Science

Experts find ancient mammal fossils in China that are a 'missing link' in modern ear development


All the better to hear you with: Experts find ancient fossils in China that are a ‘missing link’ in the evolution of modern mammal ears from jaw bones

  • The three tiny bones in our inner ear that allow us to hear started out differently
  • In our ancient ancestors, hearing was achieved using the bones of the jaw
  • Origolestes lii lived more than 100 million years ago near what is now Jinzhou
  • Its hearing bones had just detached from each other and the cartilage of the jaw

Experts have found the fossils of an ancient mammal species in China that is a ‘missing link’ in the evolution of modern ears from jaw bones.

The tiny bones in our ears are known to have evolved out of the jawbones of our ancient ancestors, but the steps along this process had remained elusive. 

In the newly-identified transitional species, however, the hearing bones appear to have begun their isolation from each other and the cartilage of the jaw.

This first decoupling — which took place over 100 million years ago — freed chewing and hearing from each other and let them develop into their modern forms.

Experts have found the fossils of an ancient mammal species in China — pictured centre in this artist's reconstruction — that is a 'missing link' in the evolution of modern ears from jaw bones

Experts have found the fossils of an ancient mammal species in China — pictured centre in this artist’s reconstruction — that is a ‘missing link’ in the evolution of modern ears from jaw bones

Modern mammals. including us humans. owe our sense of hearing to the three tiny but intricate bones of the middle ear — dubbed the ‘ossicles’ — which are commonly referred to as the hammer, anvil and stirrup.

These bones take sound waves passing through the air and — passing the vibrations through each of the three bones in turn — transmit such into fluid waves inside the inner ear.

Here, the waves in the snail-shaped inner ear vibrate and are detected by hair cells and turned into electric signals which our brains can interpret as sound.

Originally, however, the ossicles formed part of the jaw which — for our earliest ancestors — was used to both hear as well as to chew.

While the evolution of these jaw bones into a complex hearing apparatus is taken as fact by the scientific community, fossil examples that show the intermediate stages between the two have been elusive.

Palaeontologist Fang-Yuan Mao of the Chinese Academy of Science and colleagues report the discovery of a new species and genus of mammal from the Yixian rock formation near the Chinese city of Jinzhou.

The animal — dubbed Origolestes lii — would have lived during the Early Cretaceous Period, around 146–100 million years ago.

O. lii sports detached hearing bones that represent a key and previously unseen step in the evolution of the modern mammal ear.

This initial separation of hearing and chewing functions served to remove the constraints each imposed on the other, allowing both to be improved.

Origolestes lii, whose fossil remains are pictured, would have lived during the Early Cretaceous Period, around 146–100 million years ago

Origolestes lii, whose fossil remains are pictured, would have lived during the Early Cretaceous Period, around 146–100 million years ago

Origolestes lii, pictured in this artist's reconstruction, sports detached hearing bones that represent a key and previously unseen step in the evolution of the modern mammal ear

Origolestes lii, pictured in this artist’s reconstruction, sports detached hearing bones that represent a key and previously unseen step in the evolution of the modern mammal ear

In the newly-named transitional species the hearing bones (highlighted in pink and yellow) appear to have begun their isolation from each other and the cartilage (purple) of the jaw

In the newly-named transitional species the hearing bones (highlighted in pink and yellow) appear to have begun their isolation from each other and the cartilage (purple) of the jaw

The researchers analysed the fossil remains of O. lii in detail — taking high-resolution CT scans — allowing them to understand the nature of the mammal’s auditory bones and cartilage.

They found that the animals hearing bones lacked the bone-on-bone contact seen in species that came from earlier points in time.

The full findings of the study were published in the journal Science.

The researchers analysed the fossil remains of O. lii in detail — taking high-resolution CT scans, pictured — allowing them to understand the mammal's auditory bones and cartilage

The researchers analysed the fossil remains of O. lii in detail — taking high-resolution CT scans, pictured — allowing them to understand the mammal’s auditory bones and cartilage

Palaeontologist Fang-Yuan Mao of the Chinese Academy of Science and colleagues report the discovery of a new species and genus of mammal from the Yixian rock formation near the Chinese city of Jinzhou

Palaeontologist Fang-Yuan Mao of the Chinese Academy of Science and colleagues report the discovery of a new species and genus of mammal from the Yixian rock formation near the Chinese city of Jinzhou

HOW CAN THE CHEETAH’S INNER EAR MAKE IT FASTER?

The inset shows the cheetah's inner ear, which affects the animal's balance

The inset shows the cheetah’s inner ear, which affects the animal’s balance

Scientists have discovered one of the keys to the incredible speeds of the world’s fastest animal, the cheetah.

The balance system, in vertebrates’ inner ears, consists of three canals that are semicircular.

The canals contain sensory hair cells and fluid. 

The cells detect head movements. 

Each of the three canals is angled differently.

They are all especially sensitive to distinct movements: one is sensitive to up-and-down movements, one to side-to-side movements and the last to tilting movements that go from one side to the other.

New research from the American Museum of Natural History has found that two of the three semicircular canals in the inner ear of the modern cheetah are of different lengths than those of extinct species.

Scientists believe the animal’s inner ear design evolved over time to make it faster. 



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