Mammalian Ear Evolution: Tiny Bones in the Ear Not a Part of the Jaw
Mon, April 19, 2021

Mammalian Ear Evolution: Tiny Bones in the Ear Not a Part of the Jaw

Among the tiniest bones in the human body are the three bones that sit in the middle ear. These three tiny bones (auditory ossicles) are the malleus, incus, and stapes / Photo by: BLACKDAY via Shutterstock

 

Among the tiniest bones in the human body are the three bones that sit in the middle ear. These three tiny bones (auditory ossicles) are the malleus, incus, and stapes. When a sound wave vibrates the eardrum, it attaches to the malleus, which then transmits the vibration via incus and the stapes. These three bones form a kind of bridge and amplify the sound waves before ithey move on into the inner ear.

Contrary to birds and reptiles that only have a single middle-ear bone, mammals have unique auditory ossicles. New research conducted by Fangyuan Mao from the Institute of Vertebrate Paleontology and Paleoanthropology in the Chinese Academy of Sciences and group has found that the tiny bones in the middle ear were not a part of the jaw of mammals and that no “bone-on-bone” contact was previously seen in other species discovered.

Past vs. Present Scientific Discoveries

Prior to the team’s study, a wealth of evidence from developing embryos and fossils pointed to the theory that ear bones were once a part of jawbones. In a 2012 study titled "Evolution of the mammalian middle ear and jaw: adaptations and novel structures," for instance, the authors explained that the incus and malleus are “homologous” to the articular and quadrate (the bone that is part of the skull) and articular (the bone that is part of the lower jaw). 

Prior to the team’s study, a wealth of evidence from developing embryos and fossils pointed to the theory that ear bones were once a part of jawbones / Photo by: dudonyrud via Shutterstock

 

Unearthing the Fossilized Mammal Called Origolestes Lii

In the present study, researchers excavated a roughly 123-million-year-old rock formation in China and this allowed them to unearth the fossils of six specimens. In paleontological terms, this already presents an impressive number. The specimens were that of Origolestes lii. It has an ear that looks like that of modern mammals. Although its body already appears quite “ancient,” its ear bones are detached and moved away from the jaw. Origolestes lii is also an ancestor to the therian mammal, a class that includes all marsupials and placentals that are alive today.

To understand the auditory bones and cartilage in detail, the researchers took high-resolution CT scans of the fossil. The discovery is such an “evolutionary moment” to them because it highlights that the auditory ossicles are already decoupled from the lower jaw. Thus, the jaw and the ear evolved separately in mammals and each specializes in what they do. 

Scientists have also hypothesized that the decoupling of the chewing and hearing system removed the physical constraints in mammals to both improve their hearing as well as diversify their diet.

Co-author Jeng Meng, who is also a curator of fossil mammals at the American Museum of Natural History, explained that such a kind of separation of ear bones from the jawbones is “critical” because it also separates chewing and hearing.

The previous study co-authored by Meng was a contrast to the present study. In that research, Meng and the group analyzed the Jeholbaatar kielanae, which also lived around the same period as the Origolestes. However, Jeholbaatar’s lineage of mammals is now considered extinct. In that study, the authors said that Jeholbaatar had well-preserved middle-ear bones and provided evidence of articulation and morphology of the bony elements.

Based on their fossil evidence, the post-dentary bones were either attached on the medial side of the dentary or connected to the dentary via cartilage in the early mammals.

Evolutionary Chain

Chicago-based Field Museum of Natural History mammalogist Stephanie Smith, who was not involved in either of the studies, opined that it is common to see systems evolving independently in such a way that can be linked to the mammals’ behaviors, like chewing. It is simply not just about genetic control but how being “decoupled” allowed the mammals to evolve.
 
Nevertheless, the separation of the middle ear bones from the jawbones was so valuable, it could serve as a chain for mammalian evolution. Meng and colleagues added that they are now examining the other parts of the fossils they found, including its brain activity to help delve further into the evolution of mammals.

University of Louisville’s evolutionary biologist Guillermo Rougier commented via newspaper Japan Times that the present discovery poses new questions. For example, does the process of evolution happen in all mammals or only in a small subset of mammals? Did it happen in different groups of mammals? Did it happen only once? While these questions are not yet answered, at least the studies are moving the boundaries of things they can ask, the biologist added. Rougier was also not a part of the study.

Mammal Species, Threatened

The United Nations Environmental Program and the World Conservation Monitoring Centre shared the number of mammalian species that are now endangered or threatened per country. Countries with a high number of threatened mammal species include Madagascar (121), Brazil (80), Mexico (96), India (93), Indonesia (191), United States (40), Russian Federation (34), China (73), Congo (32), and Australia (63).