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  1. Abstract

    The Bering Land Bridge connecting North America and Eurasia was periodically exposed and inundated by oscillating sea levels during the Pleistocene glacial cycles. This land connection allowed the intermittent dispersal of animals, including humans, between Western Beringia (far northeast Asia) and Eastern Beringia (northwest North America), changing the faunal community composition of both continents. The Pleistocene glacial cycles also had profound impacts on temperature, precipitation and vegetation, impacting faunal community structure and demography. While these palaeoenvironmental impacts have been studied in many large herbivores from Beringia (e.g., bison, mammoths, horses), the Pleistocene population dynamics of the diverse guild of carnivorans present in the region are less well understood, due to their lower abundances. In this study, we analyse mitochondrial genome data from ancient brown bears (Ursus arctos;n = 103) and lions (Pantheraspp.;n = 39), two megafaunal carnivorans that dispersed into North America during the Pleistocene. Our results reveal striking synchronicity in the population dynamics of Beringian lions and brown bears, with multiple waves of dispersal across the Bering Land Bridge coinciding with glacial periods of low sea levels, as well as synchronous local extinctions in Eastern Beringia during Marine Isotope Stage 3. The evolutionary histories of these two taxa underline the crucial biogeographical role of the Bering Land Bridge in the distribution, turnover and maintenance of megafaunal populations in North America.

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  2. Abstract

    We compared the head skeleton (skull and lower jaw) of juvenile and adult specimens of five snake species [Anilios(=Ramphotyphlops)bicolor,Cylindrophis ruffus,Aspidites melanocephalus,Acrochordus arafurae, andNotechis scutatus] and two lizard outgroups (Ctenophorus decresii,Varanus gilleni). All major ontogenetic changes observed were documented both qualitatively and quantitatively. Qualitative comparisons were based on high‐resolution micro‐CTscanning of the specimens, and detailed quantitative analyses were performed using three‐dimensional geometric morphometrics. Two sets of landmarks were used, one for accurate representation of the intraspecific transformations of each skull and jaw configuration, and the other for comparison between taxa. Our results document the ontogenetic elaboration of crests and processes for muscle attachment (especially for cervical and adductor muscles); negative allometry in the braincase of all taxa; approximately isometric growth of the snout of all taxa exceptVaranusandAnilios(positively allometric); and positive allometry in the quadrates of the macrostomatan snakesAspidites,AcrochordusandNotechis, but also, surprisingly, in the iguanian lizardCtenophorus. Ontogenetic trajectories from principal component analysis provide evidence for paedomorphosis inAniliosand peramorphosis inAcrochordus. Some primitive (lizard‐like) features are described for the first time in the juvenileCylindrophis. Two distinct developmental trajectories for the achievement of the macrostomatan (large‐gaped) condition in adult snakes are documented, driven either by positive allometry of supratemporal and quadrate (in pythons), or of quadrate alone (in sampled caenophidians); this is consistent with hypothesised homoplasy in this adaptive complex. Certain traits (e.g. shape of coronoid process, marginal tooth counts) are more stable throughout postnatal ontogeny than others (e.g. basisphenoid keel), with implications for their reliability as phylogenetic characters.

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