This is how the first ancestors of living vertebrates were

July 8. () –

A study of the yunnanozoans, extinct creatures from the early Cambrian period (518 million years ago) has revealed that they are the oldest known trunk vertebrates.

The term backbone vertebrate refers to those vertebrates that are extinct, but very closely related to living vertebrates.

The research team, from the Nanjing Institute of Geology and Paleontology, the Chinese Academy of Sciences and Nanjing University, published their findings in the journal Science.

Over the years, as scientists have studied how vertebrates evolved, a key focus of research has been on the pharyngeal arches, those structures that produce parts of the face and neck, such as muscles, bones, and the connective tissue.

Researchers have hypothesized that the pharyngeal arch evolved from a non-articulating bar of cartilage in vertebrate ancestors, such as the chordate amphioxus, a close invertebrate relative of vertebrates. But it is not known for sure whether such an anatomy actually existed in ancient ancestors.

In an effort 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 the Yunnanozoans, with differing conclusions about how to interpret the creature’s anatomy. The affinity of the Yunnanozoans has been debated for around three decades, with multiple published papers supporting differing views, including four in Nature and Science.

The research team set out to examine newly collected Yunnanozoan fossil specimens in previously unexplored forms, conducting high-resolution anatomical and ultrastructural study. The 127 specimens they studied have well-preserved carbonaceous residues that allowed the team to perform ultrastructural observations and detailed geochemical analyses.

The team applied X-ray microtomography, scanning electron microscopy, transmission electron microscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and energy dispersive X-ray spectroscopy on the fossil samples. Their study confirmed in multiple ways that Yunnanozoans have cellular cartilage in their pharynx, a feature thought to be specific to vertebrates. The team’s findings support that the Yunnanozoans are backbone vertebrates. The results of their study show that the Yunnanozoans are the first and also the most primitive relatives of the crown group vertebrates.

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

“Two types of pharyngeal skeletons, the basket-shaped and isolated types, occur 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 provided the team with new insights into the detailed structures of the pharyngeal arches. The new anatomical observations the team achieved in their study support the evolutionary placement of the Yunnanozoans. in the lowest part of the vertebrate tree of life.