This is the section of the exoskeleton where separate tergites are articulated with each other to enable flexibility and enrollment. The number of thoracic segments varies between two and as many as forty-two, most frequently in the range of eight to fifteen. Each tergite consists of a center part—the axial ring—and two adjacent pleurae. These may terminate bluntly or may arch downward and backward into pleural spines of varied length. The pleurae are commonly sulcated by a pleuralfurrow, which may have served the function of strengthening the tergite. The articulation of one tergite with the other occurs through an anterior extension of the axial ring, which is inserted beneath the posterior margin of the next tergite. The interlocking mechanism between segments and the extent of rotation allowed evolved considerably between the earlier trilobite forms and more advanced ones. This function obviously determines the mode and capability of enrollment, which will be discussed in section 3.4.
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