A strange creature that roamed the oceans of Earth more than half a million years ago appears to be the oldest vertebrate relative we have encountered so far.
They are called yunnanozous, dating from the early Cambrian about 518 million years ago. Paleontologists have discovered the cartilaginous features found in their fossilized remains are comparable to those of modern vertebrates.
This suggests that animals are stem vertebrates, an extinct sister group from which modern vertebrates descend.
“Pharyngeal arches are a key innovation that probably contributed to the evolution of the jaws and vertebral box of vertebrates,” wrote the team, led by Qingyi Tian of Nanjing University and the Chinese Academy of Sciences. China.
“The pharyngeal skeleton of controversial Cambrian animals called yunnanozoans may contain the oldest fossil evidence limiting the early evolution of arcs, but their correlation with that of vertebrates is still being discussed.
“Examining additional specimens in techniques not previously explored … we found evidence that yunnanozoan gill arches consist of cellular cartilage with an extracellular matrix dominated by microfibrils, a feature that was previously considered specific to vertebrates.”
Tracking the evolution of vertebrates to their murky beginnings has been a difficult task for scientists. For hundreds of millions of years, traces of life erode and degrade; Fossils can be left behind, if conditions are right, but they are often extremely difficult to interpret, especially for the very old and very rare.
The Yunnanozoans are very old and very strange. For decades, scientists have been baffled about where this creature fits into the tree of life, relying on studies and opposing interpretations of ancient fossils recovered from Maotianshan shales in China.
It was in the hope of clarifying the problem that Tian and his colleagues embarked on a study of 127 recently collected yunnanozoan fossils.
A fossilized yunnanozou (Fangchen Zhao)
These fossils were subjected to a number of techniques that had not previously been applied to yunnanozoa, such as X-ray microtomography, scanning and transmission electron microscopy, Raman spectrometry, and mapping of spectrometry elements. energy dispersive. Their results revealed previously unknown details about the anatomy of yunnanozoans.
The pharyngeal arch is a structure that can be found during the embryonic development of vertebrate organisms, and is the precursor of various parts of the face and jaw, depending on the organism.
In fish, these bows are known as gill arches and support the gills. Scientists believe that, in the ancestors of vertebrates, the pharyngeal arch evolved from an unarticulated cartilage rod; although it is unknown when and how it arose.
Seven pairs of bilaterally symmetrical gill arches had previously been identified in yunnanozoans. Tian and his colleagues looked much more closely at the microscopic structures of these bars and how they are arranged in the body.
They found that the seven pairs of fossil arches are similar to each other, and that they are connected by horizontal dorsal and ventral bars, forming a basket-like structure.
They also found that gill arches are composed of cartilage within a densely packed array of microfibrils. They found that these microfibrils are similar to the microfibrils found in the connective tissue of vertebrates. It is a combination of features widely present in modern vertebrates: cartilage in an array of microfibrils, gill arches, and the presence of horizontal bars at the ends of gill arches.
In addition, a basket-like structure of the pharyngeal skeleton can be found in some modern jawless fish, such as lampreys and mackerel fish.
“Two types of pharyngeal skeletons, the basket types and the isolated ones, are found in the Cambrian and living vertebrates,” Tian said. “This implies that the shape of the pharyngeal skeletons has a more complex early evolutionary history than previously thought.”
The team’s evidence is compelling for vertebrate-like structures in these mysterious animals. Although they are not directly related to modern vertebrates, yunnanozous can therefore help to illuminate the evolution of vertebrates.
“While evolutionary biologists have been busy chasing the mythical ancestor that explains everything about the vertebrate body plane, perhaps the opposite is a sensible approach,” wrote paleobiologist Tetsuto Miyashita of the Canadian Museum of Nature in a scientific perspective. related. Miyashita did not participate in the investigation.
“In other words, the winding journey to modern vertebrates can be better understood by populating the family tree with divergent and discontinuous anatomical forms, guided by phylogenetic inference rather than theory.”
The team paper has been published in Science.