319-million-year-old fish fossil has preserved brain

A study led by a Brazilian researcher found a preserved brain in a fossil fish that lived 319 million years ago (Carboniferous period), in what is now England.

The condition of exceptional preservation —generally, soft parts such as organs and other internal structures are not preserved in fossils— of the find was highlighted in this fourth edition (1st) of the scientific journal Nature.

The paleontologist and doctoral student at the Department of Earth and Environmental Sciences at the University of Michigan (USA), Rodrigo Figueroa, also the first author of the study, explains that the material was actually known for over a hundred years.

The fish known as Coccocephalus wildi (something like “coconut head”, in Latin) was found in the 1920s in a coal mine, but received little attention because it was very small (about 3 cm) and difficult to identify the bony parts of the animal, such as structures of the skull and jaws.

Using computed tomography equipment, Figueroa identified the presence of an unusual mass, with a lighter color than other parts of the fossil.

The analysis of the rock where it was found indicates that the fossil lived in an estuary environment (such as a mangrove), at low depth, and when it died it was buried by very fine sediments in a context with a low oxygen rate, which may have caused brain preservation.

“At first, it was difficult to even know that it was a brain, because it has some rocky concretions and we came to think that it was an intrusion. [tipo de formação mineral que ocorre após a fossilização]. But then we were able to identify some characteristics that define the brain of a vertebrate, such as bilaterality [simetria da direita para a esquerda] and the presence of cranial nerves”, explains the paleontologist.

With that, the fossil includes the oldest known brain of a vertebrate. Other research, such as one published in 2009 with a species of fish from the chimaera group (relatives of sharks and rays) also found the brain of the fossilized animal, but the fossil in question was younger, around 300 million years old. .

Studies that indirectly identified the brains of extinct animals based on a kind of brain volume mold —called endocastes— also already exist, but in these there is an inference of the anatomy of the nervous system from the cranial box, without the presence of the organ in question. yes, evaluates Figueroa.

For comparison, it’s like making a chocolate egg by pouring the chocolate into the plastic mold, but the inside remains hollow.

In case of Coccocephalus, it was possible to identify the three portions of the fish brain: anterior, middle and posterior. Unlike mammals, fish, as well as amphibians and reptiles, have a brain divided into three parts, each with a distinct function.

The discovery could thus revolutionize the knowledge we had about the evolution of the brain in vertebrates.

THE C. wildi belongs to the group of fish known as ray-finned (or Actinopterygii), which is supported by bones. They are the most diverse group of vertebrates today, with around 30,000 species, and include all of the most well-known marine and freshwater fish.

A smaller number of bony fish species are represented by the lobe-finned ones (Sarcopterygii) –which has muscles and ligaments connected to a single bone that connects with the rest of the body–, which includes the lungs and which gave rise to terrestrial vertebrates (amphibians, reptiles, including birds, and mammals).

The rest of vertebrate diversity (the other 30,000 species) is the sum of sharks and rays (cartilaginous fish) and terrestrial vertebrates.

However, in current ray-finned fish, the brain during embryonic development is formed by eversion, that is, with the tissue from the inside out, like a sock turned inside out.

In other groups of vertebrates, brain development is by evagination, with the fold from the outside to the inside.

THE Coccocephalus it has an evaginated brain, similar to that seen in other groups of vertebrates, which indicates that this characteristic may have appeared very early in evolutionary history and changed over millions of years in ray-finned fish.

“The analysis of the structure that we identified as the brain of C. wildi it was difficult because there are many gaps in the knowledge of the evolution of the brain in these fish”, evaluates Figueroa. “As I had to compare with several fossil species [naqueles em que havia o endocasto] and current, some of the features that were once considered unique to ray-finned fish emerged much later.”

In his doctoral research at the University of Michigan, Figueroa is interested in understanding the evolution of the group of ray-finned fish and, therefore, in addition to Ç. wildihas analyzed several other fossil species of the group.

“As much as it is the most diverse group of vertebrates today, there is not that much information about the evolutionary characteristics of the brain of many of the animals in the group”, he says.

According to him, studies like this demonstrate the need to carry out studies of the so-called basic science, which seeks to understand phenomena and patterns of evolution in nature in order to generate scientific knowledge.

“Sometimes it is difficult to understand the importance of basic science, but it is what, as the name implies, provides the basis for more applied research. For example, understanding the evolution of the fish brain from fossils helps to understand the development of this organ in species used in laboratory research, such as zebrafish”, he concludes.