Yunnanozoan anatomy could explain the evolutionary leap that gave rise to vertebrates

O gill arches are an essential biological innovation that likely contributed to the evolution of the jaws and the bony cage that protects the skull of modern vertebrates. They are believed to have developed from a rigid cartilaginous bar present in ancestors such as chordate amphioxus, but this is not yet fully confirmed.

Now one analysis fossil phylogenetics yunnanozoa, creatures that crossed the seas 520 million years ago, positions them as ancestors of today’s vertebrates and claims they had a gill skeleton. The results of the analysis were published in Science.

The yunnanozoo, similar to a fish with eyes and a body as soft as a worm, would suppose the oldest fossil evidence of evolutionary leap which gave rise to the skeleton in chordates. Specifically, they are considered “mother vertebrates”, that is, although extinct, they are closely related to the living.

Examine the past with unexplored techniques

Although these prehistoric animals are probably the oldest relatives of vertebrates, they still its taxonomic position is debated and how similar their cartilaginous skeleton is compared to that of more modern ones.

The study used a variety of methods (eg, X-ray microtomography, scanning and transmission electron microscopy, and energy dispersive spectrometry element mapping) to assess the structure of 127 of these copies.

By closely observing the cellular and subcellular structures of the skeletal bars of the specimens, the authors found that these arches were composed of cartilage within an extracellular matrix of microfibrils, which is a combination of tissues inherent to vertebrates.

This fact places the Yunanozoo as the oldest known animal within this group. “The skeleton of these organisms shares three characteristics with vertebrates belonging to the crown group: predominance of microfibrils in the cartilaginous matrix, gill arches with stacked circular structures and the presence of horizontal bars at both ends of the arches”, he explains to SINC. Baoyu Jiangco-author of the study and researcher at Nanjing University (China).

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The entire specimen of a yunnanozoo. / Fangchen Zhao

In search of the origin of vertebrates

The findings support the idea that Yunnanozoans are stem vertebrates. “While evolutionary biologists have looked for the mythical ancestor that explains everything about the body plan of these organisms, perhaps the opposite is a sensible approach,” he says. Tetsuto Miyashitaindependent investigator.

This evolution expert argues that the path to modern vertebrates can be better understood if we fill in the family tree with divergent and discontinuous anatomical shapesguided by phylogenetic inference (which seeks hypotheses about patterns of relationships) rather than theory.

For the study’s researchers, the next step is “to find out what are the evolutionary steps that led vertebrates to develop their jaws as a result of the arches of the frontal parts. We need to find evidence to study this process further”, concludes Jiang.