Paleontologists use fossils to peer into the past, but on rare occasions, fossils seem to uncannily return that gaze.

That’s the case with a newly discovered trove of crane fly fossils that preserve extremely fine details of the eyes of these insects, which lived 54 million years ago. This level of preservation is unusual, especially for small animals like crane flies.

The specimens challenge established theories about the evolution of compound eyes, the most common type of visual organ in the animal kingdom, according to a paper published on Wednesday in Nature. The intricate fossils were discovered in Denmark’s Fur Formation over the past few years, said lead author Johan Lindgren, a paleontologist at Lund University in Sweden.

“For such delicate details to be retained in the fossil record, exceptional preservation conditions are required,” Lindgren noted in an email. He said that the Fur Formation is thought to have once been “a restricted marine bay, surrounded on three sides by land and with a waterway” that had an estimated water depth of at least 50 meters.

Marine habitats are particularly conducive to fossilization because carcasses are more likely to become rapidly buried in seafloor sediment, lowering the risk of damage from decomposition or scavengers.

Cliffs of the Fur Formation. Image: Jensbn

Originating some 520 million years ago, compound eyes are essential to the vision of the arthropod family, which includes insects, crustaceans, and extinct species such as trilobites.

These complex organs are made of a patchwork of tiny photoreceptor units called ommatidia that offer a wider panoramic view of an environment compared to the single-lens eyes that humans possess.

Previously studied fossils of compound eyes show evidence of calcification around the cornea and lens, especially in trilobites. As a result, scientists have suggested that trilobites and other fossilized arthropods used calcite to fortify their eye structure, even though this would make lenses more rigid and difficult to focus.

Using advanced spectroscopic techniques, Lindgren’s team examined the Fur Formation specimens, and found that the fossilized flies also had calcite lenses. But the real breakthrough emerged when the researchers identified traces of a pigment called eumelanin in both the fossil insects and living crane flies.

In the living insects, the pigment is located behind chitinous corneas, meaning the lenses are strengthened by the biopolymer chitin and not calcite. This led the team to speculate that the calcite corneas seen in trilobite fossils are actually a result of the fossilization process itself, and not a feature of the animals while they were alive. Instead, they may have had chitin-based lenses like modern crane flies.

“We need to reassess what we think we know about the optic properties of trilobite eyes,” said Lindgren. “This is important because trilobites are some of the earliest organisms known with complex eyes, and their function is immensely important when assessing the evolution of compound eyes.”

One way to resolve this question, Lindgren said, is to try to simulate the fossilization process, and see if compound eye chitin turns into calcite in real-time.