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Phenomenal sense organ: Fossil lizards had four eyes


Researchers discover lizards with four eyes

It has been known for a long time that there are lizards with three eyes: in the bridge lizard, the additional sensory organ sits in the middle of the skull plate and serves to perceive differences in brightness. A long time ago, there was even a four-eyed lizard, as researchers have now discovered.

Extraordinary sensory achievements in the animal kingdom

The sensory organs perform, which often seem unimaginable. For example, the human nose perceives over a trillion smells. In the animal kingdom, the senses are often much sharper. For example, dogs can not only hear much better than humans, but also smell significantly better. Other animals can see particularly well. For example, the outstanding visual performance of the birds of prey with the proverbial "eagle eye" has entered general awareness. What frogs are capable of is at least as impressive: they can see colors even in the dark. And the bridge lizard has a third eye, which is used to perceive differences in brightness. A long time ago there was a lizard that even had four eyes. Researchers at the Senckenberg Research Institute have now found out.

First evidence of a four-eyed higher vertebrate

As the Senckenberg Society for Natural Research reports in a communication, scientists from the Senckenberg Research Institute and an international team have provided evidence of a four-eyed lizard.

Using the extinct species Saniwa ensidens, they show that the monitor lizard had another sensory organ behind the so-called "third eye". They thus provide the first evidence of a four-eyed higher vertebrate.

The study was published in the "Current Biology" journal.

Reduced in the course of evolution

It is present in many lizards and the New Zealand bridge lizard: the apex eye, also known as the "third eye" or parietal organ.

"A vertex eye was regularly trained in the vertebrates of the Paleozoic over 250 million years ago," explains study leader Dr. Krister Smith from the Senckenberg Research Institute in Frankfurt.

He continues: "The history of the crest eye seemed to be quite simple so far: We assumed that this organ has regressed in the course of evolution in all higher vertebrates except the lizards."

This regression was accompanied by a change in function to the pineal gland. However, according to the latest findings of the US-German team around Smith, this does not seem to have been the case: the fossil monitor species Saniwa ensidens they examined even had another primitive fourth visual organ behind the third eye.

About 49 million years old

The Eocene monitor lizard fossil from North America is around 49 million years old and up to 1.30 meters long.

On its head, the two additional sensory organs are located in a central line position one behind the other on the skull cap. The position of both eyes contradicts the classic, paired model of the pineal gland.

"We therefore assume that the usual 'third eye' of the lizards has nothing to do with the pineal gland. The pineal gland from which the fourth eye developed is still present in lizards, but is located inside the skull, as in mammals, ”said Smith.

Lizards have a special position

In their study, the researchers therefore speak of a "re-evolution": the appearance of features that have already disappeared after a very long time.

“It would be a comparable process, for example, if our today's birds got teeth again,” adds Smith. The special development of the visual organs at Echsen also has consequences for future research.

“It turns out that the evolution of these two organs - pineal gland and parietal eye - is far more complicated than previously thought. We think that lizards have a special place in the development of the eyes and therefore should not - as before - serve as model organisms for other vertebrates, ”said Smith. (ad)

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