The Agua de la Peña Group of the Ischigualasto-Villa Unión Basin (northwestern Argentina) documents the evolution of archosauromorph assemblages in western Gondwana during the late Middle and Late Triassic. However, the South American archosauromorph record in the aftermath of the Permo/Triassic mass extinction (Early—early Middle Triassic) is remarkably scarce and restricted to isolated bones. Here, we describe a recently collected isolated second sacral vertebra and rib that represents one of the few fossils known from the Lower—Middle Triassic Tarjados Formation, the unit that underlies the Agua de la Peña Group. This specimen is identified as an archosauromorph because of the presence of a non-notochordal vertebra and a bifurcated distal end of the second sacral rib. A quantitative phylogenetic analysis places the new specimen as an archosauromorph more derived than protorosaurs. In particular, this specimen resembles Pamelaria, Prolacerta and early rhynchosaurs in the presence of a squared posterior projection of the bifurcated second sacral rib. The new specimen represents the first body fossil of a diapsid formally described in the Tarjados Formation and the oldest member of the group in the Ischigualasto-Villa Unión Basin. As a result, this specimen increases the high-level taxonomic richness of Archosauromorpha in South America in the aftermath of the Permo/ Triassic mass extinction.

The original material assigned to Laplatasaurus araukanicus Huene come from five different localities in northern Patagonia (Argentina) where the Upper Cretaceous (Campanian) lies exposed. This material includes several postcranial bones from multiple specimens and showing different conditions of preservation, often lacking anatomical overlap. The taxonomic status of the taxon is further obscured by the absence of quarry maps of the multiple localities (lacking the recognition of associated materials), and a proper designation of type material in its original description. After Huene, new material was assigned to this taxon, although none of it resolved the existing taxonomic issues. In 1979, the designation of a lectotype (one tibia and one fibula) was the first stage in the nomenclatural stabilization of the species. However, the assignment of the remaining material to L. araukanicus remained uncertain. Here we review all the material hitherto assigned to this taxon in order to clarify its taxonomic status. We also provide a re-description of the lectotype and discuss the taxonomic identification of material previously referred to this taxon. Lastly we include it for the first time in an updated phylogenetic data matrix. Laplatasaurus araukanicus is retained only for the lectotype. The material from Rancho de Ávila is referred to cf. Bonitasaura sp. because they share diagnostic features and are stratigraphically congruent. The remaining material is referred as Lithostrotia indet. A phylogenetic analysis nests Laplatasaurus within Titanosauria in a clade formed by ((Laplatasaurus Uberabatitan) (Bonitasaura (Futalognkosaurus Mendozasaurus))).

The small-sized Astrohippus stockii was a one-toed horse restricted to the latest Hemphillian (Hh4) NALMA (5.8–4.6 Ma). Although this species is relatively common in fossil localities of the United States and Mexico, much remains to be known regarding its feeding ecology. We applied a multidisciplinary approach to reconstruct the dietary behavior of A. stockii based on a sample from Arroyo La Carreta locality, latest Hemphillian of Guanajuato, central Mexico. Carbon isotope and meso- and microwear analyses were conducted. The δ¹³C isotope data indicate a diet consisting of both C3/C4 plants with a preference for C4 plants (average δ¹³C of -1.9‰ ± 1,3‰ [SD]), whereas the micro-(high number of scratches and a low number of pits) and mesowear (round to blunt cusps and low relief) signatures implied processing of abrasive sources (biogenic and abiogenic). The information about the feeding ecology of A. stockii from Guanajuato, Chihuahua, Florida and Texas showed that this North American horse had broad feeding habits, ranging from almost exclusive grazing to browsing. The dietary flexibility in this horse species is probably related to local environmental conditions, suggesting an adaptable dietary regime across its geographic distribution. This is the first study that uses three of the most widely applied analyses to reconstruct the feeding ecology of a Mexican fossil ungulate species.

The analysis of two-stiped dichograptids, largely of baltograptid and expansograptid taxa of the family Didymograptidae from the Floian of northwestern Argentina, provides insight into the evolutionary relationships of this group. The important proximal end characters clearly cluster the baltograptids with the derived genera Didymograptus and Aulograptus. One of the analyses provides the information that the genera Cymatograptus and Expansograptus share a common ancestor but are not closely related to the two-stiped Kiaerograptus and related taxa. Likewise, with the exception of some controversial taxa, the families Tetragraptidae and Didymograptidae can be readily differentiated, supporting previous interpretations that they represent monophyletic clades. Although the present analyses are poorly supported, they provide hypotheses that could be further tested based on new data on isolated dichograptid taxa based on isolated or relief-preserved material.

New graptolite-trilobite records provide relevant information for the biostratigraphic framework of the lower Paleozoic in western Gondwana. Aorograptus victoriae (T. S. Hall), together with Adelogratus cf. A. altus Williams and Stevens and Ancoragraptus bulmani (Spjeldnaes) are described for the first time in the Santa Victoria area. These species come from the upper beds of the Santa Rosita Formation, expanding the distribution of the A. victoriae Biozone. The complete succession of the Aorograptus victoriae, Araneograptus murrayi, Hunnegraptus copiosus, Tetragraptus phyllograptoides, T. akzharensis, Baltograptus cf. B. deflexus and Didymograptellus bifidus biozones is confirmed for the first time at a single locality in the Cordillera Oriental, indicating a late Tremadocian—late Floian age for the upper deposits of the Santa Rosita Formation and the Acoite Formation in the studied area. The co-occurrence of trilobites with the records of these graptolite biozones enables a better resolution of the upper boundary of the Notopeltis orthometopa Biozone and the definition of associations partially filling gaps in the succession of traditional trilobite biozones (Asaphellus cf. A. stenorhachis/Leptoplastides and Megistaspis (Ekeraspis) associations). It also contributes towards clarifying the distribution of some members of the well-known Thysanopyge Fauna (T. taurinus, T. victoriensis and T. clavijoi). Together with the proposed framework, we also discuss previous records of conodonts and key palynomorphs (particularly chitinozoans) related to graptolite occurrences. Additionally, we provide new insights on regional and intercontinental correlation with classic Early Ordovician sections in northwestern Argentina (e.g., Pascha-Incamayo, Los Colorados, Mojotoro range and Humahuaca area), southern Bolivia and Balto-Scandinavia.

Two new genera of late Pleistocene Mimosoideae from Argentina are described. The fossils were recovered from the Arroyo Feliciano Formation in the upper Río Gualeguay Basin, eastern Argentina. The anatomical features suggest a relationship with Cylicodiscus Harms for the first one and Abarema Pittier and Hydrochorea Barneby and J.W. Grimes for the second. Cylicodiscuxylon paragabunensis gen. nov. and sp. nov. is characterised by diffuse-porous wood, solitary vessels, in radial multiples and in clusters; alternate and vestured intervessel pits; vasicentric, aliform, lozenge-aliform and confluent axial parenchyma, strands of 5–11 cells, prismatic crystals in chains in chambered axial parenchyma cells; 1–3 (4) seriate rays and homocellular, vessel-ray parenchyma pits similar in size and shape to intervessel pits, and non-septate fibres. Abaremaxylon hydrochorea gen. nov. and sp. nov. has the following diagnostic features: diffuse-porous wood, solitary vessels, in radial multiples and in clusters; alternate and vestured intervessel pits; vasicentric, aliform and unilateral aliform, confluent axial parenchyma, strands of 4–16 cells; prismatic crystals in chains in chambered axial parenchyma cells; exclusively uniseriate and homocellular rays, and non-septate fibres. The fossils described herein represent the first report of legume woods from the Río Gualeguay Basin. The record of Cylicodiscuxylon in southern South America supports the existence of an old relationship with the African flora and Abaremaxylon reinforces the idea that Neotropical flora was widespread in the past. The analysis of fossil plants suggests that they grew under warm and humid to semi-arid climatic conditions.