A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan

Lend an ear to a classic tale of mammalian evolution

“…Elements of the ear, or the corresponding jaw bones from which they evolved, are shown in colour. Wang et al 2 propose a new terminology system for classifying fossils depending on the connection between the middle ear and the jaw. This system describes the degree of direct attachment between the middle ear and the lower jaw, classifying fossil middle ears as either detached, Meckelianattached (partially attached through Meckel's cartilage) or postdentaryattached (still part of the jaw).…”

“…Nineteenth-century biologists were the first to recognize the similarities in development between some of these extra elements of the lower jaw of non-mammalian vertebrates and the middle ear of mammals, and in doing so chronicled one of the most fundamental transitions in mammalian evolution: the transformation of lower jaw bones to form the middle ear. On the basis of new fossil evidence, Wang et al 2 now crucially revise this classic story of mammalian evolution.…”

Monotreme middle ear is not primitive for Mammalia

“…Wang et al. [ 1 ] recently reported a specimen of Vilevolodon diplomylos [ 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 specialized [ 3 ] or autapomorphic [ 4 ] in mammals.…”

“…6 in ref. [ 1 ]), is a misinterpretation—it is part of the element with debatable identity (Fig. 1 k–m).…”

“…The possibility that it is a non-incus structure, as those interpreted in Jeholbaatar and Qishou , cannot be ruled out. This could explain why both sets of the incus and malleus ‘were moved to that degree from their position in life and yet remain well preserved’ [ 1 ]. It is uncertain whether Wang et al ’s computerized tomography (CT) data with a voxel size of 32.7 μm could secure the identities of the incus and malleus; a CT-slice showing the suture between them, as we did in Fig.…”