We present a detailed description of an incomplete but excellently preserved braincase and a digitally generated endocast of a pseudosuchian archosaur from the Upper Triassic (Carnian) Evangeline Member of the Wolfville Formation of Nova Scotia (Canada). The general morphology of the braincase particularly resembles that of the rauisuchid Postosuchus kirkpatricki from the Upper Triassic (Norian) of Texas. Indeed, the quantitative phylogenetic analyses found the Wolfville specimen within Rauisuchidae (Postosuchus spp., Polonosuchus silesiacus, and Rauisuchus tiradentes) among loricatan pseudosuchians based on the presence of a deep and dorsoventrally elongate basisphenoid recess. Although the specimen described here shows differences with Postosuchus kirkpatricki and Postosuchus alisonae, its fragmentary condition and the absence of overlapping elements with other rauisuchids (Polonosuchus silesiacus and Rauisuchus tiradentes) led us to be cautious and identify it as aff. Postosuchus sp. The Wolfville rauisuchid braincase provides evidence for the presence of this clade in low paleolatitudes during the Carnian, bridging the higher paleolatitude occurrences of Rauisuchidae in southern Brazil and in Poland, respectively.

Here we explore the potential of sauropod dinosaurs to adopt a bipedal or tripodal stance using digital biomechanical modeling and Finite Element Analysis (FEA). Seven sauropod species from diverse lineages and sizes were sampled, and 3D models of their femora were analyzed under both extrinsic (body weight distribution) and intrinsic (muscular force) functional scenarios. The results indicate that smaller sauropods, like the saltasaurid titanosaur Neuquensaurus, were more capable of sustaining bipedal postures, likely due to their robust femora combined with advantageous muscle attachment areas. In contrast, larger sauropods such as Dreadnoughtus experienced higher stress levels, making bipedal postures less likely for extended periods. Our analysis provides new insights into sauropod functional evolution, highlighting that species size and morphology significantly influenced their ability to rear up, which could have played a role in behaviors such as feeding, defense, and reproduction.

Mesozoic mammals from Gondwana remain poorly understood, with most species known only from isolated teeth. Nevertheless, these tantalizing fossils point to a diversity of families, that were distinct from Laurasian forms. Among these, the enigmatic South American family Reigitheriidae, was proposed based on the unique crushing teeth of its sole representative, Reigitherium from Northern Patagonia. Here, we report a new mammal, Yeutherium pressor gen. et sp. nov., which is only the second known taxon of Reigitheriidae. This is based on a partial maxilla bearing an upper molar from the Late Cretaceous Dorotea Formation in the Río de Las Chinas Valley, Sub-Antarctic Chile. Parsimony and Bayesian phylogenetic analyses indicate a close relationship between Reigitherium and Yeutherium. Shared features between both taxa—such as expanded pre- and postparacristae, a lingual cingulum at the paracone base, interradicular crests, and enamel crenulations—help define Reigitheriidae. Cingula development and molar morphology of Yeutherium differ from that of the related Mesungulatidae, suggesting independent evolution of the crushing adaptations in Reigitheriidae. Additionally, we suggest a reinterpretation of the dental formula of Reigitheriidae. The discovery of Yeutherium highlights the specialised dentition of Reigitheriidae as a significant functional innovation in a radiation of Gondwanan mammals that is starting to be understood.

Batesian mimicry is an impressive example of convergent evolution driven by predation. However, the observation that many mimics only superficially resemble their models despite strong selective pressures is an apparent paradox. Here, we tested the ‘perfecting hypothesis’, that posits that inaccurate mimicry may represent a transitional stage at the macro-evolutionary scale by performing the hereto largest phylogenetic analysis (in terms of the number of taxa and genetic data) of ant-mimicking spiders across two speciose, but independent clades, the jumping spider tribe Myrmarachnini (Salticidae) and the sac spider sub-family Castianeirinae (Corinnidae). We found that accurate ant mimicry evolved in a gradual process in both clades, by an integration of compound traits contributing to the ant-like habitus with each trait evolving at different speeds. Accurate states were highly unstable at the macro-evolutionary scale likely because strong expression of some of these traits comes with high fitness costs. Instead, the inferred global optimum of mimicry expression was at an inaccurate state. This result reverses the onus of explanation from inaccurate mimicry to explaining the exceptional evolution and maintenance of accurate mimicry and highlights that the evolution of Batesian mimicry is ruled by multiple conflicting selective pressures.

South American savannas are a disjunct biome with an unclear evolutionary history. We tested hypotheses about their Quaternary history and evolution of savanna cores through fragmentation or dispersal from the Cerrado. We used genomic data (genotyping-by-sequencing) and ecological niche models of the Burnished-buff Tanager (Stilpnia cayana Linnaeus 1766) to evaluate intraspecific differentiation, gene flow, past range shifts, and landscape-genomics association. We found clear genomic differences between populations on each side of the Amazon basin and high admixture in the Marajó Island and Bolivia. Landscape genomics analysis indicated that the Amazon River, isolation by distance and temperature predict genomic differentiation in this bird. Taken together, the results suggest that a combination of dispersal from the Cerrado, isolation due to geographic distance and the Amazon River basin, and local adaptation shaped the species diversification. We propose that the populations on each side of the Amazon River be considered distinct species (S. cayana at the north and S. flava at the south) with subspecies huberi representing part of a hybrid zone between them, located on Marajó island at the mouth of the river.

The Rufous-collared Sparrow (Zonotrichia capensis) shows phenotypic variation throughout its distribution. In particular, the Patagonian subspecies Z. c. australis is strikingly distinct from all other subspecies, lacking the black crown stripes that characterize the species, with a uniformly grey head and overall paler plumage. We sequenced whole genomes of 18 individuals (nine Z. c. australis and nine from other subspecies from northern Argentina) to explore the genomic basis of these color differences and to investigate how they may have evolved. We detected a single ~465-kb divergence peak on chromosome 5 that contrasted with a background of low genomic differentiation and contains the ST5 gene. ST5 regulates RAB9A, which is required for melanosome biogenesis and melanocyte pigmentation in mammals, making it a strong candidate gene for the melanic plumage polymorphism within Z. capensis. This genomic island of differentiation may have emerged because of selection acting on allopatric populations or against gene flow on populations in physical and genetic contact. Mitochondrial DNA indicated that Z. c. australis diverged from other subspecies ~400,000 years ago, suggesting a putative role of Pleistocene glaciations. Phenotypic differences are consistent with Gloger’s rule, which predicts lighter colored individuals in colder and drier climates like that of Patagonia.

Sebecid notosuchians are the only terrestrial crocodyliforms to survive the Cretaceous–Paleogene extinction, 66 Ma, which eliminated large-bodied species (~>5 kg) in terrestrial ecosystems. Early sebecid evolution is unclear due to the scarcity of remains from both sides of the boundary. We present the stratigraphically earliest post-extinction notosuchian record, from the lower Paleocene Salamanca Formation of Patagonia. Tewkensuchus salamanquensis n. gen. n. sp. has unique features, including a skull roof with elevated lateral margins, and an accessory peg and socket articulation between the postorbital and posterior palpebral. Our phylogenetic analysis allies Tewkensuchus with a clade of predatorial crocodyliforms from the Eocene of Europe (and possibly of Africa, as Eremosuchus may belong to this clade). This clade forms the sister taxon of South American sebecids. We name Sebecoidea for this more inclusive clade of Eurogondwanan notosuchians and suggest that its spatial distribution reflects earlier diversification and dispersal events, which are only partially known. We estimate a body mass of ~300 kg for Tewkensuchus, one of the largest known notosuchians. Phylogenetic optimization of notosuchian body size change reconstructs a Cretaceous–Paleogene boundary-crossing sebecoidean lineage with an estimated mass between 332–443 kg. This provides the first support for survival of a large-bodied terrestrial vertebrate lineage across the K-Pg boundary.

The systematics of humble-in-appearance brown spiders (“marronoids”), within a larger group of spiders with a modified retrolateral tibial apophysis (the RTA Clade), has long vexed arachnologists. Although not yet fully settled, recent phylogenomics has allowed the delimitation and phylogenetic relationships of families within marronoids to come into focus. Understanding relationships within these families still awaits more comprehensive generic-level sampling, as the bulk of described marronoid genera remain unsampled for phylogenomic data. Here we conduct such an analysis in the family Cybaeidae Banks, 1892. We greatly increase generic-level sampling, assembling ultraconserved element (UCE) data for 18 of 22 described cybaeid genera, including all North American genera, and rigorously test family monophyly using a comprehensive outgroup taxon sample. We also conduct analyses of traditional Sanger loci, allowing curation of some previously published data. Our UCE phylogenomic results support the monophyly of recognized cybaeids, with strongly supported internal relationships, and evidence for five primary molecular subclades. We hypothesize potential morphological synapomorphies for several of these subclades, bringing a robust phylogenomic underpinning to cybaeid classification. We discover and describe a new cybaeid genus (Siskiyu gen. nov.) and species (Siskiyu armilla sp. nov.) from far northern California and adjacent southern Oregon and describe a new species in the elusive genus Cybaeozyga (C. furtiva sp. nov.) from far northern California.

Supplementary information for the article: 'Quantitative morphological comparison of rake-legged mites over the last 100 million years and the first fossil larva of Caeculidae,' published in the journal Paleobiodiversity and Paleoenvironment.File "Shapes" includes the outlines dataset used in th elliptical Fourier analysisFile "PC_Contrib_loadings" includes the graphical component loadings from the PCA analysis.File  "Caeculidae_scores" includes PC scores of each specimen from the PCA analysis.File "Supp_list_Caeculidae" includes the list of specimens included in this study.File "Caeculidae_analysis_script" includes the R-script used for the statistical analysis.

Supplementary information for the article: 'Quantitative morphological comparison of rake-legged mites over the last 100 million years and the first fossil larva of Caeculidae,' published in the journal Paleobiodiversity and Paleoenvironment.File "Shapes" includes the outlines dataset used in th elliptical Fourier analysisFile "PC_Contrib_loadings" includes the graphical component loadings from the PCA analysis.File  "Caeculidae_scores" includes PC scores of each specimen from the PCA analysis.File "Supp_list_Caeculidae" includes the list of specimens included in this study.File "Caeculidae_analysis_script" includes the R-script used for the statistical analysis.