Petrosal and inner ear anatomy and allometry amongst specimens referred to L itopterna ( P lacentalia)

New isolated petrosals from the I taboraí beds of B razil (late P alaeocene or early E ocene) are here described and referred to the early diverging litoptern M iguelsoria parayirunhor , based on phylogenetic, size, and abundance arguments. Both the external and internal anatomy of these specimens were investigated, which for the first time document many details of the auditory region of a P alaeogene litoptern. Our cladistic analysis, which included our new observations, failed to recover a monophyletic L itopterna but did not exclude it. A constrained analysis for the monophyly of this order showed that several features such as a (sub)quadrangular and anteroposteriorly elongated tensor tympani fossa and a large notch in the vicinity of the external opening of the cochlear canaliculus may constitute synapomorphies for L itopterna. The evolution of several other auditory characters amongst L itopterna is discussed and the relative dimensions of the inner ear and surrounding petrosal in the group were also investigated. This allowed detection of negative allometry of the bony labyrinth within the petrosal, which was confirmed by measurements and regression analysis across a larger sample of placental mammals. This scaling effect probably has an important influence on several characters of the bony labyrinth and petrosal, amongst which are the length of the vestibular aqueduct and cochlear canaliculus. It demonstrates that many aspects of the morphological variation of the bony labyrinth need to be thoroughly investigated before being incorporated into phylogenetic analyses.  © 2015 The Linnean Society of London