Leandro Carlos Gaetano

Viviparity is a reproductive mode in which the embryo completes their development inside the female body. Present-day mammals, with the exception of monotremes, are viviparous, as opposed to the egg-laying reproductive mode of most vertebrates. Non-therian cynodonts are regarded as oviparous although direct evidence on the reproductive modes in extinct taxa is lacking. We identified, in a palaeohistological study, a neonatal line in two bones of the early probainognathian Chiniquodon theotonicus from the Late Triassic of South America. The body mass at birth is comparable to that observed in living placentals, and notably higher than the one registered for monotremes, marsupials, lepidosaurs, crocodiles, and birds. Our results reveal that viviparity is not exclusive of therians among synapsids, pushing back its recognition in the clade ~90–95Myr.

This paper analyzes the paleoneurology (cranial endocast and maxillary canal) of Massetognathus pascuali, an iconic non-mammaliaform cynodont from the early Late Triassic of South America, using Neutron Tomography. The application of neutron tomography holds the potential for uncovering more refined anatomical and quantitative data. The newly examined cranial endocast shows a forebrain with a tubular shape without an interhemispheric fissure, presence of a pineal body (with a closed parietal foramen), and a marked unossified zone. In comparison with a smaller, putatively juvenile specimen previously studied (PVL 4016), the new endocast exhibits a similar degree of encephalization, indicating little change in relative brain size between both ontogenetic stages. In the context of cynognathian brain evolution, M. pascuali maintained a low encephalization quotient, typical of early cynognathians, contrasting with the higher values of some Late Triassic taxa. The maxillary canal of M. pascuali is described here for the first time. It is considerably ramified, although slightly less than in some early cynognathians, following the general pattern of non-probainognathians and suggesting the absence of a flexible rhinarium or mobile vibrissae. By integrating endocast data with the maxillary canal, this study offers enhanced insights into the neurosensory ecology of M. pascuali, thereby deepening our understanding of its biology and ecological interactions. Graphical Abstract

The Chañares Formation in north-western Argentina has provided an impressive number of specimens of early archosauriforms, dicynodonts, and non-mammaliamorph cynodonts. However, there is a very low diversity of cynognathian cynodonts. Herein, we report on Pontognathus ignotus gen. et sp. nov., a small new traversodontid from the traditional Late Triassic Carnian levels of the unit. It differs from other traversodontids for lacking a maxillary platform lateral to the posterior postcanines and by presenting a unique combination of characters from the premaxilla, maxilla, and dentition (i.e., the precanine diastema; the placement of the paracanine fossa; the number of upper postcanines; the general outline of the upper postcanines crown in occlusal view; the shouldering between adjacent teeth; the expansion of the crown distolateral to the sectorial crest; the extension, limits and deepness of the occlusal basin; the distal ridge-like bulging at the base of the crown; the number and relative placement and development of the cusps). It is a relatively small taxon inferred to have occupied a specialized, small herbivorous/omnivorous role, which in this fauna is otherwise only shared by small individuals of the traversodontid Massetognathus pascuali.

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

Probainognathia is a derived lineage of cynodonts which encompass Mammalia as their crown-group. The rich record of probainognathians from the Carnian of Argentina contrasts with their Norian representation, with only one named species. Here we describe a new probainognathian, Tessellatia bonapartei gen. et sp. nov., from the Norian Los Colorados Formation of the Ischigualasto-Villa Unión Basin of Argentina. The new taxon, represented by a partial cranium with associated lower jaws, was analyzed through neutron and X-rays micro-tomography (μCT). The high-resolution neutron μCT data allowed the identification of a unique character combination, including features inaccessible through traditional techniques. We constructed the largest phylogenetic data matrix of non-mammalian cynodonts. The new species and its sister taxon, the Brazilian Therioherpeton cargnini, are recovered as probainognathians, closely related to Mammaliamorpha. We conducted the first quantitative paleobiogeographic analysis of non-mammalian cynodonts, focusing in probainognathians. The results indicate that Probainognathia and Mammaliamorpha originated in southwestern Gondwana (in the Brazilian Paraná Basin), which was an important center of diversification during the Triassic. Finally, the Chinese Lufeng Basin is identified as the ancestral area of Mammaliaformes. These new findings, besides adding to the knowledge of the poorly represented Norian cynodonts from the Los Colorados Formation, are significant to improve our understanding of probainognathian diversity, evolution, and paleobiogeographic history.