Trilobites

The trilobites (Phylum Arthropoda, Class Trilobita) first appear in the fossil record in the Lower Cambrian some 540 million years ago. They persisted for the next 270 million years, always finding a way to survive as marine environments and predators challenged. The long trilobite run ended in the Great Dying at the end of the Permian. But, their emergence, successive adaptations and radiations exemplars of evolution in the history of life on earth.

When trilobites first appeared in the fossil record they were already geographically dispersed. From that time until the last families died out at the end in the Permian extinction, they attained amazing diversity. They a prodigious fossil record with some 500 genera and 20,000 known species spanning Paleozoic time, providing a sound record of how evolution by means of natural selection progresses. Trilobites adapted to occupy a broad spectrum of environmental niches and made a living in concomitantly varied ways.

The number of trilobite families actually peaked in the upper Cambrian, and thereafter gradually declined. Only one of the nine trilobite orders, Proetida, survived the Devonian extinction that mainly affected marine organisms. Proetids continued on into the Carboniferous and they too disappeared finally along with an estimated 96% of all marine species at the end of the Permian about 250 million years ago. Nonetheless, trilobites have to be ranked among the most successful of the early animals, swarming in the oceans for over 270 million years, beginning with the Cambrian Explosion.

The Nine Orders of Arthropod Class Trilobita and Nectaspida, the so-called soft bodied trilobites:
Agnostida	Redlichiida	Corynexochida	Phacopida	Lichida	Odontopleurida	Proetida	Harpetida	Ptychopariida	Asaphida	Nectaspida

The number of trilobite families actually peaked in the upper Cambrian, and thereafter gradually declined. Only one of the 10 trilobite orders, Proetida, survived the Devonian extinction that mainly affected marine organisms. Proetids continued on into the Carboniferous and they too disappeared finally along with an estimated 96% of all marine species at the end of the Permian about 250 million years ago. Nonetheless, trilobites have to be ranked among the most successful of the early animals, swarming in the oceans for over 270 million years, beginning with the Cambrian Explosion.

Trilobites as Exemplars of Evolution

Variation is the very essence of nature, without which life would perish. Diversity enables life to find a way to survive. While individuals of a species might all look the same, exhibiting the same phenotypic traits, at the genomic level they are each unique. It is at the genomic level that mechanisms of evolution and natural selection act within a population. Changing physical and biological environments over geological time altered the selective pressures to which trilobites had to adapt or perish. Evolutionary changes at the genome level manifested at the phenome level, resulting in the astonishing diversity of trilobite forms we see in their fossil record. Trilobites are truly an exemplar of evolution, and consequently an excellent animal group to use for teaching evolution.

How Trilobites Lived:

Trilobites adapted to occupy many different marine environments, essentially everywhere, and led correspondingly different life styles as predators, scavengers and bacterial farmers. Some lived a benthic life in the lowest level of the water column, including the sediment surface and some in sub-surface layers. They moved over the sea bed as predators, scavengers or filter feeders. Other trilobites lived a pelagic life, swimming and feeding on plankton. Members on the Olenidae family are believed to have evolved a symbiotic relationship with chemotrophic, sulfur-eating bacteria from which they harvested their food. They lived inshore and in deep water, and at all depths in between. Each niche they occupied required different adaptation in order to survive, thrive, compete, and eat before they were themselves eaten.

Selective pressures acting on genomic diversity drove the astonishing diversity we see in trilobite fossils. The ability to defensively enroll, amazing crystalline eyes, and sharp spiny adornments are among traits seen in some trilobites. Others developed smooth exoskeletons and eyes on stalks to slip into and hide in silt on the sea floor. Still others developed a streamlined morpholgy in order to use speed as a survival advantage.

Why trilobites are important to science:

The study of trilobites has advanced science in such diverse areas as biostratigraphy, paleontology, evolutionary biology and plate tectonics.