Paleontologists from the United States and China have examined the middle ear bones (ossicles) of Vilevolodon diplomylos, a gliding haramiyidan that lived during the Middle Jurassic epoch, some 160 million years ago, and found them remarkably similar to those of modern monotremes (egg-laying mammals).
Life reconstruction of Vilevolodon diplomylos showing a parent and offspring. Image credit: Sarah Shelley.
Paleontologists know that the three ossicles — the malleus, incus and stapes — in extant mammals’ close non-mammal relatives were attached to the lower jaw and functioned with it, serving in both chewing and hearing.
In many mammal lineages, these bones eventually detached from the lower jaw and came to serve only auditory functions. The details of this transition have been mysterious.
In the new research, Professor Shundong Bi from Yunnan University and the Indiana University of Pennsylvania and colleagues analyzed computed tomographic (CT) images of the well-preserved malleus and incus of Vilevolodon diplomylos.
They found the ancient animal’s malleus and incus remarkably similar to those of modern monotremes.
Since haramiyidans and monotremes are not thought to be closely related, this resemblance demonstrates a surprising instance of convergent evolution.
Vilevolodon diplomylos: (a) main slab; dark patches outside the skeleton between the skull, forelimbs and hind limbs indicate the patagium (gliding membrane). Scale bar – 20 mm. (b) Cranium in right oblique dorsal view, right mandible in lateral view and left mandible in medial view. Scale bars – 1 mm in (d, e), 2 mm in (c), 5 mm in (b). (c) Left mandible in medial view, showing the disposition of the auditory elements and the absence of postdentary trough and Meckelian sulcus. (d) Left incus (blue) in ventral view, left malleus (green) in dorsal view and left ectotympanic (red) in ventral view. (e) Left incus, malleus and ectotympanic restored to life position in oblique dorsal and ventral views (right and left, respectively). Abbreviations: an – angular process; api – anterior prominence of incus; apm – anterior process of malleus; cb – crus breve; co – mandibular condyle; cp – coronoid process; e – ectotympanic; fapm – facet for anterior process of malleus; fe – facet for ectotympanic; fi – facet for incus; fma – facet for malleus; fr – frontal; ju – jugal; lac – lacrimal; li – left incus; lm – left mandible; lma – left malleus; mab – mallear body; mas – mandibular symphysis; mf – mandibular foramen; mm – manubrium of malleus; mx – maxilla; na – nasal; pa – parietal; pal – palatine; pl – posterior limb of ectotympanic; pmx – premaxilla; ptf – pterygoid fossa; ri – right incus; rl – reflected lamina of ectotympanic; rm – right mandible; rma – right malleus; smx – septomaxilla; sp – stapedial process; sq – squamosal, ts – tympanic sulcus. Image credit: Wang et al., doi: 10.1038/s41586-020-03137-z.
“Although separated by 160 million years of geologic time, it is remarkable how similar these bones are in Vilevolodon diplomylos and the living monotremes, the platypus and echidna,” said Dr. John Wible, a researcher at Yunnan University and the Carnegie Museum of Natural History.
“The preservation of these ossicles in the fossil is amazing; the incus is just a little over a millimeter across!”
The malleus and incus of monotremes have long been considered to be unique adaptations as they are wholly unlike these bones in the remaining living mammals, marsupials and placentals.
Three types of middle ear in mammaliaforms: (a) in the postdentary-attached middle ear, the postdentary bones and Meckel’s cartilage are attached to the mandible via the postdentary trough and Meckelian sulcus (Late Triassic-Early Jurassic mammaliaform Morganucodon); (b) in the Meckelian-attached middle ear, the postdentary trough is absent and the postdentary bones are attached to the mandible via Meckel’s cartilage (ossified in this example) in the Meckelian sulcus; Meckel’s element is bent medially (arrow), moving the postdentary bones away from the temporomandibular joint (Early Cretaceous eutriconodontan Liaoconodon); (c) in the detached middle ear, the postdentary trough and Meckelian sulcus are absent and the auditory elements are detached from the mandible (Middle Jurassic haramiyidan Vilevolodon). Image credit: Wang et al., doi: 10.1038/s41586-020-03137-z.
“This type of malleus and incus represent an evolutionary stage between fossils with ossicles attached to and functioning with the lower jaw and those where the ossicles are detached from the lower jaw and exclusively function for hearing,” the researchers said.
“We hope the discovery advances the understanding of how mammals, including humans, developed their unique sense of hearing.”
The results were published in the journal Nature.
J. Wang et al. 2021. A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan. Nature 590, 279-283; doi: 10.1038/s41586-020-03137-z