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Focus on the “eyes” preserved in fossils! The magnificent life and workings of the Earth, as remembered by these tiny fossils.



Dr. Gengo Tanaka, Associate Professor
Marine Science Laboratory, Center for Water Cycle, Marine Environment and Disaster Management
 
When we think of fossils, we tend to think of huge dinosaurs, but Associate Professor Gengo Tanaka is making major discoveries by studying fossils of much smaller creatures. We introduce Associate Professor Tanaka, who has been looking at fossils ever since he first encountered them when he was in elementary school.
 

The central nervous system was preserved in the fossil of a small animal!

What kind of research are you doing?
 
I am researching small fossils called microfossils, as well as fossils of arthropods such as trilobites and shrimp. The most significant discovery I have made so far is that I found the world's first preserved central nervous system in a fossil of an arthropod from the Cambrian period. A paper on this discovery was published in the scientific journal “Nature”.
 
Many of the creatures that lived during the Cambrian period had very strange appearances, very different from those of organisms today. In my research on microfossils, I have been focusing on the eyes preserved in fossils, and I have found that some Cambrian creatures had four or five eyes.
 
When I was shown the fossil repository at Yunnan University in China, I saw a fossil with four eyes, and I asked for permission to bring it back to Japan and take a CT scan. This revealed a white line down the center of the body. When I analyzed it, I found that the central nervous system, including the brain, had been preserved.
 

Photo: CT images of Alalcomenaeus

Sometimes nerves are preserved in fossils!
 
It is very rare for nerves to be preserved. In vertebrates, the central nervous system is protected by the vertebrae, but in arthropods like Alalcomenaeus, which have a hard shell on the outside, the inside is soft, and it is difficult for the central nervous system to be preserved.
 
The new species of Alalcomenaeus I discovered appears to have four eyes, but actually has two eyes that are joined together, so it has one eye on each side. There was a bundle of nerves at the base of each eye, which was connected to the brain. From the arrangement of the central nerves and other factors, we were able to determine that Alalcomenaeus is classified as a chelicerate, such as scorpions and horseshoe crabs, among living arthropods.
 

Photo: A reconstructed drawing of Alalcomenaeus


Are they the ancestors of scorpions and horseshoe crabs?
 
I believe so.
 
The Cambrian period is the oldest period of the “Phanerozoic eon”, a period in which the appearance of life on Earth is clearly visible.
 
It was thought that many of the creatures from this period became extinct before the next period, and that they were not connected to the creatures of today. However, discoveries like this have shown that, although they had strange appearances, their internal anatomies were not different from the creatures of today.
 
This discovery has also led to the creation of a phylogenetic tree based on the central nervous system. My co-researcher at the University of Arizona created this phylogenetic tree.
 
Until now, phylogenetic trees that connect Cambrian creatures with those of today have been created based on appearance. However, for example, dolphins and fish are completely different creatures, but they both have a streamlined shape because they swim in water. It is actually risky to create a phylogenetic tree based on appearance alone.

In contrast, a phylogenetic tree based on the conservative feature of the nervous system is more accurate, and is still used today without any changes.
 

I also supervise TV programs and give public lectures in collaboration with museums.

Why did you start researching fossils?
 
I've always liked old things since I was little. I think it's because my grandmother used to tell me stories about the Taisho era, which was about 100 years ago.
 
At first, I collected old coins and stamps, but some of them were very expensive and I couldn't afford them as a schoolchild. But I still wanted to collect old things. Then, by chance, I came across a fossil of a Cretaceous bivalve called Inoceramus. I was playing in an open space, throwing stones around, and I found it inside one of the stones. I showed it to my elementary school science teacher, who told me it was a fossil, and that's how I got started.
 
After that, I was always surrounded by teachers who knew a lot about fossils. My junior high school teacher was the biggest influence on me, as he took me to places where I could collect fossils even after I became a university student. After I got my doctorate, I met a teacher in Kyoto who was also working on fossils, and when I went overseas, I met a teacher who was researching the Cambrian arthropods. I had some great chances to meet the right people at the right times.
 

Photo: Fieldwork in Sweden

You also do a lot of work outside of research.
 
I also appear in the NHK TV program “Darwin's Amazing Animals (ダーウィンが来た)”, and I also supervise other TV programs and exhibitions of museums. I was also involved in the supervision of the 2015 NHK anime “Pikaia! (ピカイア)”, as the story includes creatures from the Paleozoic era, including the Cambrian period. One of the students in my lab told me that they had seen “Pikaia!” and were surprised to hear that I had been involved in the supervision of the program.
 
Goshoura Town in Amakusa is famous for its fossils, and the marine fieldwork course in Amakusa offered as part of Kumamoto University's public lectures is also run in collaboration with the Kyushu University Museum and the Amakusa Museum of Goshoura Dinosaur Island. Amakusa is a rare place in Japan where you can see mudflat deposits from 50 million years ago and 100 million years ago at the same time. In addition, the Aizu Marine Station at Kumamoto University is located right next to the current tidal flats, and there are also tidal flat strata and fossils nearby, so we hold public training sessions here twice a year. All participants are students from other universities. The sessions are always very popular, as we always have more applicants than available places.
 

The main focus of our research is “the eye in fossils”
We want to clarify the origins of the eye in organisms.

How do you think your research into fossil eyes, your specialty, will progress in the future?
 
For many years I have been researching fossil bivalved crustaceans called “Ostracoda”. Ostracods are less than 1mm in size and are also called “seed shrimp”. They have inhabited the earth for the past 500 million years, right up until the present day.
 
They have developed a lens as part of their eye on their shell. The diameter of the eyes is about 50 microns. Even though it is called an eye, it only has one or two retinal cells, so it cannot form an image on the retina, and it seems that they can only sense the presence or absence of light.
 
The angle of the eyes of these ostracods differs depending on the depth of the water in which they live. Those living in deep water have their eyes on the dorsal side so that they can more easily detect light coming from above, while those living in shallow water have their eyes on the lateral side so that they are not blinded by the light. By understanding the correlation between the depth of the water where present-day ostracods live and the angle of their eyes and applying this to the angle of the eyes of fossil ostracods, it should be possible to measure the depth of the sea in the past. I would like to collect eye data on present-day ostracods from around the world and create a tool that can be used worldwide to determine the depth of the sea in the past from fossil ostracods.
 

Photo: Ostracoda viewed under a microscope

What are your future plans?
 
Organisms have various sensory organs, but I think that the eye is the organ that can most easily distinguish food from a distance. At first, it could only sense light, but gradually it became able to detect something passing by. Then, it became able to form a blurred image. If we can find evidence of this process in the fossil record, I think we will be able to clarify the origin and evolution of the eye. I want to clarify the existence of the oldest predators with the eye through fossil records.
 

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