525 Million Year Old Fossil Challenges Textbook Explanation of Brain Evolution

525 Million Year Old Fossil Challenges Textbook Explanation of Brain Evolution

Reconstruction of the armored lobopod Cardiodictyon catenulum

image: Artist’s impression of a 525 million year old Cardiodictyon catenulum on the shallow coastal seabed, emerging from the shelter of a small stromatolite built by photosynthetic bacteria.
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Credit: Nicholas Strausfeld/University of Arizona

Fossils of a tiny sea creature that died more than half a billion years ago could force a science textbook to rewrite brain evolution.

A study published in Science – conducted by Nicholas Strausfeld, Regents Professor in the Department of Neuroscience at the University of Arizona, and Frank Hirth, an evolutionary neuroscience reader at King’s College London – provides the first detailed description of Cardiodictyon cattenulum, a worm-like animal preserved in the rocks of Yunnan province in southern China. Measuring just half an inch (less than 1.5 centimeters) long and originally discovered in 1984, the fossil has until now concealed a crucial secret: a delicately preserved nervous system, including a brain.

“To our knowledge, this is the oldest fossilized brain that we know of to date,” Strausfeld said.

cardiodictyon belonged to a group of extinct animals known as armored lobopods, which were abundant at the start of a period known as the Cambrian, when virtually all major animal lineages appeared in an extremely short period between 540 million and 500 million years. Lobopods probably moved across the seafloor using several pairs of soft, stubby legs that lacked the joints of their descendants, the euarthropods – Greek for “true jointed foot”. The closest current living relatives of lobopods are the velvet worms which live primarily in Australia, New Zealand, and South America.

A debate that dates back to the 1800s

Fossils of cardiodictyon reveal an animal with a segmented trunk in which there are repeating arrangements of neural structures called ganglia. This contrasts sharply with his 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 and heated debate over 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 centipedes.

“From the 1880s, biologists noted the clearly segmented appearance of the typical arthropod trunk, and essentially 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 cardiodictyon shows that the primitive head was not segmented, nor 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 site in Yunnan province discovered by paleontologist Xianguang Hou. The soft, delicate bodies of lobopods have been well preserved in the fossil record, but apart cardiodictyon none have been examined for their heads and brains, possibly because lobopods are typically small. The most significant parts of cardiodictyon were a series of triangular saddle-like structures that defined each segment and served as attachment points for the pairs of legs. These had been found in even older rocks dating from the advent of the Cambrian.

“This tells us that armored lobopods may have been the first arthropods,” Strausfeld said, even predating trilobites, an iconic and diverse group of marine arthropods that became extinct around 250 million years ago.

“Until very recently, the common understanding was that ‘brains don’t fossilize,'” Hirth said. “So you wouldn’t expect to find a fossil with a preserved brain in the first place. And, second, this animal is so small that you wouldn’t even dare look at it in hopes of finding a brain.”

However, work over the past 10 years, largely 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 to those of known fossils and living arthropods, including spiders and centipedes. By combining detailed anatomical studies of lobopod fossils with analyzes of gene expression patterns in their living descendants, they conclude that a common pattern of brain organization has been maintained from the Cambrian to the present day.

“By comparing known gene expression patterns in living species,” Hirth said, “we have identified a common signature of all brains and how they form.”

In cardiodictyonthree brain domains are each associated with a characteristic pair of cephalic appendages and 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 genes combined, regardless of the species we examined,” Hirth added. “It suggested 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 likely apply to other creatures besides arthropods and their close relatives. This has important implications when comparing the nervous system of arthropods with those of vertebrates, which exhibit a similar 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 climate change.

“At a time when major geological and climatic events are reshaping the planet, simple marine animals like cardiodictyon gave rise to the world’s most diverse group of organisms – euarthropods – which eventually spread to all emergent habitats on Earth, but are now threatened by our own short-lived species.”

The article, “The Lower Cambrian Lobopodian cardiodictyon Resolves the Origin of Euarthropod Brains” was co-authored by Xianguang Hou of the Yunnan Key Laboratory for Paleontology at Yunnan University in Kunming, China, and Marcel Sayre, who has 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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