New fossils show what the brains of arthropod ancestors looked like

The well-preserved fossils left over from creatures that lived more than 500 million years ago reveal in great detail the same structures that researchers have long hypothesized, and these structures must be part of the prototype brain inherited by all arthropods

These fossils belong to an arthropod named lechoilia, which confirms the existence of early insect and spider embryo genetics and developmental biology studies predicting the existence of an unsegmented extreme brain that cannot be seen in modern adult arthropods.

A paper published today in the journal "Contemporary Biology" describes these fossils.

Strathfield said: "The extraordinary fossils we describe are never seen before

Ganglion refers to the reticular system that forms the nerve center in each segment of the arthropod nervous system

Fossil brain tissue is extremely rare

The Leanchoilia fossil remains found in the sediments of the Kaili Formation in Guizhou Province in southwestern China can be traced back to the Cambrian period about 508 million years ago

"The Kaili fossil opens a window for us, giving us a glimpse of the evolution of the body plane of animals that lived more than 500 million years ago," said Tian Lan of the Guizhou Paleontology Research Center, Guizhou University, China, the first author of the paper.

"The nervous system, like other soft tissues, is difficult to turn into fossils," added Pedro Martinez, a co-author of the University of Barcelona and the Catalá Institute in Spain

These fossils also reveal the evolutionary origins of two different visual systems during the evolution of arthropods: a pair of frontal eyes and a pair of side-looking eyes.

Many arthropods, including insects and crustaceans, have a unique pair of double-sided polyhedral compound eyes and another pair of less obvious eyes, which are more primitive in structure and are called unsegmented eyes or monoculars

On the other hand, Leanchoilia's lateral eye is related to the front brain, which is the segmental ganglion, which defines the forebrain of arthropods and is located behind the front brain

Straussfeld explained that in living arthropods, the protobrain, or forebrain, has to some extent merged-swallowed-the ancient center provided by the protobrain, so it is no longer a Unique anatomical entity

These fossils are so well preserved that they show that, in addition to the front eyes, the front brain also produced ganglia associated with the lips (or "upper lip") of modern arthropods

Implications for the evolution of vertebrate brains

Straussfeld said that in addition to filling a century of gaps in the understanding of arthropod brain evolution, these findings have important implications for the early evolution of vertebrate brains

Although these simple, fish-like animals co-exist with these now fossilized arthropods, there is no convincing fossil of their brains.
Therefore, there is neither fossil evidence nor anatomical evidence that vertebrates have antecedents.
Brain
.
However, modern research has shown that, for example, the genes that define the forebrain, midbrain, and hindbrain of mice correspond to the genes that define the three ganglion divisions of the arthropod brain
.
In vertebrates, certain key centers involved in decision-making, learning and memory have some genetic correspondences with the higher centers of arthropod brains, which originated in the forebrain of ancient arthropods
.

Therefore, it is possible that the common ancestor of vertebrates and invertebrates possessed the basic circuits of simple cognition and decision-making before the Cambrian, even before the evolution of the body plan of the segmental organization
.
Although there may be an ancient prebrain-like brain in the early ancestors of vertebrates, such fossils do not even provide evidence for a discrete, non-segmental area
.

Straussfeld said: "However, we can reasonably speculate that the'modern' brains of vertebrates have embedded parts of an ancient, unsegmented brain.
So far, this has only been found in early arthropods (such as Leanchoilia).
) Was confirmed
.
"

DOI 10.
1016 / j.
cub.
2021.
07.
048

Leanchoiliidae reveals the ancestral organization of the stem euarthropod brain