The artistic reconstruction of the yunnanozoan of the Chengjiang biota of the Cambrian shows basket-like pharyngeal skeletons. Credit: Yang Dinghua
Scientists have long been baffled about the gap in the fossil record that would explain the evolution of invertebrates to vertebrates. Vertebrates, such as fish, amphibians, reptiles, birds, mammals, and humans, share unique characteristics, such as a spine and a skull. Invertebrates are animals without a spine.
The process that moved invertebrates into vertebrates, and how they were the first vertebrates, has been a mystery to scientists for centuries.
A research team has conducted a study of yunnanozoans, extinct creatures from the early Cambrian period (518 million years ago), and has discovered evidence that they are the oldest known stem vertebrates. The term stem vertebrate refers to those vertebrates that are extinct but closely related to living vertebrates.
The research team, from the Nanjing Institute of Geology and Paleontology, the Chinese Academy of Sciences and Nanjing University, published their findings in the journal. Science on July 8, 2022.
Over the years, as scientists have studied how vertebrates evolved, a key focus of research has been pharyngeal arches, those structures that produce parts of the face and neck, such as muscles, bones, and tissue. connectivity. Researchers have hypothesized that the pharyngeal arch evolved from an unarticulated cartilage rod in vertebrate ancestors, such as the amphioxus cordate, an invertebrate relative close to vertebrates. But it is not known for certain whether this anatomy really existed in the ancient ancestors.
In an effort to better understand the role of the pharyngeal arc in ancient vertebrates, the research team studied the fossils of soft-bodied yunnanozoans found in Yunnan Province, China. For years, researchers have studied Yunnanozoans, with different conclusions about how to interpret the creature’s anatomy. The affinity of yunnanozous has been debated for about three decades, with several published articles supporting different opinions, including four in Nature i Science.
The yunnanozou stem vertebrate. Credit: Zhao Fangchen
The research team set out to examine recently collected fossil specimens of yunnanozoans in ways not previously explored, conducting a high-resolution anatomical and ultrastructural study. The 127 specimens they studied have well-preserved carbon residues that allowed the team to perform ultrastructural observations and detailed geochemical analyzes.
The team applied X-ray microtomography, scanning electron microscopy, transmission electron microscopy, Raman spectrometry, Fourier transform infrared spectroscopy, and energy-dispersed X-ray spectroscopy to the fossil specimens. His study confirmed in multiple ways that yunnanozoans have cellular cartilage in the pharynx, a feature considered specific to vertebrates. The team’s findings support that yunnanozous are stem vertebrates. The results of his study show that yunnanozoans are the oldest relatives and also the most primitive of the vertebrates of the crown group.
During their study, the team observed that all seven pharyngeal arches of yunnanozoan fossils are similar to each other. All arches have segments and filaments similar to bamboo. The neighboring arches are all connected by horizontal dorsal and ventral bars, forming a basket. A basket-like pharyngeal skeleton is a feature found today in live jawless fish, such as lampreys and mixina fish.
“Two types of pharyngeal skeletons are produced in the Cambrian and living vertebrates: basket types and isolated ones. This implies that the shape of pharyngeal skeletons has a more complex early evolutionary history than previously thought,” said Tian Qingyi, the first author. of the study, Nanjing University and Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences.
His research provided the team with new ideas about the detailed structures of pharyngeal arches. The new anatomical observations the team made in their study support the evolutionary placement of yunnanozoans in the very basal part of the vertebrate life tree.
How did vertebrates first evolve their jaws? More information: Qingyi Tian et al, Ultrastructure reveals the pharyngeal skeleton of ancestral vertebrates in the Yunnanozoans, Science (2022). DOI: 10.1126 / science.abm2708. www.science.org/doi/10.1126/science.abm2708
Tetsuto Miyashita, “Arc” -typing vertebrates, Science (2022). DOI: 10.1126 / science.adc9198. www.science.org/doi/10.1126/science.adc9198
Provided by the Chinese Academy of Sciences
Citation: The study reveals that yunnanozous are the oldest known stem vertebrates (2022, July 7) recovered on July 7, 2022 from
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