“Septal compass” and “septal formula”: a new method for phylogenetic investigations of the middle ear region in the squirrel-related clade (Rodentia: Mammalia)

Abstract

Here, we introduce the “septal compass” and the “septal formula” as a new method for phylogenetic investigations of the middle ear region in squirrel-related clade. The middle ear cavity is characterized by bony septa that divide the dorsally lying epitympanic recess and the ventrally lying tympanic cavity into several segments or diverticula. The distribution patterns of these septa are conservative among the squirrel-related clade and are restricted to the species, genus, and family level. In the studied outgroups represented by †Ischyromys typus and lagomorphs, no septa are found in the epitympanic recess and tympanic cavity. Therefore, the “septal compass” and the “septal formula” provide a new approach for phylogenetic interpretations of the middle ear region. It is user-optimized and can be modified for other rodent families and mammalian taxa and will facilitate phylogenetic assumptions in future investigations. Additionally, this method will enable the allocation of isolated tympanic bullae to the respective genus level.