The filter-feeding organ of some extinct brachiopods is supported by a skeletal apparatus called the brachidium. Although relatively well studied in Atrypida and Athyridida, the brachidial morphology is usually neglected in Spiriferida. To investigate the variations of brachidial morphology in Spiriferida, 65 species belonging to eight superfamilies were analyzed. Based on the presence/absence of the jugal processes and normal/modified primary lamellae of the spiralia, four types of brachidium are recognized. Type-I (with jugal processes) and Type-II (without jugal processes), both having normal primary lamellae, could give rise to each other by losing/re-evolving the jugal processes. Type-III, without jugal processes, originated from Type-II through evolution of the modified lateral-convex primary lamellae, and it subsequently gave rise to Type-IV by evolving the modified medial-convex primary lamellae. The evolution of brachidia within individual evolutionary lineages must be clarified because two or more types can be present within a single family. Type-III and Type-IV are closely associated with the prolongation of the crura, representing innovative modifications of the feeding apparatus in response to possible shift in the position of the mouth towards the anterior, allowing for more efficient feeding on particles entering the mantle cavity from the anterior gape. Meanwhile, the modified primary lamellae adjusted/regulated the feeding currents. The absence of spires in some taxa with Type-IV brachidium might suggest that they developed a similar lophophore to that in some extant brachiopods, which can extend out of the shell.

中文翻译：

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Spiriferida (Brachiopoda) 的臂部支撑结构

一些已灭绝的腕足类动物的滤食器官由一种称为腕的骨骼装置支撑。尽管在 Atrypida 和 Athyridida 中进行了相对较好的研究，但在 Spiriferida 中通常忽略了臂的形态。为了研究 Spiriferida 腕臂形态的变化，分析了属于 8 个超科的 65 个物种。基于有无颧突和螺旋体的正常/修饰的初级薄片，识别出四种类型的臂。I型（有颏突）和II型（没有颏突）都具有正常的初级薄片，可以通过丢失/重新进化颏突而相互产生。III型，没有颏突，起源于II型，通过改良的侧凸初级薄片的进化，它随后通过进化改良的内凸初级薄片产生了 IV 型。必须澄清个体进化谱系中臂的进化，因为在一个家族中可以存在两种或多种类型。III 型和 IV 型与脚的延长密切相关，代表对进料装置的创新修改，以响应嘴部位置可能向前移动，从而可以更有效地对进入地幔腔的颗粒进料从前面的缝隙。同时，修改后的初级薄片调节/调节馈电电流。某些 IV 型腕足类动物中没有尖顶可能表明它们发育出与某些现存腕足类动物相似的带骨，可以延伸到壳外。