Embryonic and fossil evidence proves that the human middle ear evolved from the spiracle of fishes. However, the origin of the vertebrate spiracle has long been an unsolved mystery in vertebrate evolution.
Some 20th century researchers, believing that early vertebrates must possess a complete spiracular gill, searched for one between the mandibular and hyoid arches of early vertebrates. Despite extensive research spanning more than a century, though, none were found in any vertebrate fossils.
Now, however, scientists from the Institute of Vertebrate Paleontology and Paleoanthropology (IVPP) of the Chinese Academy of Sciences and their collaborators have found clues to this mystery from armored galeaspid fossils in China.
According to Prof. GAI Zhikun from IVPP, first author of the study, researchers from the institute successively found over the last 20 years a 438-million-year-old Shuyu 3D braincase fossil and the first 419-million-year-old galeaspid fossil completely preserved with gill filaments in the first branchial chamber. The fossils were found in Changxing, Zhejiang Province and Qujing, Yunnan Province, respectively.
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“These fossils provided the first anatomical and fossil evidence for a vertebrate spiracle originating from fish gills,” said GAI.
A total of seven virtual endocasts of the Shuyu braincase were subsequently reconstructed.
