If you put a pair of headphones on one of the earliest creatures resembling mammals, would it start rocking out to the music, or would it hardly be able to hear anything?
This question might seem a bit silly, but it has long puzzled paleontologists. Early therapsids—a group that includes now-extinct mammal ancestors and closely related creatures—known as cynodonts had a mashup of mammalian and reptilian characteristics. One of their more reptilian traits was a lack of visible ears, and there has long been an ongoing debate about whether they had evolved anything resembling an eardrum. After all, they would have had a significant survival advantage if they were able to pick up on the subtle sounds of predators and prey.
Nowadays, it’s another story. With technological advantages like upgraded imaging technology and simulation software, paleontologists Alec Wilken and Zhe-Xi Luo from the University of Chicago set out to determine just how well Thrinaxodon liorhinus—a cynodont that lived 250 million years ago and would have competed for food with dinosaurs—could hear. After taking extensive CT scans of a well-studied Thrinaxodon skull, the team used both its features and those of living animals to create a digital model that they put through a hearing simulation.
“An abundant fossil record shows that the malleus, incus, and ectotympanic ear bones of living mammals were derived from the postdentary bones of Paleozoic therapsids and Mesozoic cynodonts through their detachment from the mandible, change in shape, and reduction in size,” Wilken and Luo said in a study recently published in the journal PNAS. “The ectotympanic ultimately provided attachment for a soft tissue ear drum, or tympanum.”