When trilobites molted or died, the librigenae (the so-called "free cheeks") often separated, leaving the cranidium (glabella + fixigenae) exposed.
Others would assert that the trilobite line itself left no descendants, but that ancestors of the trilobites would also have given rise to species that became today's horseshoe crabs.
The number of lenses in such an eye varied, however: Some trilobites had only one, and some had thousands of lenses in a single eye.
Spectacular spined trilobites have also been found in western Russia; Oklahoma, U.S.; and Ontario, Canada.
Trilobite larvae are reasonably well known and provide an important aid in evaluating high-level phylogenetic relationships among trilobites.
Trilobites are hard-shelled, segmented members of the phylum Arthropoda and the class Trilobita that appear in the fossil record for almost 300 million years—from about 540 to 251 million years ago (mya).
Trilobites are collected commercially in Russia (especially in the St. Petersburg area), Germany, Morocco's Atlas Mountains, Utah, Ohio, British Columbia, and in other parts of Canada.
According to New Scientist magazine (May 2005), "some… trilobites… had horns on their heads similar to those of modern beetles."
Trilobites had a single pair of pre-oral antennae and otherwise undifferentiated biramous (double-branched) limbs.
Pure forms of calcite are transparent, and some trilobites used a single crystallographically oriented, clear calcite crystal to form the lens of each of their eyes.
The thorax is fairly flexible—fossilized trilobites are often found curled up like modern woodlice, perhaps for protection.
Other trilobites found there include Dalmanites, Trimerus, and Bumastus.
The pygidia are still fairly rudimentary in the most primitive trilobites.
Many trilobites had eyes, while some trilobites lacked eyes, probably living too deep in the sea for light to reach them.
Trilobites are thought to represent an early stage in the step-by-step development of life on Earth, but there remain alternative views about their precise cladistic connection to current species.
When describing differences between different taxa of trilobites, the presence, size, and shape of the cephalic features are often mentioned.
Some Native Americans, recognizing that trilobites were water creatures, had a name for them which means "little water bug in the rocks."
Contrary to popular belief, it is this longitudinal tripartite division into left and right pleural lobes and a central axial lobe that gives trilobites their name, not the latitudinal division into cephalon, thorax, and pygidium.
Trilobites also had antennae that perhaps were used for taste and smell.
The tracks left behind by trilobites crawling on the sea floor are occasionally preserved as trace fossils.
The eyes of trilobites were made of calcite (calcium carbonate, CaCO3).
Within the marine paleoenvironment, trilobites were found in a broad range, from extremely shallow water to very deep water.
The closest extant (living) relatives of trilobites may be the cephalocarids, minute marine crustaceans (Lambert 1985) and the horseshoe crabs.
Some trilobites, such as those of the order Lichida, evolved elaborate spiny forms from the Ordovician period (488-444 mya) until the end of the Devonian period (416-359 mya).
During molting, the exoskeleton generally split between the head and thorax, which is why so many trilobite fossils are missing one or the other: Many trilobite fossils are actually molted exoskeletons rather than dead trilobites.
Based on morphological similarities, it is possible that the trilobites have their ancestors in arthropod-like creatures such as Spriggina, Parvancorina, and other trilobitomorphs of the Ediacaran period of the Precambrian.
Chalcedony is a generic term for cryptocrystalline quartz.
Trilobites, like brachiopods, crinoids, and corals, are found on all modern continents, and occupied every ancient ocean from which fossils have been collected.
Trilobites range in length from one millimeter to 72 cm (1/25 inch to 28 inches), with a typical size range of two to seven centimeters (1 to 3Ѕ inches).
Trilobites may have provided a rich source of food for these new arrivals.
Trilobites appear to have been exclusively marine organisms, since the fossilized remains of trilobites are always found in rocks containing fossils of other salt-water animals, such as brachiopods, crinoids, and corals.
The trilobites are considered to be the first animals to have evolved true eyes.