Trilobite Eyes

Trilobites Family Album

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 Ommatidium of Trilobite Compound Eyesoft 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

Redlichiida Crescent Shaped Holochroal EyesHolochroal 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).

Trilobite Fossil Eyes

Platyscutellum Holochrocal Eye
Olenellus chiefensis
Devonian
Order Phacopida
Family Acastidae
Oufaten, Morocco
Middle Devonian
Order Phacopida Family Phacopidae
Windom Shale, New York
Devonian
Order Corynexochida
Family Styginidae
Laghdad, Morocco
Lower Cambrian
Suborder Olenellina Family Olenellidae 
Pioche Shale Formation, Nevada
Drotops megalomanicus Schizochroal Eyes
Hatangia Rusian Trilobite
Huntoniatonia lingulifer Schizochroal Eyes
Acadoaradoxides nobilis Redlichiid Trilobite with Holochroal Eyes
Early Cambrian
Order Redlichiida
Family Paradoxididae
Mecissi, Morocco
Devonian
Order Phacopida
Family Phacopidae
Alnif, Morocco
Middle Cambrian
Order Ptychopariida
Family Proasaphiscidae
Siberia, Russia
Devonian
Order Phacopida
Suborder Phacopina
Haragan Formation, Oklahoma
Spinodontochile spinifer
Phacops araw Morocco Trilobite
Walliserops hammi
Delocare rostrata
Devonian
Order Phacopida
Family Dalmanitidae
Alnif, Morocco
Devonian
Order Phacopida
Family Phacopidae
Aatchana, Morocco
Devonian
Order Phacopida
Family Acastidae
Ziguilma, Morocco
Devonian
Order Phacopida
Subfamily Asteropyginae
Mrakib, Morocco
Struveaspis Schizochroal Eye
Gerastos Blind Trilobite
Eocryphops
Conocoryphe sulzeri Secondarily Blind Trilobites
Devonian
Order Phacopida
Family Phacopidae
TaFrawte, Morocco
 Devonian
Family Proetidae
Jorf, Morocco
Devonian
Order Phacopida
Family Phacopidae
Jorf, Morocco
Middle Cambrian
Order Ptychopariida
Family Conocoryphidae
Jince Formation, Rejkovice, Czech Republic
Drotops armatus with Schizochroal Eyes
Coltraenia oufatenensis
   
Drotops armatus with Schizochroal Eyes Coltraenia oufatenensis High Wrap-Around Schizochroal Eyes    
Devonian
Order Phacopida
Family Phacopidae
Erfoud, Morocco
Devonian
Suborder Phacopina
Superfamily Acastoidea
Family Acastidae
Oufaten, Morocco