It was soft, wormlike, lived more than 500 million years ago – and now scientists say the extinct animal was an ancient ancestor of modern vertebrates.
Scientists have long been searching for fossils of distant vertebrate ancestors to explain the evolution of invertebrates to vertebrates, including fish and mammals.
Researchers from the Chinese Academy of Sciences and Nanjing University say new analysis of 127 fossils of yunnanozoans found at the Chengjiang Fossil Site in the southwestern province of Yunnan indicates that the animal bridged that gap.
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“These analyses support that yunnanozoans are the earliest branching stem vertebrates,” the team wrote in an article published in the peer-reviewed journal Science on Friday.
The fossils from the early Cambrian period 518 million years ago showed that the fishlike creatures had an unmistakable vertebrate trait – a pharyngeal skeleton made of cellular cartilage, a key feature that likely laid the foundation for the development of the jaw and skull.
The skeleton comprised seven pairs of pharyngeal arches, structures in developing embryos that give rise to bone, nerves and muscles of the face and neck.
They said all seven pharyngeal arches in the creature were identical with bamboo-like segments and gill filaments, and the structures of its skeletal bars closely resembled the cartilaginous gill arches of modern vertebrates.
The team also said the arches were made of cellular cartilage in a matrix that had previously been considered specific to vertebrates.
“Pharyngeal arches are a key innovation that likely contributed to the evolution of the jaws and braincase of vertebrates,” the team wrote.
Lead author Jiang Baoyu, a professor in palaeontology and stratigraphy at Nanjing University, said the study offered solid evidence that yunnanozoans were a close relative and one of the ancestors of living vertebrates.
“The two key features of vertebrates are jaw and limbs, or fins in fish. The jaw is developed from a pair of pharyngeal arches,” Jiang said.
“There have long been debates about how jawed vertebrates evolved from jawless ancestors.
“Our findings on yunnanozoans support a mainstream hypothesis that ancestors of vertebrates had serially patterned, identical pharyngeal arches, with the first two or three pairs developing into the jaw and hyoid bones.”
He said the specimens analysed showed that yunnanozoans had an ancestral head with mouth, and the team would continue to study the fossils to confirm if they had structures of eyes and ears.
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