These 54-Million-Year-Old Fly Eyes Are Preserved in Astounding Detail
Eyes surprise: fossil eyes from a 54 million year old cranefly. Lindgren et al./Nature

Composition of fossil insect eyes surprises researchers

Fossilised flies that lived 54 million years ago have revealed a surprising twist to the tale of how insects’ eyes evolved. These craneflies, unveiled in Nature today, show that insect eyes trap light the same way as human eyes, using the pigment melanin – yet another example of evolution finding similar solutions to similar problems.

"We were surprised by what we found because we were not looking for, or expecting it," says Johan Lindgren, an Associate Professor at the Department of Geology, Lund University, and lead author of the study published this week in the journal Nature.

The researchers went on to examine the eyes of living crane-flies, and found additional evidence for eumelanin in the modern species as well.

By comparing the fossilized eyes with optic tissues from living crane-flies, the researchers were able to look closer at how the fossilization process has affected the conservation of compound eyes across geological time.

The fossilized eyes further possessed calcified ommatidial lenses, and Johan Lindgren believes that this mineral has replaced the original chitinous material.

This, in turn, led the researchers to conclude that another widely held hypothesis may need to be reconsidered. Previous research has suggested that trilobites -- an exceedingly well-known group of extinct seagoing arthropods -- had mineralized lenses in life.

"The general view has been that trilobites had lenses made from single calcium carbonate crystals. However, they were probably much more similar to modern arthropods in that their eyes were primarily organic," says Johan Lindgren.

Compound eyes are found in arthropods, such as insects and crustaceans, and are the most common visual organ seen in the animal kingdom. They are made up of multiple tiny and light-sensitive ommatidia, and the perceived image is a combination of inputs from these individual units.

Eyes like our own?

However, detailed chemical analysis of the fossil cranefly eyes revealed that they contained human-like melanin. When the researchers had another look at the eyes of living craneflies, they were surprised to confirm the presence of melanin (as well as lots of ommochrome). It took fossils to alert us that the eyes of humans and insects both use the same shielding pigments (melanin) - yet another example of convergent evolution.

Intriguingly, the outer layers of the fossilised eyes were full of calcite, the mineral that makes up most of limestone. Not only that, but crystals in the calcite were aligned to transmit light efficiently into the eye. Yet this apparent fine engineering (a mineralised outer eye layer optimised to transmit light) was almost certainly caused by the fossilisation process, as the eyes of living craneflies are not mineralised.

While the fossil record can reveal, it can also mislead, if not interpreted carefully. Trilobites, the hard-shelled crab-like creatures that are among the most abundant and diverse animal fossils, are frequently found with mineralised, light-transmitting outer eye layers. These have usually been assumed to faithfully reflect their life condition: predation in ancient oceans was so intense that trilobites even armoured their eyeballs.

Lindgren and colleagues warn against this interpretation: perhaps the trilobite’s “protective goggles” only appeared after fossilisation, just as in the craneflies. However, this interpretation will likely be debated. Trilobite eyes seem to have been unusually rigid and resilient in real life, as they are preserved in three dimensions much more often than eyes of other animals. They also have certain optical properties that make more sense when the rigid outer layer is accepted as real.

A disagreement between a few palaeontologists might seem a bit arcane, but these debates can have real-world relevance. Most famously, the concept of nuclear winter was directly inspired by discussion of how the dinosaurs went extinct, when a meteorite impact enveloped the world in a cloud of dust, deep-freezing the entire biosphere.

Granted, the debate over how insect and trilobite eyes functioned is unlikely to influence world peace, but it might still have useful applications. For example, the way trilobite lenses (apparently) provide constant acuity while being totally rigid has inspired bioengineers to fashion high-performance optical devices with uses spanning microscopy to laser physics.

The above story is based on Materials provided by Lund University.