Trilobites are a group of extinct marine arachnomorph arthropods that form the class Trilobita. Trilobites form one of the earliest-known groups of arthropods. The first appearance of trilobites in the fossil record defines the base of the Atdabanian stage of the Early Cambrian period, and they flourished throughout the lower Paleozoic before slipping into a long decline, when, during the Devonian, all trilobite orders except the Proetids died out. The last extant trilobites finally disappeared in the mass extinction at the end of the Permian about 252 million years ago. Trilobites were among the most successful of all early animals, existing in oceans for almost 300 million years.
Redlichiida is an order of trilobites, a group of extinct marine arthropods. Species assigned to the order Redlichiida are among the first trilobites to appear in the fossil record, about halfway during the Lower Cambrian. Due to the difficulty to relate sediments in different areas, there remains some discussion, but among the earliest are Fallotaspis, and Lemdadella, both belonging to this order. The first representatives of the orders Corynexochida and Ptychopariida also appear very early on and may prove to be even earlier than any redlichiid species. In terms of anatomical comparison, the earliest redlichiid species are probably ancestral to all other trilobite orders and share many primitive characters. The last redlichiid trilobites died out before the end of the Middle Cambrian.
Phacopidae is a family of phacophid trilobites that ranges from the Lower Ordovician to the Upper Devonian, with representatives in all paleocontinents.
Paradoxides is a genus of large to very large trilobites found throughout the world during the Mid Cambrian period. One record-breaking specimen of Paradoxides davidis is 37 cm (15 in). It has a semicircular head, free cheeks each ending with a long, narrow, recurved spine, and sickle-shaped eyes, providing almost 360° view, but only in the horizontal plane. Its elongated trunk was composed of 19-21 segments and was adorned with longish, recurved lateral spines. Its pygidium was comparatively small. Paradoxides is a characteristic Middle Cambrian trilobite of the 'Atlantic' (Avalonian) fauna. Avalonian rocks were deposited near a small continent called Avalonia in the Paleozoic Iapetus Ocean. Avalonian beds are now in a narrow strip along the East Coast of North America, and in Europe.
Olenellina is a suborder of the order Redlichiida of Trilobites that occurs about halfway during the Lower Cambrian, at the start of the stage called the Atdabanian. The earliest trilobites in the fossil record are arguably Olenellina, although the earliest Redlichiina and Eodiscina follow quickly. The suborder died out when the Lower passed into the Middle Cambrian, at the end of the stage called Toyonian. A feature uniting the Olenellina is the lack of rupture lines in the headshield, which in other trilobites assist the periodic moulting, associated with arthropod growth. Some derived trilobites have lost facial sutures again, but all of these are blind, while all Olenellina have eyes.
Redlichiina is a suborder of the order Redlichiida of Trilobites. The suborder contains three superfamilies: Emuelloidea, Redlichioidea and Paradoxidoidea. These trilobites are some of the oldest trilobites known. They originated at the beginning of the Cambrian Period and disappeared at the end of the middle Cambrian.
Dikelocephalus is a genus of very large trilobites of up to 50 cm (20 in) long, that lived during the last 3 million years of the Cambrian (Sunwaptan). Their fossils are commonly found as disarticulated sclerites, in the upper Mississippi Valley and in Canada (Alberta). The exoskeleton is rounded anteriorly, with the thorax and sides of the tailshield slightly tapering to about ⅔× of the width across the base of the spines at the back of the headshield. At the side corners of the pygidium there may be triangular or hooked spines, pointing backwards, while between the spines the posterior margin is at a 30-75° angle with the lateral margin, gently convex or nearly straight. If pygidial spines are lacking, the margin is gradually rounded. The thorax has 12 segments.
Yunnanocephalus is a genus of ptychopariid trilobite. It lived during the late Atdabanian and Botomian stages, in what are currently Antarctica, Australia and China. It was a "moderately common" member of the Chengjiang Fauna. Yunnanocephalus is the only genus currently assigned to the Yunnanocephalidae family.
Tsunyidiscus is a genus of eodiscinid agnostid trilobites. Tsunyidiscus occurs near the end of the Lower Cambrian, during the late Atdabanian stage period and some collections suggest it may have survived into the Botomian. They are very small, have eyes, and equal sized head and tail shields, with a narrow dome-shaped glabella and a narrow bullet-shape pygidial axis. It has a thorax of three segments. Tsunyidiscus is the only genus currently attributed to the family Tsunyidiscidae.
Ellipsocephalus is an extinct genus of blind trilobite that lived during the Cambrian. The genus comprises benthic species inhabiting deep, poorly lit or aphotic habitats. E. hoffi is a common trilobite mainly from central Europe.
Fallotaspis is a genus of redlichiid trilobite genus found in Early Cambrian-aged strata of the United States and Morocco.
Phytophilaspis is a phosphatized genus of trilobite-like arthropod with eyes, found in association with algal remains. It dwelt in well-lit, shallow waters.
The cephalon is the head section of an arthropod. It is a tagma, i.e., a specialized grouping of arthropod segments. The word cephalon derives from the Greek κεφαλή (kephalē), meaning "head".
Burlingia is a rare and diminutive genus of trilobite, that lived during the early to middle Middle Cambrian. Species assigned to Burlingia have been found in Norway, Sweden, Northern Siberia, and South-eastern China.
Mesolenellus is an extinct genus of trilobites that lived during the lower Cambrian (Botomian), found in Greenland and Spitsbergen.
Eodiscina is a suborder of trilobite arthropods. The Eodiscina first developed near the end of the Lower Cambrian period and became extinct at the end of the Middle Cambrian. Species are tiny to small, and have a thorax of two or three segments. Eodiscina includes six families classified under one superfamily, Eodiscoidea.
Thoracocare is a minute to very small trilobite, that lived during part of the Middle Cambrian in what are today the states of Idaho, Nevada and Utah. It is the only trilobite known with just two thorax segments outside most members of the Agnostida order. It can be distinguished from Agnostida by the very wide subquadrate glabella, parallel-side or widening forward in the largest specimen, with the full front side touching the border. Two species are known, one, T. idahoensis, only from pygidia.
Schmalenseeia is genus of trilobites of uncertain affinity, that lived during the middle Middle to earliest Upper Cambrian. Species assigned to Schmalenseeia have been found in Norway, Sweden, Northern Siberia, Eastern China, Australia (Tasmania), India (Himalayas) and the United Kingdom.
Placoparia is a genus of trilobites of average size that lived during the late Lower to the early Upper Ordovician on the paleocontinents Gondwana, Avalonia and Laurentia, now the Czech Republic, France, Germany, Morocco, Portugal, Spain and Wales. Its headshield is semi-circular to rectangular with rounded frontal corners. It lacks eyes, but eye ridges are present. The fact that the facial sutures are opisthoparian is an exception in the otherwise proparian Cheirurina. The thorax has 11 or 12 segments, with the axis slightly wider than the ribs to its sides. The tips of the pleurae are free, which resembles an old-fashion central heating radiator. The axis in the small tailshield consists of four rings and a minute endpiece. The four pleurae end in spatulate spines that fit to corresponding indentations in the cephalon.
Viaphacops is a genus of trilobites in the order Phacopida, family Phacopidae, that lived during the Middle Devonian, and is known from North and South America, Asia.
