The
Evolution of Sight Trilobite Eyes Holochroal
Eyes Schizochroal
Eyes Abathochroal
Eyes Blind Trilobites Evolutionary
Secondary Trilobite Eye Loss Trilobite
Fossil Eyes
The Evolution of Sight
The
evolution of sight has been the subject of intense scientific
research. So far, scientist believe sight (eyes) evolved
numerous time in
the history of life. And, why not, the survival advantage is
enormous, and the early animals shared the basic gene and
protein building
blocks on which natural selection could work. In fact, the
eye is considered to be a homologous organ, simply meaning
that there
exists a shared ancestry (and genes and proteins) between a
pair of structures, in this case eyes, in different species.
Nature
abounds with homologous organs. The genetic toolkit for eyes
had its etiology before the Cambrian probably from primitive
photosensitive
cells. Examples are the Opsins, a group of light-sensitive
G protein-coupled receptors of the retinylidene protein family
found in photoreceptor
cells of the retina. These are a highly conserved and ancient
proteins (Perez
2003).
There
may not have been animals with eyes to form a focused image
in Precambrian deep time, but even a sense of light and shadows
could
nonetheless
have enabled
the bearer to distinguish night from day, and detect the movement
of food and predators. From that humble beginning, a diverse
genome
with
some proteins to start, mutations, natural selection, and time
were all that were needed for eyes to rapidly evolve to sophistication,
and true images. The advantage of eyes was so profound that
acquiring better eyes likely spawned the equivalent of an evolutionary
arms race between predator and prey. In this arms race,
the trilobite
apparently excelled, as they
were probably the first animals with complex eyes, with
some species having many thousands of individual lenses per
eye.
Ostensibly,
based on the fossil record, complex eyes initially evolved
over a short span of a few million years
in the interval
known as the Cambrian explosion. The meager Precambrian fossils
exhibit no evidence of eyes, but much diversity of eyes, some
most complex, was widespread by the Middle Cambrian, as seen
in Burgess Shale fauna, and
animals from other Burgess Shale-like deposits, where soft
tissue was often preserved. The earliest of the trilobites
in the fossil
record
already had
complex, compound eyes with crystal lenses (calcite). The hypothesis
that primitive arthropod eyes evolved in the Precambrian seems
very sound.
Trilobite
Eyes
Trilobite
eyes are best viewed as an amazing evolutionary achievement,
and one that remains widespread across crustaceans, insect,
and the preponderance of arthropods. Genes dictate protein
sequence, that dictates protein shape, that dictates protein
function, and ultimately the function of the eye. While trilobite
fossils do not preserve the fine eye internal soft eye structures,
we can infer a close similarity to modern arthropods based
on survival calcite crystal lenses of the ommatidia. Trilobite
eyes were compound or composite arrays comprised of distinct
optical units called Ommatidia.
A single ommatidium contains a cluster of photoreceptor cells
around which are support cells and perhaps pigment cells,
all covered with a transparent cornea. The photoreceptors and
associated
neurons convert the visual stimulus into an image perception.
The ommatidia were in a hexagonal numbering in the hundred
to tens of thousands,
with each one focusing light on the retina to create a portion
of an composite image.
Trilobite
eyes are often cited as the
oldest preserved complex visual systems. Because they possessed
a calcified cuticle (crystal eyes), they
have left a good fossil record, and commonly the lens-bearing
surfaces of their paired compound eyes are well preserved. Trilobite
eyes appear highly developed even in the earliest fossil record
of the lower Cambrian, suggesting
development long
before in the Precambrian. The age old competition between predator
and prey in marine environments provided powerful evolutionary
selective
pressures to further improve eyesight. These
earliest trilobites already had complex, compound eyes with lenses
made of crystalline calcite, pure forms of which
are transparent. Moreover, the lense system eyes
of some trilobites were doublet structures that evolution
designed to eliminate
spherical aberration in a manner similar to desings by
Des Cartes and
Huygens in accordance with laws of optical physics. These
allowed good focus both near and far with acceptable
spherical aberration. Natural selection may have selected for
such eye
lenses in order to
maximize optic nerve response in a dim
environment (Clarkson
and Levi-Setti, 1975).
Holochroal
Eyes
Holochroal
eyes are far and away the type found in all trilobite orders
except Agnostida Suborder Eodiscina (Clarkson,
1979) trilobites that had abathochroal
eyes, and Phacopida
Suborder Phacopina
trilobites that had schizochroal compound
eyes. The number of
lens vary from a few to up to 15,000. The lenses are in a closely
packed hexagonal
lattice,
with adjacent
lenses
in direct contact with no interstitial sclera between lenses,
such
that
the cornea only covers the lens' surface. Lenses are typically
30 to 100 µm. Unfortunately, details of the holochroal
eyes are rarely preserved in lower to middle Cambrian trilobites.
Holochroal eyes likely evolved in the precambrian where there
are no fossils, and persisted throughout the long existence of
trilobites to near the end of the Permian. Among redlichids,
the holochroal eyes were large curved and crescent shaped.
Schizochroal
Eyes
Schizochroal
eyes are only found in trilobites of Order Phacopida, Suborder
Phacopina (Clarkson,
1997), appear in the fossil record in the lower Ordovician.
While
have far fewer lenses than holochroal eyes, they still can number
in the
hundreds
and
up
to about 700
larger
lenses.
The normally hexagonally arranged matrix of lenses have individual
cornea interstitially separating lenses and extending deeply
into the sclera. Schizochroal eyes putatively evolved from a
holochroal ancestor, providing a larger circumferential field
of view. The location of
schizochroal and ostensibly parallel evolvement of more
efficient enrollment ability in phacopid trilobites supports
the hypothesis that the eye type provided better defensive
warning of encroaching predators.
Abathochroal
Eyes
Abathochroal
eyes are exclusively found in Suborder Eodiscina of Order Agnostida,
a group that went extinct at
the end of the Middle Cambrian. They had far lessor number of
small lenses up to a maximum of about 70. Each lens an individual
cornea separated by interstitial sclera. Adaptation to changing
environmental conditions apparently led to secondary
loss of eyes in many members of Suborder Eodiscina.
Blind Trilobites
Members
of trilobite Order Agnostida are often called blind trilobites,
and many were, though some were not. All members
of Suborder Agnostina were
eyeless, a condition believed derived from a benthic
existence, mostly in deeper waters where light was scarce.
Secondary
Trilobite Eye Loss
Secondary
blindness in trilobites is not uncommonly observed in the fossil
record, particularly in groups such as the
Agnostida
and Trinucleioidea. Eye size reduction or even loss of eyes in
trilobites has been considered associated with a benthic lifestyle
in deep habitats where little or no sunlight penetrates. In
Proetida
and Phacopina from western Europe and especially in Subfamily
Tropidocoryphinae of family Proetidae from France (Feist,
1986) where stratigraphy was well known, there are well
studied examples of fossils showing
progressive
eye
reduction between closely related species that ultimately led
to secondary loss of visual organs
(Clarkson,
1997). Another example of eye loss is in the benthic Ptychopariid genus
Conocoryphe from
the Cambrian of the Czech Republic, France, Spain, Turkey,
and United States, that lack
eyes
except
for
in one species. (Also
see evolution of secondary blindless of trilobites).