Extinct creatures fill a puzzling gap in the fossil record

Extinct creatures fill a puzzling gap in the fossil record
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Yunnanozoan Artistic Reconstruction

Artistic reconstruction of Yunnannozoa from the Cambrian biota of Chengjiang shows basket-like pharyngeal skeletons. Credit: Dinghua Yang

Research reveals that yunnanozoans are the oldest known vertebrates with a body.

The new findings answer questions in the fossil record.

A 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 skull and include fish, amphibians, reptiles, birds, mammals, and humans. Invertebrates are animals without backbones.

The evolutionary process that turned invertebrates into vertebrates—and what those first vertebrates looked like—has been a mystery to scientists for centuries.

Now, a team of scientists has studied yunnanozoans, extinct creatures from the early Cambrian period (518 million years ago) and found evidence that they are the oldest known bodied vertebrates. Vertebrate is a term that refers to vertebrates that are extinct but 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.

Root Vertebrate Yunnanozoan

Vertebrate body Yunnanozoan. Credit: Fangchen Zhao

Over the years, as researchers have studied how vertebrates evolved, pharyngeal arches have been a major focus of study. These are the structures that make up parts of the face and neck, such as muscles, bones, and connective tissue. Scientists have hypothesized that the pharyngeal arch evolved from a jointless cartilaginous rod in vertebrate ancestors such as the chordate amphioxus, a close invertebrate relative of vertebrates. However, it is not known for certain whether such anatomy actually existed in ancient ancestors.

To better understand the role of the pharyngeal arch in ancient vertebrates, the research team studied fossils of soft-bodied yunnanozoans found in Yunnan Province, China. For years, researchers have studied yunnanozoans, reaching different conclusions about how to interpret the creature’s anatomy. The affinities of Yunnanozoans have been debated for nearly three decades, with many papers supporting different views, including four Nature and Science.

The research team set out to examine the newly collected Yunnanozoan fossil specimens in previously unstudied ways, performing high-resolution anatomical and ultrastructural studies. The 127 samples they studied contained well-preserved carbonaceous remains, which allowed the team to conduct ultrastructural observations and detailed geochemical analyses.

The group 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 had cellular cartilage in the pharynx, a feature considered unique to vertebrates. The team’s findings confirm that yunnanozoans are root vertebrates. The results of their research show that yunnanozoans are the earliest and at the same time the most primitive relatives of the crown group of vertebrates.

During the study, the team observed that all seven pharyngeal arches in Yunnanzoan fossils are similar. All arches have bamboo-like segments and threads. All the adjacent arches are connected by dorsal and ventral horizontal bars, forming a basket. The basket-like pharyngeal skeleton is a feature found today in jawless fish such as lampreys and jawfishes.

“Two types of pharyngeal skeletons—the basket-like and isolated types—are found in Cambrian and living vertebrates. This means 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 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 team’s new anatomical observations in their research support the evolutionary placement of yunnanozoans at the most basal part of the vertebrate tree of life.

Reference: “Ultrastructure reveals ancestral vertebrate pharyngeal skeleton in Yunnanozoans” by Qingyi Tian, ​​Fangchen Zhao, Han Zeng, Maoyan Zhu, and Baoyu Jiang, July 7, 2022, Science.
DOI: 10.1126/Science.abm2708

The research team included Qingyi Tian of Nanjing University (NJU) and Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS); Fangchen Zhao and Han Zeng of NIGPAS; Maoyan Zhu of NIGPAS and Chinese Academy of Sciences University; 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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