Early Cambrian – Middle Permian
Trilobita is an extinct class of mobile, primarily benthic carnivores, and detritivores.
Characteristics of the Class
- Body divided into three lobes. The lobes run along the length of the body.
- Composed of a cephalon (head shield), thorax, and a pygidium (tail shield).
- Sizes range from 2 mm to 70 cm, with an average between 2 and 10 cm.
- Exoskeleton is shed via ecdysial sutures within and around the cephalon. Therefore, many specimens lack the outer part (flexigenae) of the cephalon.
- Thorax divided into segments which run the width of the body. 2 – 40 or more segments.
- Compound eyes present.
- Highly distinctive but variable shape.
Treatise on Invertebrate Paleonotology (1959):
- Marine arthropods characterized by a generally subelliptical, arched or flat dorsal exoskeleton of mineralized chitinous composition, divided longitudinally into three distinct parts (lobes) and with a distinct, relatively large head shield (cephalon), which articulates axially with the thorax composed of articulated transverse segments, the hindmost almost invariably articulating with a tail shield (pygidium) formed by fusion of segments like those of the thorax. Margins of exoskeleton may bend inward ventrally to form a doublure. Length of average adults commonly 2 to 10 cm. but extreme range extending from approximately 2 mm. to a known maximum of 70 cm. Cephalon typically marked by a somewhat raised axial portion (glabella) that is bounded by a narrow furrow and superficially indented by transverse depressions (glabellar furrows); compound eyes present in most forms, located along lines (facial sutures) of ecdysial cleavage that divide the cephalon into a central portion (cranidium, comprising glabella and fixigenae) and lateral portions (librigenae), which tend to become dissociated. Cephalic doublure may include separate median plates such as hypostoma and also a rostral plate that exceptionally may encroach upon dorsal side. Thoracic segments 2 to 40 or more, each commonly with strongly defined axial portion and somewhat flattened and grooved lateral portions (pleurae). Pygidium highly variable in shape and size. Lateral and dorsal hollow spines may be present on cephalon, thorax, or pygidium. Ventral appendages (rarely preserved) include an anterior pair of uniramous antennae followed by a series of pairs of similarly constructed biramous limbs decreasing in size toward posterior end and distributed with one pair to each segment; in one genus (
- ) the posterior segment possesses a pair of antenniform cerci. Trilobite remains usually are found dissociated into their component skeletal elements.