An artist’s reconstruction of Yunanozoans from the Cambrian Chengjiang biota showing basket-like pharyngeal skeletons. Credit: Dinghua Yang
Research reveals Yunanozoans as the oldest known stem vertebrates.
New discoveries answer questions in fossils.
A puzzling gap in the fossil record that would explain the evolution of invertebrates into vertebrates has long puzzled scientists. Vertebrates share unique characteristics, such as a backbone and a skull, and include fish, amphibians, reptiles, birds, mammals, and humans. Invertebrates, on the other hand, are animals without a backbone.
The evolutionary process that caused invertebrates to become vertebrates—and what those earliest vertebrates looked like—has been a mystery to scientists for centuries.
A team of scientists has now conducted a study of Yunanozoans, extinct creatures from the early Cambrian period (518 million years ago), and found evidence that they are the oldest known stem vertebrates. Stem vertebrates is a term that refers to those vertebrates that are extinct but are very closely related to living vertebrates.
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 Yunanozoa. Credit: Fangchen Zhao
Over the years, as researchers have studied how vertebrates evolved, a key focus of study 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 an unfused cartilaginous rod 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 examined Yunnanzoan mollusc fossils found in Yunnan Province, China. For years, researchers have studied Yunanozoans with mixed conclusions about how to interpret the creature’s anatomy. The affinities of the Yunanozoans have been debated for about three decades, with numerous papers published in support of different opinions, including four in Nature and Science.
The research team set out to examine newly collected Yunanozoan fossil specimens in previously unexplored ways, conducting high-resolution anatomical and ultrastructural examination. The 127 specimens they examined had well-preserved carbon remains that allowed the team to conduct 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 specimens. Their study confirmed in many ways that yunanozoans had cellular cartilage in the pharynx, a feature considered unique to vertebrates. The team’s findings support that Yunanozoans were stem vertebrates. The results of their research show that Yunanozoans are the earliest and also the most primitive relatives of the crown group vertebrates.
During their research, the team noticed that all seven pharyngeal arches in the Yunanozoic fossils were similar to each other. All arches have bamboo-like segments and threads. All adjacent arcs are 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 fish.
“Two types of pharyngeal skeletons—the basket-like and isolated types—occur in Cambrian and living vertebrates. This suggests that the shape of pharyngeal skeletons has a more complex early evolutionary history than previously thought,” said TIAN Qingyi, first author of the study, from Nanjing University and the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences.
Their research provided the team with new insights into the detailed structures of the pharyngeal arches. The new anatomical observations achieved by the team in their study support the evolutionary placement of Yunanozoans at the very core of the vertebrate tree of life.
Reference: “Ultrastructure reveals pharyngeal skeleton of Yunanozoic vertebrate ancestors” by Qingyi Tian, Fangchen Zhao, Han Zeng, Maoyan Zhu, and Baoyu Jiang, 7 July 2022, Science.DOI: 10.1126/science.abm2708
The research team includes Qingyi Tian of Nanjing University (NJU) and the Nanjing Institute of Geology and Paleontology, China Academy of Sciences (NIGPAS); Fangchen Zhao and Han Zeng of NIGPAS; Maoyan Zhu of NIGPAS and the University of the Chinese Academy of Sciences; and Baoyu Jiang of NJU.
The Strategic Priority Research Program (B) of the Chinese Academy of Sciences and the National Science Foundation of China funded this research.
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