Amazing stuff!
"... Then, in 2015, Benjamin Sulser, a University of Chicago biology student on the hunt for a thesis project, took detailed 3D images of the inner ear of a bat skull. But he couldn’t find a feature common in virtually all mammals—a bony tube that encases the nerve cells and connects the ear to the brain. Thinking he’d made a mistake, he and Luo imaged the skulls of two more related species using a computed tomography scanner, with similar results. The researchers realized they might have stumbled across an answer to a mystery that had bedeviled bat biologists for 2 decades—and an explanation for why some families of bats had such a diverse echolocation arsenal. ...
But until now, no one could find a physical difference linked to the genetic split between the two groups. After Sulser’s discovery, the duo spent 5 years scanning the skulls of 39 bat species. In 24 of 26 yang bats they scanned, the bony nerve channel was missing or contained large holes making it resemble lace, Sulser and Luo report today in Nature. ..."
For years, bats were divided into two groups: big fruit bats, which don’t generally echolocate, and small bats that hunt by sound. But in 2000, a genetic analysis revealed bats had actually diverged into two different groups some 50 million years ago: one group that included the large fruit bats and some of the echolocating insect eaters (known as Yinpterochiroptera, or “yin”), and another group that included the rest of the small, sound-hunting bats (known as Yangochiroptera, or “yang”). The latter make up 82% of the 1250 known echolocating species of bats.
From the abstract:
"... Here we report the observation of highly derived structures of the spiral ganglion in yangochiropteran bats: a trans-otic ganglion with a wall-less Rosenthal’s canal. This neuroanatomical arrangement permits a larger ganglion with more neurons, higher innervation density of neurons and denser clustering of cochlear nerve fascicles. This differs from the plesiomorphic neuroanatomy of Yinpterochiroptera and non-chiropteran mammals. The osteological correlates of these derived ganglion features can now be traced into bat phylogeny, providing direct evidence of how Yangochiroptera differentiated from Yinpterochiroptera in spiral ganglion neuroanatomy. These features are highly variable across major clades and between species of Yangochiroptera, and in morphospace, exhibit much greater disparity in Yangochiroptera than Yinpterochiroptera. These highly variable ganglion features may be a neuroanatomical evolutionary driver for their diverse echolocating strategies and are associated with the explosive diversification of yangochiropterans, which include most bat families, genera and species."
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