Trilobites – those ubiquitous, half-billion-year-old armored seabugs – had eyes that were faceted like disco balls. Now, new images reveal that these eyes were remarkably similar to those of bees and dragonflies.
Trilobites were buglike, multi-legged marine arthropods that appeared during the Cambrian period (543 million to 490 million years ago) and died about 250 million years ago. Paleontologists have found many fossils of the hard exoskeletons of trilobites. And although trilobite eyes are sometimes preserved as such and resemble a surface similar to the eyes of insects, previous analysis has simply indicated their complexity.
Now, researchers have for the first time described the internal structures that made the eyes of the trilobite Aulacopleura koninckii, which lived about 429 million years ago. The composition and arrangement of photosensitive receptor cells in these lenses provided scientists with a look at trilobity vision, as well as an indication of the early evolution of compound eyes.
Related: Cambrian creatures: Primitive sea life (image gallery)
Although the specimen was collected in 1846 at Loděnice in the Czech Republic, this is the first time the preserved eye has been imaged using microscopy. The technique revealed optical units called ommatidia, hundreds of which make up the compound eyes in modern crabs and insects.
Examining the trilobite’s small eyes was challenging; A. koninckii is “fairly flat” – about 0.04 inches (1 millimeter) high, the scientists reported in a new study, published today (August 13) in the journal Scientific reports. In this species, the two protruding oval-shaped eyes are on the back of the head and are particularly vulnerable to damage; only the left eye of the specimen was still intact when the researchers found it.
In compound eyes, individual lenses – sometimes thousands – process visual input and send signals to the brain, working together to form a mosaic of images. And within the only remaining trilobite eye, the authors of the study discovered structures still present in modern arthropod eyes.
Each of the trilobite’s ommatidia contained eight light-detecting cells “forming a kind of rosette,” the scientists wrote. The cells were grouped around a transparent, light-channeled cylinder called a rhabdom, and a thick lens covered the ommatidia at the top, the scientists wrote. Dark rings of pigmented cells formed a “cellular curve” around the optical units, and there was some evidence of a thin, crystalline cone, “although the shape is not very distinct,” according to the study.
The study found that together these structures formed “a classic apposition composite eye”, which looks a bit like a disco ball. This type of compound eye is found in insects and crustaceans that are active during the day, such as bees and dragonflies.
A. koninckii was a soil dweller, sliding over the seabed and hanging particles of organic material. The diameter of the facets, like ommatidia, also told scientists about the habitat of the trilobite, said author Brigitte Schoenemann, lead study, a paleontologist at the Institute of Biology Education at the University of Cologne in Germany.
Composite eyes with large ommatidia help animals navigate in low light conditions. Mar A. koninckiiThe eye facets each measure 35.00 diameters (35 micrometers) in diameter – on the smaller side for compound eyes. This meant that the trilobite “was certainly active for days and lived in shallow, bright flooded waters,” Schoenemann told Live Science in an email.
The number of facets in each eye gave A. koninckiiThe mosaic vision is a “resolution” of about 200 “pixels”, which would be sharp enough to navigate obstacles and avoid cephalopod predators, she said.
Even the earliest trilobites, dating to about 522 million years ago, show signs of having complex compound eyes, Schoenemann said. This kind of eye probably evolved much earlier, perhaps in shell-less beings that are not fossil, they added.
The scientists’ findings show that the vision of most current active insects and crabs “is an enormously old and effective system, quite a bit has changed since our trilobite,” Schoenemann said.
Originally published on Live Science.
