This study presents a case in trunk regionalization and functional morphology of redlichiid trilobites, based on a globally distributed Bathynotus, collected from the base of the Kaili Formation in the late Cambrian Series 2 of China. This micropygous trilobite processed a body pattern with four types of thoracic tergites within two batches at maturity: a homonymous morphotype with similar thoracic tergite structure throughout the anterior portion of the thorax, and a heteronomous morphotype in the posterior portion of the thorax, with a pair of macropleurae in the 11th thoracic tergite and two pairs of tiny pleurae (micropleurae) in the 12th and 13th thoracic tergites. The unique morphology by regionalization in Bathynotusis considered closely related to its articulation structures and the ability to perform encapsulated enrolment, permitting the posterior trunk tergites with long pleural spines to fully cover the ventral cephalic shield during dorsal flexure. The regional integration of trunk tergites into structural regions provides the opportunity to trace the origin of their functional morphology and behavior in the evolutionary process of trilobite caudalization.