This Confusing Water Creature Could Be The Oldest Relative Of All Vertebrates

The strange creatures that raged in Earth’s oceans more than half a billion years ago appear to be the earliest vertebrate relatives we’ve found to date.

They are called yunnanozoans, dating from the Early Cambrian about 518 million years ago. The cartilaginous features found in their fossil remains are comparable to those of modern vertebrates, paleontologists have found.

This suggests that these animals were stem vertebrates, an extinct sister group to the original group of modern vertebrates.

“The pharyngeal curvature is a key innovation that likely contributed to the evolution of the vertebrate jaw and braincase,” wrote the team led by Qingyi Tian of Nanjing University and the Chinese Academy of Sciences in China.

“The pharyngeal skeletons of controversial Cambrian animals called yunnanozoans may contain the oldest fossil evidence limiting the early evolution of arches, but their correlation to vertebrates is debated.

“By examining additional specimens in previously unexplored techniques.. we found evidence that the Yunnanozoan branchial arches were composed of cellular cartilage with an extracellular matrix dominated by microfibrils, a feature hitherto thought to be specific to vertebrates.”

Tracing vertebrate evolution back to its grim beginnings has been a difficult task for scientists. Over hundreds of millions of years, traces of life were eroded and degraded; Fossils can be left behind, if the conditions are right, but they are often very difficult to interpret, especially for very old and very strange fossils.

Yunnanozoa is very old, and many are strange. For decades, scientists have puzzled over where this creature fits on the tree of life, based on competing studies and interpretations of ancient fossils found from the Maotianshan Shale in China.

In hopes of clarifying the issue, Tian and his colleagues began a study of 127 recently collected Yunnanozoan fossils.

A yunnanozoan fossil. (Fang Chen Zhao)

These fossils were subjected to a variety of techniques that had never been applied to Yunnanozoans before, including X-ray microtomography, scanning and transmission electron microscopy, Raman spectrometry, and element mapping energy dispersive spectrometry. Their results reveal previously unknown details about Yunnanozoan anatomy.

The pharyngeal arch is a structure that can be found during the embryonic development of vertebrate organisms, and is the precursor to a number of different parts of the face and jaw, depending on the organism.

In fish, these arches are known as branchial arches, and they provide support for the gills. Scientists believe that, in the ancestor of vertebrates, the pharyngeal arches evolved from disjointed cartilage rods; although when and how it appeared is unknown.

Seven pairs of bilaterally symmetrical branchial arches have been previously identified in the Yunnanozoans. Tian and his colleagues took a closer look at the microscopic structure of these rods, and how they are arranged in the body.

They found that the seven pairs of arches in the fossil were similar to each other, and that they were connected by horizontal dorsal and ventral trunks, forming a basket-like structure.

They also found that the branchial arch is composed of cartilage within a dense matrix of microfibrils. These microfibrils, they found, were similar to the microfibrils found in the connective tissue of vertebrates. This is a combination of features that are widely present in modern vertebrates: cartilage in the microfibril matrix, branchial arches, and the presence of horizontal bars at the ends of branchial arches.

In addition, a basket-like pharyngeal skeleton structure can be found in some modern jawless fish, such as lampreys and hagfish.

“Two types of pharyngeal skeleton – basket-like and isolated types – occur in Cambrian and living vertebrates,” Tian said. “This implies that the skeletal shape of the pharynx has a more complex early evolutionary history than previously thought.”

The team’s evidence is very convincing for a vertebrate-like structure in this mysterious animal. Although not directly related to modern vertebrates, the yunnanozoans may help explain vertebrate evolution.

“Although evolutionary biologists have been busy pursuing the mythical ancestor that explains everything about vertebrate body plans, perhaps the opposite is a reasonable approach,” wrote paleobiologist Tetsuto Miyashita of the Canadian Museum of Nature in a related report. Science Perspective. Miyashita was not involved in the study.

“In other words, the meandering journey towards modern vertebrates is perhaps best understood by filling the family tree with disjointed and disjointed anatomical shapes, guided by phylogenetic inference rather than theory.”

The team paper has been published in Science.

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