Puesto Viejo Group Research Articles

Age and Correlation of the Bone-Bearing Puesto Viejo Group, Triassic of Argentina

Age and Correlation of the Bone-Bearing Puesto Viejo Group, Triassic of Argentina

Osteohistology of non-mammalian therapsids from Puesto Viejo Group, Argentina

ABSTRACT The Puesto Viejo Group is an important unit for the knowledge Triassic faunas especially non-mammalian therapsids. This group crops out in the San Rafael Block, southwest Mendoza, Argentina. It is composed of the basal Quebrada de los Fósiles Formation (QF) which is overlaid by the Río Seco de la Quebrada Formation (RSQ). This report comprises tetrapod remains from the RSQ Formation at the Quebrada del Durazno area that includes, until now, exclusively therapsids. The aim of this study is to analyse the bone microstructure of cynodonts and dicynodonts to elucidate the histological features and growth patterns. Histological thin sections made in ribs and appendicular bones reveal a predominance of fibrolamellar bone tissue suggesting an overall fast bone growth, however, the presence of parallel-fibred bone suggest periods of slowing down of the growth. The existence of growth marks indicates periodic interruptions of growth, which is likely the expression of their ability to rapid respond to environmental changes. This contribution offers new insights into the bone microstructure of tetrapod assemblages from RSQ Formation during the Upper Triassic span.

Dicynodonts (Therapsida: Anomodontia) of South America

The dicynodont fauna of South America is comprehensively reviewed. Permian South American dicynodont body fossils are known only from the Rio do Rasto Formation of southern Brazil, and include a specifically uncertain representative of the trans-Gondwanan genus Endothiodon and the endemic Rastodon procurvidens. The majority of known South American dicynodonts are Triassic kannemeyeriiforms, with numerous records from Argentina and Brazil. Traditionally, the stratigraphically lowest Triassic dicynodont-bearing beds in South America were thought to be those of the Argentine Puesto Viejo Group. Historically considered Middle Triassic on biostratigraphic grounds, radioisotopic dating has instead suggested that they are at least partially Late Triassic. Re-examination of the dicynodont fossils from the Puesto Viejo Group provides evidence for taxonomic differentiation between its two component formations. The Quebrada de los Fósiles Formation contains two kannemeyeriiform morphotypes: a shansiodontid identified as the first South American record of Tetragonias and a kannemeyeriid (Kannemeyeria aganosteus sp. nov.). The overlying Río Seco de la Quebrada Formation contains the historically problematic “Kannemeyeria” argentinensis, here recovered as a kannemeyeriid but placed in a distinct genus, Acratophorus gen. nov. The South American endemic shansiodontid Vinceria andina is recorded only from the Argentine Cerro de las Cabras Formation, of uncertain age but possibly coeval with the Puesto Viejo Group. Definitive Late Triassic dicynodonts are well known from the Argentine Agua de la Peña Group and Brazilian Santa Maria Supersequence. The phylogenetic position and alpha taxonomy of the most abundant South American dicynodont, Dinodontosaurus, are problematic. Minimally, two species of this genus should be recognized (D. tener in Brazil and D. brevirostris in Argentina), but extensive variation in cranial morphology is present among the Brazilian specimens. Better-preserved material and more extensive study are required to determine whether this variation is taxonomically significant. In addition to the eponymous genus, the Brazilian Dinodontosaurus Assemblage Zone contains at least two stahleckeriids: Stahleckeria potens and Sangusaurus sp. No dicynodonts are known from the overlying Hyperodapedon Assemblage Zone in Brazil, but the correlative Ischigualasto Formation of Argentina yields the abundant stahleckeriid Ischigualastia jenseni. A single dicynodont genus (the stahleckeriid Jachaleria) is present in the South American Norian, and both nominal species (J. colorata from Argentina and J. candelariensis from Brazil) are here recognized as valid. Our results indicate that individual dicynodont species were more geographically and stratigraphically restricted than previously thought, but also that there is extensive overlap between South American and African assemblages at higher taxonomic (i.e., genus) levels.

Neoarthropitys gondwanaensis gen. nov. et sp. nov. from the Middle Triassic of Gondwana: an intermediate stage in the anatomical trend of Equisetalean stems

A new petrified stem from the Middle Triassic Quebrada de los Fósiles Formation (Puesto Viejo Group, San Rafael Basin) in Mendoza Province, central-western Argentina, is described. The specimen shows affinities with fossil and extant Sphenophyta, such as the presence of a medullar cavity and primary xylem with carinal canal. A new genus and species – Neoarthropitys gondwanaensis – is established based on the presence of a secondary xylem with interfascicular rays and fascicular wedges, and the development of a mesarch–endarch primary xylem. These anatomical features allow differentiating it from all previously known Equisetales taxa, showing an intermediate stage between Calamitaceae which is characterized by mesarch primary xylem and by presence of the secondary xylem, and the Equisetaceae having endarch primary xylem and lacking of secondary xylem. The presence of a mesarch–endarch primary xylem in the Triassic Argentinian specimen has phylogenetic value, appearing as an intermediate stage in the anatomical trends of Equisetales stems. Neoarthropitys gondwanaensis gen. nov. et sp. nov. has a set of equisetalean features not present in any other clade, supporting the idea that they were part of a distinct group. The presence of transfusion tracheid piths supports previous hypothesis that the plants recorded in the Quebrada de los Fósiles Formation grew in an ecologically disturbed environment by intermittent volcanic ash falls, in the floodplains of a high-sinuosity fluvial system under seasonally sub-humid climatic conditions.

New Middle Triassic Pollen Taxa of the San Rafael Basin, Mendoza Province, Argentina

New pollen grains are described from diverse palynological assemblages recorded from the Quebrada de los Fósiles Formation, lower unit of the Puesto Viejo Group, San Rafael Basin (Mendoza, Argentina), southwestern Gondwana. Two new pollen genera, Mendozapollenites gen. nov. and Crackipollenites gen. nov., are proposed. Also, three new species, Mendozapollenites variabilis sp. nov., Angustisulcites hexagonalis sp. nov. and Crackipollenites polygonalis sp. nov., as well as Crackipollenites sp. A and an indeterminate monosaccate pollen (Form A), are fully described and illustrated. The microflora contains: high proportions of sphenophyte and lycophyte spores; true ferns and bryophyte spores as minor components; non-taeniate pteridosperm bisaccate pollen grains; taeniate monosaccate and bisaccate pollen, scarce monosulcate and inaperturate grains, among other less frequent gymnosperm pollen; and the co-occurrence of chlorococcalean algae and fungal spores. The sedimentation of the Quebrada de los Fósiles Formation took place during the Ladinian (Middle Triassic), as was confirmed by recent radiometric dating of ignimbrites from the base and the top of the main profile of the unit. Thus, this is the first unequivocal and unique microflora recorded at present for the Ladinian in South America.

Middle Triassic freshwater green algae and fungi of the Puesto Viejo Basin, central-western Argentina: palaeoenvironmental implications

Diverse freshwater green algal microfossils and dispersed fungal spores among rich terrestrial palynofloras were recovered from the upper part of the Quebrada de los Fósiles Formation, Puesto Viejo Group, cropping-out in the fault-bounded San Rafael Basin, Mendoza Province, Argentina. Sedimentological data suggest that the vegetation recorded from this unit grew in floodplains of high-sinuosity fluvial systems when intense episodic volcanic activity provided great quantities of ash deposits to the sites. Small and temporary shallow lake deposits and thin peat ponds were recorded at two sections, yielding palynofloras composed of variable contents of spores, pollen, freshwater algae and fungal associations. The taxa described and illustrated herein consist of representatives of colonial Chlorophyta algae (Botryococcus), a diverse group of Zygnematales zygospores represented by the genera Lecaniella, Maculatasporites, Mehlisphaeridium, Ovoidites and the incertae sedis algae Leiosphaeridia and Retirotundia as well as representatives of fungi assigned to Microsporonites and Portalites. Two new species of Ovoidites are described herein, Ovoidites tripartitus and Ovoidites circumplicatus, and the new combination Ovoidites scissus is also presented. The terrestrial and aquatic vegetation grew associated with shallow water bodies in floodplain environments developed under warm temperate, strongly seasonal climatic conditions. The environment was stressed by volcanism that probably restricted the colonization and development of the ecosystem. The occurrence of algal microfossils in the Quebrada de los Fósiles Formation suggest shallow and episodic locally restricted aquatic environments inhabited by autochthonous freshwater green algae consistent with the sedimentological evidence. A Middle Triassic age is inferred from recent radiometric dating obtained from the basal and upper ignimbrite beds of the Quebrada de los Fósiles Formation indicate an age of 243.9 ± 2.0 Ma and 235.8 ± 2.0 Ma, respectively, thus mostly constrained to the Ladinian age. The composition of the fossil flora (palynoflora and macroflora) also indicates Middle Triassic age.Ana M. Zavattieri [amz@mendoza-conicet.gob.ar], Unidad de Paleopalinología, Departamento de Paleontología, Instituto Argentino de Nivología, Glaciología y Ciencias Ambientales (IANIGLA), CCT-CONICET-Mendoza, Av. A. Ruíz Leal s/n, Parque General San Martín, M5502IRA Mendoza, Argentina; Pedro Raúl Gutiérrez [pedroraulgutierrez@gmail.com], Museo Argentino de Ciencias Naturales “B. Rivadavia”, CONICET, Sección Paleopalinología, Área Paleontología. Buenos Aires, Argentina, Av. Ángel Gallardo 470, C1405DJR Buenos Aires, Argentina; Mariana Monti [mariana.monti@ypftecnologia.com.ar], YPF Tecnología S.A. (Y-TEC), Avenida del Petróleo s/n, entre 129 y 143, B1924CKU Berisso, Argentina.

The Large-Bodied Dicynodont Stahleckeria (Synapsida, Anomodontia) from the Upper Triassic (Carnian) Chañares Formation (Argentina); New Data for Triassic Gondwanan Biogeography

The non-marine Triassic displays distinct regional differences in tetrapod fossil assemblages even in adjacent regions, and these patterns have been hypothesized to reflect provincialism. For example, in the “Middle Triassic” of Gondwana, the Río Seco de la Quebrada Formation (Puesto Viejo Group) in western Argentina shares a number of taxa with the Cynognathus AZ of the Burgersdorp Formation (Karoo Basin) in South Africa. In contrast, the nearby Chañares Formation of northwestern Argentina is compositionally distinct and shows more affinities with the Dinodontosaurus AZ of the lower Santa Maria Formation in southern Brazil and the top of the upper Omingonde Formation of Namibia. These problems are exacerbated by recent radioisotopic dates from the Chañares Formation and the Puesto Viejo Group that suggest these units are actually Carnian in age. We provide new data for the biostratigraphy and biogeography of these units in the form of the first record of a stahleckeriine dicynodont from the Chañares Formation, an ulna referable to Stahleckeria. This new occurrence strengthens the correlation between Chañares, Santa Maria, and the top of Omingonde units but reinforces the differences with the Río Seco de la Quebrada and Burgersdorp units. Hypotheses for this provincialism include assemblages of different ages, distinct environments controlled by paleolatitude or paleotopography between basins that formed a barrier to faunal interchange.

A Middle Triassic macroflora from southwestern Gondwana (Mendoza, Argentina) with typical Northern Hemisphere elements: Biostratigraphic, palaeogeographic and palaeoenvironmental implications

A Triassic macrofloral assemblage is documented from the Quebrada de los Fósiles Formation, lower unit of the Puesto Viejo Group (Mendoza, Argentina), southwestern Gondwana. The impression-compression flora incorporates fertile and sterile remains of lycophytes, sphenophytes, and the first seed ferns recorded from this unit. Among the novel components, Ptilozamites, a typical Northern Hemisphere genus, is herein described for the first time from Gondwana. Ptilozamites longifolia sp. nov. consists of unipinnate leaves with an often forked rachis, with long, linear to sub-rectangular pinnae attached by their whole bases. The veins are distinct, inserted directly on the rachis, and run almost parallel to the margins, with few dichotomies occurring. The cuticle is thick, stomata constituted by two sunken guard cells surrounded by 4–6 subsidiary cells fused to form a ring-like structure. The association of Ptilozamites, Pleuromeia, Lepacyclotes, Equisetites and Neocalamites was common in many Middle and Late Triassic assemblages in Laurasia, whereas the Southern Hemisphere Triassic assemblages were characterized by the Dicroidium flora. A Middle Triassic age inferred from various elements constituting this macroflora is also concordant with the recent radiometric dates assigned to the unit. The lithofacies associations indicate that this flora grew and was deposited in the floodplains of a high-sinuosity fluvial system where volcanic ashes were deposited episodically. This could have affected the development of the flora by favoring only those plants able to cope with stressful environmental conditions, as exemplified by the fleshy stems and leaves with thick cuticles present in Pleuromeia and Ptilozamites.

Diagenetic analysis of tetrapod from the Upper Triassic, Puesto Viejo Group, Argentina

The Puesto Viejo Group crops out in the San Rafael Block, southwest Mendoza, Argentina. This group is an important unit for the knowledge theTriassic faunas. It is composed of basal Quebrada de los Fósiles Formation (QF) overlying by the Río Seco de la Quebrada Formation (RSQ). In this study, we report tetrapod remains from the RSQ Formation that includes, until now, exclusively therapsids. Non-mammalian therapsids predominated in terrestrial ecosystems during the Late Paleozoic and Early Mesozoic. Although cynodonts and dicynodonts have been studied from several perspectives, the relation between taxa and sedimentary context has been scarcely documented. This study is aimed to elucidate the diagenesis of tetrapods from the Puesto Viejo Group and its relation to depositional environment. Diagenetic features of therapsid bones from the RSQ Formation were analyzed through thin sections to interpret the degree of taphonomic alteration. Diagenetic processes inferred include substitution; fracturing; brittle deformation and different permineralization events. Combined analyses through scanning electron microscopy; energy-dispersive X-ray spectrometry (SEM-EDX); X-ray diffractometry (XRD) and petrographic studies reveals the substitution of hydroxyapatite by francolite. The presence of fluorine in some samples suggests a link between the elemental composition and depositional environments: floodplain and crevasse splay. Permineralization stages include infilling of vascular canals, trabeculae and fractures with iron oxides and iron carbonate minerals during the burial history. This contribution represents a relevant approach to elucidate the relation between diagenesis on tetrapods and their paleoenvironments, as exemplified in this case study from the Puesto Viejo Group during the Upper Triassic.

Lycopodiopsids and equisetopsids from the Triassic of Quebrada de los Fósiles Formation, San Rafael Basin, Argentina

In the late 1960s, rests of a low-diversity flora dominated by the lycopsid Pleuromeia were reported. The specimens were recovered from a creek near San Rafael, Mendoza Province, western Argentina, in the lower section of the Puesto Viejo Group, the Quebrada de los Fósiles Formation, considered as Early Triassic in age. Despite the importance of this flora with respect to the end-Permian mass extinction event, it was not yet described nor illustrated. Since then, the age of the Puesto Viejo Group has been revised and the Quebrada de los Fósiles Formation is now considered to be early Middle Triassic. The fossil plants are here first described and interpreted. The most common element, Pleuromeia cf. sternbergii, a species known worldwide that characterizes the post-extinction flora recovery, is accompanied by sphenopsids such as Neocalamites and Equisetostachys, and an herbaceous lycopsid. This community grew up associated with transient water bodies in flood plain environments that developed under warm temperate, strongly seasonal climatic conditions. The landscape was stressed by volcanism, and the conjunction of these factors probably restricted the colonization and development of a full ecosystem, as happened worldwide after the end-Permian mass extinction.

A new Late Triassic age for the Puesto Viejo Group (San Rafael depocenter, Argentina): SHRIMP U–Pb zircon dating and biostratigraphic correlations across southern Gondwana

The Puesto Viejo Group crops out in the San Rafael Block, southwest Mendoza, at approximately 35° S and 68°20′ W. It consists of the basal mainly grayish Quebrada de los Fósiles Formation (QF) overlying by the reddish Río Seco de la Quebrada Formation (RSQ). The basal unit includes both plant remains (pleuromeians and sphenopsids) and vertebrates (scattered fish scales, dicynodont synapsids and remains of an archosauriform). In contrast, the RSQ beds have yielded only tetrapods, although a more diverse fauna. The latter includes cynodonts as Cynognathus, Pascualognathus and Diademodon, and also dicynodonts (Vinceria and Kannemeyeria). Based on the assemblage of tetrapod taxa the bearing levels were correlated to the Cynognathus AZ of South Africa and thus referred to the Middle Triassic (Anisian). We obtained a SHRIMP 238U/206Pb age of 235.8 ± 2.0 Ma from a rhyolitic ignimbrite interdigitated between the QF and RSQ formations at the Quebrada de los Fósiles section. This new radiometric date for the Puesto Viejo Group suggests that the tetrapod fauna in the RSQ beds existed, instead, during the Late Triassic (early Carnian) some 10 Ma later than the currently accepted age. Two scenarios might explain our results: first, the Cynognathus AZ of South Africa is wrongly assigned to the lower Middle Triassic (Anisan) and should be considered younger in age, Late Triassic (Carnian); second, the relative age of the Cynognathus AZ of South Africa is correct but the inferred range of Cynognathus and Diademodon is incorrect as they were present during the Late Triassic (Carnian) at least in South America. In any case, this new date pose serious doubts about the validity of biostratigraphic correlations based solely on tetrapod taxa, a common practice for Triassic continental successions across Gondwana.

Preliminary Chemometric Study of Bone Diagenesis in Early Triassic Cynodonts from Mendoza, Argentina

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

The first spinicaudatan (Crustacea: Diplostraca) from Permo-Triassic continental sequences of South America and its palaeoecological context

Tassi, L.V., Monti, M., Gallego, O.F., Zavattieri, A.M. & Lara, M.B., 2013. The first spinicaudatan (Crustacea:Diplostraca) from Permo-Triassic continental sequences in South America and its palaeoecological context. Alcheringa 37, 187–199. ISSN 0311-5518. Cornia martinsnetoi sp. nov., the first spinicaudatan from Permian–Triassic continental sequences of southern South America, is described. The new species has characters diagnostic of the typical late Palaeozoic–Early Triassic subfamily Vertexiinae from Gondwana. This new taxon was found in tuffaceous siltstones in the upper part of the Quebrada de los Fósiles Formation (Puesto Viejo Group), which may span the Permo-Triassic boundary in Argentina. The lower and coarser-grained interval of the Quebrada de los Fósiles Formation was deposited in distal alluvial fans and low- to high-sinuosity rivers. The upper fine-grained interval of this formation, which hosts the spinicaudatans and associated fossils (ostracods, megaspores, fish scales), was deposited in shallow floodplain lakes. Intense volcanic activity influenced the sedimentary evolution of this succession and was accompanied by aridification and some climatic seasonality that likely generated stressful conditions for the biota. Previous biostratigraphic studies support an Early Triassic age for the beds containing Cornia martinsnetoi sp. nov. [Lara Vaz Tassi [larinhatassi@hotmail]com], Área Paleontología (Centro de Ecología Aplicada del Litoral—Centro Científico Tecnológico Nordeste—Consejo Nacional de Investigaciones Científicas y Técnicas), Casilla de Correo 128, 3400 Corrientes, Argentina; Mariana Monti [marimontivaldes@yahoo.com.ar], Departamento de Geología, Museo de Historia Natural de San Rafael—Parque Mariano Moreno s/n°, 5600 San Rafael, Mendoza, Argentina; Oscar Florencio Gallego [ofgallego@live.com.ar], Micropaleontología (Facultad de Ciencias Exactas y Naturales y Agrimensura—Universidad Nacional del Nordeste), Corrientes, Argentina and Área Paleontología (Centro de Ecología Aplicada del Litoral—Centro Científico Tecnológico Nordeste—Consejo Nacional de Investigaciones Científicas y Técnicas), Casilla de Correo 128, 3400 Corrientes, Argentina; Ana María Zavattieri [amz@mendoza-conicet.gov.ar], Laboratorio de Paleopalinología (Instituto Argentino de Nivología y Glaciología y Ciencias Ambientales—Centro Científico Tecnológico—Consejo Nacional de Investigaciones Científicas y Técnicas), Casilla de Correo 131, 5500 Mendoza, Argentina; Maria Belén Lara [lara.maria.belen@live.com.ar], Área Paleontología (Centro de Ecología Aplicada del Litoral—Centro Científico Tecnológico Nordeste—Consejo Nacional de Investigaciones Científicas y Técnicas), Casilla de Correo 128, 3400 Corrientes, Argentina. Received 3.5.2012; revised 22.8.2012; accepted 11.9.2012.

The Triassic dicynodont Vinceria (Therapsida, Anomodontia) from Argentina and a discussion on basal Kannemeyeriiformes

In 1969, Bonaparte described a new dicynodont, Vinceria andina, from the early Middle Triassic Cerro de las Cabras Formation of central-western Argentina (Potrerillos, Mendoza). Later, this author mentioned two additional specimens from the same beds, but they were never figured and/or described until now. Years later, another specimen identified as Vinceria sp. was briefly described from a different locality and stratigraphic level (Early-Middle Triassic Puesto Viejo Group, southern Mendoza). Vinceria has alternatively been considered to be related to the South African Kannemeyeria or to the shansiodontids from China, Tanzania, South Africa and Russia. Moreover, some authors considered Vinceria andina as a junior synonym of Kannemeyeria argentinensis. A recent phylogenetic analysis on Triassic dicynodonts, including Vinceria and the shansiodontids, resulted in a basal position of this group in relation to the other Triassic dicynodonts, the kannemeyeriiforms. Nevertheless, the interrelationships of Shansiodontidae remain poorly understood. This situation is partially due to the lack of accurate descriptions of the poorly known specimens. The present paper is an exhaustive analysis of all materials once referred to Vinceria, plus some undescribed specimens, thus providing detailed anatomical descriptions and accurate figures of one of the putative basal members of the clade Kannemeyeriiformes. As a result, Vinceria is considered as a valid taxon that includes two species, V. andina and V. vieja nov. sp. Comparisons among the species of Vinceria and other shansiodontids question the characters used to justify the monophyly of the group.

Support for an “A-type” Pangea reconstruction from high-fidelity Late Permian and Early to Middle Triassic paleomagnetic data from Argentina

[1] A major disparity is observed between the late Paleozoic-early Mesozoic apparent polar wander paths (APWPs) of Laurussia and Gondwana when the landmasses are re-assembled in a conventional “A-type” Pangea. This discrepancy has endured from the earliest paleomagnetic reconstructions of the supercontinent, and has prompted discussions of non-dipole paleomagnetic fields and alternative paleogeographic models. Here we report on a joint paleomagnetic-geochronologic study of Late Permian and Early to Middle Triassic volcanic and volcaniclastic rocks from Argentina, which demonstrates support for an A-type model, without requiring modification to the geocentric axial dipole hypothesis. New SHRIMP U-Pb and 40Ar-39Ar isotopic dating has reinforced the inferred age of the sequences, which we estimate at ∼264 Ma (Upper Choiyoi Group) and ∼245 Ma (Puesto Viejo Group). Field-stability tests demonstrate that the volcanic rocks are carrying early/primary magnetizations, which yield paleopoles: 73.7°S, 315.6°E, A95: 4.1°, N: 40 (Upper Choiyoi) and 76.7°S, 312.4°E, A95: 7.3°, N: 14 (Puesto Viejo). A comprehensive magnetic fabric analysis is used to evaluate structural restorations and to correct for magnetization anisotropy. The paleomagnetic results derived from volcaniclastic rocks are interpreted to be affected by inclination shallowing, and corrections are discussed. A comparison of these new results with the existing Permian-Triassic paleomagnetic data from Gondwana suggests the presence of widespread bias in the latter. We contend that such bias can explain the observed APWP disparity, at least for Late Permian-Middle Triassic time, and that alternative paleogeographic reconstructions or non-dipole paleomagnetic fields do not need to be invoked to resolve the discrepancy.

A new basal archosauriform diapsid from the Lower Triassic of Argentina

ABSTRACT The best-known South American Early Triassic archosauriform belongs to a putative proterosuchid briefly reported by José Bonaparte in 1981, collected from the Quebrada de los Fósiles Formation (Puesto Viejo Group, Argentina). This specimen consists of well-preserved natural external molds of a partial postcranium that preserve dorsal vertebrae, osteoderms, a dorsal rib, a possible gastralium, a chevron, a humerus, an ilium, two metapodials, and an ungual. We re-describe this specimen and identify autapomorphies that allow us to recognize Koilamasuchus gonzalezdiazi, gen. et sp. nov. The presence of an iliac blade with a slightly convex dorsal margin and with a maximum length more than 3 times its maximum height places Koilamasuchus within Archosauriformes. A cladistic analysis of basal Archosauriformes positions Koilamasuchus more crownwards than Proterosuchus, Sarmatosuchus, Fugusuchus, and Osmolskina, as the sister taxon of the clade that includes Erythrosuchidae and Archosauria. Proterosuchidae is found to be paraphyletic. The presence of an iliac preacetabular process, a pubic peduncle that forms an angle lower than 45° to the longitudinal axis of the ilium, and dorsal body osteoderms positions Koilamasuchus in Archosauriformes more crownwards than proterosuchids. Koilamasuchus is more basal than erythrosuchids within Archosauriformes because of the presence of dorsal ribs with a poorly developed proximal end. Koilamasuchus importantly increases the diversity of Archosauriformes during the biotic recovery following the Permo-Triassic mass extinction.

On the postcanine dentition of Pascualgnathus polanskii Bonaparte (Cynodontia, Traversodontidae) from the Middle Triassic of Argentina

The dental morphology of Pascualgnathus polanskii Bonaparte (Therapsida, Eucynodontia, Traversodontidae), from the Middle Triassic Río Seco de la Quebrada Formation (Puesto Viejo Group, Argentina), is described and compared with that of other basal traversodontid cynodonts. Albeit Pascualgnathus is frequently used in phylogenetic analyses, neither a detailed description nor drawings of its postcanines have been published so far. The upper postcanines of Pascualgnathus are transversely wide, rectangular in occlusal view, with a lingual cusp connected to the transverse ridge which is located in the center of the crown, and one main labial cusp followed by one posterior labial, both forming the labial margin. The lower postcanines, mostly worn out in the known specimens, are quadrangular until pc6 and then they are rectangular with the major axis anteroposteriorly oriented. They have a tall mesial border, possibly constituted by one labial and one lingual cusp, and a transverse ridge. After comparisons, the number of cusps in the sectorial, labial border of the upper gomphodont teeth, frequently used in phylogenetic analyses, would not necessary implies real homologies. For example, postcanine morphologies with one or more cusps anterior to the main labial cusp would not be homologous to morphologies with one or more cusps posterior to the main labial cusp, while resulting in a similar count of labial cusps.

Diademodon tetragonus Seeley, 1894 (Therapsida: Cynodontia) in the Triassic of South America and its biostratigraphic implications

ABSTRACT We report for the first time the presence of the cynodont Diademodon tetragonus Seeley, 1894 in the South American Triassic. The specimen, represented by a fragmented skull and lower jaws, was found in levels of the Río Seco de la Quebrada Formation (Puesto Viejo Group), Mendoza Province, Argentina. It is assigned to D. tetragonus based on its overall skull shape (narrow and elongated snout with a concave outline in dorsal view), the morphology of the jugal that forms most of the dorsoventral depth of the zygoma and shows a well-excavated external auditory meatus, and a postcanine series including circular outlined anterior teeth, ovoid gomphodont teeth in the middle, and posterior sectorial teeth. The association of this taxon with the cynodonts Cynognathus crateronotus and Pascualgnathus polanskii, along with comparisons to African Triassic assemblages, suggest an Early to Late Anisian age for the Río Seco de la Quebrada Formation. These levels are most likely correlated to the subzones B and C of the South African Cynognathus Assemblage Zone, where both Cynognathus and Diademodon are known. This discovery represents the fourth report of shared cynodont genera between allegedly Lower to Middle Triassic African and South American terrestrial faunas.