Research reveals yunnanozoans as the oldest known stem vertebrates.
The new findings answer questions in the fossil record.
The perplexing 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 a 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 Nanjing University, published their findings in the journal on July 7, 2022. Sciences.
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 muscle, bone, 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 this anatomy actually existed in the ancient ancestors was 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 Greek mollusks 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 Yunnanozoans has been debated for nearly three decades, with many published research papers supporting differing opinions, including four in nature of mood and the 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 infrastructural study. The 127 samples studied contain well-preserved carbon residues that allowed the team to perform detailed ultrastructural observations and geochemical analyses.
The team applied X-ray microscopy, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, infrared spectroscopy, and X-ray energy scattering to the fossil specimens. Their study confirmed in several ways that Yunnanozoans have cellular cartilage in the pharynx, a property considered to be specific to vertebrates. The team’s findings support that yunnanozoans are leg-like vertebrates. The results of their study show that the Greeks are the oldest and most primitive relatives of the crown group of vertebrates.
During the study, the team noted that all seven of the pharyngeal arches in the Yonanozoan fossils are similar to each other. All mounts 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—the basket-like species and the isolated species—in the Cambrian and in living vertebrates. This means that the shape of pharyngeal skeletons has a much more complex 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 of the Chinese Academy of Sciences.
Their research provided the team with new insights into the detailed structures of pharyngeal arches. New anatomical observations made by the team in their study support the evolutionary position of ionozoans in the core part of the vertebrate tree of life.
Reference: “Basic Skeleton Reveals Ancestor 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 from 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 Chinese Academy of Sciences; 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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