Yang Jie / Zhang Xiguang
About half a billion years ago, in what is now Yunnan Province in China, a tiny larva was trapped in mud. Hundreds of millions of years later, after the mud had long since become the black shale of the Yuan'shan Formation, the larva resurfaced, a carefully preserved time capsule that would reveal more about the evolution of arthropods.
Youti Yuanshi is barely visible to the naked eye. About the size of a poppy seed, it is so well preserved that its exoskeleton is almost completely intact and even the outlines of what were once its internal organs can be seen through the lens of a microscope. Researchers from Durham University who examined it were able to see features of both ancient and modern arthropods. Some of these features told them how the simpler, more worm-like ancestors of living arthropods evolved into more complex organisms.
The research team also found that Y. Yuanshi, which existed during the Cambrian explosion (when most major animal groups appear in the fossil record), shares certain features with extant arthropods, such as crabs, velvet worms, and tardigrades. “The deep evolutionary position of You are Yuanshi… lighten[es] the internal anatomical changes that caused the emergence and diversification of [arthropods],” they said in a study recently published in Nature.
Inside out and outside in
While many fossils preserved in muddy environments such as the Yuan'shan Formation have been flattened by compression, Y. Yuanshi three-dimensional, making it easier to examine. What exactly did this larva look like from the outside and inside?
The research team could see that right away Y. Yuanshi was a lobopodian. Lobopodians are a group of extinct arthropods with long bodies and short legs, or lobopods. There is a pair of lobopods in the middle of each of the twenty segments, and these segments also become progressively shorter from the front to the back of the body. Although no soft tissue is preserved, bulbous outlines suggest an eye on each side of the head, although it is unknown whether these were compound eyes. This creature had a stomodeum, the precursor to a mouth, but no anus. It would have had to both take in food and expel waste through its mouth.
You are yuanshi has a cavity, known as the perivisceral cavity, that surrounds the circumference of a tube that is thought to have once been the gut. The creature's gut ends without an opening, which explains the lack of an anus. Within each segment is a pair of cavities toward the center. The researchers believe this is evidence of digestive glands, especially after comparing them to digestive glands in the fossils of other arthropods from the same era.
A ring around the mouth of the larva was once a circumoral nerve ring, which connected to nerves that extended to eyes and appendages in the first segment. Inside its head is a cavity that contained the brain. The shape of this empty chamber gives some insight into how the brain was structured. From what the researchers could see, the brain of Y. Yuanshi had a wedge-shaped frontal portion, while the rest of the brain was divided into two parts, as shown by the outline of a membrane between them.
A long, long, long time ago and now
Given the physical characteristics, the researchers believe that Y. Yuanshi shows features of both extinct and extant arthropods. Some are ancestral features that are present in all arthropods, both living and extinct. Others are ancestral features that may have been present in extinct arthropods, but are only present in some living arthropods.
One of the features present in all arthropods today is the protocerebrum; its evolutionary precursor was the circumoral nerve ring present in Y. yuanshi. The protocerebrum is the first segment of the brain in arthropods, controlling the eyes and appendages such as the antennae of velvet worms and the mouthparts of tardigrades. Another feature of Y. Yuanshi The only system present in both extant and extinct arthropods is the circulatory system, which is similar to that of modern arthropods, particularly crustaceans.
Lobopods are a morphological feature of Y. Yuanshi which are now found only in some arthropods: tardigrades and velvet worms. Many more species of lobopodians existed during the Cambrian. Lobopodians also had a remarkably structured circulatory system in their legs and other appendages, most similar to that of velvet worms.
“The architecture of the nervous system determines the early configuration of the [arthropod] brain and its associated appendages and senses, thus clarifying homologies between the brain and the senses. [arthropods]according to the researchers in the same study.
Yuti Yuanshi still holds a number of mysteries. These mainly revolve around the fact that it is a larva – what it looked like as an adult we can only guess, and it is possible that this species developed compound eyes or flaps for swimming as it matured. Whether it is the larva of an already known species of extinct lobopod is an open question. Perhaps the answers lie buried somewhere in the Yuan'shan Shale.
Nature, 2024. DOI: 10.1038/s41586-024-07756-8
