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  1. null (Ed.)
    Among extant vertebrates, mammals are distinguished by having a chain of three auditory ossicles (the malleus, incus and stapes) that transduce sound waves and promote an increased range of audible—especially high—frequencies. By contrast, the homologous bones in early fossil mammals and relatives also functioned in chewing through their bony attachments to the lower jaw. Recent discoveries of well-preserved Mesozoic mammals have provided glimpses into the transition from the dual (masticatory and auditory) to the single auditory function for the ossicles, which is now widely accepted to have occurred at least three times in mammal evolution. Here we report a skull and postcranium that we refer to the haramiyidan Vilevolodon diplomylos (dating to the Middle Jurassic epoch (160 million years ago)) and that shows excellent preservation of the malleus, incus and ectotympanic (which supports the tympanic membrane). After comparing this fossil with other Mesozoic and extant mammals, we propose that the overlapping incudomallear articulation found in this and other Mesozoic fossils, in extant monotremes and in early ontogeny in extant marsupials and placentals is a morphology that evolved in several groups of mammals in the transition from the dual to the single function for the ossicles. 
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  2. The superfamily Djadochtatherioidea is a distinctive clade of multituberculates from Upper Cretaceous beds of Mongolia and Inner Mongolia, China. Because many of the 11 included genera are known from skulls, more is known about the cranial anatomy of djadochtatherioids than any other clade of multituberculates. Within Djadochtatherioidea, the most diverse and widely accepted group is the family Djadochtatheriidae. Within the family, the basal genus, Kryptobaatar Kielan-Jaworowska, 1970, is small with a skull length of about 30 mm, whereas the other four genera, Djadochtatherium Simpson, 1925, Catopsbaatar Kielan-Jaworowska, 1994, Tombaatar Rougier et al., 1997, and Mangasbaatar Rougier et al., 2016, have skulls approximately twice as long. Here, we describe a new genus and species, Guibaatar castellanus, based on a single specimen from the Upper Cretaceous Bayan Mandahu Formation, Inner Mongolia that we refer to Djadochtatheriidae. Guibaatar is represented by a relatively complete rostrum, a partial right braincase, and partial lower jaws. As revealed by CT scanning, the specimen is a juvenile, with deciduous enlarged upper and lower incisors with permanent replacements forming, m2 erupting, and M2 forming. Based on the preserved cranial parts, we estimate the skull length to be approximately 50 mm, but as an adult, Guibaatar would have been in the size range of the larger djadochtatheriids. Phylogenetic analysis including Guibaatar, known djadochtatherioids, and outgroups places Guibaatar within Djadochtatheriidae, as sister to a clade of Mangasbaatar and Catopsbaatar. We suspect the relationships of djadochtatherioids are likely to be refined given the announcements by other researchers that skulls are known for the djadochtatheriids Tombaatar and Djadochtatherium, which were previously represented by incomplete material. 
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