Rare, Cracked Eye of 429-Million-Year-Old Trilobite Reveals Mysteries of the Ancient World

Museums display fossils of different species that lived from hundreds to million years ago. However, these pieces are not always the most informative for palaeontologists as some of the more salient secrets of ancient creatures lie inside the internal details that may somehow remain hidden.

Such is the case of the 429-million-year-old trilobite when a crack on its left eye has allowed palaeontologists to see the ancient world the creature's eye.

Trilobites

Trilobites are a group of extinct fossil arthropods characterized with their three-lobed, three-segmented form that first appeared about 521 million years ago at the beginning of the Cambrian Period and dominated the seas.

They continued to live for millions of years in the sea before they finally went extinct 252 million years ago, during the Permian Period. A trilobite's size could range from smaller than shirt buttons to bigger than a dinner plate. Given their abundance, studying their evolutionary history can offer more in-depth insights into the life of the ancient oceans millions of years ago.

Brigitte Schoenemann, a palaeontologist from the University of Cologne in Germany, said she has always liked Aulacopleura trilobites and decided to buy one of her own. As an evolution of vision expert, she immediately noticed that something was off with the specimen she purchased. For others, it might have seemed just an ordinary collector's item.

Cracked Fossil Shows the World Through Ancient Eyes

One of the eyes of the fossil Schoenemann bought was cracked, exposing its inner surface revealing its lens, receptor cells and other parts of the trilobite's eye anatomy.

This discovery is unusual in the fossil record as the preservation of the delicate cellular structures, and particularly the eyes often happen on rapid burial in environments where there is no oxygen-places like the Cambrian Burgess Shale in British Columbia.

She said that for a long time, experts believed that only limited objects were preserved in the fossil record, such as bones, teeth and other hard objects. That means the trilobite she has and to be able to distinguish cellular structures are indeed very rare and truly exceptional.

On Thursday, Schoenemann and Euan Clarksons, tudy co-author from the University of Edinburgh, published their analysis of the trilobite's eye in the journal Scientific Reports on Thursday.

Despite living millions of years ago, the trilobite has modern eyes similar to bees and dragonflies, called the apposition compound eye. This type of eyes have lens that can act independently from one another that creates a mosaic image of what they are seeing.

"The eye has about 200 facets and surely is good enough to distinguish obstacles, dark and bright zones in the environment, shelters, shapes and other organisms," Schoenemann says.

The details in the eyes of the trilobite add information to the evolutionary process that the eyes and vision in arthropods went through overtime, palaeontologist John Paterson of the University of New England said.

Based on previous research about trilobite vision, the paper also suggests the type of habitat that the creature preferred. Given their eye's anatomy, it seems the trilobite used to live in bright, shallow water.

According to physics, the relationship of the lens size and light availability would mean that the lenses of Aulacopleura would have functioned well in bright environments. Therefore, trilobites were most likely awake and active during the day.