An extinct, mouse-sized gliding animal from the Late Jurassic with unique ear bones—the earliest-known mammalian middle ear on record—has been discovered by a research team led by paleontologists at the American Museum of Natural History and the Chinese Academy of Sciences. The study, published online today in the journal Nature, resets research on the evolution of the mammalian middle ear, which gives modern mammals the sharpest hearing on Earth, and could change how we think about mammalian evolution in general.
“The middle ear is an essential part of being a mammal,” said co-author Jin Meng, a curator in the American Museum of Natural History’s Division of Paleontology. “Humans can thank the layout of their middle ears for giving them the ability to chew and hear at the same time. Whales use them to communicate infrasonically from miles away. So we are very interested in learning how and when these delicate structures evolved.”
The typical mammalian middle ear, or the area just inside the eardrum that turns vibrations in the air into ripples in the inner ear’s fluids, is ringed in shape and includes three bones: the malleus, incus, and stapes (known colloquially as the hammer, anvil, and stirrup). During the evolutionary shift from the group that includes lizards, crocodilians, and dinosaurs to mammals, two of these ear bones separated from the joint of the lower jaw and became associated with hearing.
(For more about how sensory organs in our ears help us navigate the world, visit the new exhibition Our Senses, opening this month at the Museum.)
This new species’ unique auditory apparatus consists of five bones, including the surangular, a jaw bone found in most land vertebrates, except for mammals, that has not been previously identified in any mammalian middle ear. In addition, other middle ear elements in the animal are distinctly different from corresponding bones in known mammals.
“We concluded that such a new type of middle ear may have co-evolved with the animal’s special jaw joint that is capable of posterior movement during chewing,” Meng said. “The study also shows that the middle ear of the new animal evolved independently from those of extant monotreme and therian mammals, including humans.”
In the new study, researchers describe fossils of a gliding mammal about the size of a mouse with a long tail that was discovered in northeastern China. The exinct animal was named Arboroharamiya allinhopsoni in honor of scientists Edgar F. Allin and James A. Hopson for their contribution to the study of mammalian middle ear evolution.
A. allinhopsoni belonged to a group called Euharamiyida, small mammals whose tails and feet indicate that they were tree dwellers. The A. allinhopsoni fossils include well-preserved gliding membranes and fur, which suggest that the animals likely sailed through Jurassic forests.
“The size of the gliding membrane, the long tail, and the hair pattern are highly similar to those of gliding species in extant marsupials and placental rodents, and they were likely nocturnal,” Meng said. “This research suggests that gliding was probably common among euharamiyidans and adds evidence to the idea that there was a major radiation of mammals in the mid-Jurassic.”