 carapaces, to evaluate the ontogeny and sexual dimorphism of the species. The best results were obtained from the analysis in dorsal view, discriminating four ontogenetic stages. This revealed a main ontogenetic trend related to the development of the brooding chamber. Although subtle in difference, female carapaces are wider not only at the brooding chamber, but also along the whole length, compared with the presumptive males. We provide full systematic descriptions and discussions, attempting to unify descriptive criteria for recent and fossil darwinulocopine carapaces, and suggest closeness to species previously described from the upper Permian of Russia. As a result, we recommend a review of Mesozoic records of Darwinulocopina, particularly those from the Triassic.

Evolution of ecospace occupancy by Mesozoic marine tetrapods

AbstractEcology and morphology are different, and yet in comparative studies of fossil vertebrates the two are often conflated. The macroevolution of Mesozoic marine tetrapods has been explored in terms of morphological disparity, but less commonly using ecological‐functional categories. Here we use ecospace modelling to quantify ecological disparity across all Mesozoic marine tetrapods. We document the explosive radiation of marine tetrapod groups in the Triassic and their rapid attainment of high ecological disparity. Late Triassic extinctions led to a marked decline in ecological disparity, and the recovery of ecospace and ecological disparity was sluggish in the Early Jurassic. High levels of ecological disparity were again achieved by the Late Jurassic and maintained during the Cretaceous, when the ecospace became saturated by the Late Cretaceous. Sauropterygians, turtles and ichthyosauromorphs were the largest contributors to ecological disparity. Throughout the Mesozoic, we find that established groups remained ecologically conservative and did not explore occupied or vacant niches. Several parts of the ecospace remained vacant for long spans of time. Newly evolved, radiating taxa almost exclusively explored unoccupied ecospace, suggesting that abiotic releases are needed to empty niches and initiate diversification. In the balance of evolutionary drivers in Mesozoic marine tetrapods, abiotic factors were key to initiating diversification events, but biotic factors dominated the subsequent generation of ecological diversity.

Mercury anomalies and carbon isotope excursions in the western Tethyan Csővár section support the link between CAMP volcanism and the end-Triassic extinction

The end-Triassic extinction is one of the major Phanerozoic mass extinctions and it appears to have been linked to coeval rapid and severe environmental change, thought to be triggered by volcanism in the Central Atlantic Magmatic Province (CAMP). However, direct stratigraphic evidence to substantiate this linkage and to help develop scenarios for the cascade of events is still scarce. Mercury is an increasingly widely used proxy to trace the volcanic activity associated with large igneous provinces (LIPs) in distal sedimentary sections, but so far Hg records are available from only a handful of Triassic–Jurassic boundary (TJB) sections. One of the few well-studied marine sedimentary sections with a continuous sedimentary record across the TJB is located at Csővár (Hungary) and it exposes an extended succession of carbonates deposited in an intraplatform basin on the western Tethyan shelf. Previously, this section yielded one of the first convincing records of carbon isotope excursions (CIEs) across the TJB, albeit from low-resolution sampling. Here we report a new, high-resolution δ13Ccarb curve, supplemented with Hg measurements. A series of successive negative carbon isotope excursions (termed NCIE-1 to 6) attests to carbon cycle perturbations in the TJB interval. Four excursions appear significant after statistical smoothing. Of these, NCIE-3 exhibits the highest amplitude and is biostratigraphically constrained to the topmost Triassic, hence reliably correlated with the initial CIE, a globally recognised excursion closely preceding the TJB, and coincident with the end-Triassic extinction (ETE) horizon. The Hg concentration data provide the longest record available to date from a single section across the TJB. It shows very low values below NCIE-3 that are interpreted as the pre-volcanic background, followed by a prominent Hg peak that is nearly coincident with the most significant carbon isotope spike (NCIE-3). The slight lag suggests that onset of a major extrusive phase of CAMP (marked by a significant rise in Hg) closely followed the very onset of carbon cycle perturbation at that time (expressed by an abrupt change in the δ13Ccarb signal), possibly from biogenic methane release. Subsequent and recurring smaller Hg psuggest a pulsatory nature of prolonged volcanic activity. Organic content in the section is consistently low and sedimentary Hg concentrations are therefore normalized against Fe content, a reliable proxy in the lack of significant lithological changes. The maximum sedimentary Hg concentration at Csővár is greater than that in any other TJB section, although not unprecedented if other events are considered. Three hypotheses are explored to explain the high values; i) the hit-or-miss model could suggest that deposition of the sampled beds was fortuitously coincident with major eruptions, ii) the presence and preservation of cryptotephra could account for the unusually high sedimentary Hg enrichment, and iii) changes in the proportion of Hg-carrier phases throughout the studied succession, e.g. from magnetite to pyrite dominance, could have enhanced the potential of Hg capture and deposition. Collectively, the new data provide direct stratigraphic and geochemical evidence for the link between CAMP volcanism and carbon cycle perturbations and strengthen the case of their causal relationship with the end-Triassic extinction.

New Specimens of Scutellosaurus Lawleri Colbert, 1981, from the Lower Jurassic Kayenta Formation in Arizona Elucidate the Early Evolution of Thyreophoran Dinosaurs

ABSTRACT We describe new specimens of the ornithischian dinosaur Scutellosaurus lawleri Colbert, 1981, from the Lower Jurassic Kayenta Formation of Arizona and discuss their systematic importance. The new specimens represent at least 46 individuals and include two associated skeletons that preserve regions that were poorly known until now, including the skull and pelvis. Computed tomography (CT) assisted our interpretation of these specimens. Using an ornithischian data matrix, we first tested whether the two associated skeletons were justifiably assigned to Scutellosaurus lawleri and found that they group unequivocally with the holotype and paratype specimens. This enabled scoring of 35 character states that were previously unknown, raising the scoring completeness of Scutellosaurus lawleri from 52% to 67%. The results recovered Lesothosaurus diagnosticus as the basal-most member of Neornithischia, while corroborating the monophyly of Thyreophora and Scutellosaurus lawleri as its most basally branching member. In terms of numbers of specimens, Scutellosaurus lawleri is now the most abundant dinosaur known in any Early Jurassic vertebrate fauna. The presence of a second thyreophoran in the Kayenta Formation, along with the presence of Early Jurassic thyreophorans in Europe and Asia, suggests that Thyreophora may have originated in the northern hemisphere. The ornithischians from the Kayenta Formation support a pattern of dinosaurian diversification after the end-Triassic extinction in North America, if not a broader area, that was fueled by independent northward dispersals from the southern hemisphere, supporting dispersal as an early driver of dinosaurian evolution.

The craniomandibular anatomy of the early archosauriform Euparkeria capensis and the dawn of the archosaur skull.

Archosauria (birds, crocodilians and their extinct relatives) form a major part of terrestrial ecosystems today, with over 10 000 living species, and came to dominate the land for most of the Mesozoic (over 150 Myr) after radiating following the Permian–Triassic extinction. The archosaur skull has been essential to this diversification, itself diversified into myriad forms. The archosauriform Euparkeria capensis from the Middle Triassic (Anisian) of South Africa has been of great interest since its initial description in 1913, because its anatomy shed light on the origins and early evolution of crown Archosauria and potentially approached that of the archosaur common ancestor. Euparkeria has been widely used as an outgroup in phylogenetic analyses and when investigating patterns of trait evolution among archosaurs. Although described monographically in 1965, subsequent years have seen great advances in the understanding of early archosaurs and in imaging techniques. Here, the cranium and mandible of Euparkeria are fully redescribed and documented using all fossil material and computed tomographic data. Details previously unclear are fully described, including vomerine dentition, the epiptergoid, number of premaxillary teeth and palatal arrangement. A new diagnosis and cranial and braincase reconstruction is provided, and an anatomical network analysis is performed on the skull of Euparkeria and compared with other amniotes. The modular composition of the cranium suggests a flexible skull well adapted to feeding on agile food, but with a clear tendency towards more carnivorous behaviour, placing the taxon at the interface between ancestral diapsid and crown archosaur ecomorphology, corresponding to increases in brain size, visual sensitivity, upright locomotion and metabolism around this point in archosauriform evolution. The skull of Euparkeria epitomizes a major evolutionary transition, and places crown archosaur morphology in an evolutionary context.

Dynamics of the Largest Carbon Isotope Excursion During the Early Triassic Biotic Recovery

The dynamics of the carbon cycle across different timescales is crucial for understanding past and present global climate changes. Following the Permian–Triassic boundary mass extinction (PTBME), the carbon cycle changed profoundly during the following 5.4 Myr, with magnitudes of changes comparable to those of the Precambrian. In pace with the successive cycles of the carbon budget, the recovery of the marine nekton underwent several evolutionary diversification and extinction cycles accompanied by eustatic sea-level changes and profound ecological reorganization of land plants, all indicative of climatic changes. Additional eruptive bursts of the Siberian Large Igneous Province (SLIP) are traditionally called upon as a plausible trigger for these climatic oscillations but firm evidence for coeval SLIP volcanism is still lacking. Based on new precise and accurate U-Pb zircon ages, we establish a high-resolution temporal calibration of the biggest positive carbon isotope excursion (CIE) spanning about 600 kyr in the late Smithian. The age of the Smithian-Spathian boundary (SSB) is established between 249.29 ± 0.06 and 249.11 ± 0.09 Ma. Our oldest U-Pb zircon ages indicate no overlap in time between the middle Smithian onset of the thermal maximum and the youngest available U-Pb zircon ages from SLIP volcanism. The constructed time line also indicates a duration of the global unconformity at the SSB that is compatible with glacio-eustatism. Potential cooling mechanisms such as a volcanic winter, the biological pump and the cessation of volcanism are discussed in the light of this new time line. In the low latitudes, the onset of the positive CIE remarkably predates the temperature drop by some 100 to 125 kyr. However, as long as the magnitude of such offset – if any – is unknown for the high latitudes, relations between the CIE and the cooling will remain an open question associated with largest Triassic extinction of the nekton.

Biostratigraphy of theMassospondylusAssemblage Zone (Stormberg Group, Karoo Supergroup), South Africa

AbstractThe Massospondylus Assemblage Zone is the youngest tetrapod biozone in the Karoo Basin (upper Stormberg Group, Karoo Supergroup) and records one of the oldest dinosaur dominated ecosystems in southern Gondwana. Recent qualitative and quantitative investigations into the biostratigraphy of the lower and upper Elliot formations (lEF, uEF) and Clarens Formation in the main Karoo Basin resulted in the first biostratigraphic review of this stratigraphic interval in nearly four decades, allowing us to introduce a new biostratigraphic scheme, the Massospondylus Assemblage Zone (MAZ). The MAZ expands upon the Massospondylus Range Zone by including the crocodylomorph Protosuchus haughtoni and the ornithischian Lesothosaurus diagnosticus as two co-occurring index taxa alongside the main index taxon, the sauropodomorph Massospondylus carinatus. With a maximum thickness of ~320 m in the southeastern portion of the basin, our new biozone is contained within the uEF and Clarens formations (upper Stormberg Group), however, based on vertebrate ichnofossils evidence, it may potentially extend into the sedimentary units of the lowermost Drakensberg Group. We do not propose any further subdivisions, and do not consider the Tritylodon Acme Zone (TAZ) as a temporal biostratigraphic marker within the MAZ. The MAZ is currently accepted to range in age between the Hettangian and Pliensbachian, however a faunal turnover, which observes an increase in the diversity of dinosaur clades, crocodylomorph, and mammaliaform taxa in the lower uEF, could reflect effects of the end-Triassic extinction event (ETE).

Millennial-scale ocean redox and δ13C changes across the Permian–Triassic transition at Meishan and implications for the biocrisis

The Permian–Triassic extinction was the largest Phanerozoic mass extinction event, yet its ultimate causes remain unresolved. Ocean redox conditions, global warming, and carbon cycle perturbations owing to massive volcanism and their inter-relationships are keys to resolve the origins of this event. We here assess such relationships using a method to obtain an instantaneous stratigraphic response from high-resolution (millennial-scale) records of carbonate-δ13C and pyrite framboid size distributions across the Permian–Triassic boundary (PTB) at Meishan, China. We report two gradual long-term negative δ13C shifts before and after the PTB. Late Permian ocean redox conditions varied through three successive oxic–dysoxic, dysoxic–euxinic, and high-frequency euxinic stages. Oxic and high-frequency euxinic events occurred in the early Griesbachian and middle Griesbachian, respectively. The short-term euxinia events were not associated with the negative δ13C excursions. Siberian volcanism could have produced large light-carbon inputs, global warming, and low-oxygen conditions at intermediate ocean depths, but it may not directly drive high-frequency euxinia across the PTB. Long-term dysoxia and occasional euxinia well before the PTB stressed mobile macroorganisms and caused gradual extinction before the major extinction event. High-frequency latest Permian euxinia reduced overall biotic adaptability, and caused the initial rapid mass extinction. Increasing temperatures and other deleterious environment factors then reinforced the extinction process. Oxic conditions contributed to biotic recovery immediately after the mass extinction in the early Griesbachian. Further millennial-scale dysoxic/euxinic alternations in the middle Griesbachian would have prolonged Early Triassic biotic recovery.

The evolution of respiratory systems in Theropoda and Paracrocodylomorpha, the end-Triassic extinction, and the role of Late Triassic atmospheric O2 and CO2

During the Late Triassic Period (235–201.3 Ma), Paracrocodylomorpha and Theropoda switched predatory roles, with the former filling the subsidiary predator niche and the latter filling the top predator niche at the end-Triassic extinction. Reasons for the transition in predator guilds remain unknown, but atmospheric conditions during this time, which involved high CO2 and low O2 concentration levels, may be associated with the event. Evidence of bird-like pneumatic post-crania, present in both groups as foramina and fossae, correlates with an avian-like respiratory system. This may have allowed organisms to cope with declining atmospheric O2 environments of the Late Triassic. This study estimated pO2 throughout the late Triassic using organic carbon isotope measurements from Newark-Hartford basin paleosols and examined the morphological change in skeletal remains of Theropoda, focusing on pneumatic bones, compared with Paracrocodylomorpha during the Late Triassic and across the Triassic-Jurassic boundary. With data compiled from the Paleobiology Database and published anatomical literature, the change of pneumaticity in taxa is quantified by a Pneumatic Index (PI), in which the number of pneumatized units is divided by the total number of bones examined. Patterns of pneumaticity are individually scored for the presence or absence of pneumatic bones. Morphological data are compared to corresponding femur length, estimated atmospheric O2 and CO2 level reconstructions, and an ancestral state reconstruction depicting how PI values change throughout various clades. Our results suggest that PI values in Theropoda and Paracrocodylomorpha correspond with the vacillating pO2 and pCO2 throughout the Late Triassic and into the Jurassic. High PI values are prevalent in advanced theropod taxa, while Paracrocodylomorpha PI values vary by clade with a generally negative trend throughout the Triassic. Ancestral state reconstruction analysis highlights the increasing PI values within Theropoda and clade dependent trends in Paracrocodylomorpha. This study is instrumental in reconstructing how Theropoda became evolutionarily successful and perhaps how and why the avian-like respiratory system originated.