Shuyu 3D Braincase

419-million-year-old Chinese fossils show human middle ear evolved from fish gills

Shuyu’s 3D braincase. Credit: IVPP

The human middle ear — which houses three small, vibrating bones — is key to transporting sound vibrations to the inner ear, where they become nerve impulses that enable us to hear.

Embryonic and fossil evidence proves that the human middle ear evolved from the spiracle of fish. However, the origin of the vertebrate trachea has long been an unsolved mystery in the evolution of vertebrates.

“These fossils provided the first anatomical and fossil evidence for a vertebrate trachea derived from fish gills.” † Prof. dr. GAI Zhikuno

about 20e Century researchers, who believed that early vertebrates must have a full spiracular gill, looked for a gill between the mandibular and hyoid arches of early vertebrates. However, despite extensive research spanning more than a century, none have been found in vertebrate fossils.

Now, however, scientists at the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and their collaborators have found clues to this mystery of armored Galeaspian fossils in China.

Their findings were published in the journal Frontiers in Ecology and Evolution on May 19, 2022.

According to Prof. GAI Zhikun of IVPP, lead author of the study, researchers at the institute successively found a 438-million-year-old Shuyu 3D braincase fossil and the first 419-million-year-old galeaspid fossil fully conserved with gill filaments in the first branch chamber. The fossils were found in Changxing, Zhejiang Province and Qujing, Yunnan Province, respectively.

Shuyu 3D virtual reconstruction

The 3D virtual reconstruction of Shuyu. Credit: IVPP

“These fossils provided the first anatomical and fossil evidence for a vertebrate trachea derived from fish gills,” GAI said.

A total of seven virtual endocasts of the Shuyu braincase were then reconstructed. Almost all details of the cranial anatomy of Shuyu were revealed in a fingernail-sized skull, including five brain divisions, sensory organs, and cranial nerve and blood vessel passages within the skull.

“Many important human structures can be traced back to our fish ancestors, such as our teeth, jaws, middle ears, etc. The main task of paleontologists is to find the important missing links in the evolutionary chain from fish to man. Shuyu has been considered an important missing link, as important as: Archeopteryxichtyostega and For real”, said ZHU Min, academician of the Chinese Academy of Sciences.

419 million-year-old galeaspid fossil completely preserved with gill filaments

The first 419 million-year-old galeaspid fossil fully preserved with gill filaments in the first branch chamber. Credit: IVPP

The spiracle is a small hole behind each eye that opens to the mouth in some fish. In sharks and all rays, the air coil is responsible for the absorption of water into the oral cavity before expelling it from the gills. The spiracle is often located on the top of the animal, allowing for breathing even if the animal is mostly buried under sediment.

In the polypterus, the most primitive, living bony fish, the spiracles are used to breathe air. However, fish spiracles were eventually replaced in most non-fish species as they evolved to breathe through their nose and mouth. early

tetrapods
Tetrapods are four-limbed (with a few exceptions, such as snakes) vertebrates that make up the superclass Tetrapoda which includes living and extinct amphibians, reptiles, mammals, and birds. They evolved from a group of animals known as the Tetrapodomorpha, which in turn evolved from ancient lobe-finned fishes (Sarcopterygii) about 390 million years ago in the mid-Devonian period.

” data-gt-translate-attributes=”[{” attribute=””>tetrapods, the spiracle seems to have developed first into the Otic notch. Like the spiracle, it was used in respiration and was incapable of sensing sound. Later the spiracle evolved into the ear of modern tetrapods, eventually becoming the hearing canal used for transmitting sound to the brain via tiny inner ear bones. This function has remained throughout the evolution to humans.

“Our finding bridges the entire history of the spiracular slit, bringing together recent discoveries from the gill pouches of fossil jawless vertebrates, via the spiracles of the earliest jawed vertebrates, to the middle ears of the first tetrapods, which tells this extraordinary evolutionary story,” said Prof. Per E. Ahlberg from Uppsala University and academician of the Royal Swedish Academy of Sciences.

Reference: “The Evolution of the Spiracular Region From Jawless Fishes to Tetrapods” by Zhikun Gai, Min Zhu, Per E. Ahlberg and Philip C. J. Donoghue, 19 May 2022, Frontiers in Ecology and Evolution.
DOI: 10.3389/fevo.2022.887172


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