Late Triassic Research Articles

Spatiotemporal evolution of single sandbodies controlled by allocyclicity and autocyclicity in the shallow-water braided river delta front of an open lacustrine basin

Abstract The spatial and temporal evolution of the sandbody architecture of shallow-water deltas in open lacustrine basins is controlled by the classification of allocyclicity and autocyclicity. On the southwestern margin of the Ordos Basin, a braided river system deposited a shallow-water delta in the Late Triassic Period. Based on the principle of sequence stratigraphy and the hierarchical analysis of reservoir architecture, the spatial and temporal evolution of individual sandbodies in the Chang 81 member of the Yanchang Formation in the Zhenbei Oilfield is interpreted by utilizing data from cores, wells and outcrops. The research ideas are as follows: large deposition scale architectural elements (first- to third-order cycles, as defined by Miall) of different sequence levels are affected by allocyclicity associated with changes in tectonic activity, provenance, and sea level, and small deposition scale architectural elements (fourth- to fifth-order cycles, as defined by Miall) of different sedimentary facies mainly consist of individual sandbodies that are affected by autocyclicity associated with lake-level changes caused by various river processes. Based on previous studies, the results are as follows. The sedimentary characteristics of shallow-water deltas have been verified by core and outcrop data. In addition, three ultrashort-term cycles are identified on the basis of boundary sequences and lithofacies’ sequences in the outcrop section of the Rui River, and three sedimentary evolution stages of the delta front are defined. Finally, according to well data, five types of architectural elements at the level of single sandbodies are identified. The vertical superimposition and lateral contact relationships of different architectural elements indicate that during the three sedimentary evolution stages, the hydrodynamics weakened, strengthened slightly, and finally weakened substantially. Among the 20 kinds of architectural element spatial combination patterns formed by single sandbodies, primary and secondary sandbodies have great potential for hosting remaining oil. In the process of architectural spatiotemporal evolution, the geometry and connectivity of the underwater distributary channel gradually weakened, and the spatial relationship between the underwater distributary channel and other architectural elements increased. This article proposes a new method for researching shallow-water deltas and has some guiding significance for exploiting the remaining oil in oil fields.

Onset of sedimentation near the Carnian/Norian boundary in the northwestern Sichuan Basin: New evidence from ammonoid biostratigraphy and zircon U[sbnd]Pb geochronology

Upper Triassic deposits formed at the onset of subsidence in the Sichuan foreland basin of South China, and may record a crisis of carbonate deposition related to the Carnian Pluvial Episode. However, there is no consensus yet on the precise age of these deposits in northwestern Sichuan. In this work, ammonoid biostratigraphy has been improved, and a U/Pb age from detrital zircons has been obtained from the Upper Triassic of northwestern Sichuan. New ammonoid taxa Sinotropites sichuanensis n. gen., n. sp. and Hadrothisbites hanwangensis n. sp. are described from the upper part of the Ma'antang Formation in Hanwang and Jushui area and are assigned to the uppermost Tuvalian (Anatropites spinosus Zone, Gonionotites italicus Subzone). Ammonoid and conodont biostratigraphy, combined with U/Pb concordant ages of 227.2 ± 1.1 Ma obtained from youngest detrital zircons, provide a robust constrain on the initial sedimentation phases of the foreland basin in northwestern Sichuan, and suggest that the terrigenous turnover was not related to the Carnian Pluvial Episode.

A large, pathological skeleton of Smilosuchus gregorii (Archosauriformes: Phytosauria) from the Upper Triassic of Arizona, U.S.A., with discussion of the paleobiological implications of paleopathology in fossil archosauromorphs

A large, pathological skeleton of Smilosuchus gregorii (Archosauriformes: Phytosauria) from the Upper Triassic of Arizona, U.S.A., with discussion of the paleobiological implications of paleopathology in fossil archosauromorphs

Aetosaur pes from the Upper Triassic of Krasiejów (Poland), with remarks on taxonomy of isolated bones

The incomplete, articulated, right pes of an aetosaur, extracted from the lower bone-bearing horizon of the Krasiejow Late Triassic site, is larger than any other, known from among the Aetosauria. Its individual bones resemble those of at least three genera: Desmatosuchus (astragalus), Typothorax (ungual phalanges), and Stagonolepis (metatarsals). This underscores the highly speculative nature of the taxonomical classification of isolated postcranial bones and makes it impossible to assign the limb to any particular genus. The phalangeal formula is 2-3-4-5-?; for the fifth finger: 2/3/4 is possible. The anatomy of the pes indicates adaptation for digging.

Chinlechelys from the Upper Triassic of New Mexico, USA, and the origin of turtles

Chinlechelys from the Upper Triassic of New Mexico, USA, and the origin of turtles

Mechanism of wellbore instability in continental shale gas horizontal sections and its water-based drilling fluid countermeasures

The wellbore instability in the horizontal sections of continental shale gas wells in the Ordos Basin is a major engineering and technical problem that restricts the exploration and development of the shale gas resources in the Yanchang Formation of Upper Triassic of Mesozoic in the Yanchang Block. To solve this problem, this paper analyzed the characteristics of mineral components in shale by means of X-ray diffraction. In addition, its physical and chemical characteristics, specific surface area and microstructure were analyzed. On this basis, a shale water-based drilling fluid system (PSW-2) of low free water activity based on nano plugging was developed. And it was applied on site at five horizontal wells in continental shale gas reservoirs to ensure the wellbore stability of their long horizontal sections. And the following research results were obtained. First, the Yanchang Formation shale in this area has a high content of clay mineral, and it is a fractured formation of weak expansion, easy dispersion and multiple bedding, so the wellbore instability here is the result of the comprehensive action of mechanical factors, physical and chemical factors, drilling mechanical disturbance, etc. Second, the shale has average pore diameter is 4.494–8.502 nm and is characterized by obvious capillary action, strong water absorption capacity and uneven hydration, which result in the decrease of local shale strength, so sudden collapse tends to happen easily. Third, the API loss of PSW-2 system is less than 2.8 mL, the rolling recovery rate is 95.15% (close to the recovery rate of oil-based drilling fluid 98.25%), the linear expansion rate is as low as 1.38%, and the wetting angle increases from 26° of dry rock samples to 56.5°. Fourth, the compressive strength of the system after immersion increases to 95.806 MPa, which is close to the original rock strength (110.70 MPa). It is concluded that this water-based drilling fluid system can effectively ensure wellbore stability by blocking the pores of shale with micro nano components, reducing the activity of liquid phase to increase the inhibition, and weakening the multi-element collaboration of the capillary imbibition effect of shale.

TRACKING DINOSAURS IN COARSE-GRAINED SEDIMENTS FROM THE UPPER TRIASSIC OF ARDÈCHE (SOUTHEASTERN FRANCE)

ABSTRACTCoarse-grained sediments deposited in high-energy environments are usually considered unfavorable to the preservation of fossil tracks. Here we report dinosaur footprints showing good physical preservation, despite being found in coarse-grained sandstones of alluvial origin from the Upper Triassic of Ardèche, southeastern France. The ichnoassemblage, dominated by Grallator isp., raises questions about the processes leading to the formation and preservation of tracks in coarse-grained sediments. The track-bearing surface is a medium- to coarse-grained quartz arenite that is microconglomeratic locally. The tracking surface grain size ranges from 0.2 to 2 mm and numerous pebbles are present. It is overlain by a succession of thin, intercalated layers of claystones and siltstones, themselves covered by a mix of siltstones and coarse-grained sandstones. We interpret this succession as a progressive decrease in energy due to channel migration culminating in channel abandonment, and the establishment of a lower energy setting where the tracking surface formed. Sedimentological and taphonomic observations indicate that the trackmakers walked on fine-grained layers (clay, silt) in which true tracks formed. The passage of the animals along the tracking surface deformed the older coarse-grained sand layers and resulted in the formation of the transmitted undertracks. The fine-grained layer helped record the pedal anatomy of the trackmakers and contributed to protecting the transmitted undertracks from destruction. Overall, we suggest that the fossil footprints were preserved by abiotic processes only, the main factor being the lithological contrast between successive sedimentary layers. The exceptional preservation of those relatively high quality undertracks in coarse-grained deposits contrasts sharply with the prevailing models of true track formation involving fine-grained sediments and microbial mats present in low-energy environments. This mode of undertrack formation may have been relatively frequent elsewhere but potentially overlooked in previous studies.

Comments on additional dinosaur specimens from the Janner site (Upper Triassic of the Paraná Basin), southern Brazil

The Janner site (Upper Triassic of Southern Brazil) has been intensely explored, yielding constant new findings in recent years. Among these some fragmentary dinosaur specimens still not published were sampled. In this contribution, we describe these specimens, which represent five individuals, in order to increase our understanding on the anatomy of the dinosaurs that compose the paleofauna of the Janner site. The specimen CAPPA/UFSM 0270 is comprised of a series of six articulated presacral vertebrae, plus two isolated vertebrae associated with a right femur. UFRGS-PV-1232-T comprises a sacrum and a left ilium. CAPPA/UFSM 0271 comprises a right humerus that lacks the middle portion of the diaphysis. CAPPA/UFSM 00272 and 2073 are both comprised of an isolated femur each. The fragmentary condition of the materials hampers any less inclusive assignation. However, the morphology of the elements resembles those of coeval dinosaurs. Furthermore, the new specimens increase our knowledge on the morphology of some structures poorly understood from the dinosaurs of Janner site, such as the hindlimb and axial skeleton. Keywords: Hyperodapedon Assemblage Zone, Carnian, Saurischia, Sauropodomorpha, Archosauria, Dinosauria. O sitio Janner (Triassico Superior do Sul do Brasil) tem sido explorado intensamente, produzindo constantes novos achados nos ultimos anos. Dentre estes, restos de dinossauros ainda nao publicados foram recuperados. Nesta contribuicao nos descrevemos estes especimes, que totalizam cinco individuos, com o objetivo de aumentar o entendimento sobre a anatomia e amostragem de dinossauros que compoem o conteudo paleofaunistico do sitio Janner. O especime CAPPA/UFSM 0270 e composto por uma serie de seis vertebras pre-sacrais articuladas, alem de duas vertebras isoladas e um femur direito. UFRGS-PV-1232-T corresponde a um sacro e um ilio esquerdo. CAPPA/UFSM 0271 compreende um umero direito que carece de sua diafise. CAPPA/UFSM 0272 e 0273 compreendem um femur isolado cada. A condicao fragmentaria dos especimes impede uma atribuicao taxonomica menos inclusiva. Porem, a morfologia dos elementos lembra a de dinossauros coevos. Alem disso, os novos especimes aumentam nosso conhecimento sobre a morfologia de estruturas ainda pouco conhecidas de dinossauros do sitio Janner, como o membro anterior e o esqueleto axial. Palavras-chave: Zona de Associacao de Hyperodapedon, Carniano, Saurischia, Sauropodomorpha, Archosauria, Dinosauria.

150 million years of sustained increase in pterosaur flight efficiency.

The long-term accumulation of biodiversity has been punctuated by remarkable evolutionary transitions that allowed organisms to exploit new ecological opportunities. Mesozoic flying reptiles(the pterosaurs),which dominated the skies for more than 150million years, were the product of one such transition. The ancestors of pterosaurs were small and probably bipedal early archosaurs1, which were certainly well-adapted to terrestrial locomotion. Pterosaurs diverged from dinosaur ancestors in the Early Triassic epoch (around 245million years ago); however, the first fossils of pterosaurs are dated to 25million years later, in the Late Triassic epoch. Therefore, in the absence of proto-pterosaur fossils, it is difficult to study how flight first evolved in this group. Here we describe the evolutionary dynamics of the adaptation of pterosaurs to a new method of locomotion. The earliest known pterosaurs took flight and subsequently appear to have become capable and efficient flyers. However, it seems clear that transitioning between forms of locomotion2,3-from terrestrial to volant-challenged early pterosaurs by imposing a high energetic burden, thus requiring flight to provide some offsetting fitness benefits. Using phylogenetic statistical methods and biophysical models combined with information from the fossil record, we detect an evolutionary signal of natural selection that acted to increase flight efficiency over millions of years. Our results show that there was still considerable room for improvement in terms of efficiency after the appearance of flight. However, in the Azhdarchoidea4, a clade that exhibits gigantism, we test the hypothesis that there was a decreased reliance on flight5-7 and find evidence for reduced selection on flight efficiency in this clade. Our approachoffers a blueprint to objectively study functional and energetic changes through geological time at a morenuanced level than has previously been possible.

3D models related to the publication: A new prozostrodontian cynodont (Eucynodontia, Probainognathia) from the Upper Triassic of southern Brazil

3D models related to the publication: A new prozostrodontian cynodont (Eucynodontia, Probainognathia) from the Upper Triassic of southern Brazil

Middle–Upper Triassic marine vertebrates of Mallorca (Balearic Islands, western Mediterranean)

ABSTRACT During the Triassic, many groups of predatory marine reptiles appeared and diversified. On Mallorca, fossils of marine reptiles have been found in Muschelkalk (Anisian–Ladinian) and Keuper (Carnian–Norian) facies. Here we describe an anterior caudal vertebra of a basal ichthyosauriform similar to Grippidia (Reptilia: Ichthyosauriformes) from the Ladinian carbonate ramps of the Muschelkalk of Mallorca, which fills a biogeographic gap for the group, hitherto only known from the eastern and western margins of Panthalassa. The most plausible hypothesis to explain the fact that the Mallorcan specimen is younger than all the other related species described so far is supported by a rather isolated condition of the carbonate ramps surrounding eastern Iberia during the Anisian, which might point to a relict character of the Mallorcan species, representing the youngest record of basal ichthyosauriforms to date. Moreover, an epiphysis of an indeterminate reptile and four vertebrae assigned to Nothosaurus sp. (Reptilia: Eosauropterygia) from the Keuper sabkhat provide a new record of the latter genus on Mallorca, which falls within its known distribution area. Therefore, the present work provides an exhaustive review of all the marine vertebrate fossils found so far in the Middle–Upper Triassic of Mallorca, some of them with remarkable palaeobiogeographical implications.

The importance of trace element analyses in detrital Cr‐spinel provenance studies: An example from the Upper Triassic of the Barents Shelf

AbstractInvestigations of sandstone provenance often involve U–Pb dating and chemical/mineralogical investigations of detrital minerals that are stable in sediments. As most stable detrital minerals are from felsic–intermediate rocks, investigations of the only mafic–ultramafic mineral considered stable in sediments, chromian spinel (Cr‐spinel), can reveal contributions from mafic–ultramafic sources. Cr‐spinel chemical compositions are tied to petrogenesis, making it possible to identify the nature of, and differentiate between, potential sources. Earlier detrital Cr‐spinel studies have focused on major and minor element compositions, however, the advent of laser‐ablation analytical techniques now allow routine mineral trace element analyses. Here, we integrate major, minor and trace element compositions of detrital Cr‐spinel from sandstones with a well‐characterised provenance from the Triassic (Anisian to Early Norian) Snadd and De Geerdalen formations of the Barents Shelf. The analysed Cr‐spinel compositions are depleted in the major element cations Fe3+, Al and Mg and enriched in Cr and Fe2+. Relative to MORB chromite, the minor and trace element data show high concentrations of Zn, Co and Mn, low concentrations of Ni and Ga and variable concentrations of Ti, V and Sc. The major element compositions of the detrital Cr‐spinel are similar to ophiolite‐associated Cr‐spinel, while the trace element compositions indicate a more complex petrogenesis influenced by metamorphic alteration. The compositional variations between sample locations are small, suggesting similar source rocks for the detrital Cr‐spinel throughout the study area. The most likely sources of the Cr‐spinel grains are metamorphosed ophiolite complexes in the Uralian Orogen, in accordance with earlier provenance studies. The novel addition of trace element compositions to detrital Cr‐spinel studies adds significant source‐sensitive information.

Astragalar anatomy of an early dinosaur from the Upper Triassic of southern Brazil

ABSTRACT The morphology of the ankle is a crucial topic towards our understanding of the evolutionary history of Archosauria. The morphology of these bones is directly correlated with posture, and subsequently this provides an important phylogenetic signal. The astragalar anatomy is widely adopted in phylogenetic studies of dinosaurs, however, this bone is largely missing from some of our geologically oldest dinosaurian specimens. This is the case for Buriolestes schultzi, an early sauropodomorph from the Upper Triassic (Carnian) of southern Brazil. Here, a new astragalus is reported from the type locality of B. schultzi. The presence of a markedly rimmed and elliptical fossa posterior to the anterior ascending process corroborates saurischian affinities for this new specimen. Moreover, the short anteromedial corner of the element differs from that of several other post-Carnian sauropodomorphs. Additionally, the absence of the astragalar posterior crest in this specimen also differentiates it from several theropods. Unfortunately, there are no autapomorphic traits that support a reliable alpha taxonomic assignation. Therefore, the specimen in question cannot be positively assigned to B. schultzi. Conversely, there are no significant differences between the new specimen and B. schultzi, which could alternatively be supportive of a taxonomic assignment based on topotypical principles.

Geochemical markers of paleoenvironments, weathering, and provenance in Permian–Triassic terrestrial sediments

ABSTRACTThis study compares several geochemical factors (major and minor rare earth elements, Ti/Al ratios, and chemical index of alteration, CIA, values) in the Early Triassic Cañizar Formation (Fm) (Buntsandtein facies) of E Iberia with those of adjacent Middle Permian and Middle Triassic units (Alcotas and Eslida fms, respectively). According to significant differences detected, it seems that most geochemical perturbation occurred during the Early Triassic. Variations in Ti/Al ratios suggest changes in source areas between the studied units and even within the Cañizar Fm. These provenance changes correlate with successive tectonic pulses during the opening and development of the Iberian Basin, as they can be linked to major sedimentary surfaces and unconformities, as well as major sedimentological variations. Ti enrichment in the lower and middle part of the Cañizar Fm, together with high Sr and P concentrations, may be indicative of environmental alterations related to acid meteoric waters. Moreover, this acid alteration took place under arid conditions as reflected by CIA values, indicating that during the deposition of the Cañizar Fm, variable but predominantly physical weathering prevailed in contrast to the chemical weathering that took place when the Alcotas and Eslida formations were deposited. Our data along with the known fossil record of the study area indicate that during the Middle–Late Permian and Early Triassic, conditions in this tectonically active area changed from humid to arid-acid, hampering biotic recovery. Then, during late Early–Middle Triassic times, the return of more humid and less acid environments promoted biotic development. Geochemical markers emerged as useful tools complementary to sedimentological, paleontological, and tectonic data for unveiling paleoenvironmental events, especially in a setting of significant regional change.

Origin of hydrocarbon and non-hydrocarbon (H2S, CO2 and N2) components of natural gas accumulated in the Zechstein Main Dolomite carbonate reservoir of the western part of the Polish sector of the Southern Permian Basin

The origin and migration of hydrocarbon and non-hydrocarbon (hydrogen sulphide, carbon dioxide and molecular nitrogen) components of natural gas accumulated within the Zechstein PZ2 (Stassfurt) Main Dolomite (Ca2) carbonate reservoir in the western part of the Polish sector of the Southern Permian Basin (SPB) were established based on molecular and isotopic compositions (δ13C[CH4, C2H6, C3H8, n-C4H10, i-C4H10, n-C5H12, i-C5H12 and CO2], δ2H[CH4], δ15N[N2] and δ34S[H2S]) related to geological settings and 1-D modelling of generation processes. Hydrocarbon gases were generated during both microbial and thermogenic processes. Primary microbial processes ended at the turn of the Late Permian and Early Triassic, when oil and thermogenic gas generation processes began. The reconstruction of the thermal evolution and transformation of organic matter in this part of the SPB shows that thermogenic processes ended in the Late Jurassic in the Grotów Peninsula (Zone C), and in the Late Cretaceous in the western part of the Wielkopolska Platform and Gorzów Platform (zones A and B). The presence of the microbial component in analysed accumulations indicates that traps within the Ca2 carbonate strata were formed and sealed by the overlying impermeable anhydrites and salts of PZ2 (Stassfurt), PZ3 (Leine) and PZ4 (Aller) cycles preventing its escape. Accumulation of natural gas in Ca2 traps took place in at least two stages and had a cumulative character. First, microbial gases accumulated, then thermogenic gases associated with oil were generated from type II kerogen in the “oil window”. An insignificant methane component was generated from type III kerogen and migrated to Ca2 reservoir from Carboniferous strata. H2S was most probably generated during both microbial (MSR) and thermochemical sulphate reduction (TSR) of anhydrites in the PZ1 (Werra) cycle, and additionally from thermal decomposition of organic sulphur components of oil and kerogen in the Ca2 carbonate rocks. MSR ended during the Late Permian. TSR was temporarily halted as a result of Early Cretaceous uplift and probably resumed on a minor scale after the Late Cretaceous. CO2 was predominantly generated during thermogenic processes of kerogen decarboxylation, carbonate dissolution processes, and TSR and MSR together with H2S. A lesser contribution of CO2 from microbial fermentation was also observed. To a large extent N2 was mainly generated during thermal transformation of type II kerogen dispersed in the Ca2 carbonate strata and probably partly migrated from Carboniferous high-maturity type III kerogen to Ca2 reservoirs through fault systems.

Earthquake‐induced soft‐sediment deformation structures from the Upper Triassic of the Jiyuan district, southern North China Craton: Implications for the Qinling Orogeny

Soft‐sediment deformation structures are abundant in the upper part of the Upper Triassic Tanzhuang Formation in the Jiyuan district, southern North China Craton. These deformation structures include large load casts, flame structures, ball‐and‐pillow structures, lenticular sand bodies, synsedimentary faults, and slump structures. The lenticular sand bodies, synsedimentary faults, and slump structures developed in the deep lake environment, while large load casts, flame structures, ball‐and‐pillow structures occurred in the deltaic environment. Based on the geometrical characteristics of the soft‐sediment deformation structures and criteria for recognizing the effects of seismic events, we exclude two major autogenic triggers (including slumping and sediment overloading) and infer that a seismic event was responsible for triggering deformation. Seismic activity was likely associated with activity on the Luanchuan Fault in the Qinling Orogen. The large size and complexity of the soft‐sediment deformation structures, together with their long distance from the inferred epicentre, indicate a high‐magnitude earthquake that is associated with the intensive uplift in the orogenic setting. Comparison of our data with soft‐sediment deformation structures from the Ordos Basin and the Yima district suggests that the Qinling Orogen was subjected to at least two phases of uplift during Late Triassic orogenesis.

Fossil dipterid fern Thaumatopteris in China: new insights of systematics, diversity and tempo-spatial distribution patterns

ABSTRACT Thaumatopteris (Goeppert) Nathorst is a typical dipterid fern in the early Mesozoic, showing global significances for palaeoclimate, phytogeography and biostratigraphy. However, the detailed understanding on the relationship between species diversity and the environmental background of this genus is still unclear. Here, we reinvestigated new collections and type specimens of Thaumatopteris contracta Li et Tsao collected from the Upper Triassic of Yunnan Province, and analysed the systematic, diversity variations and tempo-spatial distribution patterns of Thaumatopteris in China. An emendation was proposed for species diagnosis of Thaumatopteris contracta based on both sterile and fertile pinnae. Our analysis demonstrates that the genus Thaumatopteris reached a higher diversity and differentiation with 11 species during the Late Triassic in China and suffered a severe loss of species diversity across the Triassic-Jurassic transition. Changes in the spatial distribution pattern of Thaumatopteris demonstrate an ecological response to local environment degradation in the Upper Yangtze region. The destruction of habitat and the block of the transport channel of warm moist flow from the Tethys Ocean caused by the Indosinian tectonic movement, especially, the uplift of the Longmen Mountains may represent the potential environmental trigger.

To be or not to be: The Hidden Side of Cargninia Enigmatica and Other Puzzling Remains of Lepidosauromorpha from the Upper Triassic of Brazil

ABSTRACT In this contribution, we re-describe the holotype (i.e., the posterior fragment of a left dentary with dentition) of Cargninia enigmatica from the Riograndia Assemblage Zone, Candelária Sequence, Santa Maria Supersequence (Upper Triassic, Brazil), originally considered a member of the Lepidosauria. In addition, two other specimens of lepidosauromorphs from the same locality are described and compared. Broad comparisons suggest that C. enigmatica possesses a unique combination of features, but several key features are widely distributed among basal non-rhynchocephalian lepidosauromorphs and kuehneosaurids. Thus, two possible scenarios are proposed: (1) Cargninia is a basal non-lepidosaurian lepidosauromorph, or (2) it is a basal lepidosaurian. The new specimens are a portion of a right maxilla with teeth (in two fragments) and a fragment of a dentary with teeth. The first specimen is identified as a lepidosauromorph because the maxilla cannot be directly compared with the holotype of C. enigmatica and it is difficult to evaluate if the differences in the dentition (upper versus lower teeth) are the result of intra- or interspecific variation. The dentary, on the other hand, is tentatively referred to C. enigmatica. These specimens together add new clues to understanding the early evolution of Lepidosauromorpha.

Birth and death of seamounts in the Panthalassa Ocean: Late Triassic to Early Jurassic sedimentary record at Mount Sambosan, Shikoku, Southwest Japan

Abstract Mid-oceanic shallow water carbonate platforms are usually characterized by long-term deposition due to the continuous subsidence of the underlying volcanic edifices and thus provide good records of biotic evolution and environmental changes. However, since most of the pre-Jurassic oceanic crust has been subducted, very few remains of Triassic atoll-like platforms are preserved in the Circum-Pacific region. Therefore, it is crucial to precisely document these Triassic carbonate occurrences to better understand the global evolution of climates and ecosystems during the Early Mesozoic in the huge Panthalassa Ocean. In the Sambosan Accretionary Complex (Southwest Japan), several Upper Triassic shallow water limestone blocks are especially well exposed at Mount Sambosan in Shikoku Island. Microfacies analyses associated with biostratigraphic data (reef assemblages, foraminifer associations and conodonts) allow us to reconstruct the geological history of these seamount fragments from the first settlement of reef biota in the Ladinian-Early Carnian to platform growth during the Late Carnian to Rhaetian. Finally, the demise of the carbonate platform occurred during the Latest Triassic to Early Jurassic, followed by platform collapse and accretion to the East Asia margin in the Late Jurassic-Early Cretaceous. Furthermore, cathodoluminescence microscopy reveals that the Sambosan limestone experienced several diagenetic episodes including early cementation, dissolution and dolomitization. At the global scale, the comprehensive sedimentary record at Mount Sambosan shows that evolution of shallow water ecosystems in the Panthalassa Ocean is consistent with biotic and environmental changes documented in coeval carbonate platforms from the Tethys Ocean.

A New Prozostrodontian Cynodont (Eucynodontia, Probainognathia) from the Upper Triassic of Southern Brazil

ABSTRACT Probainognathian cynodonts are well represented in the fossil record from the Middle and Upper Triassic of South America, especially in Brazil and Argentina. In this contribution, we describe a new genus and species of non-mammaliaform prozostrodontian cynodont from southern Brazil. The new taxon comes from the Niemeyer Site, a locality in which the traversodontid cynodont Siriusgnathus niemeyerorum is numerically dominant, whereas probainognathians and other tetrapods are comparatively scarce. The fauna from the Niemeyer Site was putatively assigned to the Riograndia Assemblage Zone (Norian age) recently, although none of the index fossils for that biozone (e.g., Riograndia, Clevosaurus, Jachaleria) have so far been discovered at this locality. The new cynodont taxon is based on a left lower jaw with the canine and six (pc2–pc7) well-preserved postcanines (CAPPA/UFSM 0262, holotype), and a second referred specimen (CAPPA/UFSM 0208, paratype), which includes a right lower jaw with incisors, canine, and seven (pc1–pc7) postcanines, with pc6–pc7 being the best preserved. These specimens have a robust dentary, a long and dorsoventrally tall Meckelian groove, unserrated canines, and unserrated, sectorial postcanine teeth with posteriorly inclined cusps and a poorly developed lingual cingulum. This combination of features is unknown in other Carnian and Norian non-mammaliaform cynodonts. The new taxon contributes to our knowledge of the evolutionary radiation of small prozostrodonts that occurred in western Gondwana during the Late Triassic and led to the emergence of several important cynodont groups, including Mammaliaformes. http://zoobank.org/urn:lsid:zoobank.org:pub:FDC21881-532B-490A-96A1-A65EA5F62737