Yunnanozoans Are Oldest and Most Primitive Stem Vertebrate, Researchers Conclusively Identify

A group of scientists have identified that the fossil records of yunnanozoans, an extinct creature that lived 518 million years ago in the Cambrian Period, are the oldest known stem vertebrates. Stem vertebrates refer to creatures that were vertebrates, i.e. possessing a spinal column, but have now gone extinct though have closely related living vertebrates. Vertebrates include all mammals, birds, reptiles, amphibians, and fish but the vast majority of complex animal life are invertebrates, i.e. not possessing a spinal column. Scientists for years have been puzzling over the missing link of the evolution from invertebrates to vertebrates.

Now, the research team from the Nanjing Institute of Geology and Paleontology, the Chinese Academy of Sciences, and the Nanjing University, have identified what is the oldest known vertebrate. The team published their findings in the journal Science, on July 8.

The research team examined newly collected yunnanozoan, named so because of their presence in the Yunnan Province of China, fossil specimens using high-resolution anatomical and ultrastructural study, and geochemical studies using the well-preserved carbonaceous residues on the fossils.

Using X-ray microtomography, scanning electron microscopy, transmission electron microscopy, Raman spectrometry, Fourier-transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy, the team has confirmed that the fossils had cellular cartilages in the pharynx, a distinguishing feature of vertebrates. The scientists found that the pharynx in the yunnanozoans had seven similar arches, sharing bamboo-like segments and filaments connected by dorsal and ventral horizontal rods, forming a basket – a feature that is today seen in living jawless fishes.

Researchers have long hypothesized that the arches in the pharynx, responsible for creating structures of the face and neck, had a large role to play in the evolution of early vertebrates.

“Two types of pharyngeal skeletons—the basket-like and isolated types—occur in the Cambrian and living vertebrates. This implies that the form of pharyngeal skeletons has a more complex early evolutionary history than previously thought,” said Tian Qingyi, the first author of the study.