A new study shows that the middle ear structure of modern mammals began to evolve as early as the Cretaceous period.

The mammalian middle ear is a classic example of progressive evolution. The middle ear of modern mammals has three auditory ossicles including malleus, incus and stapes, whereas in reptiles there is only one cylindrical bone.

Paleontologists believe that the two small auditory bones in the middle ear of mammals, the incus and the malleus, evolved from the quadrate bone and the mandibular joint bone of reptiles.

Since delicate auditory ossicles are difficult to preserve as fossils, the direct fossil evidence of middle ear evolution is very limited among mammal ancestors closely related to human evolution.

Researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) under the Chinese Academy of Sciences discovered an eutherian fossil, dating back about 120 million years, in northeast China's Liaoning Province. This fossil represents a "small mouse" with a body length of about 12 centimeters.

After restoration, analysis and research, the scientists concluded that the fossil was a new species of early eutherian, and found that its middle ear bone morphology included the external tympanic bone, the malleus bone, the incus bone and the stapes. They named it Microtherulum oneirodes.

"The specific name, oneirodes, pays tribute to the dreamlike nature of this fossil's discovery, which fills a critical gap in our understanding of the middle ear evolution in mammals," said Wang Haibing, one of the researchers from the IVPP.

The middle ear of modern mammals is usually divided into six functional morphologies, and the Microtherulum oneirodes' is most closely related to the Microtype ear of modern mammals.

Modern mammals with Microtype ears are adapted to high-frequency hearing, which means early eutherians probably had more sensitive hearing than other mammals of the same period, particularly in terms of high-frequency auditory sensitivity.

The study was recently published in the journal Nature Communications. (Xinhua)