Wang et al. 2021: Vilevolodon had monotreme-like ear bones

We’ve heard this before.
Links below.

From the Wang et al. 2021 abstract:
“Recent discoveries of well-preserved Mesozoic mammals have provided glimpses into the transition from the dual (masticatory and auditory) to the single auditory function for the ossicles, which is now widely accepted to have occurred at least three times in mammal evolution.”

Wang et al. are not working from a valid phylogenetic context. They are not considering the possibility, hypothesized in the large reptile tree (LRT, 1593+ taxa) of a phylogenetic reversal in which the inner ear bones, which recapitulate phylogeny during embryonic ontogeny in placentals, could have stopped developing and stopped migrating to the typical placental position posterior to the mandible.

“Here we report a skull and postcranium that we refer to the haramiyidan Vilevolodon diplomylos (dating to the Middle Jurassic epoch (160 million years ago)) and that shows excellent preservation of the malleus, incus and ectotympanic (which supports the tympanic membrane).

See figure 1. We covered this issue earlier here, here and here.

Figure 1. Basal mammals and Vilevolodon as figured by Meng et al. Note in the other taxa the two jaw joints are nearly coincident. Not so in Vilevolodon.

Figure 1. Basal mammals and Vilevolodon as figured by Meng et al. Note in the other taxa the two jaw joints are nearly coincident. Not so in Vilevolodon.

From the Wang et al. abstract (continued)
“After comparing this fossil with other Mesozoic and extant mammals, we propose that the overlapping incudomallear articulation found in this and other Mesozoic fossils, in extant monotremes and in early ontogeny in extant marsupials and placentals is a morphology that evolved in several groups of mammals in the transition from the dual to the single function for the ossicles.”

Unfortunately
Wang et al. are pinning all their phylogenetic hopes on the inner ear bones. Therefore they are  “Pulling a Larry Martin.” Don’t do that. When placed into a phylogenetic analysis that considers traits from the entire skeleton and a wide gamut of mammals and pre-mammals, Vilevolodon nests within the placental clade Glires (the gnawers = rodents, rabbits, shrews, aye-ayes, multituberculates, etc.) We’ve known this for several years.

Wang et al. 2021 provide four prior analyses
in their SuppData, (references below) all of which employ suprageneric taxa, none of which test pertinent members of Glires.

In summary:
When tested against more taxa Vilevolodon is recovered as a derived member of Glires (rodents, rabbits, shrews, etc.) sharing with other multituberculates a neotonous retention of the embryonic condition, prior to the migration of the inner ear bones to the base of the skull, posterior to the mandibles. Evidently in their typical adult placental position typical ear bones interfered with the long slide of the mandible during gnawing and mastication, so retained the embryonic condition. The authors noted this ‘transition” in placentals in their abstract, but did not consider the possibility of a reversal or neotony.


References
Han G, Mao F, Bi S., Wang Y and Meng JA2017. Jurassic gliding euharamiyidan mammal with an ear of five auditory bones. Nature 551, 451–456.
Luo Z.-X. et al. 2017. New evidence for mammaliaform ear evolution and feeding adaptation in a Jurassic ecosystem. Nature 548, 326–329.
Wang H, Meng J and Wang Y 2019. Cretaceous fossil reveals a new pattern in mammalian middle ear evolution. Nature 576, 102–105.
Wang J, Wible JR, Guo B. et al. 2021. A monotreme-like auditory apparatus in a Middle Jurassic haramiyidan. Nature. https://doi.org/10.1038/s41586-020-03137-z

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