Researchers studying the 160-million-year-old fossils of a marine arthropod called Dollocaris ingens reveal how surprisingly sophisticated their huge eyes were. These little visual predators had more lenses on each eye than any extinct or living arthropod except for modern dragonflies, according to findings published in Nature Communications this week. 

Direct evidence of how ancient animals perceived their environment has been hard to come by because internal eye structures are almost never fossilized. The only arthropod visual systems that have been preserved in their entirety in the fossil record are that of a modern-type fly embedded in Eocene amber and the receptor cells of Devonian trilobites. 

Dollocaris ingens was a tiny marine predator from the Middle Jurassic with a pair of huge compound eyes (making up a fourth of the animal’s entire body length) and a set of powerful, raptorial appendages with spiny tips that converge towards the mouth. Their fossilized gut contents include undigested remains of mobile prey, suggesting how the animal had predatory and hunting habits.

A team led by Jean Vannier of Université Lyon 1 and Brigitte Schoenemann from University of Cologne used scanning electron microscopy and x-ray spectroscopy to visualize the structure of the eyes and other internal organs of Dollocaris ingens fossils collected in southeastern France. Their heart, circulatory system, and network for gas exchange suggest high oxygen requirements consistent with an active lifestyle, and their abdominal region bore eight pairs of very short appendages that likely helped them swim and ventilate. 

The team generated a full 3D reconstruction of their eyes, from external optics to internal cell receptors. Each eye consists of about 18,000 units called ommatidia (pictured below), and each of these had a corneal lens, a crystalline cone, and elongated receptor cells clustered around a central rod-like structure called the rhabdom. Additionally, each eye had a wide field of view that provided the animal with panoramic vision, like that found in modern insects and crustaceans that evolved large faceted eyes that are good for hunting. Some dragonflies have about 30,000 facets per eye, while the praying mantis and mantis shrimps have 9,000 and up to 10,000, respectively.

The size of their compound eyes, the panoramic field of vision, and the extremely high number of lenses suggest that Dollocaris had the ability to detect and track moving objects. It was likely a visual hunter adapted to illuminated environments, ambushing small crustaceans in shallow waters. 

^{Clockwise from top left: Preserved Dollocaris specimen, eye surface with corneal lenses, reconstruction of the eye structure, and a section through eye. The reconstruction shows the corneal lens, crystalline cone, and the sensory and pigmented cells around the rhabdom. Jean Vannier}