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The Karoo Basin of South Africa is renowned for its abundance and diversity of therapsid fossils. Among the most ubiquitous and persistent of the Permian fauna is the small herbivorous dicynodont Diictodon feliceps . Intraspecific variation in Diictodon is historically confounding, and while ontogeny is frequently cited as a potential source of variation, observable developmental changes have never been calibrated. The present study revisits this issue, comparing three-dimensional landmark configurations of 82 Diictodon crania to investigate the association between shape, size and dimorphism. Beyond the statistically significant relationship between shape and allometry, our results determine the shape differences between juvenile and adult skulls of Diictodon , aligned with common craniofacial features documented in other tetrapod taxa. Functionally, these changes are attributed to development of the jaw musculature for feeding on larger, tougher plant matter during later ontogeny. Cranial morphological variation owing to sexual dimorphism is negligible, but distinct differences are noted in the allometric trajectories of each morphotype. A component of non-allometric variation cannot be accounted for, and we propose that this represents natural variation, rather than an artefact of taphonomic deformation.

Current hypotheses of early tetrapod evolution posit close ecological and biogeographic ties to the extensive coal-producing wetlands of the Carboniferous palaeoequator with rapid replacement of archaic tetrapod groups by relatives of modern amniotes and lissamphibians in the late Carboniferous (about 307 million years ago). These hypotheses draw on a tetrapod fossil record that is almost entirely restricted to palaeoequatorial Pangea (Laurussia) 1,2 . Here we describe a new giant stem tetrapod, Gaiasia jennyae, from high-palaeolatitude (about 55° S) early Permian-aged (about 280 million years ago) deposits in Namibia that challenges this scenario. Gaiasia is represented by several large, semi-articulated skeletons characterized by a weakly ossified skull with a loosely articulated palate dominated by a broad diamond-shaped parasphenoid, a posteriorly projecting occiput, and enlarged, interlocking dentary and coronoid fangs. Phylogenetic analysis resolves Gaiasia within the tetrapod stem group as the sister taxon of the Carboniferous Colosteidae from Euramerica. Gaiasia is larger than all previously described digited stem tetrapods and provides evidence that continental tetrapods were well established in the cold-temperate latitudes of Gondwana during the final phases of the Carboniferous-Permian deglaciation. This points to a more global distribution of continental tetrapods during the Carboniferous-Permian transition and indicates that previous hypotheses of global tetrapod faunal turnover and dispersal at this time 2,3 must be reconsidered., (© 2024. The Author(s), under exclusive licence to Springer Nature Limited.)

Gorgonopsians are among the most recognizable groups of synapsids from the Permian period and have an extensive but mostly cranial fossil record. By contrast, relatively little is known about their postcranial anatomy. Here, we describe a nearly complete, semi-articulated skeleton of a gorgonopsian (identified as Gorgonops torvus ) from the late Permian Endothiodon Assemblage Zone of the South African Karoo Basin and discuss its paleobiological implications. Known gorgonopsian postcrania indicate morphological conservatism in the group, but the skeletal anatomy of Gorgonops does differ from that of other gorgonopsians in some respects, such as in the triangular radiale and short terminal phalanges in the manus, and a weakly developed distinction between pubis and ischium in ventral aspect of the pelvic girdle. Similarities between the specimen described herein and a historically problematic specimen originally referred to " Scymnognathus cf. whaitsi " confirm referral of the latter specimen to Gorgonops . Since descriptions of gorgonopsian postcrania are rare, new interpretations of the lifestyle and ecology of Gorgonopsia can be drawn from our contribution. We conclude that gorgonopsians were likely ambush predators, able to chase their prey over short distances and pin them down with strong forelimbs before using their canines for the kill. This is evidenced by their different fore- and hindlimb morphology; the former stouter and more robust in comparison to the longer, more gracile, back legs. Furthermore, the completeness of the study specimen facilitates calculation of an estimated body mass of approximately 98 kg, similar to that of a modern lioness., Competing Interests: The authors declare that they have no competing interests., (© 2023 Bendel et al.)

The latest Permian mass extinction (LPME) was triggered by magmatism of the Siberian Traps Large Igneous Province (STLIP), which left an extensive record of sedimentary Hg anomalies at Northern Hemisphere and tropical sites. Here, we present Hg records from terrestrial sites in southern Pangea, nearly antipodal to contemporaneous STLIP activity, providing insights into the global distribution of volcanogenic Hg during this event and its environmental processing. These profiles (two from Karoo Basin, South Africa; two from Sydney Basin, Australia) exhibit significant Hg enrichments within the uppermost Permian extinction interval as well as positive Δ 199 Hg excursions (to ~0.3‰), providing evidence of long-distance atmospheric transfer of volcanogenic Hg. These results demonstrate the far-reaching effects of the Siberian Traps as well as refine stratigraphic placement of the LPME interval in the Karoo Basin at a temporal resolution of ~10 5 years based on global isochronism of volcanogenic Hg anomalies., (© 2023. The Author(s).)

All modern mammals are descendants of the paraphyletic non-mammaliaform Synapsida, colloquially referred to as the "mammal-like reptiles." It has long been assumed that these mammalian ancestors were essentially reptile-like in their morphology, biology, and behavior, i.e., they had a small brain, displayed simple behavior, and their sensory organs were unrefined compared to those of modern mammals. Recent works have, however, revealed that neurological, sensory, and behavioral traits previously considered typically mammalian, such as whiskers, enhanced olfaction, nocturnality, parental care, and complex social interactions evolved before the origin of Mammaliaformes, among the early-diverging "mammal-like reptiles." In contrast, an enlarged brain did not evolve immediately after the origin of mammaliaforms. As such, in terms of paleoneurology, the last "mammal-like reptiles" were not significantly different from the earliest mammaliaforms. The abundant data and literature published in the last 10 years no longer supports the "three pulses" scenario of synapsid brain evolution proposed by Rowe and colleagues in 2011, but supports the new "outside-in" model of Rodrigues and colleagues proposed in 2018, instead. As Mesozoic reptiles were becoming the dominant taxa within terrestrial ecosystems, synapsids gradually adapted to smaller body sizes and nocturnality. This resulted in a sensory revolution in synapsids as olfaction, audition, and somatosensation compensated for the loss of visual cues. This altered sensory input is aligned with changes in the brain, the most significant of which was an increase in relative brain size., (Copyright © 2023 Elsevier B.V. All rights reserved.)

The sternum is a stabilizing element in the axial skeleton of most tetrapods, closely linked with the function of the pectoral girdle of the appendicular skeleton. Modern mammals have a distinctive sternum characterized by multiple ossified segments, the origins of which are poorly understood. Although the evolution of the pectoral girdle has been extensively studied in early members of the mammalian total group (Synapsida), only limited data exist for the sternum. Ancestrally, synapsids exhibit a single sternal element and previously the earliest report of a segmental sternum in non-mammalian synapsids was in the Middle Triassic cynodont Diademodon tetragonus. Here, we describe the well-preserved sternum of a gorgonopsian, a group of sabre-toothed synapsids from the Permian. It represents an ossified, multipartite element resembling the mammalian condition. This discovery pulls back the origin of the distinctive "mammalian" sternum to the base of Theriodontia, significantly extending the temporal range of this morphology. Through a review of sternal morphology across Synapsida, we reconstruct the evolutionary history of this structure. Furthermore, we explore its role in the evolution of mammalian posture, gait, and ventilation through progressive regionalization of the postcranium as well as the posteriorization of musculature associated with mammalian breathing., (© 2022. The Author(s).)

Sauropodomorph dinosaurs dominated the herbivorous niches during the first 40 million years of dinosaur history (Late Triassic-Early Jurassic), yet palaeobiological factors that influenced their evolutionary success are not fully understood. For instance, knowledge on their behaviour is limited, although herding in sauropodomorphs has been well documented in derived sauropods from the Late Jurassic and Cretaceous. Here we report an exceptional fossil occurrence from Patagonia that includes over 100 eggs and skeletal specimens of 80 individuals of the early sauropodomorph Mussaurus patagonicus, ranging from embryos to fully-grown adults, with an Early Jurassic age as determined by high-precision U-Pb zircon geochronology. Most specimens were found in a restricted area and stratigraphic interval, with some articulated skeletons grouped in clusters of individuals of approximately the same age. Our new discoveries indicate the presence of social cohesion throughout life and age-segregation within a herd structure, in addition to colonial nesting behaviour. These findings provide the earliest evidence of complex social behaviour in Dinosauria, predating previous records by at least 40 My. The presence of sociality in different sauropodomorph lineages suggests a possible Triassic origin of this behaviour, which may have influenced their early success as large terrestrial herbivores., (© 2021. The Author(s).)

Earth's largest biotic crisis occurred during the Permo-Triassic Transition (PTT). On land, this event witnessed a turnover from synapsid- to archosauromorph-dominated assemblages and a restructuring of terrestrial ecosystems. However, understanding extinction patterns has been limited by a lack of high-precision fossil occurrence data to resolve events on submillion-year timescales. We analyzed a unique database of 588 fossil tetrapod specimens from South Africa's Karoo Basin, spanning ∼4 My, and 13 stratigraphic bin intervals averaging 300,000 y each. Using sample-standardized methods, we characterized faunal assemblage dynamics during the PTT. High regional extinction rates occurred through a protracted interval of ∼1 Ma, initially co-occurring with low origination rates. This resulted in declining diversity up to the acme of extinction near the Daptocephalus - Lystrosaurus declivis Assemblage Zone boundary. Regional origination rates increased abruptly above this boundary, co-occurring with high extinction rates to drive rapid turnover and an assemblage of short-lived species symptomatic of ecosystem instability. The "disaster taxon" Lystrosaurus shows a long-term trend of increasing abundance initiated in the latest Permian. Lystrosaurus comprised 54% of all specimens by the onset of mass extinction and 70% in the extinction aftermath. This early Lystrosaurus abundance suggests its expansion was facilitated by environmental changes rather than by ecological opportunity following the extinctions of other species as commonly assumed for disaster taxa. Our findings conservatively place the Karoo extinction interval closer in time, but not coeval with, the more rapid marine event and reveal key differences between the PTT extinctions on land and in the oceans., Competing Interests: The authors declare no competing interest.

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., Competing Interests: The authors declare no competing financial or non-financial interests., (© 2020 The Authors.)

The Pedra de Fogo Formation in the Parnaíba Basin of northeastern Brazil hosts a recently discovered lacustrine fauna and provides the only known record of the Captorhinidae in South America. Here, new captorhinid remains from this unit are described. Two partial mandibles, including one formerly ascribed to the genus Captorhinus , are here referred to Captorhinikos sp. a genus previously described from North America. The natural mould of a large mandible probably represents a new taxon within the captorhinid subclade Moradisaurinae, and a small skull roof is regarded as Captorhinidae indet. Captorhinids are generally considered to have been herbivores or omnivores. The Pedra de Fogo captorhinids likely played an important ecological role as primary consumers in the palaeoenvironment of this geological unit, which is also known for its extensive record of petrified forests. The new finds reinforce the close relationships between the continental faunas of palaeotropical western Gondwana and palaeoequatorial North America during the Cisuralian., Competing Interests: Claudia A. Marsicano is an Academic Editor for PeerJ., (© 2020 Cisneros et al.)

Two new species of therocephalian therapsids are described from the upper Permian Teekloof Formation of the Karoo Basin, South Africa. They include two specimens of a whaitsiid, Microwhaitsia mendrezi gen. et sp. nov., and a single, small whaitsioid Ophidostoma tatarinovi gen. et sp. nov., which preserves a combination of primitive and apomorphic features. A phylogenetic analysis of 56 therapsid taxa and 136 craniodental and postcranial characters places the new taxa within the monophyletic sister group of baurioids-Whaitsioidea-with Microwhaitsia as a basal whaitsiid and Ophidostoma as an aberrant whaitsioid just outside the hofmeyriid+whaitsiid subclade. The new records support that whaitsioids were diverse during the early-late Permian (Wuchiapingian) and that the dichotomy between whaitsiid-line and baurioid-line eutherocephalians was established early on. The oldest Gondwanan whaitsiid Microwhaitsia and additional records from the lower strata of the Teekloof Formation suggest that whaitsioids had diversified by the early Wuchiapingian and no later than Pristerognathus Assemblage Zone times. Prior extinction estimates based on species counts are reflected in an analysis of origination/extinction rates, which imply increasing faunal turnover from Guadalupian to Lopingian (late Permian) times. The new records support a growing body of evidence that some key Lopingian synapsid clades originated near or prior to the Guadalupian-Lopingian boundary ca. 260-259 million years ago, but only radiated following the end-Guadalupian extinction of dinocephalians and basal therocephalian predators (long-fuse model). Ongoing collecting in older portions of the Teekloof Formation (e.g., Pristerognathus Assemblage Zone) will shed further light on early eutherocephalians during this murky but critical time in their evolutionary diversification., Competing Interests: The authors declare there are no competing interests.

Late Permian Karoo Basin tectonics in South Africa are reflected as two fining-upward megacycles in the Balfour and upper Teekloof formations. Foreland tectonics are used to explain the cyclic nature and distribution of sedimentation, caused by phases of loading and unloading in the southern source areas adjacent to the basin. New data supports this model, and identifies potential climatic effects on the tectonic regime. Diachronous second-order subaerial unconformities (SU) are identified at the base and top of the Balfour Formation. One third-order SU identified coincides with a faunal turnover which could be related to the Permo-Triassic mass extinction (PTME). The SU are traced, for the first time, to the western portion of the basin (upper Teekloof Formation). Their age determinations support the foreland basin model as they coincide with dated paroxysms. A condensed distal (northern) stratigraphic record is additional support for this tectonic regime because orogenic loading and unloading throughout the basin was not equally distributed, nor was it in-phase. This resulted in more frequent non-deposition with increased distance from the tectonically active source. Refining basin dynamics allows us to distinguish between tectonic and climatic effects and how they have influenced ancient ecosystems and sedimentation through time.

The relationship between dinosaurs and other reptiles is well established, but the sequence of acquisition of dinosaurian features has been obscured by the scarcity of fossils with transitional morphologies. The closest extinct relatives of dinosaurs either have highly derived morphologies or are known from poorly preserved or incomplete material. Here we describe one of the stratigraphically lowest and phylogenetically earliest members of the avian stem lineage (Avemetatarsalia), Teleocrater rhadinus gen. et sp. nov., from the Middle Triassic epoch. The anatomy of T. rhadinus provides key information that unites several enigmatic taxa from across Pangaea into a previously unrecognized clade, Aphanosauria. This clade is the sister taxon of Ornithodira (pterosaurs and birds) and shortens the ghost lineage inferred at the base of Avemetatarsalia. We demonstrate that several anatomical features long thought to characterize Dinosauria and dinosauriforms evolved much earlier, soon after the bird-crocodylian split, and that the earliest avemetatarsalians retained the crocodylian-like ankle morphology and hindlimb proportions of stem archosaurs and early pseudosuchians. Early avemetatarsalians were substantially more species-rich, widely geographically distributed and morphologically diverse than previously recognized. Moreover, several early dinosauromorphs that were previously used as models to understand dinosaur origins may represent specialized forms rather than the ancestral avemetatarsalian morphology.