Artist’s reconstructions of Yunanozoans from Cambrian Chengjiang organisms show basket-like pharyngeal skeletons. Credit: Dinghua Yang
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Research reveals yunnanozoans as the oldest known stem vertebrates.
The new findings answer questions in the fossil record.
The baffling gap in the fossil record that would explain the evolution of invertebrates into vertebrates has long puzzled scientists. Vertebrates share unique features, such as a backbone and skull, and include fish, amphibians, reptiles, birds, mammals, and humans. On the other hand, invertebrates are animals that do not have a backbone.
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 found evidence that they are the oldest known vertebrates. Stem vertebrates is a term referring to extinct vertebrates, but they are closely related to living vertebrates.
The scientists, from the Nanjing Institute of Geology and Paleontology, the Chinese Academy of Sciences and the Nanjing University, published their findings on July 7, 2022 in the journal. Sciences.
Haste Yunanozoan vertebrate. Credit: Fangchen Zhao
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Over the years, while researchers have studied how vertebrates evolved, the main focus of research has been the pharyngeal arches. These are the structures that produce parts of the face and neck, such as muscles, bones, and connective tissue. Scientists hypothesized that the pharyngeal arch evolved from a disjointed cartilaginous rod in the ancestors of vertebrates such as chordates, a close relative of vertebrate invertebrates. However, whether such anatomy actually existed in ancient ancestors was not known with certainty.
In an effort to better understand the role of the pharyngeal arch in ancient vertebrates, the research team studied fossils of Greek molluscs found in Yunnan Province, China. For years, researchers have studied yananozoans, with varying conclusions about how to interpret the creature’s anatomy. The affinity of the Yunnanozoans has been debated for nearly three decades, with numerous published research papers supporting differing opinions, including four in temper nature It’s at Sciences.
The research team set out to examine the newly collected yunnanozoa fossil samples in previously unexplored ways and to conduct a high-resolution anatomical and infrastructure study. The 127 samples studied contain well-preserved carbon residues that allowed the team to perform ultrastructural observations and detailed geochemical analyses.
The team applied X-ray microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and energy scattering X-ray spectroscopy to the fossil specimens. Their study confirmed in several ways that Yunnanozoans have cellular cartilage in the pharynx, a characteristic considered to be specific to vertebrates. The team’s findings support that yunnanozoans are stem-like vertebrates. The results of their study show that the Greeks are the oldest and most primitive relatives of the crown group vertebrates.
During the study, the team observed that all seven pharyngeal arches in Yonanozoan fossils are similar to each other. All brackets have bamboo-like slats and strings. All adjacent arches are connected by dorsal and ventral horizontal rods that form a basket. The basket-shaped pharyngeal skeleton is a feature found today in live, jawless fish such as lampreys and hagfish.
There are two types of pharyngeal skeletons – basket-like species and isolated species – in the Cambrian and living vertebrates. This means that the shape of pharyngeal skeletons has a much 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 of the Chinese Academy of Sciences.
Their research provided the team with new insights into the detailed structures of the pharyngeal arches. The new anatomical observations the team made in their study support the evolutionary position of ionozoans at the most basic part of the vertebrate tree of life.
Reference: “Basic Structure Reveals Ancestral Vertebrate Skeleton in Yunnanozoans” by Qingyi Tian,Fangchen Zhao, Han Zeng, Maoyan Zhou, and Baoyu Jiang, July 7, 2022, available here. Sciences.
DOI: 10.1126 / science.abm2708
The research team includes Chengyi Tian of Nanjing University (NJU) and Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (NIGPAS). Fangchen Zhao and Han Zeng of NIGPAS; Maoyan Zhu of NIGPAS and the Chinese Academy of Sciences University; and Baoyu Jiang from NJU.
This research was funded by the Strategic Priority Research Program (B) of the Chinese Academy of Sciences and the National Science Foundation of China.
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