Ancient fossils shed light on evolution of mammalian middle ear

Photo taken on January, 2018 shows the haramiyidan fossils dating back about 160 million years. (Photo provided by Bi Shundong with Yunnan University)

BEIJING, Feb. 4 (Xinhua) -- Chinese paleontologists have found mammalian middle ears in fossils dating back about 160 million years, shedding new light on the evolution of the mammalian auditory structure.

The study, which was recently published online in the journal Nature, deals with a fossil of a haramiyidan, a small squirrel-like animal from the Middle Jurassic period.

In modern mammals, the middle ear, composed of three auditory bones called ossicles, is detached from the jaw joint and is only responsible for hearing, said Bi Shundong, the paper's corresponding author and a researcher at Yunnan University.

But in early mammals and reptiles, two of these auditory bones form part of the jaw joint and serve for both hearing and chewing, he added.

The fossilized middle-ear bones described in the study are detached from the jaw joint, meaning that they have evolved from having the dual functions of chewing and hearing to a single auditory function, according to Bi.

Simulated image of the haramiyidan fossils dating back about 160 million years, Januray 2018. (Image photo provided by the Sarah Shelley of Carnegie Museum of Natural History)

The research shows that the auditory osscicles are overlapped, which allows for jaw movement when chewing.

The evolution of the middle ear into its modern form provided mammals with better hearing, helping them escape predators, according to Bi.

The ancient fossils have provided a glimpse of a transitional stage in the evolution of the mammalian middle ear, he said.  ■