Yang Jie / Zhang Xiguang
About 500 million years ago, tiny larvae were trapped in mud in what is now Yunnan, China. Hundreds of millions of years later, long after the mud had turned into the black shale of the Wonshan Formation, the larvae re-emerged to the surface — a meticulously preserved time capsule that could reveal more about arthropod evolution.
Yuti Yuanxi Barely visible to the naked eye: about the size of a poppy seed, its exoskeleton is so perfectly preserved that even the outlines of its internal organs can be seen through the lens of a microscope. When Durham University researchers examined it, they were able to see features of both ancient and modern arthropods. Some of these features tell us how the simpler, worm-like ancestors of modern arthropods evolved into more complex organisms.
The research team also Y. Motoishi, This animal, which existed during the Cambrian explosion (when most of the major animal groups began to appear in the fossil record), shares characteristics with modern arthropods such as crabs, velvet worms and tardigrades. Yuuti Genshi… Illuminate[es] Internal anatomical changes [arthropods]”In a recent study published in Nature, they state:
From inside to outside, from outside to inside
Many fossils preserved in muddy environments such as the Wonsan Formation are flattened by compression, Y. Motoishi It maintains its three-dimensional shape, making it easy to observe. So what did this larva look like on the outside and inside?
The research team immediately Y. Motoishi It was a lobopod. Lobopods are a group of extinct arthropods, or lobopods, with long bodies and short legs. There was a pair of lobe feet in the middle of each of the 20 segments, which also gradually shortened from the front to the back of the body. No soft tissue was preserved, but the spherical outline suggests that it had eyes on either side of the head, although it is unclear whether these were compound eyes. The creature had a proboscis, the precursor to a mouth, but no anus. It would have had to both ingest and excrete food through its mouth.
Uti Former There is a cavity called the visceral cavity that surrounds the outline of a tube that may have once been an intestine. The creature’s intestine has no opening, which is why it has no anus. Inside each segment, toward the center, is a pair of cavities. The researchers believe these are evidence of digestive glands, especially after comparing them to those in other arthropod fossils from the same era.
The ring around the larva’s mouth was once the perioral nerve ring, which connected nerves to the eyes and appendages of the first segment. The head has a cavity that contains the brain. The shape of this cavity reveals the structure of the brain. The researchers observed that the larval brain is Y. Motoishi The frontal portion is wedge-shaped, and the rest of the brain is divided into two parts, as evidenced by the outline of the membrane between them.
Long ago and now
Considering its physical properties, the researchers Y. Motoishi It displays characteristics of both extinct and modern arthropods. Some are ancestral characteristics present in all modern and extinct arthropods. Other ancestral characteristics may have been present in extinct arthropods but are only present in some modern arthropods.
One of the features found in all arthropods today is the protocerebrum. Its evolutionary precursors were Y. Motoishi. The protocerebrum is the first part of the arthropod brain and controls the eyes and appendages such as the antennae in velvet worms and the mouthparts in tardigrades. Y. Motoishi Extant and extinct arthropods have a circulatory system that is similar to that of modern arthropods, especially crustaceans.
Robopod is Y. Motoishi These are found today only in some arthropods, namely tardigrades and velvet worms. Many more species of lobopods existed during the Cambrian. Lobopods also have a unique circulatory system in their legs and other appendages that is most similar to that of velvet worms.
The structure of the nervous system is [arthropod] Revealing the homology between the brain, its appendages, and its sensory organs, [arthropods]” the researchers said in the same study.
Yuti Yuanxi Mysteries remain, most of which have to do with the fact that it is a larva; we can only speculate what it would look like as an adult, and by the time it reaches adulthood, the species may have developed compound eyes and flapping wings for swimming. Whether it is the larva of a known extinct lobopod is also an open question. Perhaps the answer lies buried somewhere in the Wonsan Shale.
Nature, 2024. DOI: 10.1038/s41586-024-07756-8
