Shuyu 3D Braincase

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

Shuyu 3D Braincase

The 3D brain envelope of Shuyu. Credit: IVPP

The human middle ear – which holds three small, vibrating bones – is the key to transporting sound vibrations into the inner ear, where they become nerve impulses that allow us to hear.

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

“These fossils provided the first anatomical and fossil evidence for a vertebrate spiral derived from fish gills.” – Prof. GAI Zhikun

About 20th The researchers of the century, who believed that early vertebrates must have a completely spectacular gill, searched for one between the mandibular and hyoid arches of early vertebrates. Despite extensive research spanning more than a century, no fossils of vertebrate animals were found.

But now researchers from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) at the Chinese Academy of Sciences and their associates have found clues to this mystery from armored insane fossils in China.

Their results were published in the journal Boundaries in ecology and evolution on May 19, 2022.

According to Prof. GAI Zhikun from IVPP, first author of the study, researchers from the institute have gradually found over the past 20 years a 438 million year old Shuyu 3D cerebral fossil and the first 419 million-year-old gallbladder fossil completely preserved 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 virtual 3D reconstruction of Shuyu. Credit: IVPP

“These fossils provided the first anatomical and fossil evidence of a vertebral spiral derived from fish gills,” GAI said.

A total of seven virtual endocasts by Shuyu brain case was subsequently reconstructed. Almost all details of the cranial anatomy of Shuyu was revealed in its fingernail-sized skull, including five brain compartments, sensory organs and cranial nerves, and blood vessel passages in the skull.

“Many important structures in humans can be traced back to our fish’s ancestors, such as our teeth, jaws, middle ears, etc. Paleontologists’ main task 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 Archeopteryx, Ichthyostega and TiktaalikSaid ZHU Min, a graduate of the Chinese Academy of Sciences.

419 million year old galespid fossil completely preserved with gill filament

The first 419 million year old galespid fossil completely preserved with gill filament in the first branch chamber. Credit: IVPP

The spiral is a small hole behind each eye that opens to the mouth of some fish. In sharks and all rays, the spiracle is responsible for the intake of water in the oral cavity before it is expelled from the gills. The spiral is often placed towards the top of the animal, which allows breathing even when the animal is mostly buried under sediment.

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

Tetrapods are four-limbed (with a few exceptions, such as snakes) vertebrates that make up the superclass Tetrapods that include living and extinct amphibians, reptiles, mammals and birds. They evolved from a group of animals called Tetrapodomorpha, which in turn evolved from ancient fin-finned fish (Sarcopterygii) about 390 million years ago during the Middle 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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