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Xiphosurids are aquatic chelicerates widely viewed as examples of so-called 'living fossils' due to their apparent morphological conservatism and limited diversity since at least the Jurassic. However, earlier representatives were much more diverse and morphologically disparate. Particularly striking are hypertrophied genal spines and reduced thoracetrons of the Triassic austrolimulids, possibly related to their colonization of brackish or freshwater habitats. Here we describe Polonolimulus zaleziankensis gen. et sp. nov., a new austrolimulid genus from the Early Triassic of Holy Cross Mountains, Poland. Geometric morphometric analysis positions the new find among the morphologically most 'extreme' austrolimulids, extending the geographic range of those forms to Central Europe. A palaeobiogeographic reconstruction of Triassic xiphosurids reveals their surprisingly wide distribution already in Early Triassic, suggesting either an earlier dispersal in the Late Permian or a rapid diversification in the earliest Triassic. The reconstruction of most austrolimulid occurrences within or proximal to the shallow marine areas casts doubts on the hypothesis they inhabited fully freshwater palaeonvironments, which should be reinvestigated in the future. The new material further adds to the growing understanding of xiphosurid diversity and evolution in the early Mesozoic.

Archosauromorphs are a diverse clade of vertebrates that originated in the Early Triassic and persist today as crown birds (ornithodirans) and crocodilians (pseudosuchians). They exhibit markedly different life-history strategies, with ornithodirans characterized by rapid growth, and more crown-ward pseudosuchians with slower growth rates. Understanding how these patterns emerged requires further insight into growth dynamics among early archosauromorphs. We use synchrotron X-ray micro-computed tomography to investigate the bone microstructure in an assemblage of archosauromorphs from the Early Triassic locality Driefontein Farm 11 in the Olenekian-Anisian Langbergia-Garjainia subzone of the Cynognathus zone in the main Karoo Basin of South Africa. Our findings reveal that growth strategies among Early Triassic archosauromorphs already encompass both rapid and slow growth rates. We further deduce that the diversity in growth strategies observed in crown-ward archosaur lineages were not lineage-specific derived characteristics but rather inherited growth traits already present among their Early Triassic members.

The Permian-Triassic mass extinction represents the most severe loss of biodiversity in Earth history and profoundly reorganized terrestrial ecosystems. On land, this crisis was followed by a marked floral turnover, with herbaceous lycophytes dominating Early Triassic vegetation. Here we show that these pioneer (so-called disaster) taxa that rapidly colonized stressed post-extinction environments, possessed specialized physiological traits that promoted survival under extreme conditions. Independent phylogenetic analyses show that Early Triassic lycophytes are closely related to modern Isoetales, a group characterized by exceptional ecophysiological flexibility. Their carbon isotope signatures resemble those of extant Isoetes that use crassulacean acid metabolism (CAM) photosynthesis, indicating a similar physiological strategy in deep time. Coupling these results with climate simulations suggests that CAM photosynthesis could have conferred a substantial advantage under Early Triassic super greenhouse conditions. Together, our findings identify CAM physiology as a potential mechanism enabling plant survival and ecosystem recovery following Earth's largest mass extinction.

Climate highly influenced the distribution of major animal groups that emerged during the Triassic throughout the supercontinent Pangea. The earliest dinosaurs and associated assemblages (Carnian Stage, Late Triassic, ~230 Ma) are recovered along a paleolatitudinal climate belt spanning southern Pangea. However, developmental responses of different amniotic clades to climate across these environments remain unknown. Characterizing developmental variability, presumably driven in part by climatic variability within this belt, helps constrain the climatic ranges that shaped the earliest dinosaurs prior to their worldwide dispersal, while also providing insights into developmental plasticity of Triassic fauna. We analyzed the bone histology of five vertebrate taxa from the Carnian of Zimbabwe, including sauropodomorph and herrerasaurid dinosaurs, a gomphodontosuchine cynodont, a hyperodapedontine rhynchosaur, and a suchian archosaur (?aetosaur) as a proxy to assess probable climatic influence by comparing various growth strategies/metabolisms of these clades across southern Pangea. The dinosaurs are continuously growing individuals that exhibit well-vascularized woven-parallel and parallel-fibered cortical bone with no apparent growth marks. The cynodont is an immature specimen with a well-vascularized fibrolamellar matrix that has anastomosing vascular canals throughout the cortex and a single growth mark, indicating rapid growth. The rhynchosaurian femur and suchian tibia show moderate to poorly vascularized parallel-fibered and woven-fibered matrixes possessing multiple lines of arrested growth along with an external fundamental system, suggesting these were slow-growing individuals that had reached skeletal maturity at the time of death. Comparing this histology to that of similar assemblages from current-day Brazil, Argentina, and India (which fell along the same paleolatitudinal zone during the Triassic), the mid-continent (i.e., Zimbabwean) dinosaurs exhibit continuous uninterrupted growth in contrast to other histologically sampled dinosaurs from different localities along this climatic belt, whereas the Zimbabwean rhynchosaur and suchian exhibit patterns with more frequent cessation of growth than their South American counterparts. Slower-growing Zimbabwean taxa-presumably, more susceptible to climatic stressors-exhibit characteristics suggestive of a less resource-rich environment (e.g., frequent cessations, more interrupted growth) compared with rhynchosaurs and aetosaurs from South America. Taxa with faster growth rates and higher metabolic regimes (i.e., dinosaurs, cynodont) from this same assemblage apparently grew rapidly-and roughly continuously-throughout the year. This is consistent with an overall more arid but less seasonal climate in Zimbabwe compared to the signals recovered in Brazil, Argentina, and India.

Hyperodapedontine rhynchosaurs possessed a unique oral apparatus, long interpreted as an adaptation for processing abrasive and resistant plant material. However, the microanatomical and histological evidence supporting this interpretation remains poorly documented. Here, we investigate the maxillary tooth plates and dentition of Brazilian Late Triassic hyperodapedontines across ontogenetic stages using histological thin sections, scanning electron microscopy, and high-resolution computed microtomography, and compare them with Middle Triassic rhynchosaurs to assess functional implications. Hyperodapedontine exhibits clear dental adaptations for processing harder foods, including accelerated secondary dentin deposition and three special dental features (serrations, beaded edges, and flutes), which together enhance tooth functionality under intense wear. Patterns of secondary dentin deposition indicate a dietary shift from juveniles to adults, with adults exhibiting more structurally reinforced teeth. Osteohistology of the maxilla reveals an inner and mid cortex composed of woven-fibered bone and woven-parallel complexes undergoing extensive remodeling. The outer cortex on the occlusal surface is mainly composed of an avascular lamellar bone, except for the longitudinal groove floor and walls, which are formed by a nearly avascular parallel-fibered bone. The presence of an avascular to nearly avascular bone layer in the occlusal surface is consistent with a protective role against occlusal loading stress. Variation in the thickness of the parallel-fibered bone lining the longitudinal groove reveals that the medial region was structurally reinforced, indicating that the food breakdown would primarily occur in this portion of the maxillary groove-dentary blade occlusion system. Together, these characteristics reinforce previous hypotheses that Late Triassic Hyperodapedontinae were better adapted for processing tougher food resources than Middle Triassic rhynchosaurs.

The Anisian Stage (Middle Triassic) marks a pivotal interval for biotic recovery following the Permian-Triassic mass extinction, yet its environmental drivers remain poorly understood. This study presents carbon isotopes, mercury (Hg) concentrations, and Hg isotope data from Anisian (mainly Pelsonian substage) marine and terrestrial sections across the Tethys. Our results reveal a widespread Hg enrichment event that is synchronous with the Pelsonian negative carbon isotope excursion (PENCIE). Hg isotopic compositions support an origin from enhanced regional volcanism, coupled with contemporaneous terrestrial input. New high-precision zircon U-Pb ages (244.200 ± 0.037 Ma and 243.451 ± 0.049 Ma), integrated with chemostratigraphic and biostratigraphic data, constrain the duration of the PENCIE to ∼0.5 Myr. This interval coincided with major biodiversification, revealing that recovery occurred alongside environmental perturbations rather than in stable conditions as traditionally thought. These global disturbances in carbon and mercury cycles, linked to volcanism and terrestrial input, likely accelerated Middle Triassic biotic recovery.

Silesaurids are Triassic dinosauromorphs with debated phylogenetic affinities, recently recovered as a paraphyletic assemblage along the stem leading to ornithischian dinosaurs. Despite their uncertain relationships, silesaurids provide key insights into early dinosaur evolution. Most specimens are fragmentary and small, limiting understanding of their body size and ecological roles. Here, I describe an almost complete femur from the Middle Triassic of southern Brazil, representing the largest silesaurid known from South America and one of the largest worldwide, with an estimated femoral length of 227-269 mm. The new specimen demonstrates that silesaurids attained large body sizes in southwestern Gondwana, suggesting they functioned as primary to secondary herbivorous or omnivorous consumers within Middle Triassic terrestrial ecosystems. Comparison with coeval taxa indicates that these large forms were unlikely prey for most contemporaneous predators once fully grown. The discovery also highlights the widespread geographic distribution and temporal persistence of large silesaurids across Pangea, despite faunal turnovers and environmental events such as the Carnian Pluvial Episode.

Triassic pseudosuchians had highly diversified cranial morphologies. These archosaurs occupied diverse ecological roles, ranging from terrestrial predators and herbivores to semiaquatic ambush predators and possible waders. Here, we apply linear cranial morphometrics to assess possible convergences with other sauropsids from the clades Theropoda, Squamata, and Plesiosauria and discuss their paleoecology, based upon what was known for the aforementioned clades. Thus, a total of 34 specimens of Triassic pseudosuchians and theropod dinosaurs were added to a morphometrics dataset originally used to infer the ecology of the enigmatic theropod Spinosaurus aegyptiacus. Principal component analysis and permutational multivariate analyses of variance were used to evaluate morphospace occupation and group differentiation. Our results indicate that linear variables are capable of distinguishing semiaquatic and aquatic taxa from terrestrial taxa, but fail to distinguish between semiaquatic and fully aquatic taxa, and could not distinguish between the different niches of the sampled terrestrial taxa. Phytosaurs cluster close to marine squamates, Qianosuchus mixtus occupies an intermediate position between the phytosaurs and the crocodylomorphs, erpetosuchids plotted near varanids. Beaked pseudosuchians did not cluster close to beaked theropods, with Effigia okeeffeae overlapping with Baryonyx walkeri. The loricatans, and possible synonymous, Decuriasuchus quartacolonia and Prestosuchus chiniquensis overlapped. Some loricatans plot near ornithomimids, and Saurosuchus galilei cluster close to Acrocanthosaurus atokensis. We conclude that convergent morphologies do not necessarily reflect convergent function in Triassic pseudosuchians. The research of their paleoecology would benefit from the integration of morphology and function rather than morphology and convergence, especially with obscure taxa such as the poposauroids.

In order to clarify the heterogeneity of organic sources in the Upper Triassic fifth member of the Xujiahe Formation (T3x5) on the northeastern margin of the Sichuan Basin, three typical wells (DS2, LQ1, PA3) were selected. Using petrology, organic geochemistry, biomarkers, kerogen maceral analysis, and rock pyrolysis, the sedimentary environment and organic matter (OM) sources were investigated, leading to a multi-source OM enrichment model. Results distinguish three sedimentary facies and corresponding OM assemblages. Well DS2 (east) deposited in delta plain swamp microfacies; well PA3 (south-central) in delta front and littoral-shallow lake transitional facies; well LQ1 (mid-west) in littoral-shallow lake facies. These facies control multiple OM sources (higher plants, cyanobacteria, algae). OM type is ultimately determined by a ternary coupling of depositional environment, biological assemblage, and preservation conditions. DS2 is dominated by vitrinite (93%), no sapropelinite, low HI-typical type III kerogen from higher plants. PA3 shows coexistence of sapropelinite (35-49%) and vitrinite (45-53%), average HI 111 mg/g (range 0.9-279.7), indicating mixed type II and III kerogen with local algae-rich intervals. LQ1 is enriched in sapropelinite (41-45%) and liptinite (8-11%), hydrogen-rich components > 50%, average HI 126.7 mg/g, representing stable type II kerogen. The enrichment model thus controlled by multiple organic sources and facies: in the eastern delta plain swamp, oxidation and degradation formed hydrogen-poor type III OM; in the central transitional environment, fluctuations produced mixed type II-III; in the mid-western lacustrine facies, a stable reducing environment promoted hydrogen-rich type II source rocks. The humid tropical-subtropical Late Triassic climate provided abundant biological sources, and the sedimentary facies-through proximity to provenance, redox conditions, and organisms-ultimately determined OM composition.

The end-Triassic mass extinction (ETE) was triggered by Central Atlantic Magmatic Province (CAMP) volcanism, which released voluminous carbon dioxide, sulfur dioxide, and halogens into the atmosphere, affecting marine and terrestrial ecosystems, but the mechanism driving terrestrial crisis remains unclear. Here, we investigate two terrestrial Triassic-Jurassic sections in high- and low/middle- paleolatitudes, finding synchronous anomalies of mercury concentration, sulfur (S) isotopes, S-associated molecular fossils, and biomarkers (retene, pimanthrene, and coronene), indicating that peak volcanic sulfur deposition coincided with floral diversity loss and fern spikes, and intensified high-temperature wildfires over an interval of ~60,000 years. We propose that the pulse of maximum CAMP eruptions rapidly increased volcanic-S influxes (acid rain) to terrestrial basins, leading to catastrophic plant dieback during the ETE. The consequent creation of moisture-free biomass supplied fuel for increasingly frequent and widespread intense wildfires, occurring even on a global scale.

Sauropterygia (including Placodontia and Eosauropterygia) is the most species-rich group of marine reptiles in the Mesozoic. Panzhousaurus rotundirostris is a rare eosauropterygian from the early Middle Triassic (Anisian) marine deposits of Panzhou, Guizhou Province, China. Until recently, P. rotundirostris has been known only by two specimens, and its phylogenetic position within Eosauropterygia remains much controversial. A taxonomic revision of P. rotundirostrisis is provided here based on a comparative study of the type material and a new, juvenile specimen from the same locality and horizon as the holotype. The revised description accommodates significant changes to the reconstruction of P. rotundirostris, including the skull roof, circumorbital bones, and jaws. The juvenile status of the new specimen is well confirmed by a series of features (e.g., smaller body size, relatively larger orbit, unfused neural arches, and fewer carpal ossifications and caudal ribs). The wealth of new anatomical data for this species has facilitated a phylogenetic analysis, the results of which recover Panzhousaurus as a keichousaurid and favour the sister group relationships between Keichousauridae and Pachypleurosauridae. The topology provides new insights into the origin and evolution of Keichousauridae, a major clade of eosauropterygian marine reptiles endemic in the Middle Triassic of China.

Postosuchus kirkpatricki was a large pseudosuchian archosaur from the Late Triassic period in North America. It is among several pseudosuchians proposed to have had derived aspects of locomotor function such as bipedalism or digitigrady, rather than plesiomorphic quadrupedalism or plantigrady, but disputes and inconsistencies about these propositions remain. These lingering disputes need resolution in order to formulate broader inferences about the evolution of bipedalism, limb posture, athleticism, and the end-Triassic mass extinctions. Here, we use 3D musculoskeletal modelling to address the disputes via a deep critical review of available evidence via multiple methods. We conclude that it is uncertain if Postosuchus spp. was quadrupedal or bipedal, plantigrade or digitigrade, due to conflicting evidence. Our analyses also reconstruct pelvic limb musculature that was relatively three times as massive as that in a similar-sized Nile crocodile, whereas the caudofemoralis was smaller than expected due to the gracile tail of Postosuchus. Aspects of hindlimb myology and morphofunctional analyses of the hindlimb joints suggest a mix of traits that are plesiomorphic archosaurian, derived "rauisuchian" and singular for Postosuchus. Our extensive modelling procedure and synthesis of current evidence forms a foundation for future studies such as predictive simulations or ichnological evidence of locomotor function.

This paper includes a series of new records, nomenclatural changes and changes in stratigraphic and geographic ranges of Brachiopoda from the Triassic and Jurassic of New Zealand and New Caledonia. Following a suggestion in the Treatise on Invertebrate Paleontology, the Early Jurassic species Vincentirhynchia uitoeensis and Vincentirhynchiapomeyroli are assigned to the genus Prionorhynchia. Specimens of Aparimarhynchia dunrobinensis from the Kaka Point Fold Belt, South Otago are recorded and figured, and the time range of the genus Aparimarhynchia is reviewed. Two species of Norellidae are now recognised, and are tentatively placed in the subfamily Diholkorhynchiinae. Clavigera drotae is reviewed on New Caledonian material in University of Auckland collections. A single related specimen is recorded from Kawhia. The New Zealand material identified by Trechmann (1918) as Spiriferina fragilis is assigned to Punctospirella Dagys, 1974, of which S. fragilis is the type species. The use of Punctospirella cf. fragilis is recommended for the Zealandian material. A newly found spiriferinide from the Late Triassic (Otapirian / Rhaetian) of Marokopa, Kawhia Syncline is identified as Dispiriferina cf. davidsoni. A spiriferinide previously noted as Psioidiella from the Hettangian of New Caledonia is doubtfully identified as Spiriferina arakiwa. New records of Linguithyris ageri, Murihikurhynchia malingi and Herangirhynchia awakinoensis extend the stratigraphic and geographic ranges of these species.

Interplanetary dust particles are highly enriched with 3He implanted by the solar wind, enabling the reconstruction of extraterrestrial dust fluxes to Earth from sedimentary 3He records. However, the fluxes of extraterrestrial dust during periods predating the Cretaceous remain poorly constrained. In this study, we present the first comprehensive reconstruction of extraterrestrial 3He fluxes for much of the Triassic period (approximately 246-212 Ma) based on pelagic-bedded cherts preserved in Jurassic accretionary complexes in Japan. We reconstructed extraterrestrial 3He fluxes after correcting for dilution by radiolarian-derived biogenic silica. The records reveal long-term flux enhancements during the Ladinian and late Carnian (Tuvalian) intervals, reaching amplitudes approximately 2-3 times the background level and persisting for ∼3-4 Myr. These features are inconsistent with short-lived influx events, such as comet showers or asteroid breakups, and are instead comparable to long-term background variations in the Cretaceous and Cenozoic records. A sharp 3He flux peak at ∼215-216 Ma is accompanied by elevated 3He/4He ratios, indicating a sudden influx of fine-grained extraterrestrial material. The timing of this excursion closely coincides with the Manicouagan impact event (215.4 Ma), suggesting a link between asteroid disruption and enhanced dust production during the Triassic.

From the Lowermost Triassic locality Alisa quarry (Krasnoyarsk Territory, Tura village vicinity, Nidym Formation, Lower Triassic) insect-bearing strata of a yellowish-grey tuffaceous siltstone was located 0.5 m below the basalt flow that demarcates the Permian-Triassic border. Sixteen beetle specimens: five belonging to three Permocupedidae (Permocupedinae) species, Archicupes storozhenkoisp. nov., Cytocupes magnussp. nov., Tatarocupes brevicellularissp. nov.; one specimen belongs to new monotypic Asiocoleidae genus, Alisacupes dunaevigen. et sp. nov. Ten specimens were distributed between two Schizocoleidae genera: Uskatocoleus Rohdendorf 1961, U. turensissp. nov. and three species of Schizocoleus Rohdendorf 1961: S. frumentumsp. nov., S. maximussp. nov. and S. subtilissp. nov. Coleoptera composition in Alisa quarry locality is typical for the end Permian, showing no signs of rapid evolutionary changes at the Permian-Triassic boundary. In addition to insects, the Alisa quarry yielded abundant fern and conifer fossils, along with occasional fish scales and conchostracan shells. The site represents a paleoenvironment of shallow, ephemeral freshwater pools.

Cynodontia is an important subclade of Therapsida that first occurred in the late Permian. It includes extinct subclades which are the non-mammaliaform cynodonts and Mammaliaformes, with the latter ultimately giving rise to crown mammals. The systematics of non-mammaliaform cynodonts has been extensively studied and is relatively well-resolved, however, there are still many problematic taxa that are difficult to identify and place confidently into the cynodont phylogenetic tree. Cistecynodon parvus is one such taxon, known only from a single specimen, the holotype skull which was found in the Middle Triassic Burgersdorp Formation of South Africa, in strata assigned to the Trirachodon-Kannemeyeria Subzone of the Cynognathus Assemblage Zone. Over the past century Cistecynodon has been variously referred to Eucynodontia (which includes the two major subclades Probainognathia and Cynognathia) and to non-eucynodont cynodonts, without any emerging consensus. Here the holotype of Cistecynodon parvus (BP/1/2520) is re-described based on computed tomography (CT) digital reconstruction of the specimen. This new data is applied to determine a phylogenetic matrix which supports that C. parvus is a basal (non-eucynodont) cynodont. This basal position is reflected in the anatomy of its secondary palate, which is not closed despite the specimen being a subadult. The inner ear, trigeminal canal, parietal foramen and carotid foramina are uniquely derived, likely as adaptations to an obligate fossorial lifestyle, confirming the validity of the taxon and its inferred subterranean habitat.

The Upper Triassic is an important target interval for oil and gas exploration in the Qiangtang Basin and contains well-developed dark shales. Among these units, the Tumengela Formation coal-associated shale exposed in the Central Uplift Zone and its two sides is the most extensive. In this study, we selected the Tumengela Formation coal-associated shale in the Woruoshan and Chaiduochaka areas as the key research objects and systematically evaluated the quality and hydrocarbon generation potential of the coal-associated shale in the central part of the Qiangtang Basin based on previous achievements. Meanwhile, the microscopic characteristics, mineralogy, and elemental geochemistry of the typical shale in the Woruoshan area were conducted to understand its reservoir properties, adsorption properties, and formation mechanisms. The results show that the coal-associated shale in the central part of the Qiangtang Basin is primarily medium to poor source rocks. Among them, its quality in the Woruoshan-Dopulenai-Chalangla areas is relatively good, and well-developed source rocks are located locally. Organic matter is dominated by high to overmature type II2-III kerogen. In the Woruoshan area, coal-associated shale is characterized by high clay mineral content, medium brittle mineral contents, and low carbonate mineral contents, while the coal-associated shale in the Chaiduochaka area is characterized by high brittle mineral contents, medium clay mineral contents, and low carbonate mineral contents. In these shales, reservoir space is characterized by microscopic interlayer cracks, structural cracks, intergranular pores, erosion pores, and organic pores. The above coal-associated shale is formed in a warm-humid climate, in a low-salinity, oxidized water column, and associated with medium to high primary productivity level. High primary productivity, rapid sedimentation, and clay-mineral adsorption chiefly controlled the formation of these shales. Based on the organic matter abundance, type, maturity, thickness of the coal-associated shale in the central part of the Qiangtang Basin, and the paleogeographic pattern, we infer that the depression between Woruoshan and Jiangaidarina likely represents a favorable hydrocarbon area and a significant target for future oil and gas exploration. These findings provide valuable insights into the hydrocarbon potential and organic matter enrichment mechanism of coal-associated shale in a marine-terrestrial transitional environment.

Hyperodapedontine rhynchosaurs, a group of specialized herbivorous archosauromorphs, are restricted to the Late Triassic Lower Maleri Formation of the Pranhita-Godavari (P-G) basin, India and their remains are found mostly within the thick red mudstone unit, calcirudite and mixed mud-sand lithologies. A good number of specimens (~ 210) of these rhynchosaurs, belonging to a minimum of 30 individuals, had been collected historically since 1960s, dominantly from six key accumulation sites near Kothapalle, Bheemini, Narlapuram, Venkatapur, Achlapur and Rampur, Telangana, India. Classical taphonomic studies on these specimens indicated highly fragmentary, transported, weathered and altered bones. This implied significant effects of high energy fluvial transportation on the bones, punctuated by episodes of long subaerial exposures in floodplains. Here, multivariate and spatial statistics have been employed to objectively infer a compact taphonomic framework, conduct a robust taphofacies analysis and precisely reconstruct the palaeoenvironment for the entire rhynchosaur assemblage of the Lower Maleri Formation, P-G basin. Five taphofacies and three taphofacies-lithofacies association (TL1, TL2, TL3) are derived from the combined analyses. TL1 is the dominant association, with a mix of articulated and weathered remains preserved within intercalated parallel-laminated and massive sandstone, indicating considerable transport followed by rapid burial, accompanying the waning of high-energy pulses. TL2 is characterized by fragmented and abraded bones preserved in a finely laminated and stratified mudstone, with occasionally articulated elements, indicating deposition from waning floodwaters in abandoned channel settings. TL3 represents the inter-flood stability phase, where prolonged subaqueous exposure and low sedimentation rates facilitated biological encrustation and early cementation.

Oviparity was likely the plesiomorphic reproductive condition for non-mammalian Synapsida, the stem-mammal group. Yet, despite nearly two centuries of research, no definitive fossil eggs of late Palaeozoic or early Mesozoic synapsids have been discovered. Here, three perinate specimens of the dicynodont genus Lystrosaurus from the Early Triassic of the South African Karoo Basin are examined using high-resolution CT and synchrotron scanning. One specimen, NMQR 3636, displays a tightly curled posture suggestive of an in ovo position and completely lacks tusks. Crucially, the lower jaw symphysis remains unfused-a developmental trait found only in pre-hatching embryos of modern birds and turtles. No calcified eggshell is preserved, so the egg might have been soft and leathery. The large size of the reconstructed egg suggests a precocial, non-milk-feeding developmental strategy. As a non-cynodont synapsid, Lystrosaurus offers a rare and valuable glimpse into reproductive biology far removed from the mammalian crown group. Unlike the more derived, mammal-like cynodont Kayentatherium, whose egg size aligns with lactation, Lystrosaurus anchors the plesiomorphic condition deep within Synapsida. Its reproductive strategy may have played a crucial role in its resilience and ecological dominance following the end-Permian mass extinction.