Asaphus expansus (my image)
I must give a great credit to Richard Fortey's Trilobite! Any book about paleontology which ends with an exclamation mark is always worth reading. Also, the good people at www.trilobita.de/english/eyes.html. I have adapted and extended this from a excerpt of my entry to the Bill Bryson Prize.
Trilobites
were some of the oldest, longest lived organisms on earth. They were some of the earliest complex animals and swarmed the seas for around 300 million years before dying out at the end of the Permian,
around 252 million years ago. They look a little like the modern
woodlouse (or pillbug, if you are from the US) and are easily
dismissed as “bugs”. But they were one whole subphylum (the
Trilobitomorphs) out of the five subphyla of arthropod, meaning they
have equal taxonomical ranking with the crustaceans. Trilobites show
us some of the experiments with animal body plans, and therefore help
us understand how the body plans we now see came about or could have
been. Palaeontology is to the modern zoologist as science-fiction is
to the physicist, it gives us idea of how things could have been,
not just what they were. It gives us evolutionary possibilities.
Tissues
which can sense changes in light are some of the oldest organs in the
animal kingdom, basic
vision is even found in a type of algae called Volvox.
As
vision could not have evolved in algae then evolved directly from
that into animal vision (as they are not that closely related, in
evolutionary terms), a shared evolutionary ancestors of animals and
algae must have had the chemicals needed for sight, and therefore may
have used these chemicals for sight. The protein used in vertebrate
lenses is crystallin, the genes for which pre-date vision in any life form,
bacterial and all. The
eye probably appeared in animals before the split of the Protostomes
(including arthropods, molluscs, vertebrates and worms) and the Deuterstomes
(echinoderms and vertebrates), as both groups feature some animals
with similar eyes. This could have occurred between 750 and 1250
million years ago (which doesn't mean anything!), well before the Cambrian explosion and the Edicaran, when life was
probably little more than very simple multicellular organisms. But we
have very few fossils from this time, both because of the small
numbers of creatures and because they were mostly soft bodied. Trilobites are easily dismissed due to their abundance in the fossil record, but this is
their advantage. They had a hard exoskeleton and crystal eyes,
meaning that even a rare species has a fair chance of being fossilized due to their hard tissues. Many abundant species have
been found with soft tissues preserved, even some preserved in iron
pyrite: trilobites made of fool's gold.
A trilobite with it's soft tissue preserved in iron pyrite from hudsonvalleygeologist.blogspot.co.uk/2011/04/beecher-trilobite-beds.html
Not
all trilobites had vision, of course. None of the suborder Agnostida
ever developed eyes (known as primary blindness). But of the eyed
trilobites, there are two generally accepted categories of trilobite
eyes, the typical holochroal and the more rare schizochroal eyes (a
development of the suborder Phacopida),
but we will come to this issue later. All trilobite eyes involved
small lens, which may have given similar images that modern arthropod
(compound) eyes do.
Examples of Agnostus pisiformis (my image)
Making a lens
All eyed trilobites had hundreds of tiny lenses made of calcite
(calcium carbonate crystals) which focused light onto photoreceptors.
It is thanks to the hardness of the crystal lens that we even know
about them today, as they are easily preserved in the fossil record.
The trilobite eye is therefore one of the earliest eyes we know
about, though there could have been an incredibly diverse ways of
seeing in the Pre-Cambrian seas.
One of the first known trilobite to have vision is Fallotaspis which appeared around 540 million years ago; it had crescent shaped ocular lobes which would have given it quite complex vision. It is remarkable that such a complex sensual system is found in such an ancient creature, it is tempting to get poetic and think of what strange worlds these eyes must have seen.
One of the first known trilobite to have vision is Fallotaspis which appeared around 540 million years ago; it had crescent shaped ocular lobes which would have given it quite complex vision. It is remarkable that such a complex sensual system is found in such an ancient creature, it is tempting to get poetic and think of what strange worlds these eyes must have seen.
Fallotaspis from fossilmuseum.net
But
just being transparent doesn't make a good lens. Calcite crystals are
rhomb- shaped when cleaved. Rhombs have one major axis and three axes
perpendicular to this axis. If a light ray is introduced into the
sides of the rhomb, the light ray will be split in a process known as
double refraction. Only when light passes down the major axis
(c-axis) is it not split. When the rhomb is elongated enough in
parallel to the c-axis, only the light that passes down the c-axis
will pass clearly through the crystal.
An example of Icelandic Spar calcite, showing double refection from www.segerman.org/CoT.html
How
they managed to build these lenses is not known, calcium carbonate is
common the sea, but how they harnessed this and made crystals grow in
such a specific way is not known.
Holochroal eyes
Holochroal
eyes are the most common eyes in trilobites. They involve lots
(hundreds or up to ~ 15000) of hexagonal, convex lenses squashed
together and covered by a single cornea of calcite. If you put a
photoreceptors behind this crystal, you have a single lens of a
compound eye, which is what the trilobites probably did. Each lens
can detect light coming in from one particular direction. If you
stretch a sheath of these lenses almost completely around the eye,
then the trilobite can see from tail to (so to speak) nose. They
would have seen the world in an overlapping mosaic of images of the
Paleozoic seas.
Schizochroal eyes
These
type of eyes are found in the some of the Phacopida trilobites.
They had large crescent-shaped
eyes that stood out from the cheeks (part of the head or cephalon).
In these eyes there were comparatively large, round but slightly
tear-shaped lens, each sunken into the eye so that they were
separated from the other lenses by the opaque sclera. Each lens was
covered by it's own cornea, and appears to have operated
independently of other lenses.
A curled up Phacops with clear Schizochroal lenses from fossilmuseum.net
An illustration showing how spherical aberration is corrected from www.trilobita.de/english/eyes.html
There
were many different forms of eye in trilobites, each adapted to the
particular trilobite’s behaviour. The Agnostida were blind, as they
dwelt in the mud and had nothing worth seeing. Some of the earliest
trilobites had simpler holochroal lenses in a long, thin eye which
could only see to each side, suited to shuffling about on the sea
floor. Other eyes became more baroque. The free-swimming trilobites
must have been able to see upwards and downwards as well, so some
like the comically bulge-eyed Opipeuter
inconnivus had
very large eyes to spot predator or prey coming in three dimensions.
A 3D reconstruction of Opipeuter inconnivus from www.nickjainschigg.org/Images/Trilobite/Trilo+ExtralegsRerender.html
Another mud dweller, Asaphus
kowalewski,
ended up with eyes on stalks, so to spot predators that would swoop
in from above.
Asaphus kowalewski from fossilmuseum.net