In The Blink Of An Eye (33 page)

So eyes have been found in fossils up to 430 million years old, and their vision has been extrapolated via comparisons with the eyes of today. But can we wind the clock back further and take a look into even older eyes?

After scrambling up a slippery section of the Canadian mountainside from the camp of Des Collins's field team to the Burgess Shale quarries,
I reached a ledge that had been excavated during both earlier and current fossil expeditions. At the back of the ledge was the exposed face of the quarry, where the layers of sediment were clearly defined through their various colours. On the ledge itself was a wooden table, which supported the fossils unearthed during the current excavations.

Standing in the viewing area of the Burgess quarry, on the edge of Emerald Lake below, all that could be seen of the site above us was an indiscernible blue object. Along with the many tourists using the telescope provided, I wondered what it could be. It was, as I discovered on reaching the quarry, just an old plastic sheet, but one with an important purpose - to protect the latest fossil treasures laid out on the table from the harsh climate. These fossils were awaiting quality control, to ascertain their future in the display cases of museums all over the world. And there really were some treasures. The fossils whose photographs I had seen in coffee-table books and on the projector screens at a number of famous lectures were there in front of my very eyes. And I was one of the first people to see these specimens - they were fresh from the rock. But they were very well preserved and defined - and I could identify them all.

I picked up the largest piece of thin, flat shale on the table. It had the dimensions of a large roof slate, and its smooth surface bore a detail of the most fearsome member of the Burgess community -
Anomalocaris
. The body was big, nearly half a metre long and broad with it. Emerging from the head, the grasping forelimbs, once thought to be shrimp-like animals in their own right, were obvious. And I had already identified the front end of the body thanks to another give-away clue - the large pair of eyes that were equally obvious.

The eyes of
Anomalocaris
appeared as two buttons jutting out from the sides of the head. Their smooth, rounded outlines were obvious, although that was all to be seen with the naked eye. But their position on the sides of the head suggested these were eyes and nothing else. In Chapter 1 we learnt that no new animal phyla have evolved on Earth since the Cambrian explosion - the phyla we see today are those that existed in the Cambrian (with few possible exceptions). There is a law also that animals today live and function as did their ancestors in the Cambrian. There have been no magical periods in history since the
Cambrian explosion where things happened differently. Today it is clear that the button-like structures protruding slightly from the head of
Anomalocaris
could be only one thing - eyes.

Figure 7.6
Anomalocaris
and
Waptia
from the Burgess Shale. At around 7.5cm,
Waptia
is several times smaller than
Anomalocaris
.

Back in the laboratory at the Smithsonian Institution, I examined the well-preserved analogues of another member of the Burgess community -
Waptia
.
Waptia
was a shrimp-like animal, a member of the arthropod phylum and possibly a crustacean. It was also about the size of an average shrimp of today, and seemed to share the shrimp's eye characteristics. Like shrimps and crabs,
Waptia
had eyes on stalks. This means that its eyes could have moved independently of its head. They would have been specialists at looking within a narrow range of the Cambrian environment in detail. They would have seen
Anomalocaris
as it swam in front of them. But as
Anomalocaris
moved, the eyes of
Waptia
would have moved too, and followed their giant neighbour. Unlike the compound eyes of insects, which are known as sessile because they are fused with the head and so cannot move independently, stalked eyes can change their field of view without head movement.

I mentioned ‘compound' eyes during this discussion of
Waptia
. Although the eyes of the
Anomalocaris
I examined revealed little additional detail under the microscope, a microscopic view of a well-preserved
Waptia
specimen told a different story. The internal architecture of the
Waptia
eye became evident - and it matched that of a crustacean today. The stalked apposition compound eyes of a crustacean known as a ‘mysid' are producing images of animals swimming past them in the sea today. And
Waptia
would have seen similar pictures in Cambrian seas.
Waptia
had apposition compound eyes.

Figure 7.7
Micrographs of the heads of a living ‘mysid' crustacean and
Waptia
from the Burgess Shale. Eyes show comparable internal architectures. Scale bars represent 2mm (top picture) and 0.5mm (bottom picture).

Looking through the collection of arthropods in the Smithsonian's Burgess Shale collection, it became obvious that
Anomalocaris
and
Waptia
are not alone. They were not the sole beneficiaries of vision: far from it.

Within the Smithsonian fossil deposit, the Burgess specimens are enclosed by a large metal cage, which provides additional security in the style of a bank vault. Doug Erwin is custodian of Charles Doolittle Walcott's collection today, which embraces quite a variety of multicelled animal forms. Doug kindly allowed me use of his microscope, a key to the Burgess vault, and a large wooden tray.

Examining the invaluable fossils was time-consuming. They were stored in dozens of cabinets, with hundreds of drawers full of specimens. I looked into each drawer and tried to select the best-preserved representatives to fill my tray. This is difficult to do with the naked eye, and I probably missed some informative examples.

When I had made each selection, the individual fossil was placed in my tray and an official museum form was put in the empty space in the drawer. On the back of each fossil was painted a catalogue number, and this, along with my name and the name of the specimen and its original location was recorded on the form. The level of security and guardian-ship echoed that at the Burgess quarries themselves. The small quarries are approached by only one path - there are no back doors on the exposed mountainside. In the vicinity of the quarry, the path is policed by Des Collins's field team, whose camp is positioned just the other side of the path to the Burgess quarries. The two exits of the path, each at least three hours hike from the quarries, are patrolled by wardens from Parks Canada. And the security pays off. The world fossil trade is a considerable one. There are many private fossil collections and shops around the world. Some include complete skeletons of dinosaurs, such as
T. rex
, but none contains a single specimen from the Burgess Shale.

I examined specimens of the Burgess arthropods
Canadaspis
,
Odaraia
,
Perspicaris
,
Sanctacaris
,
Sarotrocercus
,
Sidneyia
and
Yohoia
.
All possessed eyes, varying in size with respect to body length. Again, the smaller specimens appeared to have relatively larger eyes. And all these ‘eyes' really were eyes; based on comparisons with the visual organs of living species, they would have formed images in the Cambrian. In many other Burgess arthropods, the presence or absence of eyes could not be resolved with accuracy due to imperfect preservation or unfavourable orientations within the rock. Maybe I had chosen the wrong specimens to examine. For instance, I failed to detect the eyes of perhaps the commonest Burgess arthropod,
Marrella
. Recently Des Collins and his Spanish colleague Diego Garcia-Bellido identified eyes on
Marrella
resembling those of woodlice today. But I was certain of one thing - eyes were common in the Burgess arthropods.

There are a few Burgess animals from other phyla with eyes, but not many. Actually there may be only
Nectocaris
and the weird, five-eyed
Opabinia
. But then
Opabinia
is probably an arthropod, although
Nectocaris
appears closer to the chordates than the arthropods. More specimens of these rare species are needed in order to classify them with certainty. But eyes are either rare or absent in the non-arthropod Burgess animals.

Figure 7.8
Yohoia
,
Perspicaris
,
Nectocaris
and
Sarotrocercus
- examples of Burgess animals with eyes.

The Burgess Shale community lived in the Cambrian, but more precisely they lived 515 million years ago. The question we would most like to answer in the chapter is ‘When did the first eye appear on Earth?'. Now we know that eyes were well in place on Earth some 515 million years ago, but the Cambrian explosion took place sometime between 543 and 538 million years ago. So at this point I will leave the Burgess Shale fauna and continue my search for eyes in other, older fossils (I hope) from the Cambrian period.

On the subject of weird-looking Cambrian fossils,
Cambropachycope
and its relatives are bizarre arthropods that were the ancestors of the crustaceans of today. They are known from Cambrian fossils preserved in the ‘Orsten' limestone of Sweden. Their preservation is actually quite exceptional, and in full 3D. The German palaeontologist Dieter Walossek is responsible for the excellent interpretations of these fossils, and as their guardian he obligingly sent a specimen of
Cambropachycope
to me for examination in electron microscopes. I was interested in this animal for one reason in particular - its eye. I use the singular here because
Cambropachycope
had a huge visual organ compared with its body size . . . but only one of them.

Cambropachycope
was a small arthropod, just a few millimetres long. Its body is distinct for having a big, paddle-shaped limb on either side, so swimming may have been possible. The head of
Cambropachycope
is just as unusual. It is fused with the rest of the body, and has an obvious mouth near the fusion. But then it constricts to form both a false ‘neck' and a huge bulbous projection in front of the body and mouth. The projection is basically a compound eye. Maybe it evolved following the fusion of two stalked eyes, but it certainly would have seen whatever was ahead of it with some accuracy. I drew this conclusion after studying its cornea - unfortunately that's all that remains of this eye.

Although from the Cambrian,
Cambropachycope
and the other Orsten arthropods with eyes are no older than the Burgess Shale community. But, as we learnt in Chapter 1, there is a site in China where an exceptionally well-preserved suite of Cambrian fossils has been recovered. And these ‘Chengjiang' fossils are ten million years older than those of the Burgess Shale.