Fossils of a tiny sea creature that died out more than half a billion years ago could require a rewrite of a science textbook on brain development.
A study published in Science led by Nicholas Strausfeld, a Regents Professor in the Department of Neuroscience at the University of Arizona, and Frank Hirth, a Lecturer in Evolutionary Neuroscience at King’s College London, provides the first detailed description of Cardiodiction catenulum, a worm-like animal preserved in rocks in southern Yunnan province of China. The nearly half-inch (less than 1.5 centimeters) long fossil, first discovered in 1984, had until now hid a crucial secret: a carefully preserved nervous system, including a brain.
To our knowledge, this is the oldest fossilized brain known to date.”
Nicholas Strausfeld, Regents Professor, University of Arizona Department of Neuroscience
Cardiodiction belonged to an extinct group of animals known as armored lobopods, which abounded early in a period known as the Cambrian when virtually all major animal lineages emerged in an extremely short period of between 540 million and 500 million years. Lobopods likely moved on the seafloor using multiple pairs of soft, blunt legs that lacked the joints of their descendants, the euarthropods—Greek for “true articulated foot.” The closest living relatives of lobopods today are velvet worms, living mainly in Australia, New Zealand and South America.
A debate that dates back to the 19th century
fossils of Cardiodiction show an animal with a segmented proboscis in which there are repeating arrays of neural structures known as ganglia. This is in stark contrast to his head and brain, both of which show no signs of segmentation whatsoever.
“This anatomy was completely unexpected, since the heads and brains of modern arthropods and some of their fossil ancestors have been thought to be segmented for over a hundred years,” Strausfeld said.
According to the authors, the find sparks a long and heated debate about the origin and composition of the head in arthropods, the world’s most biodiverse group of animals. Arthropods include insects, crustaceans, spiders and other arachnids, as well as some other lineages such as centipedes and centipedes.
“Since the 1880s, biologists have noticed the clearly segmented appearance of the trunk, which is typical of arthropods, and essentially transferred this to the head,” says Hirth. “So the art has come to believe that the head is an anterior extension of a segmented trunk.”
“Target Cardiodiction shows that the early head was not segmented, nor was its brain, suggesting that the brain and trunk nervous system likely evolved separately,” Strausfeld said.
Cardiodiction was part of the Chengjiang fauna, a famous fossil site in Yunnan Province discovered by paleontologist Xianguang Hou. The soft, delicate bodies of lobopods have survived well in the fossil record, but apart from that Cardiodiction None have been examined for their heads and brains, possibly because lobopods are generally small. The most prominent parts of Cardiodiction were a series of triangular, saddle-shaped structures that defined each segment and served as attachment points for pairs of legs. These had been found in even older rocks, dating back to the Cambrian advent.
“This tells us that armored lobopods may have been the earliest arthropods,” said Strausfeld, who lived even before trilobites, an iconic and diverse group of marine arthropods that became extinct about 250 million years ago.
“Until recently, the common understanding was that ‘brains don’t petrify,'” Hirth said. “So, first of all, you wouldn’t expect to find a fossil with a preserved brain. And secondly, this animal is so small that you wouldn’t even dare look at it in hopes of finding a brain.”
However, work over the last 10 years, largely by Strausfeld, has identified several instances of preserved brains in a variety of fossilized arthropods.
A shared genetic blueprint for making a brain
In their new study, the authors not only identified the brain of Cardiodiction but also compared to those of known fossils and living arthropods, including spiders and centipedes. By combining detailed anatomical studies of the lobopod fossils with analyzes of the gene expression patterns of their living descendants, they conclude that a common blueprint of brain organization has been maintained from the Cambrian to the present.
“By comparing known patterns of gene expression in living species,” Hirth said, “we identified a common signature of all brains and how they are formed.”
in the Cardiodictionthree brain domains are each associated with a characteristic pair of head appendages and one of the three parts of the anterior digestive system.
“We noticed that each brain domain and its corresponding traits are specified by the same combination genes, regardless of the species we studied,” Hirth adds. “This suggests a common genetic blueprint for making a brain.”
Lessons for the evolution of the vertebrate brain
Hirth and Strausfeld say the principles outlined in their study are likely to apply to other creatures outside of arthropods and their immediate relatives. This has important implications when comparing the nervous system of arthropods to that of vertebrates, which have a similarly distinct architecture in which the forebrain and midbrain are genetically and developmentally 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 climatic changes.
“At a time when major geological and climatic events were reshaping the planet, simple sea creatures such as Cardiodiction gave rise to the world’s most diverse group of organisms – the euarthropods – which eventually spread into every emerging habitat on Earth, but are now threatened by our own ephemeral species.
The treatise The Lower Cambrian Lobopodian Cardiodiction Resolves the Origin of Euarthropod Brains” was co-authored by Xianguang Hou from the Yunnan Key Laboratory for Paleontology at Yunnan University in Kunming, China, and Marcel Sayre, who works 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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