evolution: three inner ear bones (Introduction)

by David Turell , Wednesday, November 01, 2023, 19:01 (178 days ago) @ David Turell

In reptiles and early mammals:

https://ecoevocommunity.nature.com/posts/hearing-from-our-cretaceous-ancestors-a-remark...

"The evolution of the middle ear in early therians remains enigmatic. Our recent discovery of a detached, microtype ear in a newly uncovered Early Cretaceous eutherian mammal not only addresses this mystery, but also suggests independent decoupling of hearing and chewing apparatuses.

"At the heart of this story lies a pivotal question: where did the tiny but crucial bones of the middle ear, the malleus, incus, and stapes, originate? Contrasting with reptiles, which possess a single ear bone known as the columella, we explore the remarkable transformation of the mammalian middle ear, a subject that has been a textbook example of evolution and fascinated evolutionary biologists since the 19th century. Early embryologists made pioneering contributions by examining the intricate embryonic structures of vertebrates, establishing the homology of middle ear bones between mammals and reptiles.

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"Within mammaliaforms, which include crown mammals and stem clades, there are three pivotal evolutionary and phylogenetic stages in the development of the mammalian middle ear: Mandibular middle ear (MME) (or Postdentary-attached ear), Meckelian-attached middle ear (MaME), and Detached middle ear (DME).

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"The MaME is characterized by the absence of the postdentary trough and the shrinking postdentary bones connected to the mandible through the Meckel’s cartilage. The DME emerges as the Meckelian cartilage and sulcus diminish, leading to the detachment of middle ear from the mandible.

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"Our discovery was triggered by a nearly complete fossil skeleton of a small mammal preserved in counterpart slabs. This fossil, likely representing a therian mammal, was a rare find in the Early Cretaceous terrestrial Lagerstätte Jehol Biota. The excitement of the discovery was followed by meticulous preparation and high-resolution micro-CT scan. After an in-depth comparative morphological analysis, we named it as a new genus and species, Microtherulum oneirodes. The specific name, "oneirodes", pays tribute to the dreamlike nature of this fossil's discovery, which fills a critical gap in our understanding of the middle ear evolution in mammals. Our comprehensive phylogenetic analyses revealed that Microtherulum stands as one of the earliest branching eutherians at the dawn of eutherian evolution.

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"The most exciting discovery was the saddle-shaped articulation of two incomplete elements, identical to the malleus-incus articulation in modern therians. This discovery marks the earliest known record of the saddle-like incudomallear joint in any known Mesozoic mammaliaforms. It also confirms the presence of the typical saddle-shaped incudomallear joint in Early Cretaceous eutherians, representing a significant innovation in the mammalian middle ear. We offer a partial glimpse into the appearance of the middle ear in early therians together with the identification of the C-shaped ectotympanic and the stapes with an oval-shaped footplate in the specimens.

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"...the new described middle ear is most similar to the microtype ear. Notably, this marks the first identification of the "microtype ear" in Mesozoic mammaliaforms. The discovery aligns with observations that microtype species are small mammals (e.g., bats, shrews and mice), not vice versa. The detachment of the ear, along with its functional characteristics (microtype), suggests that Microtherulum probably developed an enhanced capacity for high-frequency hearing.

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"The detached middle ear in Microtherulum adds a layer of complexity to our understanding of middle ear variation in early therians. Notably, another Jehol eutherian, Cokotherium, discovered from the same geological formation, retained an ossified Meckelian cartilage (Wang et al., 2022). Our discovery of the microtype ear in early eutherians challenges Fleischer's hypothesis that the ancestral type ear preceded the true mammalian ear and evolved into other functional types. The ancestral states of therian ears remain an open question, raising further inquiries about how the evolutionary transformation occurred among different types of middle ears in early therians. These questions underscore the need for additional specimens to provide a comprehensive picture of the middle ear and morphological variation across various clades."

Comment: see the article, filled with marvelous illustrations and photos, to better follow the evolutionary road to our three ear bones, which very sensitively carry high- and low-pitched sound waves to the drum. Irreducible complexity is obvious.