A Good Eye For a Fossil

What did a Jurassic sea predator look like? For the first time, scientists have described the three-dimensional eye-structure of Dollocaris ingens, a 160-million-year-old arthropod from the Jurassic of southeast France.

Fossils typically do not preserve delicate, soft organs such as the eyes, leaving palaeontologists none the wiser about how visual systems evolved through geological time. However, on rare occasions, exceptional preservation of fossils can occur. In the localities that boast exceptional preservation of fossils—so-called fossil Lagerstätte—details of skin, eyes, spines, fur and even colour can stand the test of time.  When palaeontologists come across a Lagerstätte, they strike gold. Luckily for Dr Jaen Vannier of Universite´ Lyon and his colleagues, Dollocaris was preserved in precisely such an environment—the La Voulte Lagerstätte in southeast France.

This fortuitous kind of preservation, combined with the development of sophisticated X-ray visualization techniques, allowed the scientists to describe the eye structure in Dollocaris in a paper published in Nature Communications this year. According to the scientists, its huge eyes, occupying nearly a quarter of the length of the animal’s entire body, were compound (made up of several visual units or ommatidia) and each eye consisted of roughly 18,000 ommatidia—a record number among extinct and living arthropods, surpassed only by modern dragonflies. Both the external and internal structures of the ommatidia are preserved in the fossils. Such eyes would have allowed for a panoramic field of vision, say the scientists, and a mosaic-like vision—surely suiting the needs of a predator.

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A reconstruction of Dollocaris, a visual predator of the Jurassic oceans. Credit: Jean Vannier.

The size, field of vision, extremely high number of ommatidia and visual parameters suggest this critter possessed the ability to detect and track moving objects. But what exactly was this big arthropod scrutinizing? Additional fossil evidence, such as a set of three powerful grasping appendages, and gut content containing the undigested remains of mobile prey indicate that Dollocaris was a visual predator.

While some eyes seem to have been adapted for catching prey, others seem to have been better suited to escaping predators. Dr Brigitte Schoenemann, a researcher at the University of Bonn and a co-author on the paper, has been working on understanding fossil vision for many years. In research published in Proceedings of the Royal Society B in 2012, she described the eyes of Henningsmoenicaris scutula, a tiny (less than 2-millimetres in size) crustacean living during the Cambrian Period, about 300 million years before Dollocaris graced the oceans.

Based on an analysis of six isolated fossil eyes from the Orsten Shale in Sweden, the researchers discovered that H. scutula had compound eyes that could detect moving objects with ease, including lurking objects that may have been approaching from behind.

Three-dimensional reconstruction of the orientation of the optical axes in the eye of H. scutula. Four visual areas are highlighted. Credit: Schoenemann et al. 2012, Proceedings of the Royal Society B.
Three-dimensional reconstruction of the orientation of the optical axes in the eye of H. scutula. Four visual areas are highlighted. Credit: Schoenemann et al. 2012, Proceedings of the Royal Society B.

Dr Schoenemann and her colleagues analysed the orientation of the optical axes within the eyes and discovered that the visual units that made up the compound eye were probably too small to give a clear image. However, as in Dollocaris, the size, velocity, direction and distance of a moving object could be determined. In H. scutula, the optical axes formed four visual areas. The most intriguing of these is by far that which was orientated inwardly, allowing this crustacean to have, quite literally, eyes on the back of its head.

These physical characteristics, as well as pointing to the existence of an advanced visual system early on in crustacean evolution, suggest that H. scutula probably fed by catching visually-selected food items, and could easily spot incoming predators, even when these were approaching from behind. This was a clever crustacean, commented Dr Schoenemann. Insightful research indeed.

By Dr. Anthea Lacchia, postdoctoral research fellow at University College Dublin and freelance science writer. 

Cover image: A fossil compound eye, around 515 million years old, from the Emu Bay Shale on Kangaroo Island, South Australia. Image credit: John Paterson

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