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Monotreme development reveals ancestral mechanisms of mammalian middle ear and jaw evolution

Rawson, James1; Athwal, Neal22; Johnston, Stephen3; Renfree, Marilyn4; Fenelon, Jane4; Martin-Silverstone, Elizabeth1; Rayfield, Emily1; Tucker, Abigail2

Published Sep 23, 2026 on Dryad. https://doi.org/10.5061/dryad.9zw3r22w3

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Abstract

During the origin of mammals the ancestral jaw joint separated from the mandible to form the detached mammalian middle ear, requiring substantial remodelling of musculoskeletal elements including the loss of a persistent Meckel’s cartilage connecting the ear and dentary. Evidence suggests a conserved mechanism underpins loss of Meckel’s in marsupials and placentals, but the mechanism behind the multiple transformations in fossil Mesozoic mammals remains unclear. Here we exploit the fact that a similar transformation occurs in extant monotremes during post-hatching development. Using micro-CT scanning and histology, we map changes to musculoskeletal anatomy and determine the mechanism of Meckel’s cartilage breakdown in the developing short-beaked echidna. Meckel’s cartilage is initially the main structural component of the jaw, persisting for >50 days before resorption. Early clast cell recruitment is responsible for Meckel’s breakdown in echidnas, strongly suggesting conservation of ossification and resorption mechanisms across all living mammal lineages. We argue that this shared mechanism underpins the multiple instantiations of the mammalian detached middle ear in living and fossil mammals. Furthermore, our results demonstrate that the intermandibular muscles show prolonged attachment to Meckel’s cartilage in monotremes and suggest that this attachment was a key function of the persistent Meckel’s cartilage in extinct mammals.