Fossils of a tiny sea creature that died more than half a million years ago may force a rewrite of a science textbook on how brains evolved.
A study published in Science, led by Nicholas Strausfeld, Regents Professor in the Department of Neuroscience at the University of Arizona, and Frank Hirth, Reader in Evolutionary Neuroscience at King’s College London, provides the first detailed description of Cardiodictyon catenulum, an animal preserved worm-like. in the rocks of Yunnan province in southern China. Just half an inch (less than 1.5 centimeters) long and initially discovered in 1984, the fossil had been hiding 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 lobopods, which were abundant at the beginning of a period known as the Cambrian, when virtually all major animal lineages appeared in an extremely short time between 540 and 500 million years ago millions of years Lobopods likely moved along the seafloor using several pairs of soft, stubby legs that lacked the joints of their descendants, the euarthropods—the Greek for “real jointed foot.” The closest living relatives of lobopods today are the velvet worms that live mainly in Australia, New Zealand and South America.
A debate that dates back to the 1800s
Cardiodictyon fossils reveal an animal with a segmented trunk in which there are repeated arrangements of neural structures known as ganglia. This is in stark contrast to his head and brain, which have no evidence of segmentation.
“This anatomy was completely unexpected because the heads and brains of modern arthropods, and some of their fossilized ancestors, have been thought of as segmented for over a hundred years,” 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 of the animal kingdom. Arthropods include insects, crustaceans, spiders, and other arachnids, as well as other lineages such as millipedes and centipedes.
“Starting in the 1880s, biologists observed the distinctly segmented appearance of the typical arthropod trunk, and basically extrapolated that to the head,” Hirth said. “This is how the field arrived at the assumption that the head is an anterior extension of a segmented trunk.”
“But Cardiodictyon shows that the early head was not segmented, and neither was its brain, suggesting that the brain and trunk nervous system likely evolved separately,” Strausfeld said.
Brains fossilize
Cardiodictyon was part of the fauna of Chengjiang, a famous fossil deposit in Yunnan province discovered by paleontologist Xianguang Hou. The soft, delicate bodies of lobopods are well preserved in the fossil record, but apart from Cardiodictyon none and their brains have been examined, possibly because lobopods are generally small. The most prominent parts of Cardiodictyon were a series of triangular saddle-like structures that defined each segment and served as attachment points for pairs of legs. They had been found in even older rocks dating back to the arrival of the Cambrian.
“This tells us that armored lobopods could have been the first arthropods,” Strausfeld said, even predating trilobites, an iconic and diverse group of marine arthropods that went extinct about 250 million years ago.
“Until very recently, the common understanding was ‘brains don’t fossilize,'” Hirth said. “So you don’t expect to find a fossil with a preserved brain in the first place. And secondly, this animal is so small that you wouldn’t even dare to look at it hoping to find a brain.”
However, work over the past 10 years, much of it by Strausfeld, has identified several instances of preserved brains in a variety of fossilized arthropods.
A common genetic blueprint for making a brain
In their new study, the authors not only identified the brain of Cardiodictyon, but also compared it to those of known fossil and living arthropods, including spiders and centipedes. Combining detailed anatomical studies of fossil lobopodians with analyzes of gene expression patterns in their living descendants, they conclude that a shared pattern of brain organization has persisted from the Cambrian to the present.
“By comparing known gene expression patterns in living species,” Hirth said, “we identified a common signature of all brains and how they form.”
In Cardiodictyon, three brain domains are each associated with a characteristic pair of head appendages and with one of the three parts of the anterior digestive system.
“We realized that each brain domain and its corresponding features are specified by the same combination of genes, regardless of the species we looked at,” Hirth added. “This suggested a common genetic blueprint for making a brain.”
Lessons for the evolution of the vertebrate brain
Hirth and Strausfeld say the principles described in their study likely apply to other creatures outside of arthropods and their immediate relatives. This has important implications when comparing the nervous system of arthropods with that of vertebrates, which show a similar distinct architecture in which the forebrain and midbrain are genetically and visually distinct from the spinal cord , said.
Strausfeld said their findings also offer a message of continuity at a time when the planet is changing dramatically under the influence of climate change.
“At a time when major geological and climatic events were reshaping the planet, simple marine animals like Cardiodictyon gave rise to the world’s most diverse group of organisms, the euarthropods, which eventually spread to all the emerging habitats of the Earth, but which are now being formed. threatened by our own ephemeral species”.
The paper, “The Lower Cambrian Lobopodian Cardiodictyon Resolves the Origin of Euarthropod Brains” was co-authored by Xianguang Hou at the Yunnan Key Laboratory for Paleonology at Yunnan University in Kunming, China, and Marcel Sayre, who holds appointments at Lund University in Lund. , Sweden, and in the Department of Biological Sciences at Macquarie University in Sydney.
Funding for this work was provided by the National Science Foundation, the University of Arizona Regents Fund, and the UK Biotechnology and Biological Sciences Research Council.
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