HomeSportsExtinct creatures fill a puzzling gap in the fossil record

Extinct creatures fill a puzzling gap in the fossil record

Artistic reconstruction of the Yunnanozoan

The artistic reconstruction of the yunnanozoan from the Chengjiang Cambrian biota shows basket-shaped pharyngeal skeletons. 1 credit

Research reveals that yunnanozoa are the oldest known stem vertebrates.

New discoveries answer questions in the 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 a spine 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 led invertebrates to become vertebrates — and what those early 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 (518 million years ago), and found evidence that they are the oldest known stem vertebrates. Stem vertebrates is a term that refers to extinct vertebrates, but very closely related to living vertebrates.

The scientists, from the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences and Nanjing University, published their findings on July 7, 2022 in the journal Science.

Yunnanozoan Strain Vertebrate

The stem vertebrate yunnanozoan. 1 credit

Over the years, as researchers have studied the evolution of vertebrates, the pharyngeal arches have been a focus of research. They 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 unarticulated cartilage rod in vertebrate ancestors, such as the chordate amphioxus, an invertebrate close relative of vertebrates. However, it is unclear whether such anatomy actually existed in ancient ancestors.

To better understand the role of the pharyngeal arch in ancient vertebrates, the research team studied soft-bodied yunnanozoan fossils found in Yunnan Province, China. For years, researchers have studied yunnanozoans, with differing conclusions on how to interpret the creature’s anatomy. The affinity of yunnanozoans has been debated for about three decades, with several published papers supporting various opinions, including four in Nature and Science.

The research team set out to examine newly collected yunnanozoan fossil specimens in previously unexplored ways, conducting high-resolution anatomical and ultrastructural study. The 127 specimens they studied have well-preserved carbonaceous residues 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 on the fossil specimens. Their study confirmed in multiple ways that yunnanozoans have cellular cartilage in the pharynx, a feature thought to be specific to vertebrates. The team’s findings confirm that yunnanozoa are stem vertebrates. The results of their study show that the yunnanozoans are the oldest and also the most primitive relatives of the vertebrates of the crown group.

During their study, the team observed that the seven pharyngeal arches of the yunnanozoan fossils are similar to each other. All bows have bamboo-like segments and filaments. The neighboring arches are all connected by dorsal and ventral horizontal rods, forming a basket. A basket-like pharyngeal skeleton is a feature found today in living jawless fish, such as lampreys and hagfishes.

“Two types of pharyngeal skeletons – basket-shaped and isolated types – occur in Cambrian and living vertebrates. This implies that the shape of pharyngeal skeletons has a more complex early evolutionary history than previously thought,” said said TIAN Qingyi, the 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 made by the team in their study support the evolutionary placement of yunnanozoa at the very basal part of the vertebrate tree of life.

Reference: “Ultrastructure Reveals the Pharyngeal Skeleton of Ancestral Vertebrates in Yunnanozoa” by Qingyi Tian, ​​Fangchen Zhao, Han Zeng, Maoyan Zhu, and Baoyu Jiang, July 7, 2022, Science.
DOI: 10.1126/science.abm2708

The research team includes Qingyi 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 University of 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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