Mystery of mammalian, diapsid eardrum formation solved

Brett Smith for redOrbit.com – @ParkstBrett
Scientists have long suspected that the eardrum evolved separately in mammals and diapsids, the taxonomic group that includes reptiles and birds. However, because the eardrum cannot be fossilized, this theory could not be proved through the fossil record.
Now, a new study from a team of Japanese researchers has revealed that the mammalian eardrum is based in the formation of the lower jaw, while diapsid eardrum formation is based in the upper jaw’s formation.
Both the eardrum and the middle ear allow mammals, reptiles, and birds to hear sound waves travelling through air and all eardrums are formed when the ear canal connects with the first pharyngeal pouch. However, fossil evidence shows that the middle ears in mammals and diapsids are very different.
Jawing
In the new study, published in Nature Communications, researchers started by noting that in mammals, the eardrum attaches to a lower jaw bone called the tympanic ring. They also noted that in diapsids, the eardrum connects to the upper jawbone.
Next, the team looked at eardrum growth in mice that were missing something called the Ednra receptor, an omission seen to restrict lower jaw development. They learned that these mice also didn’t develop eardrums and ear canals, which revealed that their development was determined by lower jaw formation.
The team then blocked the growth and development of the lower jaw in chickens and saw that it created two identical eardrums and ear canals, with the added set developing from upper jaw elements that had developed inside the deformed lower jaw.
To find out how the eardrum evolved separately in these two lineages and why it is linked with the jaw differently in each lineage, the scientists looked at a marker for the main jaw joint, called Bapxl, and its position in relation to the first pharyngeal pouch. They learned that in mouse embryos, Bapxl was expressed in cells just below the pouch and that in chickens it was expressed significantly lower. This distinction forces the eardrum to form below the main jaw joint in mammals and above the joint in diapsids, the team said in their report.
While researchers still don’t know how or why the main jaw junction moved upwards in mammals, the study reveals that the middle ear developed after this change and must therefore have took place separately after mammal and diapsid lineages split from their common ancestor.
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