Trilobita agnostids

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Class TRILOBITA (the trilobites)

Trilobites are the first successful arthropod experiment and are extensively used to zone the Paleozoic. 

As a matter of fact, the first appearance of the trilobite genus Ollenellus marks the classical beginning of the Paleozoic.  They are characterized by a body that is divided in three parts, hence their name.  Some were blind, others had well developed compound eyes.  Many were mud or filter feeders, while still others may have been predators.  Their ultimate demise may well have been due to being outcompeted by fishes.

Trilobites are one of the most successful Paleozoic groups. Because of their diversity and continued evolution they are one of the better groups for time zonation.

Perenopsids are a group of small blind trilobites. This specimen is actually less than  1.5 cm long (about 1/2 in.)
Others were much larger. This trilobite is some 20cm  (~8 in.) in length.

This is a detail of the trilobite above. Note that the trilobites had already evolved compound eyes, still used today by their relatives, the insects.

Others developed bizarre and ornate horns.

This is Phacops rana a common North American trilobite

Trilobites are hard-shelled, segmented creatures that existed over 300 million years ago in the Earth's ancient seas. They went extinct before dinosaurs even existed, and are one of the key signature creatures of the Paleozoic Era, the first era to exhibit a proliferation of the complex life-forms that established the foundation of life as it is today. Although dinosaurs are the most well-known fossil life forms, trilobites are also a favorite among those familiar with Paleontology (the study of the development of life on Earth).



were among the first of the arthropods, a phylum of hard-shelled creatures with multiple body segments and jointed legs (although the legs, antennae and other finer structures of trilobites only rarely are preserved). They constitute an extinct class of arthropods, the Trilobita, made up of eight orders, over 150 families, about 5000 genera, and over 15,000 described species. New species of trilobites are unearthed and described every year. This makes trilobites the single most diverse group of extinct organisms, and within the generalized body plan of trilobites there was a great deal of diversity of size and form. The smallest known trilobite species is just under a millimeter long, while the largest include species from 30 to 70 cm in length (roughly a foot to two feet long!). With such a diversity of species and sizes, speculations on the ecological role of trilobites includes planktonic, swimming, and crawling forms, and we can presume they filled a varied set of trophic (feeding) niches, although perhaps mostly as detritivores, predators, or scavengers. Most trilobites are about an inch long, and part of their appeal is that you can hold and examine an entire fossil animal and turn it about in your hand. Try that with your average dinosaur!


Whatever their size, all trilobite fossils have a similar body plan, being made up of three main body parts: a cephalon (head), a segmented thorax, and a pygidium (tail piece) as shown at left. However, the name "trilobite," which means "three lobed," is not in reference to those three body parts mentioned above, but to the fact that all trilobites bear a long central, or axial lobe, flanked on each side by right and left pleural lobes. These three lobes that run from the cephalon to the pygidium are what give trilobites their name, and are common to all trilobites despite their great diversity of form. You can examine the trilobite body plan in more detail using the links ( TRILOBITE BODY )




The Trilobite Eye Trilobite Eyes | last revised 02 FEB 2002 by S. M. Gon III

Trilobites developed one of the first advanced visual systems in the animal kingdom. The majority of trilobites bore a pair of compound eyes (made up of many lensed units). They typically occupied the outer edges of the fixigena (free cheeks) on either side of the glabella, adjacent to the facial sutures.  At least one suborder of trilobites, the Agnostina, are thought to be primarily eyeless. None have ever been found with eyes. In contrast, a few secondarily eyeless species (in which a clear evolutionary trend toward reduced eye size with eventual disappearence of eyes altogether) have developed within several groups, even those known for large, well-developed eyes (e.g., Phacopina). 

Holochroal eyes of the Asaphoid trilobite Isotelus

The advantage of good eye design
Compound eyes in living arthropods such as insects are very sensitive to motion, and it is likely that they were similarly important in predator detection in trilobites. It has also been suggested that stereoscopic vision was provided by closely spaced, but separate eyes. Vertebrate lenses (such as our own) can change shape (accomodate) to focus on objects at varying distances. Trilobite eyes, in contrast, had rigid, crystalline lenses, and therefore no accomodation. Instead, an internal doublet structure (two lens layers of different refractive indices acting in combination) corrected for focusing problems that result from rigid lenses. The shapes of some trilobite lenses, in fact, match those derived by optical scientists over 300 million years later to answer similar needs. Compare, for example, the optical designs of the 17th century physicists Descartes and Huygens shown below, with those of two trilobite species. The result is that, even without the benefit of accomodation, the rigid trilobite doublet lens had remarkable depth of field (that is, allowed for objects both near and far to remain in relatively good focus) and minimal spherical aberration (distortion of image).

Descartes' lens design for minimal aberration (above left) is found in the lens of the trilobite Crozonaspis (right)
Light ray paths (yellow) entering the lens from the left come into focus a short distance to the right of the lens (blue).
In the eye of Crozonaspis, an intralensar body (white) further corrects focus after passing through the outer lens layer (blue).

Huygens' lens design for minimal aberration (above left) is found in the lens of the trilobite Dalmanitina (right)
both images ©1999, 2000 by S. M. Gon III, modified from Clarkson and Levi-Setti 1975

Three types of trilobite eyes
There are three recognized kinds of trilobite eyes: holochroal, schizochroal, and abathochroal. The first two are the major types, with the great majority of trilobites bearing holochroal eyes, and the distinctive schizochroal eye a recognized innovation of the Phacopida. Holochroal eyes are characterized by close packing of biconvex lenses beneath a single corneal layer that covers all of the lenses. These lenses are generally hexagonal in outline and range in number from one to more than 15,000 per eye! Schizochroal eyes on the other hand are made up of a few to more than 700 relatively large, thick lenses, each covered by a separate cornea. Each lens is positioned in a conical or cylindrical mounting and is separated from its neighbors by sclera (cuticular exoskeleton material) that extends deeply, providing an anchor for the corneal membrane, which extends downward into the sclera, where it is called intrascleral membrane.The abathochroal eye is seen in only a few Cambrian trilobites and is somehat similar to the schizochroal eye, but differs in some important respects: the sclera is notthick,and the corneal membrane does not extend downward, but ends at the edge of the lens.
The table below illustrates and contrasts the characters of the three eye types.

Holochroal eye
from Clarkson 1975

Schizochroal eye
from Levi-Setti 1993

Abathochroal eye
from Zhang & Clarkson 1990

found in nearly all Orders
few to very many lenses (to >15,000!)
lenses typically small, numerous
one corneal layer covers all lenses
lenses in direct contact with others
no sclera between lenses
corneal membrane covers surface only

found in some Phacopida only
typically fewer lenses (to ca 700)
lenses much larger, fewer
each lens bears an individual cornea
lenses separated from each other
sclera between lenses very deep
corneal membrane extends into sclera

found in Cambrian Eodiscina only
few lenses (to ca 70)
lens size small, not numerous
each lens bears an individual cornea
lenses separated from each other
interlensar sclera not deeper than lenses
corneal membrane ends at lens margin

cross section reveals:
no sclera between lenses (blue)
single cornea (pink) covers all lenses
corneal membrane on surface only

cross section reveals:
sclera (brown) between lenses very deep
one cornea (pink) per lens (blue)
corneal membrane extends into sclera

cross section reveals:
sclera (brown) not deeper than lenses
one cornea (pink) per lens (blue)
corneal membrane ends at lens edge

had holochroal eyes

had schizochroal eyes

had abathochroal eyes

How did schizochroal eyes evolve?
All early trilobites (Cambrian), had holochroal eyes and it would seem hard to evolve the distinctive phacopid schizochroal eye from this form. The answer is thought to lie in ontogenetic (developmental) processes on an evolutionary time scale. Paedomorphosis is the retention of ancestral juvenile characteristics into adulthood in the descendent. Paedomorphosis can occur three ways: Progenesis (early sexual maturation in an otherwise juvenile body), Neoteny (reduced rate of morphological development), and Post-displacement (delayed growth of certain structures relative to others). The development of schizochroal eyes in phacopid trilobites is a good example of post-displacement paedomorphosis. The eyes of immature holochroal Cambrian trilobites were basically miniature schizochroal eyes. In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.

Variation in trilobite eyes As with other aspects of the trilobite body, there was a huge variation of size and form among trilobite eyes, which in many cases seems related to the ecological life style of different species. The figures below show some of these variations. Many of the earliest trilobite eyes were cresentic, such as those of the Corynexochid Polypleuraspis. A conical section of schizochroal eyes gave species such as Phacops an excellent field of vision. In some trilobites, such as the free-swimming pelagic trilobite Opipeuter, the eyes were so large that they dominated the cephalon, providing a 360 degree visual field. Planktonic forms, such as Agnostus, seem to have been entirely blind. Others, such as the Trinucleoid Cryptolithus were bottom feeders with a large, pitted sensory fringe, and eyes were reduced or lost. In species moving through a benthic layer of loose debris or algal growth, eyes raised above the body on stalks could peer about for danger, such as in the strange Russian Asaphoid Neoasaphus (left). Species living on the bottom in deeper waters would have little or no need for eyes at all, and species with reduced eyes, such as Trimerus and secondarily lost eyes, such as Conocoryphe are the result. 

Phacops species

cresentic eyes

schizochroal eyes

large holochroal eyes

primarily eyeless

secondarily eyeless

stalked eyes

reduced eyes

secondarily eyeless

Evolutionary Loss of Eyes
Although eyes are normally an extremely important survival feature, there are situations underwhich loss of eyes might occur. For example, trilobites that took advantage of deep-water benthic (bottom-feeding) habitats where light was dim or lacking might have gradually lost their eyes without suffering an adaptive disadvantage. Such eyeless trilobite assemblages are called atheloptic. Such evolutionary trends are repeatedly seen in a variety of trilobite orders, and two examples are shown below. In both cases, these are Devonian trilobites that started with ancestors bearing large, functional eyes. In one sequence, eyes of a Phacopid clade were lost, and facial sutures associated with eyes were also reduced and marginalized. In the other example, involving a Proetid clade, eyes were also reduced and lost, but the basic facial suture pattern was retained. In the figures below (after Fortey & Owens 1999), the eyes are show blue and facial sutures in red

The ancestral Phacops species had large eyes and typical phacopid proparian facial sutures

The proetid Pterocoryphe had large eyes associated with opisthoparian sutures.

Eyes large and typical

Reduction of eyes and a migration forward on the cephalon is seen in the descendant Cryphops.

Greatly reduced eye size marked the genus Pteroparia, descendant of Pterocoryphe.

Eyes reduced in size

Eventually the eyes were lost althogether and the sutures were left along the anterior margin of the cephalon in the genus Trimerocephalus.

Although the eyes are entirely lost in this Pteroparia species, the facial suture patterns are largely unchanged.

Eyes lost entirely

all line drawings this page ©1999, 2000, 2001 by S. M. Gon III SEM images of the trilobite schizochroal eye

courtesy of Dr. Riccardo Levi-Setti from his superb book "Trilobites"


Clarkson, E. N. K. 1975. The evolution of the eye in trilobites. Fossils and Strata 4:7-31.
Clarkson, E. N. K. & R. Levi-Setti. 1975. Trilobite eyes and the optics of Des Cartes and Huygens. Nature 254 (1975): 663-667.
Fortey, R. A. & R. M. Owens. 1999. The Trilobite Exoskeleton. in: Functional Morphology of the Invertebrate Skeleton. John Wiley & Sons.
Levi-Setti, R. 1993. Trilobites (2nd Ed.). University of Chicago Press.
Moore, R. & Kaesler 1997. Treatise on Invertebrate Paleontology, Part O, Arthropoda 1, Trilobita, Revised.
Zhang, Xiguang & E. N. K. Clarkson. 1990. The eyes of lower Cambrian eodiscid trilobites. Palaeontology 33:911-932.

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