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Multituberculates are an extinct mammalian order that lived in Mesozoic and early Cenozoic eras. Lambdopsalis, a Paleocene multituberculate recovered in China, preserves cranial remains that allow in this study: (1) a description of its skull morphology; (2) a reconstruction of its nonfossilized structures such as the cranial nerve system and major cranial vasculature; (3) an analysis of functional adaptation of its auditory system; and (4) an interpretation of phylogenetic relationships within multituberculates themselves and among other major mammalian groups.

Character analysis reveals that a number of previously used cranial features in reconstructions of mammalian phylogeny are unreliable. These include premaxillo-frontal contact, exclusion of septomaxilla from face, number of infraorbital foramina, extent of orbital exposure of palatine, presence versus absence of jugal, lacrimal, and parasphenoid, extent of cranial process of squamosal, and reduction of alisphenoid.

The bulla-like structure of Lambdopsalis is the expanded vestibular apparatus, not an enlarged tympanic bulla. The expanded vestibular apparatus, flat incudomalleal joint, and absence of a well defined fossa muscularis minor in Lambdopsalis suggest that Lambdopsalis (possibly a burrower) adapted to low-frequency perception.

Lambdopsalis possesses a large alisphenoid (perforated by the trigeminal foramina) and a slender “anterior lamina of the petrosal.” The discovery supports Presley’s (1981) argument of close affinity between “nontherian” and “therian” mammals, and invalidates the hypothesis of fundamental nontherian/therian dichotomy. Contrary to general consensus, available paleontological evidence does not indicate the existence of a uniform structural pattern of the braincase for nontherian groups.

Cranial characters coupled with dental features document monophyly for nonharamiyid multituberculates. The skull morphology of Lambdopsalis shows phylogenetic unity with taeniolabidids, and invalidity of Lambdopsalidae Chow and Qi, 1978.

Assuming monophyly of Mammalia, the class is divisible into a crown group and a stem group. The crown group includes all living mammals plus the fossil therians that shared the latest common ancestor with monotremes. The stem group consists of all remaining extinct mammals. Multituberculates belong to the paraphyletic stem group, and diverged from the main lineage leading to living mammals prior to emergence of the latest common ancestor of modern mammals. More intimate relations among members of the stem group remain uncertain, but are limited to but a few reasonable alternatives.

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