Trilobita (trilobites; phylum Arthropoda) The most primitive arthropod class, known from more than 3900 fossil species. Inhabitants of Palaeozoic seas, the trilobites appeared first in the early Cambrian, had their widest distribution and greatest diversity in the Cambrian and Ordovician Periods, and became extinct in the Permian. The body was divided into three regions: an anterior cephalon, comprising at least five, fused segments; a mid-body or thorax, with a varying number of segments; and a hind region or pygidium. All three regions were divided by a pair of furrows running the length of the body, giving a trilobite appearance (i.e. a median or axial lobe, flanked on either side by a lateral lobe). The mouth was situated in the middle of the central surface of the cephalon. Paired gill-bearing limbs were attached to the membranaceous, pleural skeleton. X-ray studies show the eyes to have resembled the compound eyes of living arthropods (see TRILOBITE EYE). Trilobites ranged in size from 0.5mm long planktonic (see PLANKTON) forms to those nearly 1m in length; most species were 3–10 cm long. There were 9 orders: Redlichiida; Agnostida; Naraoiidae; Corynexochida; Lichida; Phacopida; Ptychopariida; Asaphida; and Proetida.

Trilobita (trilobites; phylum Arthropoda) The most primitive arthropod class (or in some classifications a phylum, where the Arthropoda rank as a superphylum), known from more than 3900 fossil species. Inhabitants of Palaeozoic seas, the trilobites appeared first in the early Cambrian, had their widest distribution and greatest diversity in the Cambrian and Ordovician Periods, and became extinct in the Permian. The body was divided into three regions: an anterior cephalon, comprising at least five, fused segments; a mid-body (thorax), with a varying number of segments; and a hind region (pygidium). All three regions were divided by a pair of furrows running the length of the body, giving a trilobite appearance (i.e. a median or axial lobe, flanked on either side by a lateral lobe). The mouth was situated in the middle of the central surface of the cephalon. Paired gill-bearing limbs were attached to the membranous, pleural skeleton. X-ray studies show the eyes to have resembled the compound eyes of living arthropods. Trilobites ranged in size from 0.5 mm-long planktonic forms to those nearly 1 m in length; most species were 3–10 cm long.

trilobite An extinct marine arthropod belonging to the class Trilobita (some 4000 species), fossils of which are found in deposits dating from the Precambrian to the Permian period (590–280 million years ago). Trilobites were typically small (1–7 cm long); the oval flattened body comprised a head (covered by a semicircular dorsal shield) and a thorax and abdomen, which were protected by overlapping dorsal plates with a raised central part and flattened lateral portions, presenting a three-lobed appearance. The head bore a pair of antenna-like appendages and a pair of compound eyes; nearly all body segments bore a pair of Y-shaped (biramous) appendages – one branch for locomotion and the other fringed for respiratory exchange. Trilobites were bottom-dwelling scavengers.

trilobite Any of an extinct group of arthropods found as fossils in marine deposits, dating from the Cambrian to the Permian period. The body was oval, tapering towards the rear, and covered by a chitinous skeleton. Transverse divisions show segmentation, with each segment bearing a pair of jointed limbs. Most species were bottom-crawling, shallow-water forms, and ranged in size from 6mm (0.25in) to 75cm (30in).

tri·lo·bite / ˈtrīləˌbīt/ • n. an extinct marine arthropod (subphylum Trilobita) that occurred abundantly during the Paleozoic era, with a carapace over the forepart, and a segmented hindpart divided longitudinally into three lobes.