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Although several well-preserved crania are known for the Mesozoic Eutriconodonta, three-dimensional reconstructions of the character-rich inner ear and basicranial region based on high-resolution computed tomography scans have previously only been published for the Late Jurassic Priacodon. Here we present a description of the petrosal and inner ear morphology of a triconodontid eutriconodontan from the Lower Cretaceous Cloverly Formation, which we provisionally assign to Astroconodon. The bony labyrinth of Astroconodon is plesiomorphic for mammaliaforms in lacking a primary osseous lamina, cribriform plate, and osseous cochlear ganglion canal. However, as in Priacodon and the zhangheotheriid Origolestes, Astroconodon has a secondary osseous lamina base that extends nearly the complete length of the cochlear canal. The cochlear canal is straighter in Astroconodon and other eutriconodontans compared to several basal mammaliaform clades (e.g., morganucodontans, docodontans), that exhibit varying degrees of cochlear canal curvature. The pars cochlearis of the petrosal was well vascularized in Astroconodon, exhibiting a network of venous canals that crossed the cochlea transversely on its ventral and dorsal aspects. Of particular note are several canals that passed along the base of the secondary osseous lamina. As in Priacodon and Origolestes, those canals do not show the extensive connections to the cochlear labyrinth as seen in the basal mammaliaforms Morganucodon and Borealestes. The inner ear of Astroconodon thus highlights the complex history of the mammaliaform cochlear canal, in which different clades appear to follow independent evolutionary trajectories and various key morphological features (e.g., cochlear canal length, curvature, vascularization and osseous supports for the basilar membrane) exhibit considerable homoplasy. 

Abstract Island ecosystems provide natural laboratories to assess the impacts of isolation on population persistence. However, most studies of persistence have focused on a single species, without comparisons to other organisms they interact with in the ecosystem. The case study of moose and gray wolves on Isle Royale allows for a direct contrast of genetic variation in isolated populations that have experienced dramatically differing population trajectories over the past decade. Whereas the Isle Royale wolf population recently declined nearly to extinction due to severe inbreeding depression, the moose population has thrived and continues to persist, despite having low genetic diversity and being isolated for ∼120 years. Here, we examine the patterns of genomic variation underlying the continued persistence of the Isle Royale moose population. We document high levels of inbreeding in the population, roughly as high as the wolf population at the time of its decline. However, inbreeding in the moose population manifests in the form of intermediate-length runs of homozygosity suggestive of historical inbreeding and purging, contrasting with the long runs of homozygosity observed in the smaller wolf population. Using simulations, we confirm that substantial purging has likely occurred in the moose population. However, we also document notable increases in genetic load, which could eventually threaten population viability over the long term. Overall, our results demonstrate a complex relationship between inbreeding, genetic diversity, and population viability that highlights the use of genomic datasets and computational simulation tools for understanding the factors enabling persistence in isolated populations. 

The fossil record of mammaliaforms (mammals and their closest relatives) of the Mesozoic era from the southern supercontinent Gondwana is far less extensive than that from its northern counterpart, Laurasia. Among Mesozoic mammaliaforms, Gondwanatheria is one of the most poorly known clades, previously represented by only a single cranium and isolated jaws and teeth. As a result, the anatomy, palaeobiology and phylogenetic relationships of gondwanatherians remain unclear. Here we report the discovery of an articulated and very well-preserved skeleton of a gondwanatherian of the latest age (72.1–66 million years ago) of the Cretaceous period from Madagascar that we assign to a new genus and species, Adalatherium hui. To our knowledge, the specimen is the most complete skeleton of a Gondwanan Mesozoic mammaliaform that has been found, and includes the only postcranial material and ascending ramus of the dentary known for any gondwanatherian. A phylogenetic analysis including the new taxon recovers Gondwanatheria as the sister group to Multituberculata. The skeleton, which represents one of the largest of the Gondwanan Mesozoic mammaliaforms, is particularly notable for exhibiting many unique features in combination with features that are convergent on those of therian mammals. This uniqueness is consistent with a lineage history for A. hui of isolation on Madagascar for more than 20 million years.