Extinct creatures fill puzzling gap in fossil record

Yunnanozoan Artistic Reconstruction

Artistic reconstruction of the Yunnanozoan from the Cambrian Chengjiang biota shows basket-like pharyngeal skeletons. Photo credit: Dinghua Yang

Research shows that Yunnanozoa are the oldest known basal vertebrates.

New finds answer questions in the fossil record.

The puzzling gap in the fossil record that would explain the evolution from invertebrates to 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. Invertebrates, on the other hand, are animals without a backbone.

The evolutionary process that turned invertebrates into vertebrates — and what those earliest vertebrates looked like — has puzzled 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 progenitors. Progenitor vertebrate is a term that refers to those vertebrates that are extinct but are very 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 Science.

Phylum Vertebrate Yunnanozoan

The Yunnanozoan stem vertebrate. Photo credit: Fangchen Zhao

Over the years, as researchers studied how vertebrates evolved, a focus of research has been on the pharyngeal arches. These are structures that produce parts of the face and neck, such as muscle, bone, and connective tissue. Scientists have hypothesized that the pharyngeal arch evolved from an inflexible cartilaginous rod in vertebrate ancestors such as B. the Chorda amphioxus, a close invertebrate relative of the vertebrates. However, whether such an anatomy actually existed in ancient ancestors is not known with certainty.

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

The research team set out to examine newly collected Yunnanozoan fossils in previously unexplored ways, conducting a high-resolution anatomical and ultrastructural study. The 127 samples examined show well-preserved carbonaceous residues that have enabled the team to perform 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 samples. Their study confirmed in several ways that Yunnanozoa have cytocartilage in the pharynx, a feature thought to be specific to vertebrates. The team’s findings support that Yunnanozoa are ancestral vertebrates. The results of their study show that the Yunnanozoa are the earliest and at the same time most primitive relatives of the crowned vertebrates.

During their study, the team observed that all seven pharyngeal arches in the Yunnanozoan fossils are similar to one another. All arches have bamboo-like segments and filaments. Adjacent 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 hagfish.

“Two types of pharyngeal skeletons—the basket-like and isolated types—are found in the Cambrian and in living vertebrates. This implies that the shape of the pharyngeal skeletons has a 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, Chinese Academy of Sciences.

Their research gave the team new insights into the detailed structures of the pharyngeal arches. The new anatomical observations the team made in their study support the evolutionary placement of yunnanozoans in the very basal part of the vertebrate tree of life.

Reference: “Ultrastructure reveals pharyngeal skeleton of vertebrate ancestors 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 from Nanjing University (NJU) and Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences (NIGPAS); Fangchen Zhao and Han Zeng from NIGPAS; Maoyan Zhu from NIGPAS and the University of the Chinese Academy of Sciences; and Baoyu Jiang of 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.