525-Million-Year-Old Fossil Defies Textbook Explanation for Brain Evolution
Published:08 Dec.2022 Source:University of Arizona
A study published in Science provides the first detailed description of Cardiodictyon catenulum, a wormlike animal preserved in rocks in China's southern Yunnan province. Measuring barely half an inch (less than 1.5 centimeters) long and initially discovered in 1984, the fossil had hidden a crucial secret until now: a delicately preserved nervous system, including a brain. “To our knowledge, this is the oldest fossilized brain we know of, so far,” Strausfeld said. Cardiodictyon belonged to an extinct group of animals known as armored lobopodians, which were abundant early during a period known as the Cambrian, when virtually all major animal lineages appeared over an extremely short time between 540 million and 500 million years ago.
Fossils of Cardiodictyon reveal an animal with a segmented trunk in which there are repeating arrangements of neural structures known as ganglia. This contrasts starkly with its head and brain, both of which lack any evidence of segmentation. “This anatomy was completely unexpected because the heads and brains of modern arthropods, and some of their fossilized ancestors, have for over a hundred years been considered as segmented,” Strausfeld said. According to the authors, the finding resolves a long and heated debate about the origin and composition of the head in arthropods, the world’s most species-rich group in the animal kingdom.