Eyes from the deep past

by Matthew Cobb

[I just posted this over at z-letter.com – it’s reposted here with Jerry’s permission]

The most dramatic early eyes known up to now have been, not surprisingly, the calcite eyes of trilobites. These mineral lenses, formed into a compound eye, lend themselves to fossilisation, plus there are millions of fossilised trilobites lying around in rocks. In the latest issue of Nature, there is a dramatic description of an incredibly tiny fossil from Australia that reveals that at least one organism from the early Cambrian – 515 MY ago – had eyes that were incredibly complex and modern.

The work was carried out by a group from Australia (with help from the Natural History Museum in London), led by Michael Lee and John Paterson. They studied rocks from the Emu Shale in Kangaroo Island in South Australia. This is a famous layer of shale which contains some exceptionally preserved organisms, and is a useful comparison with the Burgess Shale in Canada and Chenjiang in China.

The researchers found a number of isolated fossilised eyes that had apparently come from some kind of arthropod. They are about 7 mm long, are curved, and are composed of up to 3,000 individual lenses or ‘ommatidia’. They are not made of calcite, and they are not from a trilobite. They are incredibly beautiful. The eye of the robber fly is included for comparison. These were NOT from a fly! They are from a marine organism!

a–d, Three fossils of compound eyes from a large arthropod from the Emu Bay Shale, South Australia (a–c), shown in similar hypothesized orientation to the compound eye of a living predatory arthropod, the robberfly Laphria rufifemorata (d; anterior view of head). All fossil eyes have large central ommatidial lenses forming a light-sensitive bright zone, b, and a sclerotized pedestal, p. Because the fossil eyes are largely symmetrical about the horizontal axis, it is not possible to determine dorsal and ventral surfaces, and thus whether the eyes are left or right. All fossils are oriented as if they are left eyes (medial is to the left of the figure). In b there is a radial tear (white line) with the top portion of the eye displaced downwards to overlie the main part; extensive wrinkling causes some central lenses (arrow) to be preserved almost perpendicular to the bedding plane.

The fossilised eyes were all about the same size, suggesting they had all come from adults. Sadly, there are no clear animal remains associated with them. They presumably became separated from the body either because the animals were predated in some odd way (predators spitting out the eyes?) or because they are in fact the cast of the organism as it grew in size. They say:

“One possibility is that the fossils reported here are of previously shed corneas. The corneal surfaces of living arthropods detach during ecdysis and remain loosely connected to the rest of the exuvia; moulted corneas might be more prone to decay and thus more susceptible to early diagenetic mineralization (in this case phosphatization) than complete eyes attached to intact organisms.”

The fine detail of the fossils made it possible to calculate the precise distance between the ommatidia. Note that the lenses are hexagonal, just as in modern arthropods:

Cambrian arthropod eye. a, Entire specimen showing the positions of close-ups in c and f. b, Relief-map three-dimensional reconstruction of a. c, Close-up of large ommatidial lenses in the bright zone, with white line and numbers referring to the cross-section shown in e. d, Relief-map three-dimensional reconstruction of c. e, Cross-section through four large lenses indicated by the white line in c; numbers refer to individual lenses represented by concavities. f, Close-up of small marginal lenses, with white line and numbers referring to the cross-section shown in h. g, Relief-map three-dimensional reconstruction of f. h, Cross-section through four small lenses indicated by the white line in f; numbers refer to individual lenses represented by concavities.

The authors then go on to look at the optics of these eyes, in terms of the density of the ommaditidia. And report that this kind of complexity and density has previously been found only in the the Ordovician, around 40 MY later.

So – what animal do they belong to? They are too small to be from everyone’s favourite Cambrian predator, Anomalocaris. The authors reckon that they could be from a bivalve arthropod found in the Emu Bay shale called Tuzoia:

The large, unnamed Tuzoia species from the Emu Bay Shale has stalked compound eyes that are ovoid to round and 6–9 mm in diameter: very similar to the fossil eyes described here. However, no detailed structure of the visual surface is preserved in the articulated eyes of Emu Bay Shale or Burgess Shale Tuzoia specimens.

Here’s a picture of Tuzoia:

Reconstruction of Tuzoia.

The authors conclude:

The specimens described here represent the first microanatomical evidence confirming the view that highly developed vision in the Early Cambrian was not restricted to trilobites. Furthermore, in possessing more and larger lenses, plus a distinct bright zone, they are substantially more complex than contemporaneous trilobite eyes, which are often assumed to be among the most powerful visual organs of their time. The new fossils reveal that some of the earliest arthropods had already acquired visual systems similar to those of living forms, underscoring the speed and magnitude of the evolutionary innovation that occurred during the Cambrian explosion.