Fossils of a tiny marine creature that died more than half a billion years ago could force a scientific textbook rewrite on how the brain evolved.
A study published in Science – led by Nicholas Strausfeld, a Regents Professor in the University of Arizona Department of Neuroscience, e Frank Hirt, a reader in evolutionary neuroscience at King’s College London – provides the first detailed description of Cardiodictyon catenulum, a worm-like animal preserved in rocks in the southern Chinese province of Yunnan. Measuring 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.
cardiodiction it belonged to an extinct group of animals known as armored lobopodians, 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 million and 500 million years ago. Lobopodians probably moved on the seabed using multiple pairs of soft, stumpy legs that lacked the joints of their descendants, the euarthropods – Greek for “true jointed foot”. Today’s closest living relatives of the lobopodians are the velvet worms living mainly in Australia, New Zealand and South America.
A debate that dates back to the 1800s
Fossils of cardiodiction reveal an animal with a segmented trunk in which there are repeating arrangements of neural structures known as ganglia. This contrasts sharply with the 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 been considered segmented for over a hundred years,” Strausfeld said.
According to the authors, the discovery resolves a long-running debate about the origin and composition of the head in arthropods, the world’s most species-rich group in the animal kingdom. Arthropods include insects, crustaceans, spiders, and other arachnids, as well as other lineages such as centipedes and millipedes.
“Since the 1880s, biologists have noticed the clearly segmented appearance of the trunk typical of arthropods, and they’ve basically extrapolated it to the head,” Hirth said. “This is how the field came to assume that the head is an anterior extension of a segmented trunk.”
“But cardiodiction shows that the early head was not segmented, nor was its brain, which suggests that the brain and the trunk nervous system likely evolved separately,” Strausfeld said.
cardiodiction it was part of the fauna of Chengjiang, a famous fossil deposit in Yunnan province discovered by paleontologist Xianguang Hou. The soft, delicate bodies of lobopodians are well preserved in the fossil record, but separately cardiodiction none have been examined for the head and brain, possibly because lobopodians are generally small. The most important parts of cardiodiction they were a series of saddle-shaped triangular structures that defined each segment and served as attachment points for pairs of legs. Those had been found in even older rocks dating back to the advent of the Cambrian.
“This tells us that armored lobopods may have been the first arthropods,” Strausfeld said, even anticipating trilobites, an iconic and diverse group of marine arthropods that went extinct about 250 million years ago.
“Until recently, the common understanding was ‘brains don’t fossilize,'” Hirth said. “So you wouldn’t expect to find a fossil with a preserved brain in the first place. And secondly, this animal is so small that you won’t even dare look at it in hopes of finding a brain.”
However, work over the past 10 years, much of it done by Strausfeld, has identified several cases of preserved brains in a variety of fossilized arthropods.
A common genetic plan to create a brain
In their new study, the authors not only identified the brain of cardiodiction but he also compared it to those of known fossils and living arthropods, including spiders and millipedes. By combining detailed anatomical studies of lobopod fossils with analyzes of gene expression patterns in their living descendants, they conclude that a shared blueprint of brain organization has been maintained 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 cardiodictionthree 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 every brain domain and its corresponding features are specified by the same combination of genes, regardless of the species we examined,” Hirth added. “This suggested a common genetic plan for making a brain.”
Lessons for the evolution of the vertebrate brain
Hirth and Strausfeld say the principles described in their study probably apply to other creatures besides arthropods and their immediate relatives. This has important implications when comparing the nervous systems of arthropods to that of vertebrates, which exhibit a similar distinct architecture in which the forebrain and midbrain are genetically and evolutionarily distinct from the spinal cord, they 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 cardiodiction gave rise to the world’s most diverse group of organisms – the euarthropods – which eventually spread to every emerging habitat on Earth, but which are now threatened by our own ephemeral species.”
The article, “The Lower Cambrian Lobopodian cardiodiction Resolves the Origin of Euarthropod Brains” was co-authored by Xianguang Hou at the Yunnan Key Laboratory for Paleontology of Yunnan University in Kunming, China, and Marcel Sayre, who has fellowships at Lund University in Lund, China. 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’s Biotechnology and Life Sciences Research Council.
Early Cambrian Cardiodictyon lobopodium solves the origin of euarthropod brains
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The authors declare that they have no competing interests.