Research reveals yunnanozoans as the oldest known vertebrates.
New findings answer questions in fossil record. The puzzling gap in the fossil record that would explain the evolution of invertebrates into vertebrates has long puzzled scientists. Vertebrates share unique features, such as the backbone and skull, and include fish, amphibians, reptiles, birds, mammals, and humans. Invertebrates, on the other hand, are animals without backbones. The evolutionary process that led invertebrates to become vertebrates—and what those first vertebrates looked like—has been a mystery to scientists for centuries. A team of scientists has now conducted a study of yunnanozoans, extinct creatures from the early Cambrian period (518 million years ago) and discovered evidence that they are the oldest known vertebrates. Stem vertebrate is a term that refers to those vertebrates that have become extinct but are very closely related to living vertebrates. The scientists, from the Nanjing Institute of Geology and Paleontology, the Chinese Academy of Sciences and Nanjing University, published their findings on July 7, 2022, in the journal Science. The stem vertebrate yunnanozoan. Credit: Fangchen Zhao Over the years, as researchers have studied how vertebrates evolved, a key focus of research has been the pharyngeal arches. These are structures that produce parts of the face and neck, such as muscles, bones and connective tissue. Scientists have hypothesized that the pharyngeal arch evolved from a disjointed rod of cartilage in vertebrate ancestors such as the chordate amphioxus, a close invertebrate relative of vertebrates. However, whether such anatomy actually existed in ancient ancestors is not known for certain. In an effort to better understand the role of the pharyngeal arch in ancient vertebrates, the research team studied the fossils of soft-bodied yunnanozoans found in China’s Yunnan Province. For years, researchers have studied yunnanozoans, with differing conclusions about how to interpret the creature’s anatomy. The relatedness of yunnanozoans has been debated for about three decades, with many publications supporting different views, including four in Nature and Science. The research team set out to examine recent yunnanozoan fossil specimens in unexplored ways by conducting a high-resolution anatomical and ultrastructural study. The 127 samples they studied have well-preserved carbonate remains that allowed the team to perform ultrastructural observations and detailed geochemical analyses. The team applied X-ray microtomography, scanning electron microscopy, transmission electron microscopy, Raman spectrometry, Fourier transform infrared spectroscopy and energy dispersive X-ray spectroscopy to the fossil samples. Their study confirmed in many ways that yunnanozoans have cellular cartilage in the pharynx, a feature considered unique to vertebrates. The team’s findings support that yunnanozoans are vertebrates. The results of their study show that yunnanozoans are the oldest and also the most primitive relatives of crown group vertebrates. During their study, the team noticed that all seven pharyngeal arches in the yunnanozoan fossils are similar to each other. All arches have bamboo-like sections and filaments. Adjacent arcs are all connected by dorsal and ventral horizontal bars, forming a basket. A basket-like pharyngeal skeleton is a feature found today in living jawless fish such as lampreys and minnows. “Two types of pharyngeal skeletons—basketiform and isolated types—occur in the Cambrian and in living vertebrates. This means that the form of pharyngeal skeletons has a more complicated early evolutionary history than previously thought,” said TIAN Qingyi, the study’s first author, from Nanjing University and the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences. Their research gave the team new insights into the detailed structures of the pharyngeal arches. The new anatomical observations the team achieved in their study support the evolutionary placement of yunnanozoans in the very basal part of the vertebrate tree of life. Citation: “Ultrastructure Reveals Ancestral Vertebrate Pharyngeal Skeleton in Yunnanzoans” by Qingyi Tian, Fangchen Zhao, Han Zeng, Maoyan Zhu, and Baoyu Jiang, 7 July 2022, Science.DOI: 10.1126/science.abm2 The research team includes Qingyi Tian from Nanjing University (NJU) and the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (NIGPAS). Fangchen Zhao and Han Zeng from NIGPAS. Maoyan Zhu from NIGPAS and the University of the Chinese Academy of Sciences. and Baoyu Jiang from NJU. The Strategic Priority Research Program (B) of the Chinese Academy of Sciences and the National Science Foundation of China funded this research.
