(ORDO NEWS) — The fossil of a tiny marine worm that lived 525 million years ago has settled an age-old debate about the evolution of the arthropod brain.
The study showed that the brain of the first arthropods was not segmented, and the nervous system of the head and trunk evolved independently.
Scientists from the University of Arizona (USA) have provided the first detailed description of the worm-like creature Cardiodictyon catenulum, which lived about 525 million years ago. His remains are perfectly preserved in the rocks of the southern Chinese province of Yunnan.
It turned out that the fossil, less than one and a half centimeters long, first discovered in 1984, had been hiding a well-preserved nervous system all this time.
The brain of C. catenulum proved to be the oldest fossilized brain and helped resolve the debate about the evolution of the arthropod nervous system.
C. catenulum belonged to an extinct group of armored lobopods ( Scleronychophora ) abundant in the early Cambrian.
These animals moved along the seabed with the help of several pairs of soft short legs, devoid of joints.
Their closest living relatives are the onychophora, or velvet worms, found in Australia, New Zealand, and South America.
C. catenulum had a segmented body, each part of which contained ganglia – nerve nodes. However, there was no evidence of segmentation in his brain.
This discovery was completely unexpected for scientists, since the heads and brains of modern arthropods and some of their ancestors for a long time were considered only segmented.
According to the authors of the work, their find will be able to resolve long disputes about the origin and structure of the head in arthropods.
Indeed, since the 1880s, biologists have noted the segmented appearance of the body of arthropods and extrapolated this to the head.
A study of C. catenulum showed that the heads of the first arthropods were not segmented, nor were their brains. Thus, the brain and nervous system of the torso probably developed separately.
The researchers also compared the brains of C. catenulum to those of known fossils and living arthropods, including spiders and centipedes.
A combination of detailed anatomical studies of the fossil and analysis of gene expression patterns has shown that the overall organization plan of the arthropod brain has been preserved from the Cambrian to the present day.
The scientists noted that the principles described in this study likely apply to animals other than arthropods and their closest relatives.
This is important for comparing the nervous system of arthropods with that of vertebrates, in which the forebrain and midbrain are genetically and evolutionarily distinct from the spinal cord.
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