Data from: Absolute axial growth and trunk segmentation in the early Cambrian trilobite Oryctocarella duyunensis

A short stratigraphic interval near Bulin in western Hunan (China) yields multiple specimens of the ~514 Ma old oryctocarine trilobite Oryctocarella duyunensis. Size data obtained from these specimens indicates that, from meraspid degree 1 onwards, degrees represent successive instars. Meraspid growth persisted until a terminal stage was reached, providing the first example of determinate growth in trilobites and, notably, in an early Cambrian species. The sample contains three varieties of such terminal stages, recognized as holaspids, with 9, 10, or 11 thoracic segments respectively. During the meraspid phase growth rates were not constant in this species. The pattern of growth seen in the Bulin assemblage differs modestly from that reported in the same species from two other localities, attesting to microevolutionary variation in developmental patterns among these collections.

中国湖南西部布林附近的一处短地层间隔产出了约514 Ma（百万年）前的掘头虫亚科（oryctocarine）三叶虫（trilobite）都匀掘头形虫（Oryctocarella duyunensis）的多件标本。对这些标本的尺寸数据分析显示，自甲期（meraspid）1期起，甲期分期依次对应连续的龄期（instars）。甲期生长持续直至达到终末阶段，这为三叶虫的确定性生长（determinate growth）提供了首个实例，尤为值得关注的是，该物种作为寒武纪早期类群，亦是此类生长模式的首个发现案例。该标本群包含三类此类终末阶段个体，被鉴定为成体期（holaspids），其胸节（thoracic segments）数量分别为9、10或11节。该物种在甲期阶段的生长速率并非恒定。布林标本群所呈现的生长模式，与另外两处产地报道的同种物种生长模式存在细微差异，这证实了这些标本群在发育模式上存在微进化（microevolutionary）变异。