A group of paleontologists has concluded in a study published in Science that the Yunnanozoans could be the oldest relatives of vertebrates . These strange creatures, which look like fish but had a worm-like texture, lived in our planet’s oceans more than 500 million years ago, in the early Cambrian.
Experts have reached this conclusion due to the discovery in its fossil remains of cartilaginous features comparable to those of modern vertebrates. They would be a sister group, now extinct, of the group from which modern vertebrates descend. ” Pharyngeal arches are a key innovation that likely contributed to the evolution of vertebrate jaws and ribcage,” the team wrote in the publication.
“The pharyngeal skeleton of the controversial Cambrian animals, called Yunnanozoans, may contain the oldest fossil evidence limiting the early evolution of bows, although its correlation with that of vertebrates remains disputed,” the paper reads. Researchers have found microfibrils in the cartilage of the gill arches of Yunnanozoans, something that is specific to vertebrates.
Investigating the evolution of vertebrates from their beginnings is a very complicated task. The traces of life are damaged and degraded over hundreds of millions of years and, although fossils may exist if the right conditions are met, these are usually very difficult to interpret, especially the oldest and strangest.
The Yunnanozoans are precisely very ancient and very strange creatures . Scientists have spent decades trying to place them on the tree of life using studies and interpretations of ancient fossils recovered from the Maotianshan schists in China.
The team led by Qingyi Tian of Nanjing University and the Chinese Academy of Sciences set out to study 127 recently collected Yunnanozoan fossils with the aim of shedding light on these rare creatures and their place in the evolution of life.
The researchers subjected the Yunnanozoan fossils to techniques that had not been used on them before . They used X-ray microtomography, scanning and transmission electron microscopy, Raman spectrometry, and energy dispersive spectrometry mapping of elements.
The pharyngeal arch is a structure that appears during the embryonic development of vertebrates. Thanks to it, a series of parts of the face and jaw are subsequently formed. In fish, the pharyngeal arch is known as the branchial arch and is responsible for supporting the gills. Scientists believe that in the ancestors of vertebrates, the pharyngeal arch evolved from a non-articulated cartilaginous rod. It is unknown when and how it arose.
Seven pairs of bilaterally symmetrical gill arches were previously identified in the Yunanozoans. Examining these structures more precisely, Tian and his team found that the seven pairs of arches in the fossils are similar to each other, and that they are connected by dorsal and ventral horizontal bars , forming a basket-like structure.
They also saw that the gill arches are composed of cartilage within a densely packed matrix of microfibrils. These microfibrils, they discovered, are similar to the microfibrils found in the connective tissue of vertebrates. This is a combination of features found in modern vertebrates: cartilage in a matrix of microfibrils, gill arches, and the presence of horizontal bars at the ends of the gill arches.
The basket-shaped pharyngeal skeleton has also been seen in some modern fish that do not have a jaw, such as lampreys and hagfish.
“In the Cambrian and in living vertebrates, there are two types of pharyngeal skeleton, the basket-type and the isolated one,” Tian said. “This implies that the shape of the pharyngeal skeletons has a more complex early evolutionary history than previously thought.”
The evidence that has been obtained in the study is convincing regarding the structures shared by yunnanozoa and vertebrates. However, the latter are not directly related to modern vertebrates . Despite this, the yunnanozoans could help to better understand how vertebrates evolved.
Referencia: Tian, Q. et. al. 2022. Ultrastructure reveals ancestral vertebrate pharyngeal skeleton in yunnanozoans. Science. DOI: 10.1126/science.abm2708
