Monotreme middle ear is not primitive for Mammalia

Abstract

The study on evolution of the mammalian middle ear has been fueled by continuous discoveries of Mesozoic fossils in the last two decades. Wang et al.1 recently reported a specimen of Vilevolodon diplomylos (IMMNH-PV01699)2 that adds to the increasing knowledge about the auditory apparatus of haramiyidans, an extinct Mesozoic group of mammaliaforms. The authors hypothesized that a middle ear with a monotreme-like incus and malleus and incudomallear articulation was primitive for mammals, which challenges the convention that the monotreme middle ear is specialized3 or autapomorphic4 in mammals. We raise concerns about terminology and identification of the incus presented by Wang et al. and show that their analysis does not support their hypothesis; instead, it supports the one by Mao et al.5,6.